JIALINGBO

In parallel with the tremendous progress in the efficiency of perovskite solar cells, research on the issue of instability has attracted enormous attention. In this review, the strategies to enhance the stability from the perspectives of the device structure, the photoactive layer, hole‐ and electron‐transporting layers, electrode materials, and device encapsulation are portrayed.

Abstract

In this review, the factors influencing the power conversion efficiency (PCE) of perovskite solar cells (PSCs) is emphasized. The PCE of PSCs has remarkably increased from 3.8% to 23.7%, but on the other hand, poor stability is one of the main facets that creates a huge barrier in the commercialization of PSCs. Herein, a concise overview of the current efforts to enhance the stability of PSCs is provided; moreover, the degradation causes and mechanisms are summarized. The strategies to improve device stability are portrayed in terms of structural effects, a photoactive layer, hole‐ and electron‐transporting layers, electrode materials, and device encapsulation. Last but not least, the economic feasibility of PSCs is also vividly discussed.

Causes of intrinsic and extrinsic instability of perovskite materials and related mechanisms are discussed in terms of their chemical‐bonding nature. Understanding the critical mechanisms rationalizes the chemical approaches to mitigate the degradation in perovskite solar cells.

Abstract

Chemical bonding dictates not only the optoelectronic properties of materials, but also the intrinsic and extrinsic stability of materials. Here, the causes of intrinsic and extrinsic instability of perovskite materials are reviewed considering their correlation with the unique chemical‐bonding nature of perovskite materials. There are a number of key standardized stability tests established by the International Electrotechnical Commission for commercialized photovoltaic modules. Based on these procedures, the possible causes and related mechanisms of the material degradation that can arise during the test procedures are identified, which are discussed in terms of their chemical bonds. Based on the understanding of the critical causes, promising strategies for mitigating the causes to enhance the stability of perovskite solar cells are summarized. The stability of the state‐of‐the‐art perovskite solar cells implies a need for the development of improved stability‐testing protocols to move onto the next stage toward commercialization.

Despite the record efficiency exceeding 25%, the long‐term operational stability of perovskite solar cells is limited by the degradation mechanisms accelerated by the presence of vacancies and defects. In this review, recent engineering strategies ranging from grains to interfaces that mitigate degradation and improve efficiencies are discussed.

Abstract

Lead halide perovskite solar cells now show excellent efficiencies and encouraging levels of stability. Further improvements in performance require better control of the trap states which are considered to be associated with vacancies and defects at crystallite surfaces. Herein, a reflection on the ways in which these traps can be mitigated is presented by improving the quality of the perovskite layer and interfaces in fully assembled device configurations. In this review, the most recent design strategies reported in the literature, which have been explored to tune grain orientation, to passivate defects, and to improve charge‐carrier lifetimes, are presented. Specifically, the advances made with single‐cation, mixed‐cation and/or mixed‐halide, and 3D/2D bilayer‐based light absorbers are discussed. The interfacial, compositional, and band alignment engineering along with their consequent effects on the open‐circuit voltage, power conversion efficiency, and stability are a particular focus.

A rapid layer‐specific annealing on perovskite active layers enabled by UV LED is developed, and efficiency close to 19% in a simple planar inverted structure of ITO/PEDOT:PSS/MAPbI₃/PC₇₁BM/Al without any device engineering is demonstrated. The results demonstrate that if the UV dosage is well managed, UV light is capable of annealing perovskite into high‐quality film rather than simply damaging it.

Abstract

A rapid layer‐specific annealing on perovskite active layer enabled by ultraviolet (UV) light‐emitting diode (LED) is demonstrated and efficiency close to 19% is achieved in a simple planar inverted structure ITO/PEDOT:PSS/MAPbI₃/PC₇₁BM/Al without any device engineering. These results demonstrate that if the UV dosage is well managed, UV light is capable of annealing perovskite into high‐quality film rather than simply damaging it. Different in principle from other photonic treatment techniques that can heat up and damage underlying films, the UV‐LED‐annealing method enables layer‐specific annealing because LED light source is able to provide a specific UV wavelength for maximum light absorption of target film. Moreover, the layer‐specific photonic treatment allows accurate estimation of the crystallization energy required to form perovskite film at device quality level.

