New Pathways toward Sustainable Sn‐Related Perovskite Solar Cells

Sn‐related perovskite solar cells (PSCs) have emerged as one of the most promising lead‐free, environmentally viable photovoltaic technologies. Recent years have witnessed rapid development in terms of soaring photovoltaic performances of Sn‐related PSCs, progressively narrowing their power conversion efficiency (PCE) gaps to the Pb‐based counterparts. However, further enhancement of PCE and lifespan are largely limited by the easy oxidation of Sn2+ and by‐products‐induced defects. Beyond the stereotyped antioxidation strategies using reducing agents, in this perspective, several novel chemical pathways, which are able to simultaneously boost the PCE and stability of Sn‐related PSCs, are summarized and highlighted. In addition, the impact of molecular design on the antioxidation, de‐doping, defect passivation effect, and other optoelectronic properties of Sn‐based perovskite films and devices is elucidated. Last but not least, associated challenges and future research directions are also discussed and proposed for fabricating efficient, stable, and sustainable Sn‐related optoelectronic devices.

device. In addition, SnF 2 could also serve as nucleation sites to modulate perovskite crystal growth. [23] Similarly, metallic Sn is also an effective reducing agent to decrease Sn 4þ content in mixed Sn-Pb perovskites. Lin et al. introduced Sn powder into MA 0.3 FA 0.7 Pb 0.5 Sn 0.5 I 3 precursor solution and found that the color of the solution remained unchanged after being exposed to air for 2 weeks. [24] Moreover, the target film exhibited similar crystallinity and crystallographic orientation to the control one with substantially reduced concentration of Sn 4þ , leading to improved photoluminescence (PL) intensity and increased carrier-diffusion length up to 3 μm. Hydrazine and its derivatives with strong reducing ability are wildly used with high reproducibility. Song et al. utilized the high volatility of hydrazine to produce a reducing atmosphere when preparing Sn-based perovskite films to suppress the Sn 4þ species. [25] As a result, enhanced carrier lifetime and suppressed carrier recombination which were comparable to that of their lead-based counterparts were observed. Likewise, Wang et al. used phenylhydrazine hydrochloride (PHCl) as the antioxidant to stabilize FASnI 3 perovskite film. [26] The PHCl additive plays the synergistically reductive and hydrophobic effects, boosting the PCE to 11.4%. Recently, Huang and co-workers introduced a reducing agent benzylhydrazine hydrochloride (BHC) into narrow bandgap perovskite to stabilize Sn 2þ during film deposition. Combining the hot gas-assisted blade-coating technique, monolithic all-perovskite tandem solar modules with an area of 14.3 cm 2 showed a champion efficiency of 21.6%. [27] In addition, some reductive organic small molecules such as ascorbic acid (AA) and caffeic acid (CA) have also been applied as additives in Sn-based and mixed Sn-Pb perovskites to preventing Sn 2þ from being oxidized to Sn 4þ . [28,29] Beyond reducing agents, very recently, other novel chemical pathways with the assist of bespoke molecular/device structure design have been demonstrated to be more effective to prevent the destruction of Sn-related perovskites, while simultaneously played multifunctional roles in modulating the crystallization kinetics, managing the defect profiles, and optimizing the carrier dynamics of Sn-based perovskite films, thus affording more efficient and stable Sn-related PSCs.
In this perspective, we summarized and highlighted the recent advance on boosting the photovoltaic performance and stability of pure Sn and mixed Sn-Pb PSCs by using novel chemical pathways ( Table 1), such as surface engineering, novel metallic agent-assisted reduction and/or in situ protection, synergistic modification, and carrier-transport layer engineering. By elaborating the chemical mechanism and effectiveness of these novel approaches, we aimed to shed light on the significance of molecular/interfacial design on the optoelectronic properties of Sn-based perovskite films and devices. We also pointed out the remaining challenges, proposed possible future directions, and provided an insightful perspective toward making more sustainable and environmentally viable PSCs.

