Copolymer Mediated Engineering of Halide Perovskites and Associated Devices: Current State and Future

The field of halide perovskites has advanced significantly within a decade, as engineering strategies are addressing many of the challenges and as a result improved environmental stability and electro‐optical properties have been achieved. The use of the copolymer additive strategy has received significant attention in recent years as a variety of polymers with significant differences in properties such as water affinity, polarity, elastic modulus, ion conductivity and basis for interaction with perovskite can be selected and combined with halide perovskite. As a result, there has been a rapid increase in publications reporting the effectiveness of the inexpensive and readily available copolymer additives in altering the physicochemical, mechanical, and electro‐optical properties of the halide perovskites‐based high‐performance devices. This article is an effort to provide insight into the current state of copolymer‐mediated engineering of halide perovskites with a perspective on the reported improvements in the properties and performance of the perovskite‐based devices. Critical analysis is done on the potential of the copolymer–perovskite materials toward realizing commercial applications.


Introduction
The field of halide perovskites has evolved rapidly since their first report of photosensitive ability in 2009. [1] Within roughly a decade, the power conversion efficiency of the perovskite-based single junction solar cells has surpassed 25% and is rivaling the conventional silicon-based photovoltaics. [2] Further, the excellent electro-optical properties of the halide perovskites have enabled the material's application in other devices and sensors. The photoactive material has thus been researched in diverse fields such as light-emitting diodes, photodetectors, display devices, sensors, self-powered devices, and so forth. [3] Despite its promising ability for technological advancements, the cost-effective scalable solution processability, and preliminary steps into commercial scale DOI: 10.1002/apxr.202200088 production of perovskite-based devices, the use of a simple strategy for complete eradication of the stability issues associated with the halide perovskite material and associated devices still plagues the field. [4] The halide perovskite material has poor ambient atmospheric stability and readily undergoes degradation in presence of external factors such as moisture, oxygen, light, and elevated temperature. [5] Recently, several other degradation pathways in extreme atmospheres such as carbon monoxide have also been reported. [6] The migration of constituent ions under an electric field due to their low activation energy further adds to the perovskite stability issues. [7] The grain boundaries, interfaces, and associated defects are primarily responsible for the perovskite instability. [8] These serve as the recombination and scattering centers, and preferential sites for moisture-associated degradation and seepage and pathways for ion migration. [9] Hence, in an attempt to capitalize on the excellent electro-optical properties of halide perovskites, the advancement in the performance of the perovskite-based devices has been accompanied by the development of multiple strategies to address such stability issues in halide perovskites. [10] Among the diverse strategies that are being actively researched, the polymer additive engineering strategy holds significant value as many of the polymers are inexpensive and readily accessible. [11] Polymers comprise a chain of repeat units (monomers) and can have diverse direct or indirect chemical interactions with perovskite precursors. [12] The presence of polymer can also modulate the crystallization kinetics and affect the growth of perovskite grains leading to alteration in the grain size. [13] The presence of specific molecular level interactions can also improve perovskite stability through grain boundary passivation and the formation of a crosslinked polymer network. [14] This may further prevent the migration of ions possessing low activation energy, across the perovskite grain boundaries. [15] Furthermore, viscoelastic polymers increase the flexibility of perovskite films due to the ability of the polymer chains' to entangle with each other and improve the dissipation of energy in the composites. [16] These properties make polymers promising passivating agents for achieving long-term stability and mechanical flexibility in perovskite devices. [17] Copolymers (polymers comprising more than one chemically distinct monomer) take the polymer additive engineering strategy one step forward and promise to combine the advantages www.advancedsciencenews.com www.advphysicsres.com offered by each monomeric group in the resultant composite. A copolymer can be categorized as a block copolymer or a random copolymer. Unlike random copolymers where different monomers are distributed within the polymer chain in an arbitrary manner or alternating fashion, in the case of a block copolymer, the different monomeric units are grouped in discrete homogeneous sections (or blocks) of the chain. [18] Just as polymer mixtures can separate into different phases, the segments in the block copolymer can also form morphologically distinct configurations. The covalent bonding between the block segments, however, prevents the macroscopic phase separation in block copolymers. [19] This leads to the nanoscale structural organization of each block with the domain size varying between 5 and 100 nm. The number of distinct homopolymer sections present in a block copolymer governs its molecular architecture and block, triblock, and even higher multiblock copolymers can be realized. The use of copolymers especially block copolymers is not new to the electronics industry. Block copolymers have been long utilized for high-resolution patterning because of their simplicity, elegance, and high throughput. [20] The early work of block copolymers as a microlithographic photoresist can be traced back to the late 1980s where F-or Si-containing block copolymers were being explored. [21] Copolymer structure-directed nanomaterials since then have been realized for utilization in photovoltaics, batteries, and fuel cells. [22] In the past few years, literature highlighting efforts related to the use of copolymers for stabilizing and improving the properties of perovskites have emerged. It is debatable if the copolymers have in a true sense been able to effectively address the stability issues and improve the properties of the perovskite materials by combining properties of the parent homopolymers. Hence, critical analysis and discussion on the reported results and the future direction of copolymer-perovskite composites are required to realize the full potential of this strategy. While strictly restricting to the reports with direct integration or interaction of copolymer with halide perovskites, this article presents a detailed summary of the progress made in this field and identifies areas that have been overlooked. The article is divided into sections discussing the utilization of copolymers across the diverse field of perovskite solar cells (PSCs), light-emitting diodes (LEDs), quantum dots (QDs), and nanocrystals, and other miscellaneous applications.

Integration of Copolymers Additive in Perovskite Layer of Solar Cells
The early reports of the utilization of copolymer in the perovskite layer of solar cells can be drawn back to 2014 when Tan et al. used the block copolymer of poly(isoprene)-block-poly(styrene)block-poly-(ethylene oxide) (PI-b-PS-b-PEO) triblock terpolymer derived mesoporous alumina superstructures to form a porous composite with methylammonium mixed halide perovskite. [23] The work emphasized the structural evolution of the perovskite film, and the role of the copolymer was as a template. The first true additive engineering of a copolymer can arguably be accredited to Xiang and co-workers. The authors embedded the block copolymer of poly(dimethylsiloxane)-urea (PDMS-urea) in the perovskite layer of a solar cell. [24] The authors capitalized on the ability of urea to strongly interact with perovskite (CH 3 NH 3 PbI 3 ) through hydrogen bonding (Figure 1a). While the hydrophobic dimethylsiloxane and carbamide backbone of PDMS was accredited for improving the stability of the as-fabricated devices by preventing moisture invasion, it was hypothesized that the potential chemical interaction between the ─H atoms of the hydroxyl group with ─N atoms of the organic moiety and interaction of ─O atoms of carbamide with ─I atoms of PbI 2 moiety in the perovskite was responsible for tailoring the nucleation rate leading to alteration in grain size and morphology as a function of copolymer added as well as its chain length. The best overall photovoltaic performance of the devices was observed when PDMSurea with an intermediate concentration of 10 mg mL -1 and 2500 Mw was added to the perovskite precursor solution. The authors also showed low leakage current in dark. However, despite the insulating nature of the copolymer additive, the hysteresis effect was not completely alleviated. Since the hysteresis effect in perovskites is usually credited to the presence of large defects and trap states and also, the migration of constituent ions, it appears that the copolymer was partially successful in passivating the defect sites at grain boundaries and reducing ion migration. This result may point to that the strength of the interaction between the copolymer constituents with the perovskite precursors (PbI 2 and CH 3 NH 3 I) may be critical for device performance. The optimized concentration of 10 mg mL -1 PDMS-urea additive led to a 27% increase in the PCE (16.15%) in comparison to the pristine perovskite solar cell (PSC) with a PCE of 12.68%. Furthermore, the devices fabricated with 20 mg mL -1 additive exhibited unchanged efficiency (13.77%) for more than 2500 h when stored at 50% relative humidity in dark. On the other hand, the pristine perovskite devices lost 60% of their initial device efficiency (12.68%) within 168 h.
