Wangyikai

Three hole transport materials (HTMs) based on a substituted triphenylamine moiety have been synthesized and employed in perowskite solar cells, reaching efficiencies of 19.4 %. Although all these HTMs show very similar chemical and physical properties, they provide different carrier recombination kinetics.

Abstract

Three hole transport materials (HTMs) based on a substituted triphenylamine moiety have been synthesized and successfully employed in triple‐cation mixed‐halide PSCs, reaching efficiencies of 19.4 %. The efficiencies, comparable to those obtained using spiro‐OMeTAD, point them out as promising candidates for easily attainable and cost‐effective alternatives for PSCs, given their facile synthesis from commercially available materials. Interestingly, although all these HTMs show similar chemical and physical properties, they provide different carrier recombination kinetics. Our results demonstrate that is feasible through the molecular design of the HTM to minimize carrier losses and, thus, increase the solar cell efficiencies.

There has been rapid progress in halide perovskite device performance but further improvements require a firm understanding of charge carrier photophysics. This article details the recent uses of time‐resolved optical microscopy techniques to understand nanoscale charge carrier transport and recombination mechanisms. Ongoing technical developments and future strategies to fill gaps in understanding of carrier behavior in perovskites are discussed.

Abstract

Halide perovskites have remarkable properties for relatively crudely processed semiconductors, including large optical absorption coefficients and long charge carrier lifetimes. Thanks to such properties, these materials are now competing with established technologies for use in cost‐effective and efficient light‐harvesting and light‐emitting devices. Nevertheless, the fundamental understanding of the behavior of charge carriers in these materials—particularly on the nano‐ to microscale—has, on the whole, lagged behind empirical device performance. Such understanding is essential to control charge carriers, exploit new device structures, and push devices to their performance limits. Among other tools, optical microscopy and spectroscopic techniques have revealed rich information about charge carrier recombination and transport on important length scales. In this progress report, the contribution of time‐resolved optical microscopy techniques to the collective understanding of the photophysics of these materials is detailed. The ongoing technical developments in the field that are overcoming traditional experimental limitations in order to visualize transport properties over multiple time and length scales are discussed. Finally, strategies are proposed to combine optical microscopy with complementary techniques in order to obtain a holistic picture of local carrier photophysics in state‐of‐the‐art perovskite devices.

Soft routed benzimidazole clubbed phenoxazine‐based organic ionic plastic crystals with iodide and bromide anions successfully introduced as hole transporting materials in perovskite solar cells yield power conversion efficiencies exceeding 18%, which represents the best alternative to existing spiro‐OMeTAD due to high conductivity and hole mobility with a safer, stable, and efficient system.

Abstract

Organic ionic plastic crystals (OIPCs) are synthesized through a simple metal‐free, cost‐effective approach. The strategized synchronization of electron‐rich phenoxazine with benzimidazolium iodide (OIPC‐I) and bromide (OIPC‐Br) salts lead to enhanced hole mobility and conductivity of OIPCs which is suitable for an efficient alternative to conventional organic hole transporting materials (HTMs) for stable perovskite solar cells (PSCs). The fabricated PSCs with OIPC‐I as hole transporting layer yielded a power conversion efficiency of 15.0% and 18.1% without and with additive (Li salt) respectively, which are comparable with spiro‐OMeTAD based devices prepared under similar conditions. Furthermore, the PSCs with OIPCs show good stability compared to the spiro‐OMeTAD with or without additives. Here, first time benzimidazolium‐based OIPCs have been used as an alternative organic HTM for perovskite solar cells, which opens a window for the design of effective OIPCs for highly efficient PSCs with long‐term stability.

Difluorinated pentathiophene derivatives are synthesized and used as electron donors in all‐small‐molecule organic solar cells. The different substitutional positions of the fluorine atoms in the conjugated backbone of the donor molecules lead to various material and photovoltaic properties. A champion power conversion efficiency of 9.36% is obtained with central fluorinated isomer‐based device.

