Self‐assembled monolayers (SAMs) in inverted perovskite solar cells and their tandem photovoltaics application

Self‐assembled monolayers (SAMs) employed in inverted perovskite solar cells (PSCs) have achieved groundbreaking progress in device efficiency and stability for both single‐junction and tandem configurations, owing to their distinctive and versatile ability to manipulate chemical and physical interface properties. In this regard, we present a comprehensive review of recent research advancements concerning SAMs in inverted perovskite single‐junction and tandem solar cells, where the prevailing challenges and future development prospects in the applications of SAMs are emphasized. We thoroughly examine the mechanistic roles of diverse SAMs in energy‐level regulation, interface modification, defect passivation, and charge transportation. This is achieved by understanding how interfacial molecular interactions can be finely tuned to mitigate charge recombination losses in inverted PSCs. Through this comprehensive review, we aim to provide valuable insights and references for further investigation and utilization of SAMs in inverted perovskite single‐junction and tandem solar cells.

During the past decade, the research of inverted PSCs has made rapid progress.Currently, inverted PSCs have achieved a record PCE exceeding 26%, suggesting the extraordinary potential for practical applications. [36][43][44][45][46][47] The main roles of SAMs in inverted PSCs are interfacial modifications to inhibit the corresponding defects and as HTL materials for holeselective contact. [48]The introduction of SAMs has sparked a revolutionary advancement for inverted PSCs deployed as single-junction devices, as well as for tandem PVs (Figure 1C). [49,50]Currently, SAMs have exhibited remarkable achievements in the application of inverted perovskite tandem devices, with corresponding tandem devices achieving a PCE of 32.5%. [51,52]Whether in inverted perovskite single-junction or tandem solar cells, the positive effects of SAMs are deserving of profound attention and comprehensive analysis.Until now, there are few reports to systematically dissect the functional role and working mechanism of SAMs in inverted PSCs.In this review, we summarize the noteworthy achievements of the use of SAMs for inverted PSCs in recent years, especially in terms of charge transport, interface modification, energy-level matching, and so forth, and provide a brief outlook on the future directions of SAMs for achieving highly efficient and stable inverted PSCs.

| SELF-ASSEMBLED MONOLAYERS 2.1 | The molecular structure of SAMs
SAMs are highly ordered organic molecules capable of spontaneous deposition on the surface of a substrate by molecular composition in the vapor or liquid phase. [53]he formed SAMs will have intermolecular interactions on the surface of the substrate, and the strength of these interactions mainly depends on the structural composition of the SAMs and the chemical properties of the substrate.Generally, the SAMs are composed of anchoring, spacer, and terminal functional groups, with each group playing separate roles based on their chemical composition (Figure 2A).The anchoring groups of SAMs adsorb to the substrate surface and mainly influence the electronic state of the substrate surface via chemical bonding.Common anchoring groups used are silanes, carbonyl acid, and phosphate groups. [41]The presence of anchoring groups facilitates the molecular interaction of SAMs with the substrate surface and has a critical influence on parameters such as the work function (WF) and surface contact resistance.The strength of the intermolecular interaction between SAMs and the substrate surface mainly depends on the electron affinity of the anchoring groups and the chemical properties of the interface.The spacer groups function as the backbone of SAM molecules by linking the anchoring and terminal functional groups together.They are commonly composed of alkyl chains.The length of the alkyl chains of SAM molecules determines the strength of the interchain van der Waals interactions, crucial for the synthesis of highly ordered supramolecular structures.The length of the branched chain of the spacer group has a corresponding effect on the geometry and surface coverage of the formed SAMs. [54]This branched chain design of the spacer group is also beneficial for adjusting the charge transport properties of SAMs, especially for the interfacial charge extraction and transport in inverted PSCs. [54]More common terminal functional groups that have been reported include amino, sulfhydryl, and carbonyl groups. [55]For SAMs, the terminal functional groups are mainly responsible for the interfacial chemistry with the material on its subsequent interface.Therefore, SAMs have an important influence on the preparation of subsequent materials such as the perovskite layer and its corresponding morphology.In summary, the molecular structure of SAMs presents a wide scope for design due to its versatile properties for the interfacial modulation of inverted PSCs. [56]2 | The anchoring mechanism of SAMs

| Anchoring groups
The effective adsorption of SAMs is highly correlated with the chemical properties of the anchoring group and the substrate surface.The different types of groups induce different binding modes on the surface of the target substrate.Currently, some of the most widely anchoring groups used in the field of optoelectronics are silane, carboxylic acid, and phosphate. [53]The adsorption of SAMs with silane anchoring groups generally undergoes three stages, starting with the hydrolysis reaction of the organosilane SAM (RSiX 3 ; X = Cl, OCH 3 ) on the metal oxide surface and the generation of the corresponding hydroxysilane. [41]Subsequently, the formed hydroxysilanes undergo aggregation and orderly arrangement on the oxide surface of the substrate.Finally, a condensation reaction occurs between the ─OH groups to form Si─O─M (M: metal) and Si─O─Si covalent bonds, which creates a cross-linked network for the SAMs to adhere to the substrate surface (Figure 2B).Furthermore, the carboxylic acid groups, which possess both a hydroxyl group and an oxygen atom, exhibit the capability to form C─O─M covalent bonds on the metal oxide surface through monodentate binding.Alternatively, bidentate coordination can be achieved by transferring hydrogen atoms to the surface hydroxyl group (Figure 2C).Similarly, the phosphate groups comprising two hydroxyl groups and an additional oxygen atom can achieve mono-, bi-, and tridentate binding on the substrate surface.The phosphate group can be attached to the metal oxide by forming a strong P─O─M covalent bond, where the presence of two hydroxyl groups further facilitates coordination with different metal atoms (Figure 2D). [57]For tridentate binding, the bonding coordination is mainly achieved by hydrogen atoms being transferred to the surface hydroxyl groups of the substrate and generating oxygen atom vacancies in the corresponding SAMs.Thus, the anchoring of SAMs to the substrate surface is highly dependent on the effective conduct of the condensation reaction.

| Tailoring alkyl linkers
The length of the space linking group is also an important factor in determining the ordered arrangement of the SAMs.Longer alkyl chains generally lead to denser molecular packing and better ordering of SAMs due to their stronger van der Waals interactions.The strength of intermolecular van der Waals interactions refers to the intensity of interactions occurring between molecules through van der Waals forces, which arise due to the transient, induced, and dispersion-induced polarizations between molecules.In organic molecules, the length of alkyl chains can influence the strength of these interactions. [58,59]However, for lengthy branched chains, the molecular interactions between the SAMs and substrate surface are weakened.Liu and Miao found that with an increase in the number of carbon atoms of n-alkylphosphonic acid from 6 to 18, the ordering of SAMs on the substrate surface changes from a disorderly to an orderly arrangement.In comparison with SAMs such as n-octylphosphonic acid, n-decylphosphonic acid, n-tetradecylphosphonic acid, and octadecylphosphonic acid, the n-dodecyl pentacene organic semiconductor prepared on the surface of phosphonic acid exhibits higher carrier mobility due to the uniform surface formed by the SAM material. [44]The connection branches are determined by the effective ordering of SAM, which correspondingly affects the film quality and charge transport of perovskite films.The rational design of branch chains is also an important direction in the research of inverted PSCs. [44]

| Terminal functional groups
The electrical characteristics such as surface energy are modified by adjusting the terminal functional groups of the SAMs.The strength of the surface energy greatly restricts the growth mechanism of the organic SAM molecules on the substrate surface.In general, a higher surface energy would correspond to the SAMs forming via a two-dimensional (2D) layer-by-layer growth mode due to the strong interaction between the substrate and the SAMs.In contrast, lower surface energies would prompt the SAMs to exhibit a three-dimensional (3D) growth pattern originating from the stronger intermolecular interactions. [53]In the realm of inverted PSCs, the 3D expansion of SAMs proves counterproductive to the initiation and crystallization processes of perovskite films, significantly impeding the transport of charge carriers.Consequently, the preferred growth mechanism is the 2D layer-by-layer approach.This mode not only facilitates the systematic arrangement of SAMs but also fosters favorable charge transport dynamics within a perovskite device.Therefore, the cultivation of SAMs in a 2D model emerges as a critical determinant of PV performance in inverted PSCs.To explore the effect of surface energy on SAMs, Liu and Miao measured the surface energy of different terminal functional groups, such as 1H,1H,2H,2H-perfluorodecyltrichlorosilane (0.0186 N m −1 ), (3-aminopropyl)trimethoxysilane (0.0514 N m −1 ), phenyltrichlorosilane (PTS, 0.0452 N m −1 ), phenylhexyltrichlorosilane (0.0329 N m −1 ), and hexamethyldisilazane (0.0318 N m −1 ).Studies have shown that preparing 6,13-bis((triisopropylsilyl) ethynyl)pentacene (TIPS-PEN) semiconductor film on PTS would result in obtaining the best crystal structure in comparison with other films.This is attributed to the matching surface energy from the combination of PTS and TIPS-PEN. [44]

| Deposition methods of SAMs
The current techniques for the preparation of SAMs are mainly differentiated between vapor-phase and liquidphase deposition.The vapor-phase deposition method involves exposing the substrate surface to a vapor atmosphere of SAMs and enhancing the effective and rapid adsorption of SAM molecules onto the substrate surface by thermal annealing (Figure 3A). [53]The key advantage of this method is that no additional organic solvent is required for the preparation of the SAMs.For the liquid-phase deposition process, it can be further subdivided into either immersion or spin-coating methods.The dip-coating method involves immersing the target substrate in a solution of SAMs and allowing the uniform and controlled deposition of SAM molecules by adjusting the solvent, the solution concentration, and the immersion time (Figure 3B).Notably, precise regulation of the solution concentration of SAMs and the immersion time is crucial for achieving optimal SAM deposition.A low concentration of SAM solution results in a longer immersion time. [41]However, the use of highconcentration SAM solutions results in cross-linking of end groups.Thus, reducing the ordered molecular assembly and surface reaction.Therefore, optimization of the solution concentration and immersion time is essential for the formation of highly ordered SAMs.The two main drawbacks of this solution-assisted growth method of SAMs are that it is time-consuming and toxic solvents are frequently used on the surfaces of oxide substrates.To deposit SAMs using the dip-coating technique, the substrate must be immersed in the SAMs solution for a duration ranging from 5 to 12 h to ensure thorough coverage.However, it is worth noting that this method involves a time-consuming process and necessitates the use of more toxic solvents, including isopropyl alcohol, when compared with the spin-coating method. [60]For the spin-coating method, SAM solutions are deposited based on adjusting the speed of solution deposition on the substrate surface followed by a heat treatment to remove the additional solvent, resulting in a fast and uniform SAM preparation (Figure 3C).The spin coating of SAMs requires evaluation of the strength of the SAMs-substrate interaction to ensure that the SAM molecules are effectively anchored to the substrate surface.The postannealing treatment is employed to enhance the bonding between the SAMs and the substrate, facilitating the removal of excessive nonbonded molecules from the layer.The thermal treatment of the formed SAMs is necessary for these techniques, and the selection of a suitable annealing temperature is equally critical for the rate of formation and ordered assembly of SAMs. [55]o compete effectively with traditional solar cells, it is crucial to minimize both the materials and manufacturing costs associated with PSCs.While the perovskite layer in the functional stack of PSCs is already cost-effective, certain charge transport layers remain economically challenging due to intricate synthesis and purification processes.The adoption of HTLs based on SAMs significantly reduces materials consumption compared with traditional thin-film HTLs (Figure 4A).Specifically, while approximately 100 g of organic HTLs is required per square meter for a device, only a few grams (1-5) of SAMs are sufficient for a device of the same size. [48]Moreover, the molecular structures of the reported high-performance SAMs are notably straightforward, enabling straightforward synthesis in most cases without the need for costly metal catalysts or complex reaction conditions.Consequently, the integration of SAMbased HTLs in PSCs holds significant promise for reducing material costs, primarily due to the decreased layer thickness.Beyond the reduction in material consumption and cost, the exceptionally low thickness of SAM-based HTLs offers advantages in minimizing losses associated with the optical and electrical performance of PSCs (Figure 4B). [48]SAMs typically exhibit thicknesses in the nanometer scale, specifically ranging from a few nanometers to several tens of nanometers. [61]Due to their monolayer thickness, SAMs exhibit minimal parasitic absorption, outperforming traditional HTLs, such as nickel oxide and PEDOT:PSS (Figure 4C).The slim profile of SAM-based HTLs enables effective mitigation of resistive losses within the device.Compared with conventional inorganic compounds and polymers (e.g., NiO, PTAA, and PEDOT:PSS), SAM molecules provide a convenient avenue for precise molecular engineering to fine-tune energy levels. [48]Additionally, the interface dipole formed by SAM layers serves as a versatile tool for controlling the WF of transparent conductive oxide and adjusting the energy-level alignment at the interface in PSCs.The robust chemical anchoring groups inherent in SAM molecular structures empower them to achieve conformal coatings on various rough substrate surfaces.These advantageous characteristics of SAMs position them as highly favorable for constructing efficient and stable single-junction and tandem PSCs.