In the same year, Meng et al. spin-coated an ultrathin interfacial copolymer layer of perylene diimide and dithienothiophene (PPDI-DTT) on top of the perovskite to passivate the trap sites and enhance charge transport across the interface of perovskite and electron transport layer (ETL) in an inverted solar cell configuration. [25] The authors combined the classical n-type nature of PPDI polymer with the ability of the electron-rich DTT (due to the presence of ─S) to form Lewis adduct with the undercoordinated Pb 2+ in mixed halide perovskite (CH 3 NH 3 PbI 3-x Cl x ). This work claimed direct evidence of a reduction in shallow and deep defects in copolymer passivated perovskite films at room temperature by measuring the energetic profile of the trap density of states (tDOS) by thermal admittance spectroscopy. The tDOS of the PPDI-DTT modified perovskite films decreases by around 50% in the energy depth of 0.40-0.45 eV, while the tDOS in the energy depth below 0.40 eV decreases slightly, which indicates that the passivation of the PPDI-DTT interfacial layer mainly occurred at the surface of perovskites leading to efficient passivation of deep traps. This was also supported by faster quenching and blue shift of the photoluminescence (PL) peak in the PPDI-DTT passivated films and efficient charge extraction ability in the PPDI-DTT/PC 61 BM bilayer compared with the pristine [6,6] phenyl-C61-butyric acid methyl ester (PC 61 BM) ETL. The efficient trap passivation, high absorption extending up to the near-infrared range, high electron mobility, and matching of the LUMO energy level of the copolymer with the conduction band of perovskite and LUMO of PC 61 BM were accredited for improved extraction of photogenerated electrons leading to improved PCE (16.5%) in the PPDI-DTT Figure 1. a) Chemical structure of PDMS-urea and the proposed mechanism of its interaction with the perovskite precursors. Reproduced with permission. [24] Copyright 2017, The Royal Society of Chemistry. b) Low magnification TEM image of MAPbI 3 -0.5 P123 film and high magnification TEM images of thin films: c) MAPbI 3 and d) MAPbI 3 -0.5 P123. Reproduced with permission. [26] Copyright 2018, Elsevier Inc. ToF-SIMS elemental depth profiles of e) pristine and f) PS-PAN-incorporated MAPbI 3 half-cells before (solid) and after (dash) the thermal aging with normalized logarithmic intensities for probed elements. Analyzed depth profiles of volume fractions of MA + and Pb + ions for g,i) pristine and h,j) PS-PAN-incorporated MAPbI 3 half-cells before g, h) and after i,j) thermal aging, respectively. Brown dashes indicate the surface of the perovskite structure, and blue dashes represent the surface of MA + -rich components. Reproduced with permission. [31] Copyright 2021, American Chemical Society. k) Inspiration of brick-mortar structure where the top inset shows the SEM image of nacre (the scale bar is 10 mm). l) Configuration of the wearable PSCs and chemical structures of insoluble SBS and soluble PU. m) Photographs of wearable PSCs as a power source to power a smartwatch, demonstrating the fitting skin and bendable, stretchable and twistable properties of wearable PSCs. n) Normalized average PCE measured after bending 100 cycles within a curvature radius from 5 to 2 mm. o) Normalized average PCE of PSCs as a function of bending cycles with a radius of 2 mm. p) J-V curves of the champion PSCs measured under 0%, 10% and 20% stretching, respectively. q) Normalized average PCE of PSCs as a function of stretching cycles with 10% and 20% stretching. Reproduced with permission. [33] Copyright 2019, The Royal Society of Chemistry. modified solar cell with respect to the pristine device (15.3%). Under an inert atmosphere, the unencapsulated PPDI-DTT copolymer-modified PSC maintained 110% of its initial device efficiency after 480 h in contrast to the pristine solar cells where the performance remained unperturbed at 100% of its initial value. It will be interesting to further research the effect of this copolymer strategy towards stability in ambient atmosphere and humidity needs to be further researched.
Zong et al. demonstrated continuous chemical functionalization of perovskite grain boundaries using a lower molecular weight commercial triblock copolymer of Pluronic P123. [26] The authors emphasized an ideal copolymer additive satisfying three key requirements to achieve improved device performance and long-term stability. The authors noted the preliminary need for a hydrophilic polymer component to ensure the dissolution and dispersion of the polymer additive in the polar solvents of perovskite precursor solutions. Second, the ability of the polymers to form a hydrogen bond with organic perovskite precursor to enable the formation of heterogenous perovskite/polymer phase at the grain boundaries. Lastly, the authors stressed on the presence of a hydrophobic polymer constituent to avoid moisture ingression. The poly(ethylene glycol)-poly(propylene glycol)-poly(ethylene glycol) (PEG-PPG-PEG) triblock copolymer satisfies the above three requirements owing to the presence of hydrophilic PEG tails with ─O atom capable of forming hydrogen bonds with MAI, and hydrophobic PPG core. Interestingly, the crystallinity and grain size reduced with an increase in the concentration of the P123 additive signifying increased heterogeneous nucleation in the perovskite film. This structural and morphological alteration was deemed beneficial unlike the majority of other reports. The pinhole-free dense packing of small perovskite grains along with passivation of defect sites was considered the reason for observing an increase in the PCE (17.6% to 19.3%) and V oc values (1.03 to 1.11 V) when 0.5 wt% P123 was added in the perovskite precursor solution. The authors further claimed that the amorphous walls observed between the www.advancedsciencenews.com www.advphysicsres.com methylammonium lead iodide (MAPbI 3 ) grains in transmission electron micrographs can be considered direct evidence of the P123 passivating the grain boundaries exclusively (Figure 1b-d). It is undeniable that the long-range macromolecules settle preferentially at the grain boundaries, however, since sample preparation for TEM differs from the method for fabricating 3D perovskite films further evidence may be valuable to confirm that the presence of P123 is restricted at the grain boundaries. Unlike the previous report by Meng et al., in this work authors observed copolymer-integrated films to show a longer PL lifetime. They ascribed it to reduced nonradiative recombination in the modified films. The authors used the evolution of space charge limited current (SCLC) in dark as a function of bias voltage in a capacitor-like device (perovskite layer sandwiched between two ETL) to calculate the trap density in the as-prepared films. The P123-perovskite composite film showed a 30% reduction in the SCLC values signifying the role of the copolymer in reducing the trap density. The PSCs based on MAPbI 3 -P123 thin films (with 0.5 wt%) exhibited significantly higher PCE retention (92%) than those based on pristine MAPbI 3 (80%) in the ambient atmosphere (25% RH, room temperature) over 30 d.
Based on a similar strategy, Wang et al. utilized another Pluronic triblock copolymer F127. [27] Similar to P123, the motivation for the addition of the poly(ethylene oxide)-co-poly(phenyl oxide)-co-poly(ethylene oxide) (PEO-PPO-PEO) triblock copolymer was to utilize its amphiphilic nature. The authors believed that the ─OH group in the PEO tails may facilitate hydrogen bond formation with the perovskite precursors while the hydrophobic backbone of PPO can avoid moisture ingression and improve thermal resistance in the composite films. However, merely the addition of the copolymer additive led to reduced crystallinity and grain size in the polymer-perovskite (F127-MA 0.7 FA 0.3 PbI 3 ) composite films. The enhanced density of the grain boundaries and associated defects in the composite films was reflected in the reduced photovoltaic performance of <18% in comparison to the 19% PCE observed for the pristine PSC. The device efficiency for the copolymer-perovskite films based solar cells increased only when the authors utilized the solvent annealing technique. Since the perovskite precursor solution used in the fabrication of the films discussed in the work involved multiple solvents and another additive (Pb(SCN) 2 ), there is a possibility of the interplay between the Pb(SCN) 2 and the F127 when the films were annealed in the presence of limited DMF during the extra step of solvent annealing. This also adds to the long-running debate if structural properties such as enlarged average grain size and improved crystallinity govern the overall device performance and are more crucial than the passivation of defect sites at the grain boundaries. The solvent annealing technique indeed improved the grain size (>1 μm) in the composite films although still not to the level of the control film which exhibited a grain size of 3 μm. The resultant solar cell with 2.5 mg mL -1 F127 exhibited a champion PCE of 21.01% with a high V OC of 1.160 V. The best flexible PSC achieved a PCE of 18.71%.