Three symmetrically difluorinated organic semiconductors (namely D5T2F‐P, D5T2F‐S, and D5T2F‐T) containing rhodanine‐flanked pentathiophene structures are synthesized and used as donors in all‐small‐molecule organic solar cells (ASM‐OSCs) prepared with the small‐molecule acceptor 2,2′‐((2Z,2′Z)‐((4,4,9,9‐tetrahexyl‐4,9‐dihydro‐s‐indaceno[1,2‐b:5,6‐b′]dithiophene‐2,7‐diyl)bis(methanylylidene))bis(5,6‐difluoro‐3‐oxo‐2,3‐dihydro‐1H‐indene‐2,1‐diylidene))dimalononitrile (IDIC‐4F). The different substitutional positions of fluorine atoms (–F) in the conjugated backbone of the donor molecule lead to various material and photovoltaic properties being exhibited. Among the three isomers, the centrally fluorinated D5T2F‐P exhibits a redshifted absorption spectrum, downshifted highest occupied molecular orbital (HOMO) energy level, and improved miscibility with IDIC‐4F in the blend films, all of which result in superior device performance. The power conversion efficiency (PCE) of the ASM‐OSCs based on D5T2F‐P:IDIC‐4F reaches an impressive value of 9.36% with an open‐circuit voltage (V _OC) value of 0.86 V and a short‐circuit current density (J _SC) value of 16.94 mA cm⁻², whereas those of D5T2F‐S (6.11%) and D5T2F‐T (5.42%) are much lower. In comparison, an ASM‐OSC based on the nonfluorinated analogue DRCN5T fabricated under the same conditions exhibits poorer performance (8.03% with IDIC‐4F), revealing 16% enhancement in the PCE achieved through backbone fluorination. The PCE of 9.36% may be one of the highest efficiencies of oligothiophene‐based ASM‐OSCs reported in the literature to date.

Two electron donor (D)–electron acceptor (A)‐type polymers PBDTT and PBTTT are developed as hole‐transporting materials for perovskite solar cells (PVSCs). Both polymers endow the PVSCs promising device performance. A power conversion efficiency of 20.28% is achieved from the devices with dopant‐free PBDTT. High device stability can be expected by employing these compact and hydrophobic polymeric hole‐transporting layers.

Abstract

The rich molecular design of electron donor (D)–acceptor (A) polymers offers many valuable clues to obtain high‐efficiency hole‐transporting materials (HTMs) for use in perovskite solar cells (PVSCs). The fused aromatic or heteroaromatic units can increase the conjugation of the polymer backbone to facilitate electron delocalization, which increases the rigidity of adjacent units to prevent rotational disorder and lower the reorganization energy, leading to improved carrier mobility and optimized film morphology. In this work, fused‐ring ladder‐type indacenodithiophene and indacenodithieno[3,2‐b]thiophene are used as D units, benzodithiophene‐4,8‐dione as the A unit, and thienothiophene as a π‐bridge to form the D–A polymers PBDTT and PBTTT, respectively. Both polymers exhibit favorable properties as HTMs including suitable energy levels, high hole mobility, and excellent film quality. Both dopant‐free HTMs endow n‐i‐p PVSCs with promising performance and stability. A maximum power conversion efficiency of 20.28% is achieved for PBDTT‐based devices, which is among the highest values reported to date.

The aza[5]helicene‐based hole‐transporter is superior to its congener with the planar N‐annulated perylene π‐linker. This study has highlighted that the use of a helical π‐linker for donor−π linker−donor typed organic semiconductors can retain stronger intermolecular π⋅⋅⋅π interactions and attenuated interface charge recombination, leading to better power conversion efficiency of perovskite solar cells.

Abstract

The superior role of helical π‐linkers is demonstrated for the design of donor−π linker−donor typed molecular semiconductors in perovskite solar cells (PSCs). Flat N‐annulated perylene (NP) and contorted aza[5]helicene (A5H) are side‐functionalized with methoxyphenyl and end‐capped with dimethoxydiphenylamine electron‐donor to afford two small‐molecule hole‐transporters J3 and J4. For methoxyphenyl functionalized π‐linkers, intermolecular π⋅⋅⋅π interactions in planar NP exist more extensively than those in helical A5H. However, for the dimethoxydiphenylamine derived hole‐transporters with high highest occupied molecular orbital energy levels, a part of the π⋅⋅⋅π interaction remains for J4 with A5H, while this desirable effect for charge transport is completely deprived for J3 with NP. Thus, the theoretically predicted hole mobility of J4 single‐crystal is even over two times higher than that of J3 one. Because of the larger size of the molecular aggregate, the hole mobility of the spin‐coated J4 thin film is also over three times as high as that of the J3 analog. Due to the reduced transport resistance and enhanced recombination resistance, PSCs with J4 exhibit a power conversion efficiency of 21.0% at standard air mass 1.5 global conditions, which is higher than that of 19.4% with J3 and that of 20.3% with spiro‐OMeTAD control.

Although high power conversion efficiency of up to 23.3% is certified for perovskite solar cells (PSCs), it is still far from the theoretical Shockley–Queisser limit efficiency (30.5%). Nonradiative recombination and charge back transfer at interfaces are mainly responsible for conversion loss. Interface engineering is the most important approach toward the theoretical efficiency in PSCs.

