Stability of Sn-Pb mixed organic–inorganic halide perovskite solar cells:Progress,challenges,and perspectives

The exploration of low bandgap perovskite material to approach Shockley-Queisser limit of photovoltaic device is of great significance,but it is still challenging.During the past few years,tin–lead(Sn-Pb)mixed perovskites with low bandgaps have been rapidly developed,and their single junction solar cells have reached power conversion efficiency(PCE)over 21%,which also makes them ideal candidate as low bandgap sub-cell for tandem device.Nevertheless,due to the incorporation of unstable Sn²⁺,the stability issue becomes the vital problem for the further development of Sn-Pb mixed perovskite solar cells(PSCs).In this review,we are dedicated to give a full view in current understanding on the stability issue of SnPb mixed perovskites and their PSCs.We begin with the demonstration on the origin of instability of Sn-Pb mixed perovskites,including oxidation of Sn²⁺,defects,and interfacial layer induced instability.Sequentially,the up-to-date developments on the stability improvement of Sn-Pb mixed perovskites and their PSCs is systematically reviewed,including composition engineering,additive engineering,and interfacial engineering.At last,the current challenges and future perspectives on the stability study of Sn-Pb mixed PSCs are discussed,which we hope could promote the further application of Sn-Pb mixed perovskites towards commercialization.     

Mixed-phase Mesoporous TiO2 Film for High Efficiency Perovskite Solar Cells

Mesoporous scaffold structures have played great roles in halide perovskite solar cells(PSCs),due to the excellent photovoltaic performance and commercial perspective of mesoporous PSCs.Here,we reported a mixed-phase TiO2 mesoporous film as an efficient electron transport layer(ETL)for mesoporous perovskite solar cells.Due to the improved crystal phase,fihn thickness and nanopartMe size of TiO2 layer,which were controlled by varying the one-step hydrothermal reaction time and annealing time,the PSCs exhibited an outstanding short circuit photocurrent density of 25.27 mA/cm^2,and a maximum power conversion efficiency(PCE)of 19.87%.It is found that the ultra-high Jsc attributes to the excellent film quality,light capturing and excellent electron transport ability of mixed-phase TiO2 mesoporous film.The results indicate that mix-phase mesoporous metal oxide fihns could be a promising candidate for producing effective ETLs and high efficiency PSCs.     

Multifunctional dopamine-assisted preparation of efficient and stable perovskite solar cells

Perovskite solar cells(PSCs)show great potential for next-generation photovoltaics,due to their excellent optical and electrical properties.However,defects existing inside the perovskite film impair both the performance and stability of the device.Uncoordinated Pb²⁺,uncoordinated I^-,and metallic Pb(Pb⁰)are the main defects occur during perovskite film preparation and device operation,due to the volatilization of organic cationic components.Passivating these defects is a desirable tas k,because they are non-radiative recombination centers that cause open-circuit voltage(VOC)loss and degradation of the perovskite layer.Herein,the multifunctional bioactive compound dopamine(DA)is introduced for the first time to control the perovskite film formation and passivate the uncoordinated Pb²⁺defects via Lewis acid-base interactions.The Pb⁰ and I^-defects are effectively suppressed by the DA treatment.At the same time,the DA treatment results in a stronger crystal orientation along the(110)plane and upshifts the valence band of perovskite closer to the highest occupied molecular orbital(HOMO)of the hole transport layer(2,2’,7,7’-tetrakis(N,N’-di-pmethoxyphenylamine)-9,9’-spirobifluorene,spiro-OMeTAD),which is beneficial for charge separation and transport processes.Consequently,the stability of MAPbI₃(MA=CH₃NH₃)PSCs prepared with the DA additive(especially the thermal stability)is effectively improved due to the better crystallinity and lower number of defect trap states of the perovskite film.The optimized MAPbI3 PSCs maintain approximately 90% of their original power conversion efficiency(PCE)upon annealing at 85℃ for 120 h.The best performance triple-cation perovskite(Cs_0.05(FA_0.83MA_0.17)_0.95Pb(I_0.83Br_0.17)₃)(FA=formamidinium)solar cell with ITO/SnO₂/Cs_0.05(FA_0.83MA_0.17)_0.95Pb(I_0.83Br_0.17)₃:DA/spiro-OMeTAD/MoO₃/Ag(ITO=indium tin oxide)structure shows a PCE of 21.03% with negligible hysteresis,which is dramatically enhanced compared to that of the control device(18.31%).Therefore,this work presents a simple and effective way to improve the efficiency and stability of PSCs by DA treatment.     