The work by Silva et al. also capitalized on the hydrogen bondforming ability of the PEO polymer and used the copolymer of poly(ethylene oxide-co-epichlorohydrin) P(EO-EC) to form a barrier against moisture and stabilize the MAPbI 3 films. [28] The use of PEC in combination with the PEO and its (PEC) role with respect to perovskites needs to be further researched. The addition of copolymer affected the crystallization kinetics but deteriorated the grain size and morphology of the resultant composite films. Although the hysteresis index improved in the composite solar cells, the PCE was reduced to 13.31% (for 2 mg mL -1 P(EO-EC)) in comparison to 17.04% for control devices. This work is unique as it claimed that the addition of the copolymer due to the specific interaction of ─O with NH 3 + cation in the perovskite limited the evolution of the methylamine gas phase and led to partial restoration of the perovskite structure when exposed to a humid atmosphere. Further research in relation to the decomposition pathway would help understand the effect of the copolymer. The reported devices were prepared in an ambient atmosphere (RH 35%) which is beneficial for manufacturing. Further, the device containing 1.5 mg mL -1 P(EO/EP) retained 68% of the initial PCE values after 480 h (20 d) in comparison to 47% retained in control devices, in the same environmental conditions with a relative humidity of 53%.
Ye et al. synthesized a perfluorocarbon copolymer (PFOMA) by combining two monomers: DOPAMA and FOMA monomers possessing two phenolic hydroxyl groups and a long C─F chain, respectively. [29] The copolymerized polymer was dissolved in the antisolvent chlorobenzene as a perovskite-HTL interfacemodifying agent. The PFOMA molecule delivered a bifunctional effect. While the outward positioned hydrophobic fluorinecarbon (C-F) chain inhibited oxygen and moisture invasion, the two catecholic OH group (Ph-OH), chelated with uncoordinated Pb 2+ through the donor atom O. The polymer hence was claimed to considerably passivate the surface defects at the interface of the perovskite and HTL Since the effect of antisolvent treatment are observed preferential to the top interface of the perovskite layer, the presence of the copolymer being exclusively restricted at this interface with limited presence across the depth perovskite film cannot be ruled out. Despite that, the ion migration and humidity or oxygen-induced device degeneracy were incredibly suppressed. In RH of 70% and at room temperature, the copolymermodified unencapsulated solar cells survived 90% of initial PCE for 35 days while the efficiency in the pristine PSC was reduced to 40%.
There have been instances where contrasting effects from the same copolymer have been reported. The reports by Zhou et al. and Yang et al., both demonstrated enhanced thermal stability of perovskite films when the copolymer of polystyrene (PS) and polyacrylonitrile (PAN) was added to them. The structure of the polymers becomes important as to whether they are block or random copolymers, as it will affect their interaction with the perovskite. While in the work by Yang et al., the authors presented the lone pair coordination ability of the -CN (of PAN component) with the undercoordinated Pb 2+ and in-house synthesized PS-PAN modification led to retarded nucleation rate and enlarged grain size. [30] Zhou et al. demonstrated that the increase in the concentration of PS-PAN resulted in reduced grain size in the perovskite film as it provided heterogeneous nucleation sites. [31] The authors hypothesized that the uniform grain distribution albeit with reduced grain size and increased grain boundaries can be considered representative of effective seepage of PS-PAN copolymer in the bulk of perovskite film. It will be crucial to discuss the effect of the viscosity of the antisolvent solution containing the dissolved copolymer as it will vary as a function of the chain length of the copolymer which in turn may determine the flow and rearrangement of polymer chains within the bulk of perovskite. The improved thermal resistance in the copolymermodified perovskite was ascribed to the high decomposition temperature and glass transition temperature (T g ) of the PS-PAN copolymer. The higher T g ensured the non-volatility of the copolymer chains unlike other small organic molecules and limited their diffusion out of the perovskite film. Similar to the report by Yang et al., Zhou et al. in their work dripped the copolymer dissolved in the chlorobenzene antisolvent during the spin coating of the perovskite films. They found the antisolvent-assisted dripping of copolymer during perovskite spin coating being effective than coating a separate surface layer of the copolymer on top of the perovskite film. The higher contact angles observed for the PS-PAN modified films were ascribed to PS constituting 69% of the copolymer's molecular weight. With the help of time-of-flight secondary ion mass spectroscopy (ToF SIMS) depth profiling, authors concluded that the thinner MA + region being observed at the surface in PS-PAN treated films (in comparison to the thicker MA + region in pristine perovskite films) was an indication of reduced outbound loss of volatile MA + upon thermal aging (Figure 1e-j). ToF-SIMS depth could also be used for profiling directly the ionic clusters representing PS-PAN chains being sputtered from the bulk of the perovskite layer. While the majority of the reports emphasize favorable chemical interaction between perovskite precursors, the authors hypothesized a repulsive interaction between the cyanide group (in PAN component) of the copolymer and MAI as being responsible for their immiscibility with each other, especially in the case where PS-PAN was surface coated on top of the perovskite film. The reduced MA + outmigration led the PS-PAN integrated PSCs to show just 5-10% decay in device performance over 24 h of thermal aging at an elevated temperature of 100°C. This is a promising result. However, longterm stability results are vital to adjudicate the effectiveness of the PS-PAN as an ideal temperature-resistant copolymer, which can progress it for practical device application. Moreover, since the limited out-diffusion of the MA + from the perovskite interface was considered the primary reason behind the limited degradation of perovskite film, it will be interesting to see if the same copolymer in future can be extended to other halide perovskite systems devoid of organic cationic sites to improve their thermal strength.
Owing to the concerns surrounding the toxicity of lead, tinbased halide perovskites are emerging as potential environmentally sustainable alternatives. However, the rapid oxidation of Sn 2+ to Sn 4+ is the critical challenge that needs to be addressed to stabilize this class of halide perovskites. Poly(ethylene-co-vinyl acetate) (EVA) was used by Liu et al. to do this as it exhibited a grain encapsulation effect against moisture and oxygen. [32] The solar cell devices demonstrated a retention of 62.4% PCE after aging for 48 h in the air with an RH of 60%. This was ascribed to the formation of Lewis acid-base adduct between the C═O group of the EVA and undercoordinated Sn 2+ ions at the perovskite grain boundaries which led to concurrent improvement in the grain size, decrease in the surface defects, and impressive PCE of 7.72% in the EVS-FASnI 3 solar cell. The role of the polyethylene needs to be further clarified to understand the combined effect of the copolymer on improved performance.
Since wearable electronics have triggered extensive interest owing to their application in next-generation robotics, bionics, sensors, and other areas, the work by Hu et al. can be considered state-of-art and deserves attention. [33] To our knowledge, this is the only work that capitalized on the advantages offered by a copolymer in tandem with another homopolymer. Inspired by biological nacre, the authors demonstrated a biomimetic crystallization approach to fabricate perovskite-based wearable solar cells by introducing an antithetic soluble composite matrix (Figure 1k-m). The authors showed that insoluble poly(styrene-cobutadiene) (SBS) scaffold may reduce the nucleation. Simultaneously, another homopolymer mixed with perovskite precursor solution, polyurethane (due to the presence of a C═O bond) could undergo interaction with the lead halide component suppressing the crystallization rate. With the combined strategy of using a copolymer (in antisolvent) and homopolymer (mixed with perovskite precursor solution) in a single step, a flexible solar cell with a champion PCE of 15.01% was realized (Figure 1n-q). The SBS-PU enabled uniaxially oriented growth of perovskite grains and a reduction in Young's modulus value of 260 MPa in control devices to 193 MPa in SBS-PU perovskite devices. These improvements were reflected as the authors successfully demonstrated a large area (56.02 cm 2 ) wearable power source owing to the wearable structure design and the elastomeric polymer filling the perovskite grain boundaries.