The application of formamidinium (FA)‐based perovskite solar cells has largely been hindered by phase transition from the dark cubic phase to yellow orthorhombic phase. Here, a highly efficient and phase stable FA‐based perovskite solar cell is fabricated by using NbF₅ as a novel additive. NbF₅ can improve the quality of perovskite films and effectively suppress the formation of the yellow δ‐phase.

Abstract

The HC(NH₂)₂ ⁺(FA⁺) is a well‐known substitute to CH₃NH₃ ⁺(MA⁺) for its capability to extend light utilization for improved power conversion efficiency for perovskite solar cells; unfortunately, the dark cubic phase (α‐phase) can easily transition to the yellow orthorhombic phase (δ‐phase) at room temperature, an issue that prevents its commercial application. In this report, an inorganic material (NbF₅) is developed to stabilize the desired α‐phase perovskite material by incorporating NbF₅ additive into the perovskite films. It is found that the NbF₅ additive effectively suppresses the formation of the yellow δ‐phase in the perovskite synthesis and aging process, thus enhancing the humidity and light‐soaking stability of the perovskite film. As a result, the perovskite solar cells with the NbF₅ additive exhibit improved air stability by tenfold, retaining nearly 80% of their initial efficiency after aging in air for 50 d. In addition, under full‐sun AM 1.5 G illumination of a xenon lamp without any UV‐reduction, the perovskite solar cells with the NbF₅ additive also show fivefold improved illumination stability than the control devices without NbF₅.

Here, urea treatment of hole transport layer (e.g., poly(3,4‐ethylene dioxythiophene):polystyrene sulfonate (PEDOT:PSS)) is reported to effectively tune its morphology, conductivity, and work function for improving the efficiency and stability of inverted CH₃NH₃PbI₃ perovskite solar cells.

Abstract

Interface engineering is critical to the development of highly efficient perovskite solar cells. Here, urea treatment of hole transport layer (e.g., poly(3,4‐ethylene dioxythiophene):polystyrene sulfonate (PEDOT:PSS)) is reported to effectively tune its morphology, conductivity, and work function for improving the efficiency and stability of inverted MAPbI₃ perovskite solar cells (PSCs). This treatment has significantly increased MAPbI₃ photovoltaic performance to 18.8% for the urea treated PEDOT:PSS PSCs from 14.4% for pristine PEDOT:PSS devices. The use of urea controls phase separation between PEDOT and PSS segments, leading to the formation of a unique fiber‐shaped PEDOT:PSS film morphology with well‐organized charge transport pathways for improved conductivity from 0.2 S cm⁻¹ for pristine PEDOT:PSS to 12.75 S cm⁻¹ for 5 wt% urea treated PEDOT:PSS. The urea‐treatment also addresses a general challenge associated with the acidic nature of PEDOT:PSS, leading to a much improved ambient stability of PSCs. In addition, the device hysteresis is significantly minimized by optimizing the urea content in the treatment.

The latest progress and issues toward scalable fabrication of perovskite solar cells are reviewed in an attempt to provide insights into the development of rational fabrication methods for large‐area perovskite films and solar modules.

Abstract

With the impressive record power conversion efficiency (PCE) of perovskite solar cells exceeding 23%, research focus now shifts onto issues closely related to commercialization. One of the critical hurdles is to minimize the cell‐to‐module PCE loss while the device is being developed on a large scale. Since a solution‐based spin‐coating process is limited to scalability, establishment of a scalable deposition process of perovskite layers is a prerequisite for large‐area perovskite solar modules. Herein, this paper reports on the recent progress of large‐area perovskite solar cells. A deeper understanding of the crystallization of perovskite films is indeed essential for large‐area perovskite film formation. Various large‐area coating methods are proposed including blade, slot‐die, evaporation, and post‐treatment, where blade‐coating and gas post‐treatment have so far demonstrated better PCEs for an area larger than 10 cm². However, PCE loss rate is estimated to be 1.4 × 10⁻²% cm⁻², which is 82 and 3.5 times higher than crystalline Si (1.7 × 10⁻⁴% cm⁻²) and thin film technologies (≈4 × 10⁻³% cm⁻²) respectively. Therefore, minimizing PCE loss upon scaling‐up is expected to lead to PCE over 20% in case of cell efficiency of >23%.