Extended X-ray absorption fine structure (EXAFS) of FAPbI3 for understanding local structure-stability relation in perovskite solar cells

Perovskite solar cells (PSCs) employing formamidinium lead iodide (FAPb I₃) have shown high efficiency.However,operational stability has been issued due to phase instability of a phase FAPb I₃at ambient temperature.Excess precursors in the perovskite precursor solution has been proposed to improve not only power conversion efficiency (PCE) but also device stability.Nevertheless,there is a controversial issue on the beneficial effect on PCE and/or stability between excess FA...     

Graded 2D/3D(CF3-PEA)2FA0.85MA0.15Pb2I7/FA0.85MA0.15PbI3 heterojunction for stable perovskite solar cell with an efficiency over 23.0%

The replacement of small cations with bulkier organic cations containing long alkyl chains or benzene rings to form a thin two-dimensional(2D)perovskite passivation layer on three-dimensional(3D)perovskite(2D/3D)has become a promising strategy for improving both the efficiency and stability of perovskite solar cells(PSCs).The 2 D layer defines the interfacial chemistry and physics at the 2D/3D bilayer and endows the 2D/3D structure with better chemical and thermal stability.Herein,2D/3D(CF₃-PEA)₂ FA_0.85MA_0.15Pb₂I₇/FA_0.85MA_0.15PbI₃ planar heterojunction perovskite was produced using a facile interfacial ion exchange process.The 2 D(CF₃-PEA)₂ FA_0.85MA_0.15Pb₂I₇ capping layer can not only passivate the FA_0.85MA_0.15PbI₃ film but also act as super-hydrophobic layer to inhibit water diffusion and significantly enhance the stability.The 2D capping layer can also establish a unique graded band structure at the perovskite/Spiro-OMeTAD interface and lead to p-type doping for Spiro-OMeTAD layer which is beneficial for efficient charge transport.Optimized PSCs based on this 2D/3D heterojunction yield a champion power conversion efficiency(PCE)of 23.1%and improved stability.The device maintains 84%output for 2400 h aging under ambient environmental conditions without encapsulation,and maintains 81%for 200 h under illumination with encapsulation.This work will inspire the design of more fluorinated 2D perovskite interfaces for advanced photovoltaics and beyond.     

Excess PbI2 evolution for triple-cation based perovskite solar cells with 21.9% efficiency

The triple cation mixed perovskites(Cs FAMA) are known as one of the most efficient candidates for perovskite solar cells(PSCs). It is found that the power conversion efficiency(PCE) of triple-cation based devices would increase with the test time extending, and the maximum efficiency is normally obtained after several days aging storage. Here, the relationship between enhanced device performance, excess PbI₂ and its evolution in triple cation perovskite films of initial days was syst...     

Defect passivation by nontoxic biomaterial yields 21% efficiency perovskite solar cells

Defect passivation is one of the most important strategies to boost both the efficiency and stability of perovskite solar cells(PSCs).Here,nontoxic and sustainable forest-based biomaterial,betulin,is first introduced into perovskites.The experiments and calculations reveal that betulin can effectively passivate the uncoordinated lead ions in perovskites via sharing the lone pair electrons of hydroxyl group,promoting charge transport.As a result,the power conversion efficiencies of the p-i-n planar PSCs remarkably increase from 19.14%to 21.15%,with the improvement of other parameters.The hydrogen bonds of betulin lock methylamine and halogen ions along the grain boundaries and on the film surface and thus suppress ion migration,further stabilizing perovskite crystal structures.These positive effects enable the PSCs to maintain 90%of the initial efficiency after 30 days in ambient air with 60%±5%relative humidity,75%after 300 h aging at 85℃,and 55%after 250 h light soaking,respectively.This work opens a new pathway for using nontoxic and low-cost biomaterials from forest to make highly efficient and stable PSCs.     

Impeded degradation of perovskite solar cells via the dual interfacial modification of siloxane

It is challenging to improve the long-term stability of perovskite solar cells(PSCs) without sacrificing efficiency. The perovskite absorbers degrade from the film surface/interfaces, which follows entangled mechanisms that have not been fully revealed yet.Herein, we decouple and elaborate two distinctive pathways regarding film degradation based on FACsPbI3perovskites.Moreover, a dual interfacial modification strategy has been developed for improving the material’s intrinsic stability, thus lea...     