| Challenges of SAM coating
Currently, the majority of SAM-based perovskites still grapple with wettability issues, often resulting in subpar reproducibility and significant batch-to-batch variations. [62]The formation of SAMs poses significant challenges, primarily stemming from the intricate task of achieving high-density, tightly packed configurations that often lead to the instability of interfacial characteristics. [63]Even on ostensibly flat substrates, the uniformity The deposition methods of SAMs, (A) vapor deposition, (B) dip coating, and (C) spin coating.Reproduced with permission. [41]Copyright 2020, Wiley-VCH.SAM, self-assembled monolayer.
of SAM formation is frequently compromised due to the limited solubility of SAMs and their inadequate chemical bonding affinity with metal oxides.Various strategic approaches have been proposed to address these challenges, such as introducing a secondary component in the precursor solution or carefully combining diverse SAMs to systematically tackle the inherent wetting challenges associated with SAMs. [62]A promising avenue for improving the deposition of perovskite on underlayer involves the strategic deployment of amphiphilic SAMs, engineered to provide a superwetting foundational stratum. [64]Furthermore, the judicious application of plasma treatment serves as a potent modulator, effectively tailoring the surface morphology of substrate and thereby facilitating the seamless, conformal growth of SAMs. [65]

| APPLICATION OF SAMs IN INVERTED SINGLE-JUNCTION PSCs
The application of inverted PSCs can be divided into perovskite single-junction and tandem solar cells.The following content mainly introduces the application achievements of SAMs in inverted single-junction PSCs.

| SAMs as HTLs
Currently, the commonly used HTLs (PTAA, PED-OT:PSS, and NiO x ) in inverted PSCs are mostly prepared by the spin-coating method. [1,33]This fabrication method may not be suitable for low-cost and large-scale technology due to its high material waste percentage. [66,67]Research by Warby et al. revealed that the thickness of the HTL film is closely related to the PV performance of inverted PSCs. [68]Having a low HTL film thickness is advantageous in reducing the internal resistance of the device, ultimately improving the fill factor (FF) of the device.However, an excessively thin HTL film may result in insufficient coverage on the surface of indium tin oxide (ITO), leading to pronounced interfacial recombination and a weakened open-circuit voltage (V OC ) in inverted PSCs.To address this challenge, the utilization of SAMs on the surface of ITO as HTLs offers several benefits.SAMs ensure the formation of a uniform film with the least possible thickness, minimizing material wastage.This approach enables precise control over HTL film formation while mitigating the drawbacks associated with inadequate film coverage.As a result, SAMs as HTLs provide a solution to achieve uniform film and efficient charge transport, optimizing the performance of inverted PSCs.The following section specifically introduces the research progress of various SAMs as HTLs.

| Carboxylic acid-based SAMs
Arkan et al. designed five new SAM molecules with different electron-withdrawing and electron-donating functional groups to modify the surface of ITO and adjust the WF of ITO (Figure 5A,B). [69]These five SAM molecules all contained a ─COOH anchor group to realize self-assembly on the surface of ITO through chemical adsorption.Among all the SAMs, the device based on EA-49 modification exhibited the highest PCE of 12.03% attributed to the lower energy barrier and defect density.Yalcin et al. investigated two different organic molecules (triphenylamine [TPA] and MC-43) with efficient hole transport characteristics as SAMs to replace the most common p-type semiconductor PED-OT:PSS (Figure 5C,D). [70]These two SAMs were mainly synthesized through a series of cross-coupling reactions.Subsequently, the ITO substrates were immersed in a solution containing these molecules for several hours to induce the effective adsorption of TPA and MC-43 molecules on the surface of ITO and the formation of dense and ordered monolayer films.The facile deposition method and the strong interaction between these SAM molecules and ITO provide a clear advantage in ensuring superior interfacial contact.Cyclic voltammetry (CV) was employed to evaluate the electrochemical behavior of TPA and MC-43 on the ITO substrate surface.The Highest Occupied Molecular Orbital (HOMO) of TPA and MC-43 were measured to be 5.33 and 5.11 eV, respectively, where the electron-donating group of the The chemical structure of various SAMs and (B) energy-level alignment with perovskite.Reproduced with permission. [69]Copyright 2020, Elsevier.The Chemical structure of (C) TPA and (D) MC-43.Reproduced with permission. [70]Copyright 2019, Royal Society of Chemistry.(E) The synthetic pathways of EADR03 and EADR04 SAMs and (F) the current density-voltage (J-V) curves of the device with EADR03 and EADR04 SAMs.Reproduced with permission. [71]Copyright 2021, Royal Society of Chemistry.TPA main chain presented in MC-43 lowered its oxidation potential resulting in a 0.2 eV difference of HOMO energy for the TPA level.Therefore, the device based on MC-43 exhibited better charge transport capability and energy-level matching compared with TPA.The champion device based on MC-43 achieved a PCE of 17.3%, a V OC of 1.07 V, a short-circuit current density (J SC ) of 20.3 mA cm −2 , and an FF of 80%.
The stability of inverted PSCs is closely related to the contact layer and its interface, and it is crucial to rationally design new molecules as hole-selective contacts.Aktas   5E). [71]The EADR03 and EADR04 were not suitable for spin coating due to solubility issues.However, immersion adsorption was more compatible with the properties of these materials.The common ─COOH anchoring group in the two SAMs effectively binds to the hydroxyl groups on the surface of ITO.The formation of ester bonds and the appearance of C─N bonds were further confirmed by X-ray photoelectron spectroscopy (XPS) spectroscopic analysis of SAMs anchored on the ITO surface.The better electron-blocking properties of the two SAMs compared with PTAA were attributed to the presence of carbazole units in the molecular backbone, which exhibits excellent hole-extracting functions.As a result, the best PCE of devices based on EADR03 and EADR04 were 21.2% and 20.6%, respectively (Figure 5F).
Chang et al. prepared a functionalized SAM poly[3(6carboxyhexyl)thiophene-2,5-diyl] (P3HT-COOH) to play the role of hole transport (Figure 6A). [72]The presence of the ─COOH anchoring group was beneficial to reduce the WF on the ITO surface, thereby passivating the surface defects of ITO and reducing the interfacial energy loss.The P3HT-COOH layer was prepared by spin-coating and immersion methods, and it was found that the immersion method was more conducive to the self-assembly of P3HT-COOH on the ITO surface.The immersion method was beneficial in promoting the free rotation of the carboxyl groups of P3HT-COOH in the liquid phase and directional anchoring on the ITO surface, thereby achieving a highly ordered and uniform arrangement of the P3HT-COOH molecule on the ITO surface.In addition, the prepared P3HT-COOH SAM was also conducive to the crystallization and preferential orientation growth of the top perovskite, thereby correspondingly reducing the defect density of the device and achieving a more efficient charge transport (Figure 6B).When measured, the device based on P3HT-COOH SAM exhibited a PCE of 20.74% with negligible hysteresis.
Liao et al. designed and synthesized a series of D-A-type SAMs, namely, MPA-BT-CA, MPA-BT-BA, and MPA-BT-RA. [73]In these SAMs, MPA represents 4-methoxy-N-(4methoxyphenyl)-N-phenylbenzenamine, BT denotes benzo [c][1,2,5]thiadiazole, CA stands for cyanoacrylic acid, BA represents benzoic acid, and RA represents rhodanine-3propionic acid (Figure 6C).Each of these SAMs exhibited distinct physicochemical properties based on the anchoring groups used.The incorporation of CA or RA groups resulted in a significant increase in the dipole moments of the SAMs compared with MPA-BT-BA molecules.It is demonstrated that the introduction of CA or RA groups had a pronounced effect on the polarity of the SAMs.Interestingly, the SAM MPA-BT-RA adopted a tilted selfassembly mode, unlike the upright self-assembly observed in MPA-BT-CA and MPA-BT-BA.The tilting behavior of MPA-BT-RA was attributed to the rotational freedom of the sp 3 hybridized carbon atom in the ─CH 2 ─ moiety of the RA group (Figure 6D).However, this unique assembly behavior resulted in a stronger tendency for aggregation and poorer film morphology, adversely affecting the growth of the perovskite active layer.In contrast, MPA-BT-CA, with the CA anchoring group, exhibited a stronger dipole moment and formed a uniform monolayer on the ITO surface through upright self-assembly.Due to its higher dipole moment and a denser uniform self-assembled film, the device based on MPA-BT-CA achieved a remarkable PCE of 21.81%.Zhang et al. conducted a study focused on enhancing the intrinsic optical and electrical stability of hole-selective contacts through the use of a series of conjugated SAMs. [74]In contrast to the alkyl linker-based SAMs commonly used, Zhang et al. employed conjugated linkers and intramolecular donor-acceptor (D-A) moieties to stabilize electron-rich aromatic amines through efficient electron and charge delocalization, as well as energy-level modulation.The conjugated molecular structures not only enhanced charge transport but also stabilized the electron-rich aromatic amines through electron/charge delocalization.Furthermore, it conveniently allowed for the modulation of frontier orbital energy levels for interface band alignment, combining the conjugation and intramolecular D-A characteristics that could alter electron delocalization and frontier molecular energy levels.Different aromatic amine units including carbazole and TPA were utilized, each possessing distinct electron-donating capabilities, to gradually adjust the HOMO energy level and match the valence band edge of metal halide perovskite.Additionally, the conjugation length was varied to further manipulate the self-assembly behavior of these SAMs.Six conjugated SAMs were designed and synthesized, denoted as Cz-CA, Cz-Ph-CA, TPA-CA, TPA-Ph-CA, MPA-CA, and MPA-Ph-CA (Figure 6E).Through exploration of the conjugated linkers and hole-transporting heads, the optimal conjugated SAM (MPA-Ph-CA) was combined with the standard three-cation perovskite Cs 0.05 (FA 0.92 MA 0.08 ) 0.95 Pb (I 0.92 Br 0.08 ) 3 , resulting in a highly efficient inverted PSC with a PCE of 22.53%.Furthermore, the devices based on MPA-Ph-CA exhibited excellent stability, retaining over 95% of the initial PCE after continuous irradiation for 800 h.