Surface Engineering
The fabrication of high-quality perovskite film with uniform morphology and desirable surface properties is essential for achieving high-efficiency PSCs, especially for pure Sn and mixed Sn-Pb PSCs, which are more sensitive to the external stimulus, such as oxygen and moisture. Hence, surface engineering with multifunctional molecules that both coordinate with Sn ions and hinder their oxidation is highly desired. Very recently, Zhou and co-workers found that the detrimental Sn 4þ ions mainly aggregated at the surface region of Sn-based perovskite films, and induced unfavorable p-type doping on the film surface. Accordingly, they developed a new surface engineering method, that is, chemo-thermal surface de-doping, to suppress the deleterious Sn (IV) self-doping (Figure 1a). [30] By means of thermal evaporation, an ultrathin layer of formamidinium hydrochloride (FACl) was deposited on top surface of FA 0.75 MA 0.25 SnI 3 perovskite film. FACl then interacted with self-formed Sn 4þ and formed a coordination complex of SnI 4 ·xFACl with much lower volatilization temperature (60°C) than SnI 2 (223°C) and SnI 2 ·xFACl (157°C). The preferential release of SnI 4 ·xFACl simultaneously removed Sn 4þ self-dopants from the top surface of Sn-based perovskite film without distinct effect on Sn 2þ after a sequential thermal annealing under mild temperatures. Furthermore, it was confirmed by depth X-ray photoelectron spectroscopy (XPS) profile that the release of SnI 4 ·xFACl during annealing would drive the Sn 4þ ions to diffuse from the bulk to the top surface, which dramatically decreased Sn vacancies and thus suppressed ion migration and enhanced intrinsic structural stability ( Figure 1a). After surface de-doping, one can observe blue shift of the PL peak center from 899.9 to 898.4 nm, owing to the decrease of Sn (IV) which not only increased the lattice parameters but also alleviated the lattice micro-strain. As compared to the control counterparts, the target devices demonstrated significantly improved photovoltaic performance with the champion PCE up to 14.7% (Figure 1b). Moreover, the champion device exhibited superior long-term shelf lifetime with 92% of the initial PCE retained after 1000 h storage in inert environment.
In addition, 2D or quasi-2D perovskite capping layer was widely demonstrated to deposit on the surface of 3D perovskite layer to modify the interfacial carrier dynamics and optimize the interfacial properties, which is beneficial to simultaneously improve the efficiency and stability of PSCs. However, the majority of posttreatment methods adopted organic ammonium iodide salts with large molecular size, which easily formed large bandgap (i.e., >2 eV) 2D perovskites with poor charge-transport characteristics. In this regard, Ning and co-workers synthesized a novel 2-thiopheneethylamine thiocyanate (TEASCN) molecule for surface treatment of mixed Sn-Pb perovskite films. [31] Different to the 2-thiopheneethylamine iodide (TEAI) molecule which formed a 2D perovskite structure with n = 1 (1L) (Figure 1c), the new TEASCN molecule enabled to precisely form quasi-2D perovskite structure (n = 2, 2L) on the 3D perovskite surface (Figure 1d) without changing the bulk perovskite structure ( Figure 1e). As for the steady-state PL, the perovskite film treated with TEASCN showcased the strongest intensity (Figure 1f ), which indicated a better passivation effect. Moreover, after being covered with C 60 , the PL intensity of TEASCN-treated film decreased more significantly than both of the pristine and the TEAI-treated samples. Likewise, the time-resolved PL (TRPL) decay characterizations showed a shorter PL lifetime of 136 ns for the TEASCN-treated film covered by C 60 , which was approximately 40% shorter than that of the TEAI film with C 60 (225 ns) (Figure 1g). Both the decreased  Reproduced with permission. [30] Copyright 2022, Elsevier. c) Schematic diagrams of perovskite structure evolution upon surface treatment by 2-thiopheneethylamine iodide (TEAI) (left) and the corresponding energy diagrams of resultant 3D/2D perovskites (right). d) Schematic diagrams of perovskite structure evolution upon surface treatment by 2-thiopheneethylamine thiocyanate (TEASCN) (left) and the corresponding energy diagrams of resultant 3D/quasi-2D perovskites (right). e) UV/Vis near-infrared (UV/Vis-NIR) absorption spectra of TEAI-and TEASCN-treated perovskite films, the inset is the corresponding Tauc plots. f ) Steady-state photoluminescence (PL) spectra of TEAI and TEASCN-treated perovskite films with or without a C 60 electron-transport layer. g) Time-resolved photoluminescence (TRPL) spectra of TEAI-or TEASCN-treated perovskite films covered with C 60 . Reproduced with permission. [31] Copyright 2022, Wiley-VCH GmbH. h) Transmission electron microscope (TEM) and high-resolution TEM (HRTEM) (inset) images showing the amorphous Sn-3X film capping on top of polycrystalline CsFASnI3 perovskite layer. i) High-resolution XPS spectra of the Sn 3d 5/3 region of Sn-2X and Sn-3X film before (top) and after (bottom) being exposed to continuous simulated AM 1.5G (100 mW cm À2 ) sunlight for 1000 h, respectively. Reproduced with permission. [32] Copyright 2021, Springer Nature.