Defect suppression and energy level alignment in formamidinium-based perovskite solar cells

The vast majority of high-performance perovskite solar cells(PSCs) are based on a formamidinium lead iodide(FAPbI₃)-dominant composition. Nevertheless, the FA-based perovskite films suffer from undesirable phase transition and defects-induced non-ideal interfacial recombination, which significantly induces energy loss and hinders the improvement of device performance. Herein, we employed 4-fluorophenylmethylammonium iodide(F-PMAI) to modulate surface structure and energy level alignme...     

Recent strategies to improve moisture stability in metal halide perovskites materials and devices

At present,the stability of the new generation of solar cells based on hybrid perovskites is the bottleneck for their practical applications.Photochemical effects,high temperature,ultraviolet light,humidity and other known or still unknown factors might cause reduction of effectiveness or even irreversible loss of materials properties due to decomposition of functional layers within perovskite solar cells(PSCs).These factors alone have a serious impact on each component of the device,while their combinations lead to much more complicated effects and consequences.This review focuses on the stability of PSCs and the degradation of the device in a humid environment.We assess the instability factors and deep-seated principles of evolution of the device structure in a humidity environment with the emphasis on the influence on their interrelations.The related solutions are reviewed from the perspective of the encapsulation,perovskite active layer,carrier transport layer and electrodes.Combined with the latest research,we believe that the waterproof strategy of PSCs requires either tight encapsulation or thorough modifications in the device itself.Therefore,it is important to develop feasible strategies to improve the overall device stability over humid according to the target characteristics of various devices.     

Chemical vapor deposition growth of phase-selective inorganic lead halide perovskite films for sensitive photodetectors

Inorganic lead halide perovskites are attractive optoelectronic materials owing to their relative stability compared to organic cation alternatives.The chemical vapor deposition(CVD) method offers potential for high quality perovskite film growth.The deposition temperature is a critical parameter determining the film quality owing to the melting difference between the precursors.Here,perovskite films were deposited by the CVD method at various temperatures between 500-800℃.The perovskite phase converts from CsPb₂Br₅ to CsPbBr₃ gradually as the deposition temperature is increased.The grain size of the perovskite films also increases with temperature.The phase transition mechanism was clarified.The photoexcited state dynamics were investigated by spatially and temporally resolved fluorescence measurements.The perovskite film deposited under 750℃ condition is of the CsPbBr₃ phase,showing low trap-state density and large crystalline grain size.A photodetector based on perovskite films shows high photocurrent and an on/off ratio of ~2.5×10⁴.     

Highly efficient and stable organic solar cells with SnO2 electron transport layer enabled by UV-curing acrylate oligomers

The interfaces between the inorganic metal oxide and organic photoactive layer are of outmost importance for efficiency and stability in organic solar cells(OSCs). Tin oxide(SnO₂) is one of the promising candidates for the electron transport layer(ETL) in high-performance inverted OSCs. When a solutionprocessed SnO₂ ETL is employed, however, the presence of interfacial defects and suboptimal interfacial contact can lower the power conversion efficiency(PCE) and operational ...     

Application of an amphipathic molecule at the NiOx/perovskite interface for improving the efficiency and long-term stability of the inverted perovskite solar cells

The presence of defects and detrimental reactions at NiO_x/perovskite interface extremely limit the efficiency performance and long-term stability of the perovskite solar cells(PSCs) based on NiO_x.Herein,an amphipathic molecule Triton X100(Triton) is modified on the NiO_x surface.The hydrophilic chain of Triton as a Lewis base additive can coordinate with the Ni³⁺ on the NiO_x surface which can passivate the interfacial defects and hinder the d...     

Solvent engineering towards scalable fabrication of high-quality perovskite films for efficient solar modules

Over the last decade, remarkable progress has been made in metal halide perovskite solar cells(PSCs),which have been a focus of emerging photovoltaic techniques and show great potential for commercialization. However, the upscaling of small-area PSCs to large-area solar modules to meet the demands of practical applications remains a significant challenge. The scalable production of high-quality perovskite films by a simple, reproducible process is crucial for resolving this issue. Furthermore, t...     