New‐type 2D/3D perovskites are designed by first introducing two hydrophobic ammonium salt cations with halogen functional groups into 3D perovskite. The 2D/3D perovskite devices exhibit an optimal power conversion efficiency as high as 20.08% under 1 sun irradiation and superior stability when exposed to humidity, temperature, and continuous UV irradiation.

Abstract

2D perovskites have attracted extensive attention due to their excellent stability compared with 3D perovskites. However, the intrinsic hydrophilicity of introduced alkylammonium salts effects the humidity stability of 2D/3D perovskites. Devices based on longer chain alkylammonium salts show improvement in hydrophobicity but lower efficiency due to the poorer charge transport among various layers. To solve this issue, two hydrophobic short‐chain alkylammonium salts with halogen functional groups (2‐chloroethylamine, CEA⁺ and 2‐bromoethylamine, BEA⁺) are introduced into (Cs_0.1FA_0.9)Pb(I_0.9Br_0.1)₃ 3D perovskites to form 2D/3D perovskite structure, which achieve high‐quality perovskite films with better crystallization and morphology. The optimal 2D/3D perovskite solar cells (PSCs) with 5% CEA⁺ display a power conversion efficiency (PCE) as high as 20.08% under 1 sun irradiation. Because of the notable hydrophobicity of alkylammonium cations with halogen functional groups and the formed 2D/3D perovskite structure, the optimal PSCs exhibit superior moisture resistance and retain 92% initial PCE after aging at 50 ± 5% relative humidity for 2400 h. This work opens up a new direction for the design of new‐type 2D/3D PSCs with improved performance by employing proper alkylammonium salts with different functional groups.

In article number https://doi.org/10.1002/adma.2019035591903559, Edward H. Sargent, Zhijun Ning, and co‐workers intentionally include a secondary amine, dimethylamine, in MAPbI₃ perovskite to improve the rigidity and steric hindrance for water adsorption, giving rise to reduced defect density and enhanced hydrophobicity. NiO _x ‐based inverted perovskite solar cells based on this perovskite structure demonstrate a record certified power conversion efficiency of 20.8% with excellent operational stability under continuous light soaking.

Quantum dots are regarded as neutralized charged intermedia to transfer ligands for interfacial modification, which can significantly adjust surface electric properties to reduce V _OC loss and improve device performance. A stable V _OC enhancement with excellent reproducibility is fulfilled by simple solution‐processed QDs modification, achieving 20.6% power conversion efficiency (PCE) and enhanced stability.

The tremendous passion for inverted planar heterojunction perovskite solar cells (PSCs) is originated from their great tendency in the roll‐to‐roll process‐compatible fabrication and huge potential for integration into tandem solar cells. But the device efficiency is still lower than regular structured PSCs. Engineering of the cathode interface to efficiently control and reduce V _OC loss lights a lamp for increasing electrochemical properties and boosting overall performance. Herein, a simple interfacial modification strategy is developed by introducing a hybrid ligand interfacial layer to reduce V _OC loss in PSCs with inverted planar structure. Heavily washed QDs are used as neutral charged intermedia to enable alloying reaction to transfer ligands without damage to perovskite (PVK). A band bending is immediately generated on the top surface of PVK film after QDs modification, which is directly confirmed by ultraviolet photoelectron spectroscopy (UPS) and Kelvin probe force microscopy (KPFM). This contributes to ≈50 mV reduced V _OC loss, leading to a V _OC of 1.15 V and a power conversion efficiency (PCE) of 20.6% in inverted PSCs. Meanwhile, enhanced stability is achieved for these devices after QDs modification, in which PCE is maintained at >90% of initial value after 1000 h storage.