www.advancedsciencenews.com www.advenergysustres.com PL intensity and lifetime could be associated with the 2L quasi-2D perovskite capping layer induced by TEASCN surface treatment, which reduced energy barrier for boosting charge transfer at the perovskite/C 60 interface, thus leading to an enhanced J SC of 31.20 mA cm À2 . Consequently, the TEASCN-treated mixed Sn-Pb PSCs exhibited a high PCE of 21.26%, accompanied by improved operational stability at the maximum power point (MPP), which dropped less than 5% of the initial efficiency after 1 h. Considering the intrinsic instability issue of pure Sn and mixed Sn-Pb PSCs, a robust surface barrier (e.g., amorphous capping layers) could effectively resist the oxidation of Sn 2þ , thereby enhancing device stability. Han and co-workers reported a novel amorphous polycrystalline bi-layered structure for the as-prepared pure Sn-based perovskite films. By adding both SnF 2 and SnCl 2 into the precursor solution, a uniform tin triple-halide amorphous layer (Sn-3X, X = I, Cl, F) spontaneously formed atop the Cs 0.2 FA 0.8 SnI 3 perovskite layer, which effectively blocked the diffusion of oxygen and moisture, as well as suppressed ion diffusion across the whole device ( Figure 1h). [32] The light stability was further confirmed by XPS measurement, as the Sn-3X-capped perovskite film contained much less Sn 4þ than that of the Sn-2X-capped counterpart (X = I, F) after continuous simulated AM 1.5G 1 sun illumination for 1000 h (Figure 1i). The champion device demonstrated a certified efficiency of 10.08%. Furthermore, more than 95% of the original PCE was remained after 1000 h 1 sun illumination. Since Sn-related perovskites are prone to be oxidized, surface engineering is of great importance to enhance film stability. Traditional surface passivation strategies usually rely on solution-processed treatment, which is difficult to control since Sn perovskites are well known to be highly sensitive to solvents such as ethanol and isopropanol. Therefore, chemo-thermal volatilization and in situ protection layer could effectively prolong the lifespan of PSCs without impairing the optoelectronic properties.

Novel Metallic Agent-Assisted Reduction and/or in situ Protection
As mentioned earlier, currently, Sn powder is one of the most widely used reducing agent in fabricating high-performance pure Sn and mixed Sn-Pb PSCs. Different from the direct introduction of reducing Sn (0) species, Nakamura and co-workers scavenged Sn 4þ impurities in FA 0.75 MA 0.25 SnI 3 perovskite by in situ formation of Sn (0) nanoparticle. According to 119 Sn nuclear magnetic resonance (NMR) and 1 H NMR spectra, SnF 2 was selectively reduced by 1,4-bis(trimethylsilyl)-2,3,5,6-tetramethyl-1,4-dihydropyrazine (TM-DHP) owing to strong affinity of the trimethylsilyl group in TM-DHP with fluoride atom, thus forming Sn (0) nanoparticles (Figure 2a,b). [33] Subsequently, SnI 4 was reduced by the Sn (0) nanoparticle to form SnI 2 . Combining the utilization of TM-DHP and SnF 2 , an impressive PCE of 11.5% has been achieved for the pure Sn-based PSCs. Fang et al. explored a novel galvanic displacement reaction (GDR) method in lead-less and MA-free PSCs. Different from the common PbI 2 ingredients and Sn powder used in mixed Sn-Pb perovskites, lead powder (Pb 0 ) was for the first time introduced into the perovskite precursor solution serving as both the Pb source and reducing agent (Figure 2c). [34] According to its standard redox potential, Pb powder would spontaneously and completely reduce Sn 4þ rather than Sn 2þ . Compared to the pristine Sn perovskite film which displayed fine grains and apparent cracks (Figure 2d), the GDR-Pb 0 film exhibited compact and dense morphology with enlarged grain size (Figure 2e). Benefited from the remarkable decline of iodide/tin-related defects, the intrinsic stability of mixed Sn-Pb perovskite was improved significantly, accompanied by enhanced photovoltaic performance. As a result, the champion device displayed a PCE up to 20.01% for the mixed Sn-Pb PSCs containing only 18.7 mol% Pb 2þ . Furthermore, the GDR-derived devices showed almost no efficiency loss after storing in N 2 for more than 2300 h, and 81% of the original efficiency was retained after continuous output at MPP for 700 h.
In addition to Sn or Pb powders, simply alloying Ge (II) with Sn perovskites also exhibited great potential to construct Sn-based PSCs with improved stability. Chen et al. fabricated highly stable FASn 0.9 Ge 0.1 I 3 -based flexible PSCs in combination of NiO x hole-transport layer. [35] Interestingly, owing to the higher oxidation activity of Ge (II) than Sn (II) and the reaction between Ge 2þ and Ni 3þ in NiO x , an ultrathin and uniform metal oxide (i.e., GeO 2 ) layer was in situ formed rapidly at the NiO x /FASn 0.9 Ge 0.1 I 3 interface (Figure 2f ). The thin and dense GeO 2 (i.e., %3 nm in thickness) interlayer not only imparted greatly enhanced stability for FASn 0.9 Ge 0.1 I 3 perovskite layer, but also enabled tunneling of carriers at the NiO x /FASn 0.9 Ge 0.1 I 3 interface. The target flexible device demonstrated a best PCE of 10.43%, exceptional operational stability under continuous AM 1.5G illumination (T 80 = 700 h), and outstanding mechanical reliability (n 80 = 2500 cycles) (Figure 2g). Very recently, by applying a reducing agent BHC both in the perovskite precursor solution and in the posttreatment, Huang and co-workers reported to effectively stabilize Sn 2þ during film fabrication, which could withstand invasion from ambient air. [27] Moreover, via intentional air exposure and appropriate aging in the glove box, a dense SnO 2 thin layer could be in situ formed on the surface of BHC-modified perovskite film, which facilitated the interfacial charge transfer and prolonged the carrier recombination lifetime (Figure 2h,i), thus enhancing the device performance. A champion efficiency of 20.3% was acquired for the mixed Sn-Pb PSCs fabricated via blade-coating method. Novel metallic sacrificial/reducing agents were selected according to the metal activities and standard redox potentials and introduced into Sn-related perovskite films to continuously prohibit Sn (IV) impurities after the raw materials were purified, while metal doping/alloying strategy would finely tune the optoelectronic properties and then enhance the photovoltaic performance as well.