Progress of all-perovskite tandem solar cells: the role of narrow-bandgap absorbers

Perovskite solar cells(PSCs)have gained increasing attention due to their excellent photovoltaic performance,achieving certified power conversion efficiency(PCE)of 25.2%.To further enhance PCE and break the Shockley-Queisser limit of the single junction PSCs,great efforts have been made in tandem solar cells based on perovskite,including perovskite/Si,and perovskite/perovskite(all-perovskite).Among them,all-perovskite tandem solar cells exhibit unique advantages of both lowcost fabrication and high efficiency.They have advanced rapidly in these years,due to the realization of stable and efficient narrow-bandgap perovskites.In this work,we review the development of monolithic all-perovskite tandem solar cells and highlight the critical role of narrow-bandgap perovskites in recent progress of all-perovskite solar cells.We also propose our perspective of future directions on this subject.     

A guide to use fluorinated aromatic bulky cations for stable and high-performance 2D/3D perovskite solar cells: The more fluorination the better?

While serious stability issues impede the commercialization of perovskite solar cells(PSCs),twodimensional(2 D) perovskites based on fluorinated bulky cations have emerged as more intrinsically stable materials.However,the influence of fluorination degree of the bulky aromatic cation on the performance of resulting PSCs has not been scrutinized.Here,2 D perovskites(FxPEA)2 PbI₄(x=1,2,3,5) are grown in situ on the surface of the three-dimensional(3 D) perovskite and demonstrate effecti...     

Boosting oxygen reduction activity and CO2 resistance on bismuth ferrite-based perovskite cathode for low-temperature solid oxide fuel cells below 600℃

Developing efficient and stable cathodes for low-temperature solid oxide fuel cells(LT-SOFCs) is of great importance for the practical commercialization.Herein,we propose a series of Sm-modified Bi_0.7-xSm_xSr_0.3FeO_3-δ perovskites as highly-active catalysts for LT-SOFCs.Sm doping can significantly enhance the electrocata lytic activity and chemical stability of cathode.At 600℃,Bi_0.675Sm_0.025Sr_0.3FeO_3-δ(BSSF25) ...     

The effect of interfacial diffusion on device performance of polymer solar cells: a quantitative view by active-layer doping

The diffusion of constituent materials at interfaces is one of the key factors for device performance and stability.In this work,the effect of interfacial diffusion of a classic interfacial material PFN on device performance of polymer solar cells was studied quantitatively by doping PFN into active layer based on P3HT:PC61BM blend.The PCEs of devices with 550 ppm PFN decrease to half compared to those of the control devices without PFN,which are mainly attributed to the decrease of short-circuit current(Jsc)and fill factor(FF).Advanced analyses of equivalent circuit,absorption spectra,and atomic force microscopy indicates that the presence of PFN in the active layer increases the leakage current,decreases the aggregation of P3HT,and reduces the phase separation.This research reveals the physical mechanism of interfacial diffusion in device performance and provides a basis for further improving the performance and stability of PSCs.     

Back contact interfacial modification mechanism in highly-efficient antimony selenide thin-film solar cells

Antimony selenide(Sb₂Se₃) is a potential photovoltaic(PV) material for next-generation solar cells and has achieved great development in the last several years. The properties of Sb₂Se₃ absorber and back contact influence the PV performances of Sb₂Se₃ solar cells. Hence, optimization of back contact characteristics and absorber orientation are crucial steps in raising the power conversion efficiency(PCE) of Sb₂Se_3<...     

Enhanced efficiency and stability of perovskite solar cells by 2D perovskite vapor-assisted interface optimization

Organic–inorganic perovskites solar cells(PSCs)have attracted great attention due to their rapid progress in power conversion efficiency(PCE).However,there is still an enormous challenge to achieve both high efficiency and stability devices as the decomposition of perovskite materials under humid and light conditions.Herein,we demonstrate that high efficiency and stability of PSCs can be obtained by the reaction of three-dimensional(3D)perovskite with 1,4-butanediamine iodide(BEAI2)vapor.The incorporation of BEAI2 intensively promotes the crystallization of perovskite film with large grain size(~500 nm).Further characterization reveals that the post-treatment perovskite film delivered low interface trap density with long carrier lifetime(>200 ns),long carrier diffusion length(>600 nm)and large carrier mobility(>1.5 cm^2 V-1S-1).Solar cells employing such post-treatment films demonstrated 19.58%PCE without hysteresis.Moreover,the post-treatment devices can retain over 90%original efficiencies stored under ambient atmospheric conditions and exhibit better stability under 85℃and continuous illumination as a two-dimensional(2D)perovskite thin layer is formed on the surface/or at the grain boundaries of 3D perovskite.This study offers an effective way to obtain PSCs with high efficiency and stability.