Role of Copolymer Additive in LEDs
The role of copolymers in light emitting diodes (LEDs) has been more often to serve as a matrix and embed luminescent perovskite QDs or nanocrystals. The initial report draws back to 2018 when Lin et al. used the electrospinning technique to incorporate CsPbX 3 (X = Cl, Br, and I) nanocrystals in poly(styrenebutadiene-styrene) (SBS) to prepare moisture-resistant fibers and realized multicolor optical layers. [34] The QDs embedded in the hydrophobic fiber membranes continued to emit bright fluorescence for over 1 h when immersed in water while the unprotected nanocrystals completely degraded within 10 s of their immersion in water (Figure 2a-f). It will be interesting to see if other inexpensive hydrophobic polymer system can be used for the same effect. The work reported combining a blue chip with green and red perovskite/copolymer optical layers. The optimized LEDs produced warm white light emissions with a luminous efficacy (LE) of up to 9 lm W -1 and a broad color gamut of 105% of the NTSC standard under an applied current of 10 mA.
The hydrophobic nature, transparency, and flexibility of the poly(ethylene-co-vinyl acetate) (EVA) copolymer were utilized by Li et al. to embed CsPbBr 3 perovskite QDs. [35] Authors reported a drop in photoluminescence quantum yield (PLQY) with an increase in the concentration of perovskite precursor solution mixed with EVA, as quenching occurs with higher concentration loading of QD due to their aggregation. The prepared flexible CsPbBr 3 PQDs/EVA films exhibited a negligible change in PL intensity when repeatedly bent for 1000 cycles. A white LED device was constructed by combining the green-emitting perovskite/EVA film with (Sr,Ca)-AlSiN 3 :Eu 2+ red phosphor on an InGaN blue LED chip. The assembled white LED showed a colorrendering index (CRI) of 74.7, a low correlated color temperature (CCT) of 2347 K, and an LE of 37.7 lm W -1 . It is interesting to note that unlike reports of the utilization of EVA as a copolymer and its role in coordinating with the Lewis acid characteristics of Pb 2+ Reproduced with permission. [34] Copyright 2018, American Chemical Society. g) Steady-state PL spectra and h) absorption spectra of perovskite films stored in high humidity (60 ± 10% RH) for 45 d. The inset of g) shows the photographs of the perovskite films under 365 nm excitation wavelength before and after being stored in high humidity (60 ± 10% RH) for 45 d. i) Steady-state PL spectra, j) absorption spectra for the as-spun perovskite films and the annealed films prepared with or without EVA. Reproduced with permission. [36] Copyright 2022, The Royal Society of Chemistry. k) Relative PL intensity of pure CsPbBr 3 QDs and CsPbBr 3 QDs@PDPEP-co-S composites as a function of different time intervals under 365-nm UV light irradiation. The insets are the original spectra with different time intervals for the pure CsPbBr 3 QDs and CsPbBr 3 QDs@PDPEP-co-S composites and the corre-in perovskite solar cells (PSCs), this modulation of crystallization and growth of perovskite QDs as a function of the strength of such interaction needs to be further researched. The recent work by Dong et al. did provide some perspective on this issue. The EVA copolymer was used as an antisolvent to treat the quasi-2D perovskite (PEA 2 ((Cs 0.25 FA 0.7 GA 0.05 )PbBr 3 ) n-1 PbBr 4 ) and the chemical interaction between the C═O and Pb 2+ ion was claimed to alter the nucleation and growth of perovskite grains. [36] The quasi-2D perovskite films often face the issue of the undesirable formation of small (n = 2, 3) and large n phases, the degradation of which leads to the overall degradation of the quasi-2D perovskite phase. The EVA copolymer antisolvent treatment reduced the formation of the n = 2 phase in the as-spun and annealed films (Figure 2g-j). The hydrophobic alkyl chain of the EVA molecule was credited to prevent moisture invasion, producing a more stable perovskite and thus a high device operating stability. In addition, the EVA-passivated surface passivated the defect and trap sites at grain boundaries and the perovskite/TPBi interface leading to the EVA-modified LEDs showing enhanced device stability (20% decay in initial luminescence in 80 min) at a relative humidity of 45% and device efficiency (22.9 cd A -1 ) when compared with the control LEDs (20% decay in 47 min and 17.4 cd A -1 , respectively).
The work by Yang et al. covered the embedding of CsPbBr 3 QDs in the polymer matrix of poly-diphenyl vinyl phosphinestyrene (PDPEP-co-S) copolymer. [37] The work utilized a one-pot hot injection strategy for the synthesis of the QDs and did not utilize any polar solvent. The presence of the phosphorus atom in the DPEP segment in the copolymer was believed to reduce the surface traps on the CsPbBr 3 PQDs owing to the coordination interaction with Pb 2+ ions in perovskites and increased the solubility of the lead precursor. The styrene segment on the other hand, with a long-chain hydrophobic structure, was believed to provide a space skeleton for the lead precursors during the progress of the reaction and prevent the encapsulated QDs from direct contact with the external environment. The composite powders exhibited extremely high PLQY of 90% and retained 60% of the initial PL intensity after 20 d in water. Finally, a white light-emitting diode with a high LE of 90 lm W -1 at 5 mA was fabricated by combining the fabricated polymer-QD composite powder with commercial red light-emitting phosphor powder. The color coordinates of the white LEDs were optimized at (0.31, 0.32) in the Commission Internationale de l'Elcairage (CIE) 1931 diagram (Figure 2k-m).
More recently, novel copolymers such as amphiphilic silicone copolymers have gained interest. In the work by He and coworkers, an ABA-type block copolymer (Si 14 -PEG 14 -QAC) was designed and used to passivate the defects in the quasi-2D perovskite film. [38] The perovskite films with Si 14 -PEG 14 -QAC additive exhibited small sized crystals, high coverage, and low defect density, leading to enhanced PLQYs. Consequently, the optimal composite LEDs exhibited a high LE of 17.32 cd A -1 and external quantum efficiency (EQE) of 4.50% in comparison to 3.32 cd A -1 and 0.86%, respectively observed in the control devices. This improved performance was accredited to multiple chemical interaction mechanisms offered by the functional sites present in the copolymer. While the ─O and ─N atom of the copolymer could form dual Lewis acid-base adducts with undercoordinated Pb 2+ rich grain boundaries, the possibility of strong hydrogen bond interaction between the H atom of the copolymer and halide ion was also demonstrated. The SEM images however showed that despite the improvement in the quality of the films with the addition of copolymer, the presence of pinholes was still evident which means that the device fabrication strategies need to be further optimized.