Operationally stable mixed‐cation‐halide perovskite solar cells are fabricated by halogen‐engineering concept via a Br‐rich seeding growth method. Bromine anions are effectively incorporated into the final perovskite film with larger grains and better vertical columnar alignment. Photovoltaic devices based on the film show a power conversion efficiency (PCE) of 21.5% and significantly enhanced operational stability for over 500 h.

Abstract

The performance of perovskite solar cells (PSCs) relies on the synthesis method and chemical composition of the perovskite materials. So far, PSCs that have adopted two‐step sequential deposited perovskite with the state‐of‐art composition (FAPbI₃)_{1− x} (MAPbBr₃) _x (x < 0.05) have achieved record power conversion efficiency (PCE), while their one‐step antisolvent dripping counterparts with typical composition Cs_0.05FA_0.81MA_0.14Pb(I_0.85Br_0.15)₃ with more bromine have exhibited much better long‐term operational stability. Thus, halogen engineering that aims to elevate bromine content in sequential deposited perovskite film would push operational stability of PSCs toward that of antisolvent dripping deposited perovskite materials. Here, a Br‐rich seeding growth method is devised and perovskite seed solution with high bromine content is introduced into a PbI₂ precursor, leading to bromine incorporation in the resulting perovskite film. Photovoltaic devices fabricated by Br‐rich seeding growth method exhibit a PCE of 21.5%, similar to 21.6% for PSCs having lower bromine content. Whereas, the operational stability of PSCs with higher bromine content is significantly enhanced, with over 80% of initial PCE retained after 500 h tracking at maximum power point under 1‐sun illumination. This work highlights the vital importance of halogen composition for the operational stability of PSCs, and introduces an effective way to incorporate bromine into mixed‐cation‐halide perovskite film via sequential deposition method.

Interfaces between the photoactive layer and electrodes play a critical role in ultimate device behaviors in organic bulk heterojunction solar cells (OSCs) and hybrid halide perovskite solar cells (PSCs). Recent progress in interface modification for OSCs and PSCs aimed at improving interfacial charge extraction and mitigating surface recombination, and at enhancing trap passivation and device stability is presented.

Abstract

Organic bulk heterojunction solar cells (OSCs) and hybrid halide perovskite solar cells (PSCs) are two promising photovoltaic techniques for next‐generation energy conversion devices. The rapid increase in the power conversion efficiency (PCE) in OSCs and PSCs has profited from synergetic progresses in rational material synthesis for photoactive layers, device processing, and interface engineering. Interface properties in these two types of devices play a critical role in dictating the processes of charge extraction, surface trap passivation, and interfacial recombination. Therefore, there have been great efforts directed to improving the solar cell performance and device stability in terms of interface modification. Here, recent progress in interfacial doping with biopolymers and ionic salts to modulate the cathode interface properties in OSCs is reviewed. For the anode interface modification, recent strategies of improving the surface properties in widely used PEDOT:PSS for narrowband OSCs or replacing it by novel organic conjugated materials will be touched upon. Several recent approaches are also in focus to deal with interfacial traps and surface passivation in emerging PSCs. Finally, the current challenges and possible directions for the efforts toward further boosts of PCEs and stability via interface engineering are discussed.

Hole‐transport material based on dibenzo[b,d]thiophene (DBTMT) is synthesized with low costs. A champion power conversion efficiency of the optimized p–i–n planar perovskite solar cells based on dopant‐free DBTMT reaches 21.12% with a high fill factor of 83.25%, due to good hole‐transport properties and the passivation effect of DBTMT.