| Phosphoric acid-based SAMs
Previous studies have shown that molecules with phosphate groups facilitate the formation of dense and uniform monolayers via covalent linkages on various oxide surfaces.On the basis of this, Magomedov et al. designed a new molecule V1036 containing a phosphonic acid group and a hole transport segment (Figure 7A). [75]he V1036 material was designed to self-assemble on the ITO surface as a charge-selective contact.The synthesis procedure of V1036 was based on a series of reactions of two commercially available materials dibromomocarbazole and 1,2-dibromoethane to form V1036, and its molecular structure was further confirmed by 1 H and 13 C nuclear magnetic resonance spectroscopy.The formation of SAM was done by immersing the ITO substrate into a certain concentration of V1036 in an isopropanol solution for 20 h.Subsequently, the substrate was dried using nitrogen gas and annealed on a heating plate at 100°C for 1 h.The resulting V1036 SAM exhibited a measured thickness of approximately 1.5 nm and demonstrated excellent hole transport capability.Meanwhile, a small aliphatic molecule butylphosphonic acid (C4) was chosen to further improve the monolayer quality.The corresponding device based on V1036 SAM achieved a PCE of 17.8%, showing a promising direction by applying SAMs as HTLs in inverted PSCs.
The development and utilization of SAMs with carbazole functional groups and phosphonic acid anchoring groups have significantly improved the contact quality in inverted PSCs, leading to minimal interface recombination and enhanced power PCE for both singlejunction and tandem solar cells.However, the deposition of these materials has primarily relied on solution-based methods.Therefore, it is crucial to explore alternative and scalable deposition techniques, such as vacuum evaporation, to enhance process flexibility.Addressing this need, Farag et al. introduced a pioneering approach by employing physical vapor deposition via thermal evaporation to deposit well-known SAMs (2PACz, MeO-2PACz) and incorporated them into inverted PSCs (Figure 7B). [76]Through the comparison of characteristic binding energy peaks and molecular vibrational bands of the evaporated 2PACz with its solution-processed counterpart using XPS and infrared spectroscopy analysis, it was conclusively demonstrated that the evaporated 2PACz retained its chemical integrity.Notably, the evaporated 2PACz formed a monolayer at the ITO interface, independent of the final film thickness.Remarkably, the champion device utilizing the evaporated 2PACz exhibited performance on par with their solution-processed counterparts, showcasing a PCE of 19.5%, a J SC of 20.1 mA cm −2 , a V OC of 1.214 V, an FF of 80.1%, and a hysteresis index of 1.2% (Figure 7C).The advancement of a suitable manufacturing process for high-throughput production of PSCs necessitates the development of deposition techniques capable of largescale and rapid fabrication of thin films, which is a pivotal component of the actual commercialization process.Among the various methods available, inkjet printing has emerged as a widely employed highthroughput technique for PSC fabrication, which enables the fast deposition of carbazole-SAM HTLs.Cassella et al. pioneered the deposition of MeO-2PACz SAM using ultrasonic spray deposition combined with spin-coating of the MAPbI 3 layer, resulting in devices with an efficiency surpassing 20% (Figure 7D).Subsequently, a novel gas-assisted spray deposition (GASP) approach was devised to optimize the deposition process. [77]By carefully controlling the solid concentration in the precursor solution and introducing a delay time between precursor deposition and gas jet application, the GASP process achieved the deposition of highly crystalline and uniform MAPbI 3 perovskite films on hydrophobic MeO-2PACz SAM.Notably, devices with a stable efficiency of 20.8% were achieved by simultaneously spraying MeO-2PACz SAM and MAPbI 3 perovskite.This remarkable achievement represents the highest reported stable efficiency for spray-deposited PSCs to date.
Al-Ashouri et al. synthesized two new SAM materials, MeO-2PACz and 2PACz, as HTLs of inverted PSCs to achieve hole-selective contacts.Carbazole derivatives are an attractive future class of materials as hole-selective contacts stemming from the possibility of incorporating lossless interfaces (Figure 7E). [78]Previous studies have shown that organophosphonic acid (PA) in the studied anchor group has stronger binding energy compared with others, which is beneficial for strong interaction with the ITO substrate surface by forming stable chemical bonds. [78]These two SAMs can be prepared by spin coating with high reproducibility on the substrate surface.Reflection-absorption infrared spectra were collected to explore the interaction of two SAMs with ITO, confirming that the SAMs exhibited monolayer characteristics and were covalently linked between ITO.
Photoluminescence (PL) measurements revealed that SAMs as HTLs displayed better energy-level alignment with perovskite compared with PTAA, which significantly reduced the interfacial defect density and nonradiative recombination loss of the devices.The devices based on these two SAMs outperformed conventional polymer PTAA in terms of efficiency and stability, with devices based on MeO-2PACz and 2PACz exhibiting PCEs of 21.1% and 20.8%, respectively (Figure 7F). [78]or HTLs in inverted PSCs, their electron blocking and hole transport ability is strongly dependent on the compactness of the film.Li et al. designed a series of phenothiazine-based molecules TPT-S6, TPT-C6, and TPT-P6 with different anchor groups having various bonding strengths (Figure 8A). [79]The anchor groups of TPT-S6, TPT-C6, and TPT-P6 are ─SO 3 H, ─COOH, and ─PO 3 H 2 , respectively, and all of them were prepared on an ITO substrate via spin-coating.Theoretical calculations revealed that the ─PO 3 H 2 groups were more easily anchored on the ITO substrate than the ─SO 3 H and ─COOH groups due to the greater adsorption energy, confirming that the chemical interaction between TPT-C6 and the ITO surface is the strongest.The HOMO energy levels of the three SAMs were measured at −5.20 eV by CV, indicating an optimal match with the perovskite energy level.It was worth noting that the adsorption rate and loading capacity of TPT-P6 molecules on the ITO surface are easily judged by the characterization of the light absorption curve, which determined a high coverage and compactness of TPT-P6 SAM on the ITO surface (Figure 8B).A device configuration of ITO/HTLs/perovskite/C 60 /bathocuproine (BCP)/Ag was employed to evaluate the effect of three SAMs on device performance.Related measurements showed that groups with high binding strength were beneficial to promote the efficient assembly of SAMs, thereby inhibiting the nonradiative recombination losses in inverted PSCs.Inverted PSCs based on TPT-P6 produced the highest PCE of 21.43% (0.09 cm 2 ) and 20.09% (1.0 cm 2 ), significantly higher than the other two SAMs (TPT-S6 and TPT-C6).
Regulating the structural arrangement of SAMs molecules is of great significance to varying the optoelectronic properties of SAMs.Ullah et al. designed and synthesized a novel and cost-effective SAM called (2-(3,7-dibromo-10H-phenothiazin-10-yl)ethyl)phosphonic acid (Br-2EPT) based on halogenated phenothiazine for use as an HTL in p-i-n type PSCs (Figure 8C).The molecule spontaneously formed a SAM through a simple spin-coating method.Phenothiazine had been previously proven as an excellent building block for highperformance HTLs in PSCs. [80]The relative nonplanarity of the phenothiazine core, due to the presence of a large sulfur atom, facilitated efficient π-π stacking of the molecules and increased their solubility in various organic solvents.Compared with previously reported analogs such as MeO-2PACz and PTAA, the phenothiazine-based molecule in Br-2EPT contained electron-withdrawing bromine groups, exhibiting improved energy alignment with the top layer of the perovskite and enhanced electron-blocking ability.The PL studies indicated that Br-2EPT could more efficiently extract holes and exhibited nearly lossless interface compatibility with the perovskite layer.When integrated as a hole-selective contact in p-i-n type PSCs, the resulting device based on Br-2EPT achieved a high PCE of 21.63% and an average FF close to 81%.Ullah et al.
synthesized Br-2EPT and its variants 2-(3,7-dibromo-10hphenoxazin-10-yl)ethyl)phosphonic acid (Br-2EPO) and 2-(3,7-dibromo-10h-phenoselenazin-10-yl)ethyl)phosphonic acid (Br-2EPSe) to be used as HTLs for inverted PSCs and the device performance of different pairs of core heteroatoms (O, S, or Se) in the tricyclic aromatic ring was investigated (Figure 8C). [80]The above materials were mainly prepared by acylation and reduction reactions of initial materials such as phenoxazine, phenothiazine, or phenoselenide, and then obtained by two-step connection of phosphate groups through the Michaelis-Arbuzov reaction and McKenna reaction.The results of density functional theory (DFT) revealed that the interaction energy between Br-EX (X = O, F I G U R E 8 (A) Chemical structures of TPT-S6, TPT-C6, TPT-P6, and TPT-H6 without anchor groups and (B) schematic diagram of hole transport process based on these SAMs.Reproduced with permission. [79]Copyright 2021, Wiley-VCH.(C) Chemical structures of Br-2EPO, Br-2EPT, and Br-2EPSe and (D) normalized PL transient of perovskite films based on these SAMs.Reproduced with permission. [80]opyright 2022, Wiley-VCH.(E) Molecular design strategy and chemical structures of 2BrPTZPA and 2BrPXZPA.Reproduced with permission. [81]Copyright 2022, Royal Society of Chemistry.(F) Frame structure and cross-sectional micromorphology of the device based on single crystal perovskite.Reproduced with permission. [82]Copyright 2023, American Chemical Society.2BrPTZPA, ( 4 S, or Se) series molecules and perovskite were Br-2ESe (−1.02 eV) > Br-2ET (−0.92 eV) > Br-2EO (−0.78 eV), which was attributed to the appropriate bond distance between the Pb atom and the Se atom of the perovskite.The presence of Br-2ESe was beneficial to increasing the lifetime of charge carriers and reducing the interfacial energy barrier of inverted PSCs, thereby achieving efficient hole transport and preventing electron backflow (Figure 8D).Correspondingly, the device based on Br-2ESe SAM exhibited the highest PCE of 22.73%.
Li et al. ingeniously designed and synthesized SAMs of (4-(3,7-dibromo-10H-phenothiazin-10-yl)butyl)phosphonic acid (2BrPTZPA) and (4-(3,7-dibromo-10H-phenoxazin-10yl)butyl)phosphonic acid (2BrPXZPA) (Figure 8E). [81]hese SAMs featured butyl spacers and grafted bromine as side groups.Leveraging the enhanced electron-donating abilities of phenothiazine and phenoxazine compared with carbazole, the 2BrPTZPA and 2BrPXZPA exhibited better alignment of energy levels and minimized energy losses when paired with perovskite materials.Moreover, the application of 2BrPTZPA and 2BrPXZPA as coatings on ITO substrates effectively facilitated the growth of superiorquality perovskite crystals, exhibiting remarkable continuity in the vertical direction without the presence of discernible grain boundaries.Consequently, these SAMs led to the realization of inverted PSCs with significantly reduced trapstate density, achieving exceptional PCE of 22.06% and 22.93% for 2BrPTZPA and 2BrPXZPA, respectively.Furthermore, the device with 2BrPXZPA demonstrated remarkable stability, maintaining an impressive 97% of their initial efficiency even after enduring 600 h of continuous solar illumination.To further enhance the PV performance, the researcher employed 2BrPXZPA for the surface modification of NiO x .The resulting inverted PSCs which incorporated a bilayer of NiO x /2BrPXZPA, showcased a remarkable PCE of 23.66% coupled with a V OC of 1.21 V. Almasabi presented findings on the growth of mixed cation FA 0.6 MA 0.4 PbI 3 perovskite single crystals on a hydrophilic surface of MeO-2PACz SAM.The DFT model revealed that the hole-selective transport layer, MeO-2PACz, exhibited stronger bonding with the FAPbI 3 perovskite surface compared with PTAA, resulting in an enhanced adhesion between the HTL and single crystal film (Figure 8F).The improved mechanical adhesion was attributed to the stronger interaction between the additional functional group (─MeO) of MeO-2PACz and the perovskite surface.Consequently, the efficiency of inverted PSCs constructed using these single crystal films grown on MeO-2PACz reached an impressive efficiency of 23.1%. [82]uo et al. conducted research on the design of two selfassembling molecules for use as HTLs in p-i-n type PSCs.These molecules featured phosphonic acid anchoring groups and comprised a D-A architecture. [83]The donor unit, 4-methoxy-n-(4-methoxyphenyl)n-phenylamine, and the acceptor unit, BT, were employed (Figure 9A).To create the phosphonic acid analogs, the BT halide was subjected to the Michaelis-Becker reaction, followed by hydrolysis.This resulted in the formation of the novel phosphonic acid D-A self-assembling HTLs (PPA and phosphonic self-assembled monolayers [PPAOMe]).Phosphonic acid as an anchoring group offered improved stability over the commonly used carboxyl groups.The strong anionic properties facilitated stronger bidentate bonding with inorganic metal oxides on the ITO substrate, thereby enhancing coverage and ordering of the phosphonic acid analogs (Figure 9B).Through interface modification, it was demonstrated that the WF of ITO could be modulated.Ultraviolet (UV) photoelectron spectroscopy measurements were performed to investigate the WF modulation of ITO by PPA and PPAOMe.The results revealed an increase in the WF of ITO after modification with the self-assembling HTLs.The PPA exhibited a WF of 4.48 eV, while PPAOMe showed a WF of 4.58 eV.The effective adjustment of the ITO WF was attributed to the relatively large dipole moments of these molecules.The DFT calculations indicated dipole moments of 6.97 D and 8.86 D for PPA and PPAOMe, respectively.These dipole moments resulted in an increased positive charge density near the ITO surface and the formation of an electrostatic potential step.As a result, the movement of electrons from the Fermi level to the vacuum level became more challenging, leading to an increased WF of the ITO substrate.During the interface rearrangement process, the PPA molecule achieved a stable equilibrium state, with a calculated sulfur-indium bond length of 3.41 Å.These findings suggested a closer wrapping of PPA around the ITO surface with a preferred face-on orientation, effectively reducing the hole injection barrier at the anode/HTL interface.Furthermore, the anchoring group acted as a Lewis base, contributing to the passivation of interface defects in the perovskite.To evaluate the PV performance of the different SAMs, the researchers fabricated PV devices based on the ITO/SAMs/perovskite/PCBM/2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP)/Ag structure.The device incorporating PPA demonstrated superior V OC and FF, resulting in a remarkable PCE of 23.24%.Specifically, a V OC of 1.14 V, a J SC of 24.83 mA cm −2 , and an FF of 82.0% were achieved.The improved V OC and FF were attributed to the effective suppression of nonradiative recombination at the PPA/perovskite interface and efficient hole extraction.The PPA-based PSCs exhibited excellent operational stability under various environmental conditions.
To achieve precise control over the growth and molecular functionalities of SAMs, Deng et al. developed a novel approach called co-SAMs. [84]This innovative technique was employed to achieve efficient hole selectivity and suppress recombination at the holeselective interface in inverted PSCs.By strategically modifying the positions of methoxy substituents, Jiang et al. synthesized (2,7-dimethoxy-9H-carbazol-9-yl) methylphosphonic acid (DC-PA), which exhibited wellordered energy levels and a favorable dipole moment (Figure 9C).To further enhance surface functionalization and optimize interaction with the top perovskite layer, coassembly with alkylammonium salts was employed.A mixture of DC-PA and the alkylammonium salt intraarticular hyaluronic acid (IAHA) was spin-coated onto an ITO substrate to deposit the co-SAM.Following an acid-base reaction, SAM molecules were covalently anchored to the substrate via monodentate or bidentate coordination, while physically absorbed molecules were effectively removed through subsequent washing steps.By implementing the co-SAM strategy and carefully adjusting the ratio of DC-PA to IAHA, the anchoring strength and packing density of the co-SAM on the ITO surface were precisely controlled, resulting in excellent film coverage and effective passivation of the perovskite surface.As a result, the PSCs based on the co-SAM approach achieved an impressive PCE of 23.59%, accompanied by notable improvements in device stability.Carbazole-derived SAMs offer promising potential as hole-selective materials for inverted PSCs.However, their limited dipole moments hinder an effective WF modulation of the ITO substrate, thereby restricting the F I G U R E 9 (A) Molecular structures of PPA and PPAOMe and schematic diagrams of the corresponding devices, and (B) the interaction mode of PPA on the surface of ITO.Reproduced with permission. [83]Copyright 2023, Wiley-VCH.(C) Molecular structure of DC-PA and IAHA, and preparation process of the coassembled layer.Reproduced with permission. [84]Copyright 2022, Wiley-VCH.(D) Molecular structures and calculation of dipole moment and HOMO energy level of 4PACZ, CbzPh, and CbzNaph, and (E) the deposition schematic diagram of these SAMs and the water contact angle measurements after modified ITO.Reproduced with permission. [85]Copyright 2022, Wiley-VCH.4PACz, [4-(9h-carbazol-9-yl)butyl] phosphonic acid; BCP, bathocuproine; CbzNaph, carbazole-derived self-assembled monolayers; DC-PA, (2,7-dimethoxy-9H-carbazol-9-yl) methylphosphonic acid; HOMO, highest unoccupied molecular orbital; IAHA, intra-articular hyaluronic acid; IPA, isopropanol; ITO, indium tin oxide; Me-4PACz, [4-(3,6-dimethyl-9H-carbazol-9-yl)butyl]phosphonic acid; PCBM, [6,6]-phenyl-C 61 -butyric acid methyl ester; PPA, phenylphosphonic acid; PPAOMe, phosphonic self-assembled monolayers; SAHTM, self-assembled hole transporting molecule; SAM, self-assembled monolayer.
photovoltage of PSCs.Furthermore, even minor structural variations can significantly disrupt their assembly modes, stacking, and energy levels, ultimately compromising the overall performance of PSCs.To address these challenges, Jiang introduced two novel carbazole-derived SAMs, CbzPh and carbazole-derived self-assembled monolayers (CbzNaph), with asymmetric or helical π-extension designs (Figure 9D). [85]The DFT calculations revealed that the HOMO energy levels of 4PACZ, CbzPh, and CbzNaph were 5.61 eV, 5.49 eV, and 5.35 eV, respectively.These SAMs exhibited favorable energy alignment with the perovskite absorber, facilitating efficient hole extraction from the perovskite to the SAMs while impeding electron transfer to the anode.Notably, the helical π-extended CbzNaph exhibited a substantial dipole moment and formed densely ordered monolayers of π-framework single crystals (Figure 9E).Consequently, the champion PSC incorporating CbzNaph SAM achieved an impressive efficiency of 24.1% along with enhanced stability.
Zheng et al. developed a one-step solution coating process in which hole-selective contacts and perovskite lightabsorbing materials spontaneously formed, resulting in highly efficient inverted PSCs (Figure 10A). [86]During the processing of the perovskite thin film, it was observed that phosphonic acid or carboxylic acid additives introduced into the perovskite precursor solution had self-assembled on the ITO substrate.As the perovskite crystallized, these additives formed robust SAMs, serving as excellent hole-selective contacts.The device based on the Me-4PACz achieved a PCE of 24.5%, and retained over 90% of its initial efficiency after operating at the maximum power point (MPP) for 1200 h under continuous illumination (Figure 10B).This method was compatible with various SAM systems, perovskite materials, solvents, and processing techniques, demonstrating its excellent versatility.Similarly, Jiang et al., employed MeO-2PACz as the HTL to successfully prepare inverted PSCs with a remarkable PCE of 25.37% (Figure 10C). [37]