Synergistic Modification
Synergistic modification of Sn-based perovskite films, namely, modulating the crystallization kinetics, improving the antioxidant capability, passivating different types of defects, and optimizing carrier dynamics, have played critical roles in ensuring high efficiency and good stability of resultant PSCs. In general, the synergistic modification can be simply achieved by employing composited additive system. For instance, Wang et al.
www.advancedsciencenews.com www.advenergysustres.com adopted SnCl 2 -GA (gallic acid) complex to wrap and protect the FASnI 3 perovskite grains, achieving high efficiency (>9%) devices. [36] The core-shell structure which was identified by scanning transmission electron microscopy (STEM) and electron energy loss spectra (EELS) (Figure 3a) effectively protected the inside Sn perovskite crystals and retard Sn 2þ oxidation, yielding eminent air stability, which could maintain %80% of their initial PCE after 1000 h of storage in ambient air with a relative humidity of 20%. Likewise, Yan and co-workers adopted a composited additive system consisting of 4-hydrazinobenzoic acid (HBA) and SnF 2 . [37] An amorphous complex of HBA-SnF 2 was spontaneously formed at the grain boundaries and encapsulated the mixed Sn-Pb perovskite grains, which could effectively suppress the oxidation of Sn 2þ (Figure 3b). Very recently, Sanchez-Diaz et al. developed a synergistic chemical engineering strategy, that is, using bi-additives composed of dipropylammonium iodide (DipI) and sodium borohydride (NaBH 4 ), to improve the efficiency and stability of FASnI 3 -based PSCs (Figure 3c). [38] Benefited from the synergetic effect of the two additives, the compact perovskite films with good surface coverage, oriented crystalline domains and smooth surface were obtained. Accordingly, the co-additive method exhibited better antioxidant capacity than that of the single-additive system. Moreover, iodide-related degradation of perovskite film under illumination (i.e., SnI 4 þ hv ! SnI 2 þ I 2 ) was significantly suppressed. This can be explained, on the one hand, DipI could passivate the perovskite film surface and block the defective sites that could accommodate the migrated I 2 ; on the other hand, NaBH 4 could preferentially reduce I 2 . As a result, the synergetic effect enabled to achieve an efficiency of 10.61%, accompanied by enhanced Reproduced with permission. [33] Copyright 2020, Nature Publishing Group. c) The photograph and schematic drawing of perovskite precursor before and after lead powder was added. The scanning electron microscope (SEM) images of d) pure Sn and e) galvanic displacement reaction (GDR)-Pb 0 (8.5) perovskite films grown on the ITO/P3CT-Cs substrate. Reproduced with permission. [34] Copyright 2021, Elsevier. f ) Schematic illustration of GeO 2 oxide interlayer served as a passivation layer and fitted and deconvoluted XPS spectra of Ge 3d at different detection depths. g) Bending durability of unencapsulated FASnI 3 -based and FASn 0.9 Ge 0.1 I 3 -based flexible PSCs with a NiO X HTL and FASn 0.9 Ge 0.1 I 3 -based flexible PSC with poly (3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT: PSS) hole transport layer (HTL). Reproduced with permission. [35] Copyright 2022, American Chemical Society. Carrier recombination lifetimes of NBG PSCs h) without air exposure and i) with air exposed for 5 min. The devices were measured before and after storage in the glove box for 2 days. Reproduced with permission. [27] Copyright 2022, Nature Publishing Group. operation stability with 96% of the initial PCE retained after 1300 h at MPP under N 2 atmosphere (Figure 3d). This result represented one of the most stable Sn-based PSCs at operational conditions reported to date. Hu et al. synchronously and systematically modified the top surface and buried interface of mixed Sn-Pb perovskite films via the combination of ethylenediammonium (EDA 2þ ) and glycinium (GlyH þ ) (Figure 4a). [20] Specifically, the mixed Sn-Pb perovskite surface was decorated by EDAI 2 solution, which could wash away the detrimental impurities, passivate the surface defects and reduce the Sn 4þ contents. Ultraviolet photoemission spectroscopy (UPS) result displayed a more n-type perovskite surface upon EDAI 2 posttreatment, which facilitated the electron extraction at the perovskite/electron-transport layer interface. In addition, glycine hydrochloride (GlyHCl) was introduced as the additive in perovskite ink. GlyHCl molecules were bonded to the perovskite colloidal particles and sedimented at the buried interface to serve as nucleation sites, leading to uniform crystallization of perovskite films with reduced defect densities, regardless of the bulk film or at the buried interface. The optimization of carrier dynamics was verified by TRPL. Increased PL lifetime excited from the perovskite surface was observed for the EDAI 2 /GlyHCl-treated films, which mainly originated from the surface passivation by EDAI 2 treatment. Likewise, with the back-side excitation, the lifetimes of EDAI 2 /GlyHCl-treated films were almost fourfold longer than that of the control samples (5.5 vs 1.5 μs), as a result of high-quality buried interface with less recombination centers optimized by GlyHCl. Surprisingly, the synergistic engineering strategy successfully produced 23.6% efficient mixed Sn-Pb PSCs with a decent fill factor of 82% (Figure 4b). To the best of our knowledge, this is the highest reported efficiency for the Sn-based PSCs, narrowing the PCE gap to the Pb-based counterparts. The champion device also showcased improved operational stability, with over 80% of the initial PCE remained after tracking at MPP condition for 200 h.