Copolymer Additive for Embedding Perovskite Nanocrystals
Copolymers have been extensively utilized to serve the dual purpose of serving as a matrix and embedding perovskite nanocrystals exhibiting variable sizes and shapes. Polystyreneblock-poly-2-vinyl pyridine (PS-b-P2VP) has long been the favorite of researchers to satisfy these purposes (Figure 3). Hou and co-workers synthesized stable core-shell colloidal perovskite nanocrystals using a low-cost PS-b-P2VP copolymer templated synthesis approach. [39] The authors believed that the intramolecular metal-coordinating interaction between the P2VP block with the lead precursor (PbBr 2 ) can enhance its solubility in the solution. Moreover, the long-chain hydrophobic PS block segment can sterically stabilize the lead precursors during the reaction and increase the colloidal stability postsynthesis by protecting the enclosed perovskite nanocrystals from the unfavorable environment such as moisture and polar solvents. The resultant multidentate core/shell perovskite nanocrystal exhibited orders of magnitude improved stability in form of a colloidal solution and a thin film. While the work of Hou used PS-b-P2VP as a template for all-inorganic perovskite nanocrystals, Han et al. instead utilized the same copolymer template for fabricating organolead halide perovskite crystals with various shapes and nanodomain sizes. [40] The precursor ions dissolved in this solution coordinated preferentially with the pyridines in the 2VP units of the P2VP blocks by Lewis acid-base interaction. The perovskite nanocrystals underwent confined crystallization in the P2VP domains yielding nanostructures in the shape of cylinders, lamellae, and cylindrical mesh with controlled domain size of 40-72 nm. The MAPbBr 3 nanocrystals coated alongside the self-assembled copolymer by the single-step spin coating technique exhibited the characteristic green emission. The resultant nanocrystal-copolymer composite film was employed as color conversion layer (CCL) in blue-emitting LEDs, which facilitated facile color mixing and gave rise to cool-white emission (Figure 3d-g). Hintermayr and co-workers used PS-b-P2VP polymer micelle assembly as an effective nanoreactor for the synthesis and protection of perovskite nanocrystals against moisture. [41] The diblock copolymer spontaneously formed core-shell nanostructure micelles with the hydrophilic P2VP part forming the core and the sponding optical photographs. l) EL spectra of the white LED based on CsPbBr 3 QDs@PDPEP-co-S composites under an injection current of 5 mA. Inset is the schematic device architecture of the white LED. m) Corresponding color coordinates of the CsPbBr 3 QDs@PDPEP-co-S composites-based white LED in the CIE 1931 diagram. Inset is the working photograph of the white LED under an injection current of 5 mA. Reproduced with permission. [37] Copyright 2020, Elsevier B.V. Reproduced with permission. [45] Copyright 2021, Wiley-VCH GmbH.
hydrophobic PS part forming the shell. The encapsulation by the polymeric shell led to the realization of the long-term stability of the nanocrystals against water and prevented halide ion migration. The thin film coated using these nanocrystals showed a 15fold increase in lifespan in comparison to unprotected nanocrystals in ambient conditions and survived over 75 days of complete immersion in water. Lastly, the heterostructure consisting of MAPbI 3 and MAPbBr 3 nanocrystals showed efficient FRET (Förster resonance energy transfer), depicting a proficient strategy for nanostructure integration.
The strategy of forming stable core-shell colloidal perovskite nanocrystals was extended by Pan et al. The researchers used an amphiphilic copolymer (ap-POSS-PMMA-b-PDMAEMA) as a self-assembled "reverse micelle" template. [43] The block copolymer consisted of hydrophobic (ap-POSS and polymethylmethacrylate PMMA) and hydrophilic blocks (poly 2-(dimethylamino)ethyl methacrylate, PDMAEMA). The PDMAEMA chains would serve as multidentate capping ligands, which would strongly couple with Pb precursors. The core-shell micelles of the block copolymer can be converted into well-defined "reverse" micelles in a variety of solvents and served as a confined nanoreactor during perovskite crystallization. These nanoreactors passivate the perovskite surface by forming a multidentate capping shell and yielded the formation of multi-nanocubes with sizes between 6 and 8 nm with PLQY greater than 60%. Moreover, the formation of strong multidentate bonds effectively passivated the perovskite nanocrystal surface by serving as a barrier to polar solvents and UV light.
The main aim of the work by Kafetzi et al. was to investigate the preparation, optical properties, and stability over time for a colloidal perovskite/copolymer system. A random copolymer of methyl methacrylate (MMA) and ((dimethylamino) ethyl www.advancedsciencenews.com www.advphysicsres.com methacrylate) MAEMA was synthesized for this purpose in the lab. [44] The P(MMA-co-DMAEMA) random copolymer was chosen as a colloidal stabilizer since the MMA and DMAEMA segments can respectively form hydrophobic and hydrophilic nanodomains in aqueous or organic solvent solutions of respective polarity. The copolymer matrix acted as a protective agent to avoid the precipitation of the crystalline MAPbBr 3 and kept the hybrid perovskite/copolymer stable in the solution. The selforganization of the P(MMA-co-DMAEMA) random copolymer in toluene resulted in the formation of nanoaggregates where the DMAEMA segments made the nanoparticle core, and the MMA units formed the corona. The addition of the perovskite-DMF solution into the colloidal solution of the random copolymer, ends up in the encapsulation of the perovskite nanocrystal into the DMAEMA domains, while the MMA segment provided colloidal stability to the hybrid nanosystem. The as-prepared colloidal solution showed increased stability for more than a month. Thin films prepared via spin-coating of the perovskite/polymer solutions exhibited similar optical characteristics as the precursor solutions. These were realized to be capable of emitting light for long periods (up to 2 years).
In another work, the fabrication of a dual-shell highly stable perovskite nanosheet by using amphiphilic diblock copolymer as a robust multidentate ligand was shown. [45] The researchers developed an amphiphilic diblock-copolymer-enabled strategy for crafting highly stable anisotropic CsPbBr 3 nanosheets by in situ formation of a uniform inorganic shell (first shielding) that was intimately ligated with hydrophobic polymers (second shielding). Diblock copolymer of poly(methyl methacrylate)block-poly(tert-butyl acrylate) (PMMA-b-PtBA) was synthesized by reversible addition-fragmentation chain-transfer (RAFT) polymerization technique with well-controlled molecular weight of each block and low polydispersity (PDI). The presence of abundant functional groups (COOH − ) on the surface of PMMA-b-PAA made it act like a multidentate ligand that effectively anchored to the surface of CsPbBr 3 nanosheets. The multidentate characteristic of PMMA-b-PAA and its strong linking to CsPbBr 3 was proved by DFT calculation. Due to the multidentate nature of the surface, it directed the anisotropic growth of the TiO 2 shell in the region situated between PAA blocks via COOH − catalyzed hydrolysis of TiO 2 precursors. As a result, CsPbBr 3 /TiO2 core/shell NSs with varied TiO 2 shell thickness were crafted and ligated with PMMA of well-controlled chain length. The PMMA-capped CsPbBr 3 /TiO2 core/shell nanosheet exhibited high stabilities (i.e., thermal, photo, moisture, polar solvent, aliphatic amine, and composition stabilities). Moreover, the researchers demonstrated the feasibility of using the PMMA-capped nanosheets in white LEDs with stable green emission (Figure 3h-j).
Hung and co-workers proposed a new approach to maintain the exceptional optoelectronic properties of CsPbBr 3 nanocrystals during their transfer from colloidal dispersion to thin film devices. This was done by dispersing CsPbBr 3 nanocrystals in a polysaccharide-based maltoheptaose-block-polyisoprene-blockmaltoheptaose (MH-b-PI-b-MH) triblock copolymer matrix. [46] The hydroxyl group on the maltoheptaose (MH) segment provided coordination with the Pb 2+ ions in the CsPbBr 3 and the nanostructures of the self-assembled block copolymer significantly controlled the growth of the perovskite nanocrystals. More importantly, the selected hydrophobic and elastic polyisoprene (PI) could not only control the ordering of nanostructures but also improved their tensile properties. The as-prepared composite thin films showed excellent durability to humidity and stretchability. A stretchable photonic memory device responsive to 450 nm light stimulation was fabricated which could maintain a high memory ratio of 10 5 and exhibited long-term memory stability over 10 4 s even under 100% strain (Figure 3a-c).