| Other types of SAMs
Más-Montoya et al. synthesized a novel SAM material anthracenyl-7-azaindole (ADAI) containing azaheteroaryl groups as HTL for inverted PSCs (Figure 11A). [87]The preparation process of ADAI was mainly through the Buchwald-Hartwig cross-coupling reaction to obtain intermediates, which were then subjected to photochemical reactions to separate ADAI.The new ADAI molecule presented a centrosymmetric structure, including both complementary acceptor sites and hydrogen bond donors in the conjugated structure (Figure 11B).These sites and hydrogen bond guidance were conducive to promoting the orderly self-assembly of ADAI molecules and providing favorable channels for hole transport.The synthesis cost of ADAI was much lower than that of conventional organic HTLs, and ADAI also presented outstanding thermal stability for a wide temperature range (25-395°C).The use of ADAI was beneficial to promote the grain growth of the top perovskite and reduce the corresponding interfacial charge recombination loss compared with PEDOT:PSS.The device based on ADAI as the HTL exhibited a PCE of 15.9%.Guo et al. presented a study on the utilization of acid-weakened boric acid as an alternative anchoring agent for highly efficient SAM-based hole-selective contacts in PSCs (Figure 11C).Theoretical calculations revealed the spontaneous chemical adsorption of boric acid on the oxygen-terminated surface of ITOs, where oxygen vacancies played a favorable role in the adsorption process, leading to the formation of stable B─O─In bonds (Figure 11D,E).Spectroscopic and electrical measurements confirmed the significant reduction in ITO corrosion with the introduction of the boric acid anchor.Through screening various aromatic amine structures, the research team identified boric acidanchoring self-assembled monolayers (MTPA-BA) as the optimal boric acid anchoring SAM, which exhibited outstanding hole selectivity, improved perovskitesubstrate contact, and minimized interface defects.The device incorporating MTPA-BA achieved an impressive PCE of 22.6%, accompanied by a high FF of 85.2% (Figure 11F). [88]The device characteristics and PCEs based on SAMs as HTLs are summarized in Table 1.

| SAMs as interface layers
SAMs can be used as interface modification layers in inverted PSCs due to their multifunctional properties.The mechanism and related research progress of SAMs as interface modification layers are summarized below.

| ETL/electrode interface modification
There exists a mismatch in the energy level between the PCBM ETL and the top electrode (such as Ag) in inverted PSCs, resulting in a low interfacial charge transport efficiency.To resolve this, Sun et al. employed rho-damine101 to improve the cathode interface between PCBM and the Ag electrode (Figure 12A). [89]The introduction of rhodamine101 was beneficial in reducing the surface roughness of the bottom film and correspondingly improving the charge extraction efficiency.Organic semiconductors containing zwitterions have recently been used as interfacial interlayers for cathodes in optoelectronic devices.The functionality of zwitterionic materials lies in the enhancement of interfacial interactions through dipoles to achieve efficient charge collection.Emrick et al. used tetrabutylammonium iodide doping to enhance the conductivity of 2,3,4-tris(3-(propyl)sulfobetaine)propoxy)fulleridine (C 60 -SB) (Figure 12B). [90]ubsequently, C 60 -SB was prepared by a solution method and integrated into an inverted PSC.The study revealed the presence of a dipole energy at the C 60 -SB/Ag interface to maximize the effective WF of the cathode, which correspondingly facilitated electron transfer.Fang et al. synthesized a novel nonconjugated zwitterion 3,3-(hexane 1,6-dialkylbis(dimethylaminodiyl)) dipropionate (HDAC) as a cathode buffer layer in inverted PSCs (Figure 12C). [91]he introduction of HDAC forms a dipole with the Ag metal electrode due to the existence of the ─COOH group, which is beneficial for reducing the interface barrier and contact resistance (Figure 12D).The champion device based on the utilization of HDAC SAM achieved an The molecular structure and preparation process of Me-4PACz, and the schematic diagram of the crystallization process of perovskite based on Me-4PACz.Reproduced with permission. [86]Copyright 2023, Springer Nature.(B) The J-V curves of the device based on Me-4PACz SAM. [86](C) The J-V curves of the device with the MeO-2PACz SAM.Reproduced with permission. [37]Copyright 2022, Springer Nature.improved PCE of 17.10% (Figure 12E).This highly suggests that the development of novel SAMs for modifying the PCBM/Ag cathode interface is of great significance for obtaining high-performance inverted PSCs.
Fullerene and its derivatives are typically utilized as ETLs in inverted PSCs.However, the relatively thin fullerene layer is insufficient in protecting the perovskite film at the rear when stored in a humid environment.To address this, Bai et al. reported a method of modifying the fullerene layer by cross-linking it with the self-assembled materials, thus improving the stability of the inverted PSCs.It was revealed that the introduction of the selfassembled molecule trichloro(3,3,3-trifluoropropyl)silane into the fullerene precursor was conducive to cross-linking through chemical bonding, leading to an increase in the conductivity of the fullerene film (Figure 13A). [92]Likewise, the introduction of a silicon-oxygen (Si─O) network and trifluoromethyl (─CF 3 ) group in silane has demonstrated remarkable efficacy in mitigating the detrimental effects of water and oxygen on perovskite films.Furthermore, the device maintained 90% of its original efficiency when stored under ambient conditions for 30 days (Figure 13B).Li et al. used trichloro(3,3,3trifluoropropyl)silane (C 3 H 4 Cl 3 F 3 Si) as a SAM to passivate the surface defects of perovskite and improve the stability of inverted PSCs.The structure of the inverted PSC fabricated was ITO/PTAA/CH 3 NH 3 PbI 3 /C 60 /benzophenanthroline/Ag, in which C 3 H 4 Cl 3 F 3 Si was incorporated between the CH 3 NH 3 PbI 3 and C 60 interface. [93]The introduction of C 3 H 4 Cl 3 F 3 Si provided tunnel junction deselection at the CH 3 NH 3 PbI 3 /C 60 interface blocking holes and transporting electrons, thereby inhibiting the recombination of carriers and enabling the effective collection of charge carriers (Figure 13C).The silicon-oxygen-silicon (Si─O─Si) crosslinked structures promote the spontaneous assembly of C 3 H 4 Cl 3 F 3 Si on the perovskite surface.The presence of hydrophobic functional groups of C 3 H 4 Cl 3 F 3 Si was beneficial in reducing the detrimental effects of water and oxygen on the inverted PSCs, where the device achieved its highest PCE of 21.12% (Figure 13D).The device characteristics and PCE based on SAMs as cathode interface modification layers are further summarized in Table 2.

| ITO/HTL interface or HTL/ perovskite interface modification
ITO is a common photoanode for inverted PSCs with excellent electrical conductivity and optical transmittance.
It is common to tune the WF and surface energy of ITO via UV-ozone or chemical treatment.However, the surface modification of ITO utilizing UV-ozone treatment is temporary, and the acidic nature of PEDOT:PSS leads to a decrease in the stability of the ITO/PEDOT:PSS interface.SAMs play a significant role in regulating the WF of ITO and improving the wettability of the substrate surface.Akın Kara et al. used three fluorine-containing boronic acid derivatives to achieve molecular self-assembly by modifying  | 221 the ITO surface (Figure 14A). [94]The ─OH group of boronic acid acts as an effective anchoring group for allowing SAMs to adsorb on the ITO surface, thereby generating a permanent dipole moment.The electron-withdrawing properties of the boronic acid derivatives and variation in the number of fluorine atoms play a significant role in influencing the size of the dipole moment exhibited by the SAMs anchored on the ITO surface (Figure 14B).The corresponding device based on a 2F-SAM modification achieved the best hole collection ability and a higher PCE of 15.66%.The introduction of 2F-SAM was beneficial in preventing direct contact between the acidic PEDOT:PSS and ITO, leading to an improvement in the stability of inverted PSCs.The SAMs play a critical role, extending beyond the ITO/HTL interface as previously highlighted, to serve as essential modifiers at the HTL/perovskite interface.