Synergistic modification plays a vital role in Sn-related perovskites, especially in mixed Sn-Pb perovskites bimetallic-sourced defects, which can be ascribed to the asymmetric reaction of organic component respectively with Sn and Pb could, thus leading to inhomogeneous and defective thin films, and more often than not, severe self-p-doping caused by undesirable Sn 2þ oxidization. Liang et al. established a selective targeting anchor (STA) strategy for achieving synergistic defect passivation on mixed Sn-Pb perovskites with ideal bandgap. [39] Phenethylammonium iodide (PEAI) and ethylenediamine Figure 3. a) Scanning transmission electron microscopy (STEM)-high-angle annular dark field images and the STEM electron energy loss spectra (STEM À EELS) maps of O, Sn, and I, corresponding to the FASnI 3 film with 1% GA and 7% SnCl 2 modification. Reproduced with permission. [36] Copyright 2020, American Chemical Society. b) TEM image of the FA 0.5 MA 0.45 EA 0.05 Sn 0.5 Pb 0.5 I 3 perovskite film with 3% 4-hydrazinobenzoic acid (HBA) additive and O K-edge of EELS spectrum measured at the grain boundary region as indicated. Reproduced with permission. [37] Copyright 2021, Wiley-VCH GmbH. c) The role of NaBH 4 and dipropylammonium iodide (DipI) during the process of surface passivation affecting the chemical properties of the Sn perovskite films under illumination. d) Normalized power conversion efficiency (PCE) of the unencapsulated FASnI 3 and FASnI 3 þ DipI þ NaBH 4 devices traced at maximum power point (MPP) in N 2 atmosphere. Reproduced with permission. [38] Copyright 2022, Elsevier.
www.advancedsciencenews.com www.advenergysustres.com diiodide (EDAI) were employed as the co-modifier to selectively anchor with Pb À and Sn À species and passivate respective defects (Figure 4c). It was found that EDAI preferentially anchors with Sn-related structure, while PEAI selectively interacted with Pb-based structure. Owing to the higher electronegativity of Sn atoms than Pb atoms, the [SnI 6 ] 4À octahedral exhibited lower electron density, which more favorably interacted with EDAI molecule with less positive electrostatic potential (ESP) than that of Reproduced with permission. [20] Copyright 2022, Royal Society of Chemistry. c) Illustration diagram of selective targeting anchor (STA) strategy and passivation mechanism. d) Forward and reverse scan of the J-V curves for the reference, ethylenediamine diiodide (EDAI), phenethylammonium iodide (PEAI), and STA devices. Reproduced with permission. [39] Copyright 2022, Wiley-VCH GmbH. e) Schematic illustration of defect profile in pristine Sn perovskite film with a variety of Sn or I-related defects, and the inside-out passivation and stabilization of Sn perovskite via 2-Guanidinoacetic acid (GAA) interaction and modification. f ) X-ray diffraction (XRD) patterns of the control and GAA-modified Sn perovskite films. The contour plot of transient absorption (TA) spectra of g) control device and h) GAA-modified device. Reproduced with permission. [40] Copyright 2022, Wiley-VCH GmbH.  (Figure 4d). However, aforementioned synergistic engineering normally needs rigorous molecular design and delicate optimization to achieve the positive coordination and effective cooperation of different additives. It is reported that there are more than 12 types of defects existed in Sn-based perovskites, making it a challenge and difficult to realize comprehensive defect passivation. [21] In a more ideal case, the synergistic modification can be smartly realized by using single additive, but with multifunctionalities. For instance, incorporating one type of multifunctional molecule with multidentate functional groups, which could concurrently heal structural imperfections of Sn-based perovskites in different ways is highly desired. Very recently, our group innovatively introduced multifunctional, bio-compatible 2-Guanidinoacetic acid (GAA), a multidentate chelator to regulate the crystal growth, alleviate the lattice distortion, and optimize the carrier dynamics of pure Sn perovskite films. [40] Embedded with both amino and carbonyl functional groups, GAA displayed comprehensive passivation effect on Sn-and I-related defects in an "inside-out" manner, especially for those Sn-and I-related antisite defects (Figure 4e). The GAA molecules distributed across the whole perovskite film from the internal bulk to the top surface, which enabled favorable chemical