Recently Cueto and co-workers utilized zwitterionic block copolymers as macromolecular ligands for the synthesis of perovskite nanocrystals with excellent colloidal stability and high PLQY. [47] The researchers used a copolymer containing a poly(nbutyl methacrylate) (PBMA) hydrophobic block and a poly (sulfobetaine methacrylate) (PSBMA) or poly(2-methacryloyloxyethyl phosphorylcholine) (PMPC) zwitterionic block. These TPBMAb-PSBMA and PBMA-b-PMPC BCPs were prepared by RAFT polymerization. The researchers found that injection of a trin-octyl phosphine bromine complex (TOP·Br 2 ) into a hot solution containing the block copolymer and cesium and lead oleate salts produced strongly luminescent CsPbBr 3 nanocrystals. Perovskite nanocrystals prepared by this route exhibited morphological differences relative to those synthesized in the presence of small molecule ligands. Moreover, the copolymer additive enabled the perovskite nanocrystal dispersion in a wide range of organic solvents, including polar solvents such as isopropanol and tetrahydrofuran, which would irreversibly precipitate or dissolve small molecule-capped perovskite nanocrystals. The block polymer architecture has a profound effect on polymer solubility in hydrophobic solvents, accommodating zwitterion concentrations of up to 21 mol% while dissolving at room temperature in mesitylene and toluene enabling the use of copolymers under hot-injection conditions.

Copolymer Tailored Synthesis of Perovskite Quantum Dots
Lately, copolymers have been effectively utilized to tailor the synthesis method for fabricating perovskite quantum dots (QDs). Unlike the case of perovskite nanocrystals where the use of a copolymer template facilitates the synthesis of perovskite nanostructures with variable sizes and shapes and their embedding in a copolymer matrix, the reports focusing on the copolymer-QD rather utilize the ability of long copolymer chains to effectively coat the surface of each spherical QD independently. Both approaches however are similar in terms of their goal of protecting degradation factors such as polar solvents and moisture.
Yoon and co-workers used amphiphilic stark-like diblock copolymer of poly(acrylic acid)-block-polystyrene (PAA-b-PS) to protect the as-synthesized perovskite QDs with varied sized and compositions. [48] The diblock copolymers were synthesized by ATRP. The core of the nanoreactor consisted of hydrophilic PAA while the shell consisted of hydrophobic PS blocks of different lengths. The PS capping around the CsPbBr 3 QDs could be easily regulated to the desired length during ATRP reaction (Figure 4ad). The length of tunable PS chains provided high stability in water and harsh environment. This restrengthens the fact that the properties of the composite are heaving dependent on the molecular chain length of the polymer. The exposure of QDs to water resulted in the grafted PS chains collapsing, forming a dense Figure 4. a) Ligand loss from perovskite QD surface upon exposure to water in oleylamine/oleic acid co-capped CsPbBr 3 . b) Permanently grafted PS(7k) forms a shell layer around CsPbBr 3 QD. c) permanently tethered PS (16k) leads to a denser shell on the CsPbBr 3 surface. d) permanently capped PS (16k) forms an even denser PS shell on smaller sized CsPbBr 3 . e) PL spectra and f) CIE color diagram of PS capped CsPbBr 3 QDs based white LED. Inset in (e) shows the photograph of white LED. Reproduced with permission. [48] Copyright 2019, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. g) A pictorial illustration of the encapsulation of hydrophobic CsPbBr 3 QDs by amphiphilic polymer to make them water soluble. h) Photoluminescence stability test of CsPbBr 3 QDs with poloxamer 127/PEG 15-hydroxystearates i) QDs encapsulated with poloxamer 127/PEG(15)-hydroxystearates with oleylamine treatment. j) PL intensity as function of incubation time of CsPbBr 3 QD samples. Reproduced with permission. [50] Copyright 2018, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
PS cushion on the perovskite QD surface and thus greatly preventing water from reaching and dissolving the perovskite QD. As a result, PS-capped QDs provided a 20-fold improvement in water stability when compared to QDs synthesized by a conventional ligand-assisted approach. Moreover, the PS-capped QDs exhibited enhanced colloidal stability due to intimate and stable contact of perovskite QDs with PS chains. Therefore, PS-capped CsPbBr 3 QDs displayed excellent colloidal stability for more than 535 d with no change in QY, PL wavelength, and FWHM. Lastly, the researchers fabricated white LED using hairy perovskite QDs which demonstrated 130% and 184% color gamut over NTSC and sRGB standards (Figure 4e,f). The same copolymer strategy was later extended for rapid in-situ synthesis of monodispersed solid and hollow QDs possessing the same diameter. [49] The main emphasis of the work by Lee et al. was to develop a facile and general method for coating CsPbBr 3 per-ovskite QDs by an emulsification-solvent evaporation technique. The CsPbBr 3 QDs were prepared in an organic solvent and were then transferred into an aqueous solvent by coating with non-ionic amphiphilic triblock polymer Poloxamer 127. [50] The copolymer is composed of poly(ethylene oxide) 100 -poly(propylene oxide) 65 -poly(ethylene oxide) 100 (PEO 100 -PPO 65 -PEO 100 ) triblock (Figure 4g). Upon coating the CsPbBr 3 by Poloxamer 127, the CsPbBr 3 QDs readily dispersed in water but exhibited poor stability and readily underwent surface decomposition. The stability of the polymer composite was then enhanced by coating the surface of CsPbBr 3 with PEG (15)-hydroxystearate (Figure 4h-j). The dual polymeric shell composed of poloxamer 127 and PEG (15)-hydroxystearate bonded strongly on CsPbBr 3 . Moreover, the PEG units in the outer polymer shell exhibited low toxicity and reduced nonspecific biomolecule adsorption, representing an ideally biocompatible species. The authors believed that such dual polymeric shell QDs could open new avenues in the field of twophoton excited fluorescence imaging.
In the work by Yang et al., the researchers developed a simple and general strategy to form aqueous colloidally stable polystyrene-b-poly(ethyl oxide) (PS-b-PEO) grafted MAPbBr 3 QDs. [51] The hydrophilic PEO block of the copolymer helped in maintaining colloidal stability and cytocompatibility, while the hydrophobic PS block provides a sturdy encapsulation against water. To disperse the colloidal QDs in water researchers found hexane as a critical intermediate solvent which ensured the complete encapsulation of the perovskite QDs core by a PS shell. After dispersing the precipitate in water, the perovskite QDs dispersed well and self-assembled into well-defined vesicular nanostructures with high PLQY of up to 43%, high color purity where the full width at half maxima was reduced to 18 nm, long average PL lifetimes up to 164 ns and exhibited water stability for more than seven months. Moreover, QDs@PS-b-PEO exhibited longterm thermal, photo, and pH stability, as well as low cytotoxicity towards HeLa cells.
Recently, QDs were conjugated with ethylene vinyl acetate/terpene phenol (1%) (EVA-TPR (1%), or EVA) to form copolymer/perovskite composite thin films. [52] The thin films were tested by steady-state spectroscopic investigations. The reason for choosing EVA was due to its low cost and high band gap which provided transparency. Moreover, the compatibility of perovskite QDs with EVA was established by performing a series of analyses and computing parameters, such as the band gap, the coefficients of absorbance and extinction, the index of refractivity, and the dielectric constant. This analysis showed that EVA addition to QDs resulted in a stable and efficient composite. Hence, the authors claimed that the EVA/PQDs could be used potentially as active materials for optoelectronic devices, such as solar cells and LEDs.

Copolymer-Assisted Engineering of Perovskite-Based Photodetectors
In comparison to perovskite-based solar cells, limited attention has been paid to understanding and utilizing the advantages of copolymer systems in the field of perovskite-based photodetectors. While several reports have been published demonstrating the role of copolymers as substrate and in enhancing the conduction of charge carriers in charge transport layers, direct interaction of copolymers with perovskite layer for photodetector application has relatively been overlooked with limited number of reports appearing over the years. [53] The research work by Kim et al. showcased the patterning of MAPbI 3 thin films using a photolithographically fabricated cross-linked copolymer template on Si or SiO 2 substrates by chemical vapor deposition (CVD). [54] The researchers employed a simple single-component photo-crosslinkable polymer system, poly(glycidyl methacrylate-r-2-((((2nitrobenzyl)oxy)carbonyl)amino)ethyl methacrylate) [poly(GMAr-NBOCAEMA or PGN) for the template patterns. The inherent cross-linking ability of the copolymer system led to the formation of a stable polymer template without requiring otherwise commonly used additives such as photoacid generators and photoactive compounds. The copolymer template-based perovskite patterning approach further offered an additional advantage over the conventional device fabrication based on top-down lithography.