Gu et al. employed 3-aminopropanoic acid SAM (C 3 -SAM)
to tune the PEDOT:PSS/perovskite interface for a more efficient hole transport (Figure 14C). [95]The study revealed that the amino group (─NH 2 ) of 3-aminopropanoic acid enhanced the surface WF of PEDOT:PSS through electrostatic interaction, which was beneficial for lowering the energy barrier and establishing a better match in the energy level of the perovskite layer.In addition, the carboxylic acid group (─COOH) of 3-aminopropanoic acid was beneficial in inducing the crystallization growth of perovskite crystals, allowing perovskite films with low surface roughness and high coverage to be obtained (Figure 14D).The corresponding device based on the C 3 -SAM interface modification achieved an improvement in its PCE from 9.7% to 11.6%.PEDOT:PSS possesses some inherent problems for the application of inverted PSCs, such as poor energy-level matching with perovskite and hygroscopicity.In contrast, NiO x is a promising HTL candidate originating from the advantages of a large bandgap, suitable Fermi energy level, high mobility, and excellent stability.Unfortunately, the metal oxide NiO x film suffers from numerous defects and poor contact with the perovskite layer during lowtemperature processing, resulting in a serious deterioration of the microscopic morphology of the perovskite and inducing carrier recombination loss. [96]Interface engineering is crucial in reducing interfacial energy loss to achieve efficient and stable inverted PSCs.A promising strategy to address both the morphology of perovskite films and charge transport issues is the utilization of The molecular structure of rhodamine101 and its specific position in the inverted PSCs.Reproduced with permission. [89]Copyright 2015, Royal Society of Chemistry.(B) The molecular structure of C 60 -SB.Reproduced with permission. [90]opyright 2017, American Chemical Society.(C) The molecular structure of HDAC.(D) The energy-level diagram and (E) J-V curves of the device in the presence of HDAC.Reproduced with permission. [91]Copyright 2017, Elsevier.SAMs as interfacial modification layers.In particular, employing hole-selective SAMs to passivate the surface of NiO x has emerged as an effective approach for enhancing the performance of PSCs.Hole-selective SAMs offer several advantages, including a simplified processing method, minimal material requirements, high compatibility with various substrates, precise control over surface dipoles, improved band alignment, mitigation of surface defects, and the ability to form well-ordered monolayers through molecular self-assembly processes.With these advantages, SAMs enable an improved morphology of perovskite films deposited and an efficient charge transport.This ultimately leads to an enhanced PV performance in inverted PSCs.For instance, Mangalam et al. employed 4-bromobenzylphosphonic acid (Br-BPA) to modify the NiO x surface and tune the NiO x /perovskite interface, thereby improving the PV performance and device stability (Figure 15A). [97]The strategy employed for the adsorption of Br-BPA on the surface of NiO x involved immersing the substrate in an acetonitrile solution containing Br-BPA molecules, followed by multiple washing steps.Through this impregnation process, it was confirmed by XPS that the phosphonic acid groups of Br-BPA effectively bind to the surface of NiO x .The hydrophobicity of the NiO x surface increased, as indicated by water contact angle measurements, confirming the dense coverage of Br-BPA molecules on the NiO x surface.Moreover, Br-BPA played a beneficial role in the wetting of the perovskite solution and the crystallization of the perovskite film due to the presence of bromine (Br).Additionally, the intrinsic dipole moment of the Br-BPA molecule was measured to be 2.3 D, effectively lowering the valence band energy level of NiO x and facilitating charge extraction.Consequently, the device incorporating the Br-BPA modification achieved a significant increase in PCE from 11.2% to 12.5%, accompanied by a higher V OC of 1.019 V.
Bai et al. introduced organosilane SAMs at the TiO 2 /perovskite interface, achieving a substantial improvement in the PV performance of the device.SAMs are equally applicable to the interface modification of inverted PSCs.Yang et al. utilized diethanolamine (DEA) molecules containing hydroxyl and amine groups to modify the NiO layer, achieving rapid hole extraction (Figure 15B). [98]The results indicated that DEA molecules act as an interfacial modification layer to form strong interactions with NiO and perovskite through functional group effects, resulting in a reduced The mechanism diagram of trichloro(3,3,3-trifluoropropyl)silane and (B) the J-V curves of inverted PSCs modified by trichloro(3,3,3-trifluoropropyl)silane.Reproduced with permission. [92]Copyright 2016, Springer Nature.(C) The mechanism diagram of C 3 H 4 Cl 3 F 3 Si and (D) the J-V curves of inverted PSCs in the presence of a C 3 H 4 Cl 3 F 3 Si layer.Reproduced with permission. [93]Copyright 2020, Royal Society of Chemistry.CLCS, crosslinked C 60 -self-assembled monolayer; FF, fill factor; HTL, hole transport layer; ITO, indium tin oxide; PCE, power conversion efficiency; PSC, perovskite solar cell; PTAA, poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine]; SAM, self-assembled monolayer.
loss in energy and defect density in inverted PSCs.The NH 2 group of DEA was beneficial in combining with the NiO at the bottom by forming N─Ni chemical bonds, while the ─OH group in DEA performs Pb─OH coordination with the top perovskite.As expected, the introduction of DEA SAM was beneficial for improved NiO/perovskite interfacial contact and accelerated hole extraction due to the favorable molecular dipole layer.Consequently, the efficiency of the corresponding devices increased from the initial 11.2% to 15.9%.NiO x prepared at low temperatures often suffers from low electrical conductivity and unfavorable interfacial contact with perovskite.Considering the excellent chemical stability and unique chemical structure of ferrocene derivatives, Zhang et al. employed ferrocene dicarboxylic acid (FDA) for the first time to modify the surface properties of NiO x . [99]The study revealed that the introduction of FDA was beneficial in reducing the surface roughness of NiO x , significantly improving the crystallization of perovskite and enhancing the NiO x /perovskite interface contact (Figure 15C).The significant PL intensity reduction observed in the presence of FDA, suggests a more efficient carrier transport from perovskite to the NiO x surface and a reduced carrier recombination loss.Additionally, the effect of FDA modification was also conducive to reducing the NiO x / perovskite interface capacitance effect, further confirming an improved NiO x /perovskite interface contact in the presence of the FDA.The inverted PSCs based on FDA modification yielded a higher PCE of 18.20% compared with the initial (15.13%).Wang et al. inserted parasubstituted benzoic acid (R-BA) into the NiO x /perovskite interface to improve the efficiency of inverted PSCs (Figure 15D).Systematic studies have clarified that the introduction of R-BA SAM is beneficial in improving the adhesion between NiO x and perovskite, thereby passivating interface defects and effectively promoting charge transport.In addition, the permanent dipole moment of Br-BA could tune the WF of NiO x and induce good crystallization of perovskite through the chemical interaction of functional groups.Due to these positive effects, the target device exhibited a higher PCE of 18.4% and outstanding stability in the air. [100]ingh and Tao utilized molecular modification on the surface of nickel oxide (NiO x ) using phosphonic acids with different tail groups on the same phosphonic acid head group (Figure 16A).The reference molecule, phenylphosphonic acid, exhibited negligible changes in the WF of NiO x .However, the modification with acids bearing electron-donating methoxy groups led to a reduction in WF due to the positive dipole moment oriented away from the NiO x layer.Conversely, a contrasting effect was observed on NiO x modified with 4-cyanophenylphosphonic acid (CNPPA), where the negative dipole moment directed toward the oxide surface increased the WF.The hydrophilicity of the modified perovskite surfaces remained similar, and there were no significant alterations in the quality and thickness of the deposited perovskite films.The champion device was based on NiO x modified with CNPPA (PCE of 18.45%).CNPPA brought the WF of NiO x closer to the valence band of perovskite resulting in a higher V OC and enhanced charge transport from the perovskite to NiO x (Figure 16B).The dipole direction also contributed to an improvement of the J SC and FF. [101]A B L E 2 Summary of characteristics of inverted PSCs with SAMs as ETL/electrode interface modification layers.