interaction with Sn 2þ ions and effectively retarded their oxidation even in ambient air. The GAA-modified perovskite film demonstrated enhanced crystallinity and preferential crystal growth along (h00) orientation ( Figure 4f ). Moreover, while 2D PEA 2 SnI 4 (n = 1) formed in the control perovskite film,the GAA-modified one subtly regulated 2D/3D phase distribution, showcasing the signal of quasi-2D perovskite with n = 2, which could strongly weaken the quantum hydrazine domain-limiting effect and facilitated the charge extraction and collection. This could be confirmed by femtosecond transient absorption (fs-TA) spectroscopy measurements, as the ground-state bleaching (GSB) signal of both 2D perovskite and 3D perovskite faded much more promptly for the GAA-modified sample than the control analogue (Figure 4g,h), suggesting a highly efficient charge transfer from the GAA-modified perovskite film to the adjacent carrier-transport layer. Meanwhile, the alleviated grazing incidence X-ray diffraction (GIXRD) peak shift and smaller slope of 2θ-sin 2 (Ψ) of perovskite film after GAA modification suggested mitigated intrinsic tensile strain and less lattice distortion. As a result, the GAA-modified PEA 0.15 FA 0.85 SnI 2.85 Br 0.15 PSC enabled a champion PCE of 13.70%, accompanied by an outstanding V OC of 0.93 V and a prominent long-term stability (i.e., T 90 > 1200 h).

Carrier Transport Layer Engineering
For the pure Sn and mixed Sn-Pb PSCs which commonly adopted the inverted device structure, there have also been some concerns regarding the instability and incompatible issue induced by hole-transport layer (HTL) materials.
Poly (3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT: PSS) demonstrated poor light stability and thermal stability, [39,41,42] while nickel oxide (NiO x ) normally contained a high density of Ni 3þ which could accelerate Sn 2þ oxidation, [43,44] and poly[bis(4-phenyl) (2,4,6-trimethylphenyl) amine] (PTAA) could lead to deficient photovoltaic performances due to its poor wettability and mismatched energy-level alignment with Sn-related perovskites. [45] Recently, Hayase and co-workers employed a novel self-assembled monolayer (SAM), that is, 2PACz ([2-(9H-carbazol-9-yl) ethyl] phosphonic acid) as the HTL to fabricate high-efficiency (23.3%) mixed Sn-Pb PSCs. [41] However, the SAMs which require elaborate fabrication control are prone to form defects and pinholes, thus limiting the reproducibility of PSCs. [46] Therefore, it is highly desired to explore superior carrier-transport materials for making efficient and reproducible Sn-related PSCs. Accordingly, Huang and co-workers reported efficient and stable methylammonium (MA)free Sn-Pb PSCs via applying a new poly[(phenyl)-imino[9-(2ethylhexyl) carbazole]-2,7-diyl] (CzAn) polymer HTL (Figure 5a). [45] CzAn HTL has an energy level well matched with the mixed SnÀPb perovskite layer, and enabled to deposit high-quality perovskite films due to its improved wettability after being modified with an ultrathin poly(methyl methacrylate) (PMMA) passivation layer. Compared to PEDOT: PSS, the modified CzAn resulted in a relatively smaller thickness of perovskite film with slightly weaker absorption. Nevertheless, the thinner perovskite film deposited on CzAn exhibited more obvious PL quenching, suggesting enhanced hole extraction. Combined with BHC passivation on the surface of perovskite layer, FA 0.8 Cs 0.2 Sn 0.5 Pb 0.5 I 3based PSCs achieved the highest PCE of 22.6% and demonstrated improved long-term photostability that maintained 90% of the initial efficiency after MPP tracking for 160 h, whereas the PEDOT:PSS-based device only kept a T 90 for 55 h. Cao et al. discovered a novel 2D conjugated metalÀorganic framework (2D c-MOF) Cu 3 (HHTT) 2 (2,3,7,8,12,13-hexahy-droxytetraazanaphthotetraphene, HHTT) HTL for mixed Sn-Pb PSCs. [47] On the one hand, enlarged π-conjugation and embedded heteroatoms of the organic moiety (HHTT) enabled strong interactions among ligands and favorable π À π stacking. On the other hand, the orbitals of divalent metal cations (Cu 2þ ) match well with the radicalstate ligands in energy, leading to in-plane π-configuration ( Figure 5b). As a result, the 2D c-MOF HTL demonstrated superior hole-transport capability. Via a layer-by-layer self-assembly method, an ultrathin (%6 nm) Cu 3 (HHTT) 2 film with high compactness and uniformity was formed on the ITO substrate even in a