Since the perovskite material was directly deposited on the template, the authors were able to avoid the otherwise inevitable rapid degradation of perovskites that occurs during top-down lithography processes due to contact with developers and photoresists with detrimental chemical functionalities. With UV light illumination, the copolymer thin film could be photopatterned as a negative type photoresist, enabling formation of perovskite thin films with the desired shapes and patterns. The CVD process was first used to deposit PbI 2 on the selected pattern which was further converted into MAPbI 3 by introducing vapors of MAI. The selective nucleation of PbI 2 crystals within the copolymer template reached 99.7% selectivity, representing a maximal difference in surface coverage between PbI 2 deposited on the Si substrate and the PGN template. This was done by modulating the growth temperature of PbI 2 deposition (during CVD) in the range of 165-255°C, where the highest surface coverage difference was observed at 235°C. The patterned PbI 2 films preserved their morphology throughout the MAPbI 3 conversion process upon vapor phase intercalation of MAI. The as-fabricated MAPbI 3 photodetector displayed a responsivity of 0.0073 A W −1 , detectivity of 1.63 × 10 10 Jones, and an ON/OFF ratio of ≈100. In another work, Zhang and co-workers incorporated silver/ polyvinyl pyrrolidone core-shell nanoparticles and amphiphilic block copolymer F68 (a poly(ethylene)-co-poly(propylene glycol) structure) within the perovskite thin film for fabrication of a photodetector. [55] The incorporation of Ag nanoparticles and F68 led to enhanced light absorption through surface plasmon resonance and improved long-term stability by defect passivation, respectively. Specifically, the authors believed that the hydrophilic part (with the polyoxyethylene chain) made a complex with perovskite elements through hydrogen bonding, while the hydrophobic end with the polyoxypropylene chain being exposed, formed a dangling bond and provided protection against moisture. The authors utilized a transparent ITO back contact making the device stack bifacial. The resultant bifacial photodetector had a responsivity of 0.145 A W − 1 , a maximum detection degree of 9.73 × 10 10 Jones, and a fast ON/OFF switching time of 100 ms/153 ms, making the device one of the state-of-the-art flexible and self-powered perovskitebased photodetector. The authors accredited the incorporation of the block copolymer with the perovskite as the reason for the device maintaining 80% of the original responsivity during 7 d of high-humidity ambient exposure. In recent work, a hierarchical structuring technique for fabricating perovskites with improved stability and enhanced performance was shown by Han et al. (Figure 5). [56] A two-step method was demonstrated where the first step involved spin coating the mixture of the MAPbBr 3 precursor ions and PS-b-P2VP block copolymer on the substrate. The second step was nanoimprinting, which produced a periodic pattern. While the P2VP interacted with the perovskite precursor by Lewis acid-base interaction, the PS provided a top hydrophobic layer. The authors hypothesized that the PL properties of the nanocomposite film were not only improved by the effective trap passivation caused by P2VP but also because of the local-field enhancement induced by Mie resonance, as reported in previously published work on patterned perovskite nanostructures, giving rise to a PLQY of ≈28%. [57] In addition to improved nanocomposite thin film performance, MAPbBr 3 films with improved photoconduction were successfully employed as anisotropic photoactive layers in arrays of two-terminal parallel-type photodetec- tors. The hierarchically ordered MAPbBr 3 /PS-b-P2VP film-based photodetectors exhibited a higher photoresponsivity of 7.19 × 10 −6 mA W −1 in comparison to the photodetector based on pristine MAPbBr 3 film (3.51×10 −9 mA W −1 ) and MAPbBr 3 /PS-b-P2VP film without nanoimprinting (9.94 × 10 −8 mA W −1 ). A linear increase in photoresponsivity with respect to the intensity of irradiation was observed under a bias voltage of 10 V and laser power of 8.287 W cm −2 . The rise and decay times for the ON/OFF switching were measured as ≈49.9 and 98.7 m, respectively. The hierarchically structured photodetector displayed excellent longterm environmental stability over 90 d under ambient conditions (relative humidity of 30-50% and 25°C).

Miscellaneous Examples of Copolymer Engineering
Lately, several miscellaneous applications demonstrating the use of copolymers have appeared. This serves as suitable evidence for the diversity of the copolymer-perovskite composite approach. For instance, in the year 2019, Zhou et al. manufactured, and 3D-printed nanocomposite inks composed of brightly emitting colloidal cesium lead halide perovskite (CsPbX 3 , X = Cl, Br, and I) nanowire bundles suspended in a polystyrene polyisoprenepolystyrene (SIS) triblock copolymer matrix. [58] This author further demonstrated the designing, printing, and characterization of polarized optical architectures in several device motifs based on the luminescent composite. It was realized that during printing, the block copolymer undergoes shear-induced alignment and forms macroscopically ordered cylindrical microdomains that enhance nanowire alignment along the printing direction. Several devices were produced to highlight the versatility of this method, including optical storage, encryption, sensing, and fullcolor display.
Kim and co-workers utilized copolymers such as poly(BMAco-2-(methacryloyloxy)ethyl-sulfobetaine) (PBMA-SB) with sulfobetaine or phosphorylcholine zwitterions as pendent groups to serve a dual purpose as ligands and host matrices for CsPbBr 3 perovskite nanoparticles (PNPs). [59] By utilizing these polymers, nanocomposite films with excellent nanoparticle dispersion, optical transparency, and impressive resistance to water degradation were obtained. The multidentate interaction of zwitterioncontaining copolymer caused weakly aggregated polymer morphologies depending on the extent of zwitterionic functionality in the polymer. Moreover, PNP-polymer nanocomposites resisted degradation in the presence of polar solvents and benefited from the easy introduction of functionality that allowed patterning by photolithographic methods.
The work by Liu et al. discussed the synthesis of highly stable 1 D perovskite nanowires by using a block copolymer of polystyrene-block-poly(4-vinyl pyridine) (PS-P4VP) to preferentially modify the surface of colloidal CsPbBr 3 nanowires. [60] The perovskite/block copolymer assembly formed a core-shell type of structure where the multidentate P4VP block served as core and anchor to the surface of CsPbBr 3 nanowires, while the hydrophobic polystyrene block served as shell and provided protection from water. Moreover, a strong interaction between block copolymer and perovskite was realized owing to the lone electron pairs of the nitrogen atoms in the pyridine rings strongly coordinating with the 6p empty orbits of Pb 2+ . The resulting nanowires displayed enhanced PL emission and colloidal stability against water and anionic exchange. Lastly, the researchers applied the Langmuir-Blodgett technique to assemble monolayers of highly www.advancedsciencenews.com www.advphysicsres.com aligned nanowires and studied their anisotropic optical properties.
To our knowledge, the report by Chang et al. was the first one to demonstrate the effect of the morphology of copolymer/perovskite composite film on the photo-responsive characteristics of a photo memory device. [61] The authors chose two asymmetric linear block of polystyrene-block-poly(ethylene oxide) (PS-b-PEO), coded as BCP-1 (PS 24k -b-PEO 21k ) and BCP-2 (PS 114k -b-PEO 31k ) and two corresponding homopolymer PEO and PS as matrix to disperse perovskite. The PEO block could passivate the defects of the MAPbBr 3 precursor owing to the Lewis acid-base interaction between ─O atom and Pb 2+ . On the other hand, the polystyrene block served as an insulating matrix between the perovskite/PEO. The antisolvent properties of composite films made it possible to use solution-processed poly(3hexylthiophene-2,5-diyl) (P3HT) as an active channel. Moreover, by manipulating the interfacial area between the perovskite and active channel by controlling the volume fraction of each block, excellent charge transfer properties were obtained. The photo memory device displayed excellent properties such as a fast photo-induced charge transfer rate of 0.056 per ns, a high charge transfer efficiency of 89%, a light switching current ratio of 10 4 , and an extremely low programming time of 5 ms (Figure 6).