SAMs
Configuration V OC (V) J SC (mA cm F I G U R E 14 (A) Chemical structures of various SAMs, and (B) schematic diagram of the energy levels of the corresponding device.Reproduced with permission. [94]Copyright 2018, American Chemical Society.(C) Schematic of the device structure based on C 3 -SAM modified PEDOT:PSS, the direction of the dipole is marked by the arrow.(D) UV-Vis absorption spectra of perovskite films with or without C 3 -SAM.Reproduced with permission. [95]Copyright 2015, Royal Society of Chemistry.F I G U R E 15 (A) Schematic diagram of NiO x layer modified by Br-BPA.Reproduced with permission. [97]Copyright 2019, Springer.(B) The schematic diagram of surface modification of NiO film by DEA monolayer.Reproduced with permission. [98]Copyright 2016, Wiley-VCH.(C) PL spectra of perovskite films based on NiO x and NiO x /FDA.Reproduced with permission. [99]Copyright 2018, Royal Society of Chemistry.(D) The structural diagram of NiO x modified by various SAMs.Reproduced with permission. [100]Copyright 2017, Wiley-VCH.Br-BPA, 4-bromobenzylphosphonic acid; DEA, diethanolamine; FDA, ferrocene dicarboxylic acid; FTO, fluorine-doped tin oxide; ITO, indium tin oxide; PCBM, [6,6]-phenyl-C61-butyric acid methyl ester; PL, photoluminescence; SAM, self-assembled monolayer.16C). [102]The phosphonic acid functional groups of the SAMs facilitated the surface modification of NiO x through coordination with the metal oxide.Additionally, the SAM molecules provided a means to finely tune the energy-level structure of NiO x , bringing its valence band edge into closer alignment with that of the perovskite absorber layer.This meticulous adjustment effectively minimized the energy loss at the interface, thereby enhancing the extraction and transport efficiency of hole carriers.Furthermore, the introduction of the SAMs improved the energy-level alignment between the HTL and perovskite, leading to an optimized device performance.16D). [103]Studies have shown that the amino groups present in TSPA play a beneficial role in interacting with the hydroxyl groups on the NiO x surface by forming hydrogen bonds, resulting in a reduction of the interface defect density and improved stability of the perovskite film.In addition, the molecular structure of TSPA contained positively charged amino groups and negatively charged silane branches, which were conducive to the ordered molecular arrangement and generation of dipole moments on the NiO x surface.It is worth noting that the formation of the dipole moment is beneficial in adjusting the WF of NiO x and correspondingly establishing a better energy-level alignment with perovskite.Thus, inducing a more efficient carrier transport and suppressing the nonradiative recombination losses.The TSPA-based device achieved an improved PCE of The schematic diagram of PPA, MPPA, and CNPPA modified NiO x layer and (B) the energy-level arrangement of each layer of the device.Reproduced with permission. [101]Copyright 2021, Wiley-VCH.(C) The molecular structure of 4PACz and 7H-DC.Reproduced with permission. [102]Copyright 2023, American Institute of Physics.(D) The energy-level diagram of perovskite and NiO x under TSPA modification and (E) J-V curves of the inverted PSCs based on NiO x and NiO x /TSPA.Reproduced with permission. [103]Copyright 2021, Elsevier.4PACz, [4-(9h-carbazol-9-yl)butyl]phosphonic acid; 7H-DC, [4-(7h-dibenzo[c,g]carbazol-7-yl)butyl]phosphonic acid; CNPPA, 4-cyanophenylphosphonic acid; ITO, indium tin oxide; MPPA, 4-methoxyphenylphosphonic acid; PCBM, [6,6]-phenyl-C 61 -butyric acid methyl ester; PPA, phenylphosphonic acid; PSC, perovskite solar cell; TSPA, 3-(triethoxysilyl)propylamine.20.21% and maintained excellent PV performance under different humidity conditions for 60 days (Figure 16E).
Liu et al. developed a strategy to suppress the interfacial reaction between metal oxides and perovskites, enhancing the PV performance and stability of inverted PSCs.They introduced a self-assembled D-A binary molecule, LS1, composed of TPA and cyanoacrylic groups, onto NiO x through hydroxyl group passivation (Figure 17A). [104]This interface treatment effectively inhibited the redox reaction between NiO x and the perovskite, preventing interface degradation in PSCs.Moreover, the deposition of LS1 on top of NiO x improved hole extraction and facilitated perovskite-layer growth.The inverted PSCs based on LS1 achieved a PCE of 20.94% and maintained 93% of its initial stability after 600 h of one sun illumination.Interestingly, this strategy was easily applied to slotdie-coated PSC modules, resulting in a high PCE of 14.90% and a geometric filling factor of 93% for an area of 19.16 cm 2 .
The main obstacles hindering the development of NiO x -based inverted PSCs are interface lattice mismatch and undesirable reactions.To address these challenges, Chen et al. employed p-dodecylbenzenesulfonic acid (CBSA) as a SAM to achieve dual passivation of NiO x and perovskite crystals (Figure 17B). [105]Specifically, the sulfonic acid (─SO 3 H) groups in CBSA were able to passivate oxygen defects on the NiO x surface and the chloride (─Cl) functional groups effectively filled iodine vacancy defects at the buried interface of the perovskite.The highly electronegative chlorine atoms induced electron density withdrawal from the less electronegative iodine atoms located at the bottom of the perovskite, thereby achieving effective passivation of Pb-I defects on the perovskite surface.In contrast to traditional singlefunctional molecules such as chlorobenzene and benzenesulfonic acid, CBSA, with its dual-functional groups, formed structural components within the imperfect perovskite crystals by chemically bonding to oxygen defects.The CBSA acted like a pair of pliers, simultaneously gripping NiO x and perovskite crystals.At the molecular level, the growth sites provided by CBSA enabled the release of lattice strain at the bottom of the perovskite, suppressing interface strain and improving phase stability (Figure 17C).As a result, the NiO x / CBSA-based PSCs achieved an impressive champion PCE of 21.8%.Notably, CBSA-treated PSCs also exhibited excellent long-term stability, with efficiency retention of 84% after 3000 h of storage in N 2 and 80% after 1000 h of storage in air at 25°C with 50%-70% relative humidity.
Phosphonic acid-based SAMs have been extensively utilized for interface modification in inverted PSCs.
Nevertheless, it is crucial to consider that the surface coverage and packing density of SAM molecules can differ depending on the specific SAM material and the underlying oxide layer.Moreover, various SAM molecules exert distinct effects on interfacial energy-level alignment and perovskite film growth, leading to a complex relationship between surface modification, device efficiency, and long-term stability.Lin et al. investigated the modification of NiO x surface using 2PACz and ethanolamine (Figure 17D). [106]The deposition of a nanostructured NiO  [107] They investigated several molecular passivation methods, including polymerizing PTAA, MeO-2PACz, and 2PACz SAMs.The study revealed that devices modified with 2PACz exhibited a significant improvement in efficiency, with a maximum efficiency of 22.2% (Figure 17E).Timeresolved PL studies, dependent on flux, demonstrated that the modification of 2PACz enabled rapid charge extraction and reduced nonradiative recombination at the NiO/perovskite interface.The binding energy between NiO and 2PACz was observed to be over seven times higher than that between NiO and PTAA.Time-resolved femtosecond transient absorption measurements indicated the formation of shallow traps at the NiO/perovskite interface upon UV irradiation.Dual injection current measurements revealed a significant migration of ions following UV stress.The incorporation of 2PACz effectively suppressed interfacial traps and ion migration.The unencapsulated NiO/2PACz devices maintained over 90% of their initial efficiency at the MPP even after exposure to a cumulative UV dose of 35 kWh m −2 .The bestperforming device achieved a stable efficiency of 22.2% and retained 82% of its original efficiency after 2000 h of MPP tracking under one sun illumination (100 mW cm −2 ) at 45°C in ambient air.Zhu et al. have introduced an innovative SAM material, specifically identified as (4-(3,11dimethoxy-7H-dibenzo[c,g]carbazol-7-yl)butyl)phosphonic acid (MeO-4PADBC).This SAM is meticulously immobilized on a NiO x thin film, aiming to enhance inverted PSCs.The NiO x /MeO-4PADBC interface demonstrates optimal dipole moments and a surface conducive to facile interaction with perovskite, thereby facilitating ideal energy alignment, rapid hole extraction, and minimizing defect density (Figure 17F). [108]The architectural design of the NiO x /MeO-4PADBC interface not only securely anchors SAM molecules at the NiO x /perovskite interface but also establishes a robust hole-selective layer, contributing significantly to the thermal stability of PSCs (Figure 17G).These combined effects result in a device featuring an impressive V OC of 1.19 V and a validated PCE of 25.6% for The chemical structure of LS1 and the mechanism pathway to inhibit the degradation of the interface between NiO x and perovskite.Reproduced with permission. [104]Copyright 2022, American Chemical Society.(B) The schematic diagram of the structure of CBSA as the interface layer, and (C) the lattice strain diagram of the perovskite film on NiO x and NiO x /CBSA substrates.Reproduced with permission. [105]Copyright 2022, Wiley-VCH.(D) The molecular structure of 2PACz.Reproduced with permission. [106]Copyright 2023, American Chemical Society.(E) The J-V curves of the device in the presence of 2PACz.Reproduced with permission. [107]Copyright 2022, Elsevier.(F) Molecular structure of MeO-4PADBC.(G) Surface-anchoring mechanism of MeO-4PADBC on NiO x .(H) The J-V curves of the 1.53 eV devices employing various HTLs such as NiO x , MeO-4PADBC, and NiO x /MeO-4PADBC.Reproduced with permission. [108]opyright 2023, American Association for the Advancement of Science. the perovskite with a 1.53 eV bandgap (Figure 17H).Moreover, an extended operational stability assessment at 65°C for 1200 h attests to the durability of PSCs based on NiO x /MeO-4PADBC, showcasing their ability to maintain over 90% of their initial efficiency. [108]The device characteristics and PCE based on SAMs as anode interface modification layers are summarized in Table 3.

| SAMs IN PEROVSKITE-BASED TANDEM SOLAR CELLS
Improving the PCE of solar cells is a crucial strategy for reducing the cost of solar cell technology.To surpass the predicted efficiency of single-junction solar cells, the development of tandem solar cells represents a promising avenue. [40,109]Utilizing different bandgap perovskite materials to construct tandem solar cells has emerged as an effective strategy to enhance the efficiency limit of inverted single-junction PSCs. [110,111]The inverted perovskite tandem solar cells have the potential to more efficiently harness solar energy and achieve PCE values exceeding 40%. [102,103]In the subsequent section, we will introduce the utilization of SAMs in perovskite tandem devices.

| SAMs in perovskite-perovskite tandem solar cells
High-bandgap subcells have been identified as the primary cause of V OC losses in numerous tandem devices reported in the literature.To address this issue, Thiesbrummel et al. implemented a triple optimization strategy for highbandgap perovskites. [112]The strategy involved substituting HTL PTAA with 2PACz, incorporating oilamine into the perovskite layer, and introducing a thin LiF layer between the perovskite and ETL to enhance the performance of the high-bandgap subcell (Figure 18A).The triple optimization approach was successfully applied to threecation high-bandgap perovskites with bandgaps ranging from 1.80 eV to 1.85 eV to 1.88 eV, resulting in significant reductions in V OC losses across all measured bandgaps.This achievement was of great significance as the latter T A B L E 3 Summary of characteristics of inverted PSCs with SAMs as ITO/HTL interface or HTL/perovskite interface modification layers.

SAMs
Configuration  18B).Lai et al. proposed a comprehensive optimization strategy for various components of the device.They employed 2PACz as the HTL for 1.77 eV triple cation perovskite devices, effectively suppressing V OC losses at the HTL/perovskite interface and enabling the fabrication of high-quality and uniformly distributed perovskite absorber layers on flexible polymer foils. [113]Subsequently, solvent of the all-perovskite tandem structure and corresponding cross-sectional images.Reproduced with permission. [112]Copyright 2023, Wiley-VCH.(B) J-V curves for tandem devices with three different perovskite bandgaps.Reproduced with permission. [112]Copyright 2023, Wiley-VCH.(C) J-V curves for two-terminal all-perovskite flexible tandem solar cells.Reproduced with permission. [113]Copyright 2022, Wiley-VCH.(D) Chemical structure of the bridging molecule and its position in the device.Reproduced with permission. [114]Copyright 2022, Springer Nature.(E) Molecular structure of BCBBr-C4PA, (F) schematic structure, and (G) J-V curves of a 4 T all-perovskite tandem device in the presence of BCBBr-C4PA.Reproduced with permission. [115]Copyright 2023, Wiley-VCH.BCBBr-C4PA, ( engineering was employed to optimize the deposition of the PCBM transport layer, ensuring an optimal morphology.Additionally, the researchers introduced 2thiopheneethylammonium chloride to form a 2D perovskite layer on the perovskite film surface.This layer effectively mitigated recombination losses at the electron-selective interface and optimized the energy band alignment between the perovskite layer and PCBM, further improving charge extraction and reducing nonradiative recombination on the perovskite surface. Through the successful integration of these optimization strategies, the researchers achieved a V OC of 1.29 V and developed a near-infrared transparent wide-bandgap (WBG) PSC with a PCE 15.1% on flexible substrates.Notably, the high V OC corresponded to a record-low V OC deficit (480 mV) for the perovskite considering a bandgap of approximately 1.80 eV.Moreover, by combining flexible narrow-bandgap (NBG) (1.24 eV) PSCs, they successfully fabricated flexible all-perovskite four-terminal (4T) tandem solar cells with impressive PCEs of 22.6% and 23.8% for different configurations (Figure 18C).In a recent study by Li et al., a novel molecularly bridged hole-selective contact was introduced to tackle the challenges associated with interface recombination and hole extraction in flexible PSCs. [114]This innovative contact, known as molecule-bridged NiO (MB-NiO), anchored hole-selective molecules onto low-temperature processed NiO nanocrystalline films, creating a seamless connection with the perovskite layer (Figure 18D).To achieve optimal performance, the researchers selected two types of molecules, 2PACz and MeO-2PACz and mixed them in a 3:1 molar ratio to bridge the interface.These molecules have previously demonstrated excellent hole selectivity and reduced interface recombination in similar studies. [114]y employing the molecular bridging approach with MB-NiO, the research team successfully fabricated flexible all-perovskite tandem solar cells incorporating WBG FA 0.8 Cs 0.2 PbI 1.95 Br 1.05 (~1.75 eV) and NBG FA 0.7 -MA 0.3 Pb 0.5 Sn 0.5 I 3 (~1.22eV) perovskite compositions.The tandem devices featured an inverted device structure consisting of PET/ITO/MB-NiO/WBG perovskite/C 60 /ALD-SnO 2 /Au/PEDOT:PSS/NBG perovskite/ C 60 /BCP/Cu.Impressively, the WBG and NBG tandem cells achieved PCE of 24.7% and 23.5%, respectively, with device areas of 0.049 cm 2 and 1.05 cm 2 , respectively.Furthermore, the flexible tandem devices with molecularly bridged interfaces exhibited excellent stability, retaining their initial performance even after undergoing 10,000 bending cycles with a bending radius of 15 mm.Wang et al. reported a universal SAM-based HTL that rivaled the high-performance PTAA and PEDOT:PSS in WBG PSCs.The SAM, based on the structure of (4-(10-bromo-7H-benzo[c]carbazol-7yl)butyl)phosphonic acid (BCBBr-C4PA), combined an asymmetric conjugated backbone and bromination strategy in molecular design, thereby enhancing solubility and increasing the dipole moment (Figure 18E). [115]The BCBBr-C4PA HTL exhibited negligible light absorption and a lower HOMO energy level, promoting efficient interfacial charge transfer and suppressing nonradiative recombination losses.The WBG PSCs based on the BCBBr-C4PA SAM achieved a maximum PCE of 18.63%, with the efficiency remaining above 90% after 250 h of continuous operation.By stacking the optimized WBG PSC with an NBG PSC as the bottom cell, a 4T all-perovskite tandem solar cell achieved a remarkable PCE of 26.24% (Figure 18F,G).
The rapid efficiency enhancement of small-area (<0.1 cm 2 ) series-connected solar cells has primarily been driven by significant progress in the bottom subcell utilizing NBG perovskite materials (approximately 1.25 eV).However, challenges persist for the top subcell employing WBG perovskite materials (>1.75 eV), particularly in the context of large-area (>1 cm 2 ) tandem solar cells.Given this, He et al. introduced a novel approach by developing a SAM of 4-(7H-dibenzo[c,g]carbazol-7-yl) butyl phosphonic acid (4PADCB) as a hole-selective layer for WBG PSCs (Figure 19A). [116]The strategy aimed to address the growth of high-quality WBG perovskite films over large areas while mitigating interface nonradiative recombination and facilitating efficient hole extraction (Figure 19B).By incorporating 4PADCB into the device architecture, the researchers successfully demonstrated the growth of large-area, high-quality WBG perovskite films with a bandgap of 1.77 eV.The introduction of 4PADCB led to a remarkable V OC of 1.31 V, corresponding to an extraordinarily low V OC deficit of only 0.46 V relative to the bandgap energy (Figure 19C).The integration of these advanced WBG perovskite subcells resulted in the development of a monolithic allperovskite tandem solar cell with a total aperture area of 1.044 cm 2 , achieving a certified PCE of 27.01%.Notably, the certified tandem cell exhibited an exceptional combination of a high V OC value (2.12 V) and a remarkable FF of 82.6%.Jiang et al. demonstrated the value of gas quenching for high Br content perovskites, enabling both a high PCE and excellent operational stability, particularly with a high V OC . [117]They conducted extensive gas quenching studies using nitrogen gas (N 2 ).In contrast to conventional solvent-based methods, the gentle gas quenching approach offers a distinct methodology.This method involves the initial formation of a bromine-rich surface layer, followed by the growth of columnar structures from the top to the bottom of the material.As a result, a gradient structure is formed, with the near-surface region exhibiting higher bromine content compared with the bulk region.The device architecture employed a typical inverted configuration: glass/ITO/SAM/perovskite/LiF/C 60 /BCP/Ag.The SAM consisted of a mixture of MeO-2PACz and Me-4PACz.The resulting WBG perovskite films exhibited improved structural and optoelectronic properties, including larger grain size, longer diffusion length, and reduced defect density.To harness the benefits of the optimized WBG perovskite, an NBG (1.25 eV) FA 0.6 -MA 0.4 Sn 0.6 Pb 0.4 I 3 perovskite was integrated with the optimized 1.75 eV WBG perovskite to create an allperovskite tandem solar cell.The tandem device structure comprised glass/ITO/SAM/1.75eV perovskite/LiF/ C 60 /33 nm SnO x /1 nm Au/PEDOT:PSS/1.25 eV perovskite/C 60 /BCP/Ag, achieving an impressive PCE of 27.1% for the all-perovskite tandem cell along with a high V OC value of 2.2 V (Figure 19D,E).
F I G U R E 19 (A) Molecular structure of 4PADCB and schematic representation of the calculated electrostatic surface potential (ESP) and (B) interconnections with ITO substrates.Reproduced with permission. [116]Copyright 2023, Springer Nature.(C) QFLS values for the deposition of perovskite on 4PADCB.Reproduced with permission. [116]Copyright 2023, Springer Nature.(D) J-V curves and (E) stable power output of all-perovskite tandem solar cells based on 1.75 eV WBG perovskite.Reproduced with permission. [117]Copyright 2022, American Association for the Advancement of Science.(F) Schematic of tandem device structure based on Me-4PACz SAM, (G) external quantum efficiency (EQE) and (H) J-V curves of the devices.Reproduced with permission. [118]Copyright 2023, Springer Nature.The V OC deficit in WBG (greater than 1.7 eV) PSCs was larger compared with PSCs with a bandgap of approximately 1.5 eV.The QFLS measurements indicated that the V OC limitation arose from recombination at the ETL interface.Research findings have demonstrated that the nonuniform surface potentials and energy alignment between the perovskite layer and the ETL contribute to this observation.To address this issue, Chen et al. introduced 1,3-propanediammonium iodide (PDA) as a surface modifier to improve the surface state of perovskite, resulting in a more uniform spatial distribution of surface potentials (Figure 19F). [118]Encouraged by the reduced interface recombination and improved surface uniformity brought about by PDA treatment, WBG PSCs were fabricated.The device structure consisted of ITO/ NiO x /Me-4PACz/WBG perovskite/C 60 /ALD SnO x /Ag.With the incorporation of PDA, the QFLS increased by 90 meV, enabling the 1.79 eV PSC to achieve a certified V OC of 1.33 V and a PCE exceeding 19%.When the PDA was integrated into a monolithic all-perovskite tandem device, a V OC of 2.19 V and a PCE exceeding 27% (26.3% certified steady-state) were reported.These tandem devices retained over 86% of their initial PCE after 500 h of operation (Figure 19G,H).