large scale (%1 cm 2 ). Subsequently, this ultrasmooth HTL facilitated the formation of perovskite film with high crystallinity, larger grain size, and alleviated lattice distortion. Compared to NiO x , the energy levels of 2D c-MOF better matched with the ideal-bandgap FA 0.83 Cs 0.17 Sn 0.35 Pb 0.65 I 2.9 Br 0.1 perovskite, thereby reducing the voltage loss. Consequently, the device based on selfassembled c-MOF HTL displayed a champion efficiency of 22.01% with a V OC of 0.92 V. Moreover, the target device showcased much better light stability, retaining 85% of the initial PCE after 300 h, while NiO x -based PSCs showed a rapid degradation of PCE to 65%. Meanwhile, the self-assembly process of the 2D c-MOF HTL can be applied to large-area devices, which yielded a PCE of 19.86% for the 1 cm 2 sized PSCs, which is one of the highest efficiencies for the large-area ideal-bandgap PSCs. In addition to organic materials, very recently, Huang and coworkers innovatively employed a ternary Sn (II) alloy of SnOCl as the HTL material via a simple solution process followed by post-annealing treatment. [48] Previous calculations have indicated that SnOCl which contained %90% of SnO was favorable to hole transport as the hybridization of Sn 5s and O 2p orbitals of SnO-introduced mid-gap states. Considering its work function (4.95 eV) which was close to that of PEDOT: PSS and high conductivity (2.7 Â 10 À3 S cm À1 ) and mobility (7.4 Â 10 À2 cm 2 V À1 s À1 ), SnOCl was regarded as a promising candidate for preparing HTL especially in Sn-based PSCs. In particular, the SnOCl-based mixed Sn-Pb PSCs displayed increased J SC than that of PEDOT: PSS-based devices, which not only resulted from the enhanced EQE in the long wavelength, but also originated from the shifted EQE response in shorter wavelength range (from 350 to 520 nm) of the target device. The variation in J SC was attributed to multifaceted benefits brought by SnOCl HTL: 1) reduced optical loss due to its texture structure with value-added functions of light antireflection or scattering in PSCs; 2) avoided the formation of small grains at the buried surface of perovskite films which might contribute to high trap density; and 3) enhanced electron-diffusion length (3.63 μm) of the resultant PSCs. Subsequently, due to the declined optical loss and enhanced charge extraction and transport, the target devices obtained a high efficiency of 22.2% with a remarkably increased J SC of >32 mA cm À2 . By inserting an ultrathin neutral PEDOT buffer layer between ITO and SnOCl HTL, the efficiency of mixed Sn-Pb PSCs was further improved to 23.2%. Additionally, the SnOCl HTL greatly enhanced the long-term stability of the Sn-Pb PSCs, which maintained 85% of the initial efficiency after heating at 85°C for 1500 h (Figure 5c) and retained 81% of the initial efficiency after continuous 1 sun illumination (without cooling) for 850 h (Figure 5d). The all-perovskite tandem solar cells based on hybrid neutral PEDOT/SnOCl HTLs achieved a higher averaged J SC (16.2 mA cm À2 ) and a high efficiency of 26.3% (stabilized 25.9%). Similarly, Wang et al. employed plasma-assisted deposition of SnO x as the HTL (P-SnO x ) and top protection layer (T-SnO x ) for Sn-based PSCs (Figure 5e). [49] To prepare SnO x , Sn layer was first deposited by thermal evaporation, followed by plasma treatment under air condition. While the longer plasma treatment led to a large amount of Sn 2þ in P-SnO x with deeper Fermi level and higher hole mobility than the commercial SnO 2 nanoparticle (C-SnO 2 ) and PEDOT: PSS, respectively, the momentary treatment produced a mixture of SnO 2 and Sn metal in T-SnO x , which was conducive to protect the perovskite surface and facilitate the electron transport (Figure 5f ). Finally, (Cs 0.02 (FA 0.9 DEA 0.1 ) 0.98 ) 0.98 EDA 0.01 SnI 3 -based PSCs demonstrated a champion PCE of 14.09%. Apparently, carrier-transport layer (CTL) is the key factor which limits the performance, stability, and reproducibility of Sn-related PSCs. A superior CTL requires not only appropriate energy levels that matched well with perovskites and high carrier mobilities for efficient carrier extraction and transport, but also positive effect on modulating the growth of perovskite layer and improving the stability of resultant PSCs. Moreover, the preparation of CTL should be compatible to large-area production for perovskite solar modules in a low-cost manner.