Nah et al. reported the production of organic-inorganic hybrid perovskite nanostructure by using block copolymer micelles as scaffolds. [62] The block copolymer used as a template was PSb-P2VP which when dissolved in toluene formed a reverse micelle nanoreactor. The block copolymer micelle acted as a perovskite precursor reservoir for storing crystallization reagents. The structure of the resulting nanostructure is determined by both kinetic and thermodynamic parameters involved in micelle assembly. For instance, under rapid stirring conditions, micelles retained their shape during crystallization, and encapsulated nanocrystals were formed. While slow stirring disrupted the micelle assembly by retarding convection flow. Moreover, an increase in temperature caused softening of polymeric micelle and enhanced the micelle disassembly which resulted in size and shape-controlled nanostructure. The authors claimed that various MAPbBr 3 -based nanostructures such as encapsulated nanocrystals, diffusion-limited products, anisotropic nanostructures, and micron-sized nanoplates could be obtained by controlling the stirring rate and reaction temperature.
The work by Li and co-workers was inspired by the earlier work of Han et al. as they showcased the feasibility of improving the environmental stability of the perovskite by using PSb-P2VP and capitalized on the Lewis acid-base interaction between P2VP block and Pb 2+ ions. [40,63] The authors used a twostep method where the first layer of lead acetate and copolymer was followed by a coating of the MABr to fabricate a composite film of MAPbBr 3 and block copolymer. The as-synthesized composite exhibited an average PLQY of 58%. The color of the PL could be tuned easily from green to blue by simply controlling the Cl/Br ratio in the second step. A sky-blue emission (at 477 nm) was obtained from a CH 3 NH 3 PbBr 1.5 Cl 1.5 perovskite film with a PLQY of 23 ± 2%. The diversity of this approach was elucidated by preparing the composite film using inkjet printing where the MABr dissolved in IPA solution was loaded and printed on the precoated copolymer-PbAc 2 ·3H 2 O film by the inkjet printer.

Summary and Outlook
In this review, we have attempted to summarize the progress of copolymer/perovskite composites and the role of copolymer additive in improving the properties of halide perovskite and associated device characteristics. Although the device efficiency and long-term stability in perovskite devices have improved considerably in the last couple of years, the efforts to ensure the realization of perovskites towards real-world applications must continue. To improve device performance for commercialization, modifying the stoichiometry and composition of perovskite has proved insufficient on multiple occasions. Rapid progress has been made in utilizing polymeric materials, especially copolymers to rationally tune the properties of perovskites and enable their protection from detrimental environmental factors. This composite strategy however needs to be adapted simultaneously with the systematic engineering, structural design, and improvement of charge-transport and electrode materials of perovskite-based devices. Although copolymers have been integrated with several halide perovskites, to further realize the potential of their incorporation with perovskite research needs to continue. Some might believe that to pursue the prospects of polymeric additives, we need to look beyond the classical homopolymer-based strategies. The use of random copolymers and other multimonomeric polymers is certainly a research subject worth exploring. Developing some key fundamental concepts to guide the design of the polymer based on their interaction with perovskites, and impact on electro-optical properties and stability will help focus the research efforts. It is undeniable that it is necessary to fabricate special polymer structures exhibiting several desired additive properties by introducing certain polarity gradations through copolymerization or by subsequently modifying the polymer structures themselves. But the clear role of each polymeric constituent in a copolymer system should be straightforwardly laid out. It is extremely crucial to be able to correlate the effects observed in the composite films to the inherent properties of the copolymer constituents. The knowledge needs to be expanded in these areas regarding the orientation of different segments of the copolymers, and it is crucial to analyze it to better control perovskite crystallization. The role of tacticity of copolymers has been long overlooked and needs to be dealt with in detail. Unless a clear comparison is laid out between the effects caused by the copolymers in perovskite composites with respect to the case where the homopolymers segments are independently integrated alongside perovskite, the superiority of copolymers cannot be established. Similarly, studies comparing the effect of inherent properties of the copolymer such as their polarity, affinity towards water, thermal characteristics, and flexibility dynamic over the perovskites need to be conducted. One of the most common aspects across the entire literature on copolymer-perovskite composites has been over-dependence over the Lewis acid-base adduct or hydrogen bond formation. Although such direct interaction is undeniably strong as can be seen through many published articles, it does not necessarily improve the crystal grain size which in layman's terms can otherwise be beneficial owing to the reduced possibility of charge recombination. Hence, it is important to also focus on exploiting other potential interaction mechanisms such as cation-and utilize copolymers that may establish such interactions with perovskite constituents. While the research in thin film-based perovskites (both in solar cells and LEDs) has focused on stabilizing and enhancing the properties of organolead halide perovskites by tailoring grain size and passivation of undercoordinated ions at grain boundaries, the emphasis of the research work in the field of QDs and nanocrystals has been on encapsulating the all-inorganic perovskites. It is known that all-inorganic perovskites readily undergo phase transition when coated as a thin film. Similarly, organolead halide perovskites are more prone to undergo degradation in the colloidal system owing to the presence of a hygroscopic organic moiety. Hence, it is important to utilize copolymer strategies to stabilize both these systems irrespective of them being in the thin film or colloidal solution form. Additionally, the role of copolymers has been merely restricted to the encapsulation of perovskite QDs or nanocrystals. But in principle, it can play a much bigger role. Preferential orientation of the perovskite crystals in films by carefully tailoring the copoly-www.advancedsciencenews.com www.advphysicsres.com mer segments can provide a new scope and avenue for improving device performance. In the case of perovskite thin films and solar cells, copolymers have been demonstrated as effective defect passivation agents, however, the distribution and arrangement of polymers in the composite film needs to be further explored. This is of significance since unless the distribution of copolymers is known, it is impossible to lay the basis for the appropriate amount and chain length of each polymer segment to be included in the perovskite matrix. [64] Recent attempts have been made to establish such context by utilizing homopolymers of two of the most common polymers with contrasting natures, PS and PEG along with their block copolymers with varied molecular weights to understand this aspect. [65] Another interesting future research direction could be exploiting the chemical interaction between Sn 2+ and Ge 2+ with copolymers to prevent their rapid oxidation and realization of ambient atmosphere stable and efficient lead-free perovskite systems. It is yet to be seen if the conventional Lewis acid-base adduct policy can be effectively extended to the perovskite devices involving tin and germanium, owing to their poor Lewis acid characteristics. Although several polymers possessing high T g have been proposed, the self-healing of perovskites in humid conditions or high temperatures needs more research for reaching practical applications. We expect that careful selection and synthesis of copolymers could potentially lead to self-healing in perovskites through strong physical interactions between polymer chains without requiring any external stimuli. In conclusion, the copolymer engineering strategy has made significant gains and has brought the perovskite-related science forward. The future is promising for copolymeric-mediated engineering of perovskites as the field expands and scientists work towards realizing and capitalizing on the benefits offered by copolymers through fundamental design and understanding.
www.advancedsciencenews.com www.advphysicsres.com Saikiran Khamgaonkar received his Bachelor's degree in Metallurgical and Materials Engineering from VNIT Nagpur in the year 2020. He is currently a Ph.D. student at the Department of Chemistry at the University of Waterloo. His research interest includes organic-inorganic halide perovskites for photovoltaic and photocatalytic applications.
Vivek Maheshwari is an associate professor of Chemistry at the University of Waterloo. He is also a member of the Waterloo Institute for Nanotechnology. His group's research interests include lead halide perovskite materials for devices and sensors, 1D materials for wearable devices, and nanomaterials for electrocatalysis. He completed his Bachelor's degree from the Indian Institute of Technology, Delhi, and his doctorate from Virginia Tech.