| SAMs in perovskite-silicon (Si) tandem solar cells
Due to their covalent bonding with the substrate surface, SAMs offer increased durability during perovskite processing and can effectively maintain conformal coverage on textured surfaces.Therefore, SAM-based HTLs are excellent candidates for direct integration into monolithic perovskite-silicon tandem solar cells on textured silicon.Ou et al. successfully demonstrated the excellent stability of MA-free WBG perovskite as a light absorber (Figure 20A). [119]They introduced a novel strategy for simultaneous interface modification and surface passivation to mitigate interface barriers and recombination losses in the devices.A mixture of 2PACz and MeO-2PACz was employed as a SAM to reduce interface charge transport losses and promote the growth of larger perovskite grains (Figure 20B).The surface defects of the perovskite were passivated using 4trifluoromethylphenyl iodide ammonium.Consequently, the WBG-PSC exhibited significantly improved V OC and PCE.The devices with both opaque and transparent electrodes achieved PCE values of 20.11% and 17.80%, respectively.Remarkably, the unencapsulated opaque PSC retained over 85% of its initial efficiency after being stored in an N 2 -filled glovebox for 1750 h.Furthermore, a perovskite-Si tandem solar cell with a 4T architecture was fabricated, achieving an impressive efficiency of 26.59% (Figure 20C).
Stable and efficient perovskite-Si tandem solar cells require effective defect passivation and suppression of light-induced phase segregation in WBG perovskite materials.Isikgor's research demonstrated the utilization of phenylbiguanide hydrochloride (PhenHCl), a molecule containing both electron-rich and electron-poor parts, to fulfill these requirements independently of the surface chemistry of perovskite, grain boundaries, and interfaces (Figure 20D). [120]By passivating wide-bandgap (≈1.68 eV) p-i-n PSCs with PhenHCl, a remarkable improvement in the V OC of approximately 100 mV was achieved compared with the control device, resulting in an impressive PCE of up to 20.5%.The PhenHCl passivation approach effectively reduced ion migration and phase segregation, significantly enhancing the operational and thermal stability of WBG PSCs.Even after subjecting the devices to thermal stress for over 3000 h at 85°C in an N 2 environment, no loss in V OC was observed.Moreover, when combined with 2PACz SAM as hole-selective contacts for the perovskite layer, PhenHCl passivation further improved the PCE of perovskite-Si tandem solar cells from 25.4% to 27.4%.
To achieve high-performance inverted perovskite-Si tandem cells, Zheng et al. employed 2PACz as the surface modification material to optimize the NiO x /perovskite interface in tungsten-doped indium oxide-based inverted PSCs for tandem devices (Figure 20E). [121]The researchers conducted in-depth investigations into the underlying mechanisms that contributed to the superior performance of the NiO x /SAM HTL, which surpassed the results obtained with either a single NiO x or SAM HTL.By implementing additional light management techniques using PCBM, two key issues were successfully addressed: scattering reflection losses and parasitic absorption losses in the PSCs.This approach not only increased the energy allocated to the bifacial intrinsic thin film silicon solar cell but also effectively mitigated the losses resulting from diffuse reflection and parasitic absorption.Consequently, the perovskite-Si tandem device achieved an exceptional PCE of 27.6%.Ying et al. successfully demonstrated a monolithic perovskite/silicon tandem solar cell based on the tunnel oxide passivated contact (TOPCon) structure with nanostructured black silicon (b-Si) subcell (Figure 20F). [122]Through surface reconstruction, the passivation of TOPCon on b-Si was improved while its wide-band light absorption capability was maintained.The reconstructed b-Si exhibited a unique combination of low polarity and high dispersion of surface components, resulting in enhanced moisture resistance, improved wetting properties of the perovskite layer, and enhanced adhesion between subcells.More importantly, the perovskite layer grown on the reconstructed b-Si demonstrated highly crystalline properties with vertically aligned grain boundaries, facilitated by nanoscale confinement.The structural feature effectively reduced undesirable carrier recombination losses and promoted efficient charge collection at the interfaces.Additionally, they introduced a carbazole-based MeO-2PACz SAM as a hole-selective contact at the indium zinc oxide/perovskite interface.These advancements significantly improved the series-connected J SC and FF, ultimately resulting in a PCE of 28.5% (Figure 20G,H).
Mishima conducted a study on perovskite-Si tandem solar cells, examining the impact of blending two different SAMs containing carbazole cores (Figure 21A). [123]The SAMs chosen for investigation were MeO-2PACz and 2PACz.The MeO-2PACz was selected due to reported uncovered regions on ITO substrates coated with this SAM, while 2PACz was chosen as a companion SAM for its superior passivation quality and molecular compatibility with MeO-2PACz (Figure 21B).By employing analytical techniques such as XPS, CV, and electrochemical impedance spectroscopy, the study revealed that MeO-2PACz exhibited lower passivation quality, due to spatial effects causing exposed areas on the ITO surface.However, blending 2PACz with MeO-2PACz allowed for partial coverage of the uncovered regions of 2PACz with the smaller ligand, effectively improving passivation effects.The investigation focused on inverted perovskite-Si tandem solar cells incorporating the blended SAMs.Remarkably, the device F I G U R E 20 (A) Diagram of the four-terminal perovskite-Si tandem solar cell framework.Reproduced with permission. [119]Copyright 2023, Elsevier.(B) Full-width at half-maximum (FWHM) of a perovskite layer in a mixture of 2PACz and MeO-2PACz modified NiO x , and (C) J-V curves of tandem devices based on hybrid SAMs.Reproduced with permission. [119]Copyright 2023, Elsevier.(D) Structure of a perovskite-Si tandem solar cell and corresponding cross-sectional images.Reproduced with permission. [120]Copyright 2021, Elsevier.(E) Design of a perovskite-Si tandem device.Reproduced with permission. [121]Copyright 2022, Royal Society of Chemistry.(F) Light capture mechanism diagrams for planar and reconstructed b-Si samples, (G) J-V curves, and (H) EQE curves for planar and nanostructured perovskite-Si tandem solar cells.Reproduced with permission. [122]Copyright 2022, Elsevier.21C). [124]This extension of the conjugated system is meticulously tailored for high-bandgap PSCs and holds promise for tandem structures based on multiple-cation perovskites.The newly engineered SAM hole-selective layer exhibits a narrowed bandgap offset, precisely aligning its HOMO level with the VBM of the perovskite.Moreover, it showcases heightened surface wettability, facilitating superior perovskite film formation, an optimized perovskite/HTL interface, and accelerated charge extraction.Experimental results unequivocally affirm the effectiveness of this strategy.A p-i-n single-junction PSC with a 1.67 eV bandgap achieves an impressive efficiency of 21.3%, complemented by a V OC of 1.26 V and an outstanding FF of 82.6%.Integration of the Ph-2PACz into F I G U R E 21 (A) Structure of a perovskite-Si tandem solar cell based on mixed MeO-2PACz and 2PACz SAMs.Reproduced with permission. [123]Copyright 2022, IOP Science.(B) Surface property of mixed MeO-2PACz and 2PACz SAMs on ITO substrate, Reproduced with permission. [123]Copyright 2022, IOP Science.(C) Schematic of the perovskite/Si tandem solar cell with Ph-2PACz.Reproduced with permission. [124]Copyright 2023, Elsevier.(D) Certified J-V curve of the device based on Me-4PACz SAM.Reproduced with permission. [125]opyright 2020, American Association for the Advancement of Science.(E) J-V and (F) EQE curves of four-terminal perovskite-Si tandem solar cells based on Me-2PACz SAM.Reproduced with permission. [126]Copyright 2023, Wiley-VCH.a single-junction perovskite/silicon tandem propels the leading tandem device to a PCE of 28.9% and a V OC of 1.91 V. Crucially, postencapsulation assessments underscore the robust stability of the tandem cells.Enduring continuous 1-day illumination (680 h) and harsh humidheat conditions (280 h at 85°C), these tandem cells exhibit exceptional stability, underscoring the practical viability of the proposed technology. [124]Albrecht reported the development of a monolithic perovskite-Si tandem device, achieving a certified PCE of 29.15%. [125]The perovskite absorber used in this material possessed a bandgap of 1.68 eV.Importantly, it exhibited remarkable stability when subjected to illumination due to its ability to facilitate rapid hole extraction and minimize nonradiative recombination at the hole-selective interface.These features were achieved by incorporating a SAM of Me-4PACz as the hole-selective layer in the PSCs.The accelerated hole extraction was correlated with a low ideality factor of 1.26 and an impressive FF of up to 84%, resulting in a high V OC of 1.92 V in the tandem configuration (Figure 21D).Notably, the monolithic tandem cell without encapsulation maintained 95% of its initial efficiency after 300 h of operation in ambient air.
Tan et al. devised a novel strategy called Grain Regeneration and Bilateral Passivation (GRBP) to effectively reduce recombination losses at grain boundaries and perovskite/charge transport layer interfaces. [126]This was accomplished through a posttreatment process using a mixture of methylammonium thiocyanate (MASCN) and phenethylammonium iodide (PEAI) on the perovskite film.The addition of MASCN facilitated the regrowth of perovskite grains while promoting the infiltration of PEAI at buried bottom interfaces.The GRBP strategy successfully mitigated V OC losses at both the perovskite/ETL and HTL/ perovskite interfaces.Notably, the opaque WBG PSCs reached an impressive optimized PCE of 21.9% (V OC = 1.221V, J SC = 21.5 mA cm −2 , FF = 83.3%).Similarly, the best PCE achieved for semitransparent devices was 19.9% (V OC = 1.189V, J SC = 20.3mA cm −2 , FF = 82.0%).These devices exhibited excellent stability under continuous illumination, maintaining their initial PCE even after 500 h of exposure.Moreover, the researchers successfully fabricated perovskite-Si 4T tandem solar cells using the GRBP strategy.The perovskite top cell included a device structure of MgF 2 /glass/ ITO/MeO-2PACz/perovskite/C 60 /SnO 2 /ITO/MgF 2 .Notably, PCE of 29.8% and 28.5% was achieved for 0.09 and 1 cm 2 tandem devices, respectively (Figure 21E,F).
Chin et al. have showcased the enhanced PV performance of perovskite-Si tandem solar cells by utilizing phosphoric acid as a dual-function passivating agent for interface defects (Figure 22A).They employed Me-4PACz as the HTL and introduced 2,3,4, F I G U R E 22 (A) Schematic diagram and picture of a stack of perovskite-Si tandem device, (B) the EQE and (C) J-V curves of the device based on Me-4PACz SAM.Reproduced with permission. [127]Copyright 2023, American Association for the Advancement of Science.(D) Measurements of EQE and reflectance of perovskite-Si tandem solar cells at 100 and 40 nm IZO, and (E) the corresponding certified PV performance of the device.Reproduced with permission. [51]Copyright 2023, American Association for the Advancement of Science.EQE, external quantum efficiency; FF, fill factor; ITO, indium tin oxide; IZO, indium zinc oxide; Me-4PACz, [4-(3,6-dimethyl-9H-carbazol-9-yl) butyl]phosphonic acid; PCE, power conversion efficiency; PV, photovoltaic; SAM, self-assembled monolayer.
5,6-pentafluorobenzenephosphonic acid (FBPAc) into the perovskite precursor solution.The integration of Me-4PACz proved to be highly effective in reducing voltage losses at the interface between the perovskite layer and the HTL.Simultaneously, the incorporation of FBPAc during the perovskite deposition process successfully minimized voltage losses at the interface between the perovskite layer and the C 60 ETL, resulting in a more favorable perovskite microstructure.To further enhance the device performance, the perovskite layer was conformally coated onto a silicon substrate featuring micron-sized pyramids, which significantly improved the device photocurrent.As a result, the tandem device achieved an impressive certified efficiency of 31.25% with an active area of 1.17 cm 2 (Figure 22B,C). [127]To improve the stability and efficiency of monolithic perovskite-Si tandem solar cells, reducing recombination losses is of paramount importance.Albrecht effectively addressed this by combining interface modification of triple halide perovskite with pyridine iodide.This innovative approach resulted in improved energy-level alignment, thereby minimizing nonradiative recombination losses and enhancing charge extraction at the electron selective contact.Furthermore, Albrecht meticulously optimized the device fabrication process by incorporating Me-4PACz as the HTL for the top PSCs and introduced additives for better wettability.These refinements played a pivotal role in enhancing the overall device performance.As a result, the perovskite-Si tandem solar cell achieved an impressive V OC of 2.00 V, accompanied by a certified PCE of 32.5% (Figure 22D,E). [51]The application of SAMs in perovskite tandem devices and the corresponding PCEs are summarized in Table 4.