Conclusion
In conclusion, the aforementioned studies have shed light on the innovative passivation and stabilization strategies, including Figure 5. a) Device structure of mixed SnÀPb PSC and molecular structure of CzAn HTL. Reproduced with permission. [45] Copyright 2022, American Chemical Society. b) The fabrication process of a 2D c-MOF film based on Cu 3 (HHTT) 2 and the crystal structure of the material. H atoms are omitted for clarity. Reproduced with permission. [47] Copyright 2022, American Chemical Society. c) Long-term thermal stability for the Sn-Pb PSCs using PEDOT:PSS and SnOCl as HTLs under a thermal stress at 85°C for 1500 h. The tests were taken in a N 2 -filled glove box under dark. d) Long-term operation stability of the mixed Sn-Pb PSCs using PEDOT:PSS and SnOCl as HTLs under 1 sun illumination without cooling for 850 h. The temperature of devices under illumination is %50°C during the testing process. Reproduced with permission. [48] Copyright 2022, Wiley-VCH GmbH. e) Schematic of the device structure of Sn-based PSC, and the fabrication procedures of SnO x . f ) Energy-level diagram of Sn-based PSC with different functional layers as indicated. Reproduced with permission. [49] Copyright 2022, American Chemical Society.
www.advancedsciencenews.com www.advenergysustres.com exoteric surface engineering (chemo-thermal surface de-doping and quasi-2D bilayer surface passivation), novel metallic agentassisted reduction and in situ protection, synergistic modification, and carrier-transport layer engineering. Compared to the traditional performance optimization strategies applied in Sn-related PSCs, more flexible, comprehensive, and effective chemical passivation and stabilization strategies are imparted, yielding more prominent photovoltaic performances and stabilities. Though promising, Sn-based, and mixed Sn-Pb PSCs still encountered three critical issues: 1) the unsatisfactory long-term lifespan originating from the easy oxidation of Sn 2þ ; 2) severe V OC loss resulting from large mismatch of energy-level alignment between Sn-related perovskites and charge-transport layers; and 3) high defect density both at the interface of CTL/perovskite and across the bulk perovskite due to the uncontrollable perovskite film crystallization, which would contribute to significant carrier recombination loss. [16] Given the disparity between Sn-based and mixed Sn-Pb PSCs and ShockleyÀQueisser (SQ) limit, and the inferior stability compared to Pb-based devices, we provide an insightful perspective on the future development of Sn-based and mixed Sn-Pb PSCs, looking forward to a huge leap for the efficiency and stability. First, realizing efficient and stable Sn-related PSCs that can be prepared in ambient air is attractive for the mass production and commercialization of Sn-based PSCs. To overcome the easy oxidation of Sn 2þ , synthesizing high-quality Sn-based and mixed Sn-Pb perovskite crystals and using them to prepare perovskite inks, films and devices could be a promising way. The photovoltaic properties and stability of the Sn-related perovskites are highly correlated with the quality of raw materials and the chemical environment of precursor solution. As crystal growth process is a purification process, perovskite crystals possess accurate stoichiometric ratio, low density of trap states and excellent stability, which could be inherited by the perovskite films prepared by the redissolved crystals. [50] In addition, perovskite crystals redissolution strategy could also avoid Sn 2þ oxidation resulted from impurities and water in raw materials. Given that the precursor solution prepared by redissolved perovskite crystals contains a high content of perovskite clusters of colloidal sizes, [51][52][53] constructing in situ core-shell structure with perovskite clusters wrapped by a thin layer of insulating/amorphous materials in the precursor solution could further enhance the stability of Sn-related perovskites even during the air processing of perovskite films, thus fulfilling ambient air-processed efficient and stable Sn-related PSCs. [54] Second, one could explore the metallic redox shuttles that could synchronously enhance the stability of Sn 2þ and eliminate the B-site defects. In particular, the redox shuttle additives with suitable redox potentials could be introduced into mixed Sn-Pb perovskites to kill two birds with one stone, that is, to eliminate Pb-related and Sn-related defects in the perpetual redox-reaction cycle. According to the standard redox potentials, metallic redox shuttles that are more active than Sn 2þ could be exploited as the perfect reductant that can radically eliminate Sn 4þ . Meanwhile, Pb 0 defects could be oxidized to Pb 2þ again by metal ions with the higher redox potentials. [55] In this case, the defects are expected to be recovered back to active light-harvesting perovskites during device operation, thus ensuring remarkable PCE and exceptional stability. Third, developing adequate encapsulation technique to block the invasion of oxygen and moisture.
Adequate encapsulation could prolong the lifetime of photovoltaic device with minimized efficiency loss. This is a universal strategy to stabilize Sn-based and mixed Sn-Pb PSCs in ambient environment, which is especially important for the future practical application of Sn-related devices. A suitable encapsulation material requires good fabricability, high light transmission, superb photostability, and thermal stability, as well as excellent compatibility to Sn-related perovskites. [56] In the long run, simple encapsulation technique of low cost for large-area devices needs to be developed, in view of the disadvantages of commonly used atomic layer deposition (ALD) [57] and UV-curable epoxy for Sn-related PSCs. [58] Last but not least, further improving the efficiency and stability of large-area and flexible Sn-related PSCs is of great importance, to meet the requirements of commercial applications, such as artificial intelligence (AI), wearable electronics, and integrated photovoltaics. [59] We believe these strategies can boost the development of both Sn-based and mixed SnÀPb perovskites and photovoltaic devices, hence facilitating the commercialization in the market.