N ²,N ²,N ⁸,N ⁸‐tetrakis(4‐(methylthio)phenyl)dibenzo[b,d]thiophene‐2,8‐diamine (DBTMT) is synthesized from three commercial monomers for application as a promising dopant‐free hole‐transport material (HTM) in perovskite solar cells (pero‐SCs). The intrinsic properties (optical properties and electronic energy levels) of DBTMT are investigated, proving that DBTMT is a suitable HTM for the planar p–i–n pero‐SCs. The champion power conversion efficiency (PCE) of the optimized pero‐SCs (with structure as ITO/pristine DBTMT/MAPbI₃/C₆₀/BCP/Ag) reaches 21.12% with a fill factor (FF) of 83.25%, which is among the highest PCEs and FFs reported for planar p–i–n pero‐SCs based on dopant‐free HTMs. The Fourier‐transform infrared spectroscopy, X‐ray diffraction, and X‐ray photoelectron spectroscopy spectra of MAPbI₃ and DBTMT–MAPbI₃ films demonstrate that there is an interaction between DBTMT and MAPbI₃ at the interface through the sulfur atoms in DBTMT to passivate the defects, which is corresponding to the higher FF and PCE of the corresponding device.

Passivation project: Chelate‐assisted defect passivation is an efficient way to prepare high‐quality polycrystalline perovskites. The bidentate chelating agent controls crystallization kinetics, grain size, and crystal defects of Cs‐, methylammonium‐ and formamidinium‐based perovskite materials. The high‐quality perovskite films result in higher device efficiency and improved device stability.

Abstract

Perovskite solar cells are sensitive to subtle changes in atmospheric conditions, resulting in problems such as the collapse of the perovskite structure and sharp drops in efficiency. Internal defects are also a big obstacle for high‐quality polycrystalline perovskites. At present, it is difficult to control the density of the trapping sites. By using the bidentate chelating agent thenoyltrifluoroacetone (ttfa), the crystallization kinetics, grain sizes, and crystal defect of Cs‐, methylammonium‐, and formamidinium‐based perovskite materials can be to effectively controlled through a nucleation and growth process for the preparation of perovskite crystals. Crystalline‐state tuning during the crystallization process to obtain better quality perovskite thin films can be achieved with no additional operation, which is suitable for the needs of modern industrial production and management. The chelating agent can effectively passivate the defects in perovskite films, leading to a low defect density and a long charge carrier lifetime. As a result, the ttfa‐passivated perovskite solar cell demonstrated a high power conversion efficiency of 19.70 % with superior stability retention of 64 % of the initial power conversion efficiency after two weeks unencapsulated storage in an adverse atmosphere with approximately 50 % relative humidity.

A dopant‐free hole transporting material (HTM) named DMZ, is synthesized and applied in inverted planar perovskite solar cells (PSCs). High power conversion efficiency (PCE) (18.61%) and stable‐enhanced PSCs devices are achieved and after storage for nearly 560 h, 90% of the maximum PCE is retained in air with a relative humidity ≈ 45%–50% without any encapsulation.

Abstract

A new hole transporting material (HTM) named DMZ is synthesized and employed as a dopant‐free HTM in inverted planar perovskite solar cells (PSCs). Systematic studies demonstrate that the thickness of the hole transporting layer can effectively enhance the morphology and crystallinity of the perovskite layer, leading to low series resistance and less defects in the crystal. As a result, the champion power conversion efficiency (PCE) of 18.61% with J _SC = 22.62 mA cm⁻², V _OC = 1.02 V, and FF = 81.05% (an average one is 17.62%) is achieved with a thickness of ≈13 nm of DMZ (2 mg mL⁻¹) under standard global AM 1.5 illumination, which is ≈1.5 times higher than that of devices based on poly(3,4‐ethylenedioxythiophene)/poly(styrene sulfonic acid) (PEDOT:PSS). More importantly, the devices based on DMZ exhibit a much better stability (90% of maximum PCE retained after more than 556 h in air (relative humidity ≈ 45%–50%) without any encapsulation) than that of devices based on PEDOT:PSS (only 36% of initial PCE retained after 77 h in same conditions). Therefore, the cost‐effective and facile material named DMZ offers an appealing alternative to PEDOT:PSS or polytriarylamine for highly efficient and stable inverted planar PSCs.

This study reports a systematic study in terms of efficiency and stability by post‐treatment with alkyl ammonium iodides of different alkyl lengths on (FAPbI₃)_0.95(MAPbBr₃)_0.05 perovskite surface. As the length of the alkyl chain increases, the electron‐blocking ability and humidity stability increase, but the highest efficiency is obtained at the optimal alkyl length.