| SAMs in perovskite-CIGSe or organic tandem solar cells
Cu(In, Ga)Se 2 (CIGSe) thin-film solar cells possess desirable bandgaps, making them a promising choice for integration into perovskite-based tandem solar cells. [130]However, the rough surface of CIGSe presents challenges in the fabrication of thin hole-selective contact layers, which are essential for achieving high-efficiency perovskite top cells. [131]Al-Ashouri et al. have demonstrated the robustness of the self-assembly process even when confronted with the challenges posed by the rugged surface of CIGSe, which was realized through the immersion of a CIGSe bottom cell into a solution containing MeO-2PACz.Through the seamless integration of SAM into a monolithic perovskite/CIGSe tandem solar cell, they successfully prepared a device that consistently maintained a stable efficiency of 23.26% (area of 1.03 cm²). [78]Kafedjiska et al. conducted a comprehensive investigation into the performance of five different HTLs in monolithic perovskite and CIGSe tandem solar cells (Figure 23A). [128]The HTLs under investigation were nickel oxide (NiO x ), copper-doped nickel oxide (NiO x :Cu), NiO x +SAM, NiO x :Cu+SAM, and SAM (MeO-2PACz material).The study focused on understanding the carrier dynamics at the HTL/perovskite interface, with particular emphasis on identifying the limiting factors of the HTL/perovskite stack.To unravel these factors, the researchers employed time-resolved and absolute PL measurements, along with transient surface photovoltage analysis, on both the ITO/HTL/perovskite and CIGSe/HTL/perovskite stacks.The results revealed that the NiO x -based devices suffered from a substantial deficit in quasi-Fermi level splitting at open circuit (QFLS-V OC ).The deficit stemmed from issues such as poor electron capture, inefficient hole extraction at the NiO x / perovskite interface, and a shorter effective carrier lifetime within the majority of the perovskite layers (Figure 23B). [128]However, the incorporation of 2% copper into NiO x , coupled with the surface passivation of NiO x using MeO-2PACz, proved to be an effective strategy.The combination strategy successfully suppressed electron capture, enhanced hole extraction, mitigated nonradiative interface recombination, and improved energy band alignment.As a result, the NiO x :Cu+SAM configuration emerged as the most promising HTL for monolithic perovskite-CIGSe tandem devices, achieving an impressive PCE of 23.5%, a V OC of 1.72 V, and an FF of 71%.Brinkmann et al. analyzed the quasi-Fermi level splitting (QFLS) of perovskite under illumination, which serves as a reliable indicator for predicting V OC in solar cells (Figure 23C). [129]This analysis was instrumental in identifying potential losses in V OC associated with charge extraction layers adjacent to the perovskite.To address this, the researchers employed MeO-2PACz as the HTL, which formed a dense and pinhole-free SAM on the bottom electrode.Remarkably, the perovskite layer on MeO-2PACz as the HTL exhibited a significant increase in QFLS of 90 meV compared with its counterpart on PTAA (Figure 23D).Intriguingly, unlike PTAA, there was no observable halide segregation after several minutes when MeO-2PACz was used as the HTL, even with a Br:I ratio of 0.5:0.5.These findings suggest that an appropriate selection of the HTL could effectively mitigate halide segregation in perovskite, even with a higher concentration of Br.Furthermore, the researchers demonstrated the efficiency of the perovskite-organic tandem solar cell, by achieving an impressive PCE of 24.0% (certified at 23.1%) and a high V OC of 2.15 V. [129] These results were attributed to the optimization of the charge  extraction layer, which provided the perovskite subcells with both high V OC and FF.

| CONCLUSION AND OUTLOOK
Herein, we conducted a comprehensive review of the research progress on SAMs in inverted perovskite singlejunction and tandem solar cells, highlighting their pivotal role in interface modulation, enhancement of optoelectronic properties, and device stability.SAMs offer distinct advantages such as tunability, controllability, and stability.SAMs also offer a diverse range of functionalities through ultrathin molecular monolayers on substrates, with the reactivity between different anchoring groups and various substrates resulting in thermodynamically stable self-assembly.Leveraging their self-assembly characteristics enables the creation of highly ordered interfacial structures, ensuring uniform interface energy levels and optimized charge carrier transport pathways.This, in turn, presents a promising avenue for improving the performance of inverted PSCs.Utilizing interface engineering, SAMs facilitate the finetuning of interface energy levels, passivation of defects, and enhancement of charge carrier transfer efficiency in inverted PSCs.These modulation effects are of paramount significance in advancing the PV performance and stability of inverted perovskite single-junction and tandem solar cells.Despite the notable progress achieved in SAMs research for inverted PSCs, several challenges remain to be addressed.Future investigations can be directed toward the following areas: (1) Exploring SAMs with controlled dipole moments constitutes a crucial research aspect for realizing highly efficient PSCs.The effective modulation of dipole strength and direction can be accomplished through the integration of diverse anchoring groups and functional moieties.(2) Employing appropriate characterization techniques, such as surface-sensitive optical and scanning methods, to thoroughly investigate the microscopic structure of SAMs is imperative.Systematic exploration of the individual impacts of anchoring groups, spacing, and head groups on charge transfer processes is warranted.(3) The current mainstream SAMs primarily exhibit p-type characteristics, thereby underscoring the importance of developing n-type SAMs to enhance their widespread adoption and device efficiency.The development of dual-functional SAMs capable of facilitating both electron and hole transport holds promise F I G U R E 23 (A) Structural framework of the tandem perovskite-CIGSe solar cell and (B) tr-SPV measurements at f = 1 kHz based on the tandem device.Reproduced with permission. [128]Copyright 2023, Wiley-VCH.(C) Framework of the perovskite-organic tandem device, and (D) QFLS splitting in the case of PTAA or MeO-2PACz.Reproduced with permission. [129]Copyright 2022, Springer Nature.AZO, aluminum-doped zinc oxide; CIGSe, Cu(In, Ga)Se for reducing manufacturing costs and simplifying processing techniques.(4) To expand the repertoire of SAMs, it is crucial to explore novel core building blocks that provide a broader range of choices for different perovskite compositions and diverse device architectures.Emphasis should be placed on designing and optimizing the structure and properties of SAMs to enable more efficient bandgap tuning, superior charge carrier transport, and extended spectral responsiveness.Further advancements in the fabrication methods of SAMs are required to enhance precision, reproducibility, and scalability.( 5) Delving into the interactions and interface engineering strategies between SAMs and other functional layers is essential.Exploring the design and implementation of multilayer structures will enable more sophisticated interface engineering and performance optimization.( 6) Strengthening research on the stability and durability of SAMs in complex environments is vital for facilitating their commercial application in the field of PSCs.
In summary, the study of SAMs in inverted PSCs presents a promising pathway for improving device performance and stability.Future research should prioritize material design and optimization, interface engineering, improvements in fabrication techniques, stability investigations, and the exploration of multifunctional materials.Through sustained and in-depth research efforts, the application of SAMs will promote the commercialization of inverted perovskite single-junction and tandem solar cells.
x layer at low temperatures offered compatibility with flexible substrates.Additionally, the use of 2PACz (2-(2-(2-(4aminophenyl)ethylamino)−5-(4-bis(2-ethylhexyl) amino)phenyl)−1,4-diphenyl-1,3-butadiene) demonstrated excellent hole selectivity and the potential to fill any remaining uncovered areas on the NiO x surface.The surface modification with ethanolamine and 2PACz significantly enhanced the stability of these devices compared with those modified solely with 2PACz.Although the PCE between a modification with only 2PACz and the ethanolamine-2PACz combination was similar (PCE of rigid champion devices, 21.6%-22.0%;PCE of flexible devices, 20.2%-21.0%), the T 80 lifetimes of both rigid and flexible devices based on ethanolamine-2PACz were improved by more than 15 times under simulated solar light exposure attributing to the augmented defect passivation and improved perovskite film quality imparted by ethanolamine.Zhu et al. observed strong UV-induced deprotonation at the interface between NiO and perovskite, resulting in the generation of voids/ vacancies near the interface.
The structural properties and PCE summary of inverted PSCs based on different SAMs as HTLs.