Abstract

Recently, two‐dimensional (2D) structure on three‐dimensional (3D) perovskites (graded 2D/3D) has been reported to be effective in significantly improving both efficiency and stability. However, the electrical properties of the 2D structure as a passivation layer on the 3D perovskite thin film and resistance to the penetration of moisture may vary depending on the length of the alkyl chain. In addition, the surface defects of the 2D itself on the 3D layer may also be affected by the correlation between the 2D structure and the hole conductive material. Therefore, systematic interfacial study with the alkyl chain length of long‐chained alkylammonium iodide forming a 2D structure is necessary. Herein, the 2D interfacial layers formed are compared with butylammonium iodide (BAI), octylammonium iodide (OAI), and dodecylammonium iodide (DAI) iodide on a 3D (FAPbI₃)_0.95(MAPbBr₃)_0.05 perovskite thin film in terms of the PCE and humidity stability. As the length of the alkyl chain increased from BA to OA to DA, the electron‐blocking ability and humidity resistance increase significantly, but the difference between OA and DA is not large. The PSC post‐treated with OAI has slightly higher PCE than those treated with BAI and DAI, achieving a certified stabilized efficiency of 22.9%.

The acid treatment of TiO₂ weakens the bonding of octahedral chains in anatase TiO₂, rendering the formation of amorphous TiO₂ buffer layer on the surface of anatase TiO₂. This amorphous TiO₂ buffer layer contains rich oxygen vacancies, which increase the donor density of TiO₂.

Abstract

The ability to effectively transfer photoexcited electrons and holes is an important endeavor toward achieving high‐efficiency solar energy conversion. Now, a simple yet robust acid‐treatment strategy is used to judiciously create an amorphous TiO₂ buffer layer intimately situated on the anatase TiO₂ surface as an electron‐transport layer (ETL) for efficient electron transport. The facile acid treatment is capable of weakening the bonding of zigzag octahedral chains in anatase TiO₂, thereby shortening staggered octahedron chains to form an amorphous buffer layer on the anatase TiO₂ surface. Such amorphous TiO₂‐coated ETL possesses an increased electron density owing to the presence of oxygen vacancies, leading to efficient electron transfer from perovskite to TiO₂. Compared to pristine TiO₂‐based devices, the perovskite solar cells (PSCs) with acid‐treated TiO₂ ETL exhibit an enhanced short‐circuit current and power conversion efficiency.

Perovskite solar cells have reached certified efficiencies of 25.2% within just ten years due to their excellent optoelectronic properties. Nonradiative recombination at the interface between the perovskite absorber and charge‐transporting layers is identified as the major source of open‐circuit‐voltage losses in state‐of‐the‐art devices, requiring advanced strategies to study and to control efficiency‐limiting interfacial processes.

Abstract

Perovskite solar cells combine high carrier mobilities with long carrier lifetimes and high radiative efficiencies. Despite this, full devices suffer from significant nonradiative recombination losses, limiting their V _OC to values well below the Shockley–Queisser limit. Here, recent advances in understanding nonradiative recombination in perovskite solar cells from picoseconds to steady state are presented, with an emphasis on the interfaces between the perovskite absorber and the charge transport layers. Quantification of the quasi‐Fermi level splitting in perovskite films with and without attached transport layers allows to identify the origin of nonradiative recombination, and to explain the V _OC of operational devices. These measurements prove that in state‐of‐the‐art solar cells, nonradiative recombination at the interfaces between the perovskite and the transport layers is more important than processes in the bulk or at grain boundaries. Optical pump‐probe techniques give complementary access to the interfacial recombination pathways and provide quantitative information on transfer rates and recombination velocities. Promising optimization strategies are also highlighted, in particular in view of the role of energy level alignment and the importance of surface passivation. Recent record perovskite solar cells with low nonradiative losses are presented where interfacial recombination is effectively overcome—paving the way to the thermodynamic efficiency limit.