Abstract

Organic–inorganic hybrid perovskite materials are receiving increasing attention and becoming star materials on account of their unique and intriguing optical and electrical properties, such as high molar extinction coefficient, wide absorption spectrum, low excitonic binding energy, ambipolar carrier transport property, long carrier diffusion length, and high defects tolerance. Although a high power conversion efficiency (PCE) of up to 22.7% is certified for perovskite solar cells (PSCs), it is still far from the theoretical Shockley–Queisser limit efficiency (30.5%). Obviously, trap‐assisted nonradiative (also called Shockley–Read–Hall, SRH) recombination in perovskite films and interface recombination should be mainly responsible for the above efficiency distance. Here, recent research advancements in suppressing bulk SRH recombination and interface recombination are systematically investigated. For reducing SRH recombination in the films, engineering perovskite composition, additives, dimensionality, grain orientation, nonstoichiometric approach, precursor solution, and post‐treatment are explored. The focus herein is on the recombination at perovskite/electron‐transporting material and perovskite/hole‐transporting material interfaces in normal or inverted PSCs. Strategies for suppressing bulk and interface recombination are described. Additionally, the effect of trap‐assisted nonradiative recombination on hysteresis and stability of PSCs is discussed. Finally, possible solutions and reasonable prospects for suppressing recombination losses are presented.

An autonomously longitudinal scaffold constructed by the interspersion of in situ polymerized methyl methacrylate in PbI₂ is introduced to effectively eliminate the dependence of sequential deposition on mesoporous TiO₂, and is applied in planar perovskite solar cells, with excellent performance. Moreover, this scaffold's cross‐linking grains are capable of releasing mechanical stress, impeding ion migration, and water/oxygen permeation.

Abstract

Sequential deposition is certified as an effective technology to obtain high‐performance perovskite solar cells (PVSCs), which can be derivatized into large‐scale industrial production. However, dense lead iodide (PbI₂) causes incomplete reaction and unsatisfactory solution utilization of perovskite in planar PVSCs without mesoporous titanium dioxide as a support. Here, a novel autonomously longitudinal scaffold constructed by the interspersion of in situ self‐polymerized methyl methacrylate (sMMA) in PbI₂ is introduced to fabricate efficient PVSCs with excellent flexural endurance and environmental adaptability. By this strategy perovskite solution can be confined within an organic scaffold with vertical crystal growth promoted, effectively inhibiting exciton accumulation and recombination at grain boundaries. Additionally, sMMA cross‐linked perovskite network can release mechanical stress and occupy the main channels for ion migration and water/oxygen permeation to significantly improve operational stability, which opens up a new strategy for the commercial development of large‐area PVSCs in flexible electronics.

A multifunctional conjugated benzene ammonium halide is introduced to enhance phase purity, reduce trap‐state density, and suppress nonradiative charge recombination. Blade‐coated solar cells based on stabilized formamidinium‐dominant perovskite compositions deliver an impressive efficiency of 22.0% and an improved operational stability.

Abstract

Currently, blade‐coated perovskite solar cells (PSCs) with high power conversion efficiencies (PCEs), that is, greater than 20%, normally employ methylammonium lead tri‐iodide with a sub‐optimal bandgap. Alloyed perovskites with formamidinium (FA) cation have narrower bandgap and thus enhance device photocurrent. However, FA‐alloyed perovskites show low phase stability and high moisture sensitivity. Here, it is reported that incorporating 0.83 molar percent organic halide salts (OHs) into perovskite inks enables phase‐pure, highly crystalline FA‐alloyed perovskites with extraordinary optoelectronic properties. The OH molecules modulate the crystal growth, enhance the phase stability, passivate ionic defects at the surface and/or grain boundaries, and enhance the moisture stability of the perovskite film. A high efficiency of 22.0% under 1 sun illumination for blade‐coated PSCs is demonstrated with an open‐circuit voltage of 1.18 V, corresponding to a very small voltage deficit of 0.33 V, and significantly improved operational stability with 96% of the initial efficiency retained under one sun illumination for 500 h.

Hole‐Transporting Materials

In article number 1900182, Zong‐Xiang Xu and co‐workers synthesize dopant‐free hole‐transporting material containing hexamethyl‐mono‐n‐butyl‐substituted zinc phthalocyanine that demonstrates fully face‐on molecular orientation when spin‐coated on perovskite. The as‐fabricated n‐i‐p planar perovskite solar cells exhibit a high power conversion efficiency of 17.41% and long‐term stability, retaining over 90% of their initial efficiency after storage in humidity (about 75%) for 1400 h without encapsulation.

Surface trap–mediated nonradiative charge recombination is a major limit to achieving high-efficiency metal-halide perovskite photovoltaics. The ionic character of perovskite lattice has enabled molecular defect passivation approaches through interaction between functional groups and defects. However, a lack of in-depth understanding of how the molecular configuration influences the passivation effectiveness is a challenge to rational molecule design. Here, the chemical environment of a functional group that is activated for defect passivation was systematically investigated with theophylline, caffeine, and theobromine. When N-H and C=O were in an optimal configuration in the molecule, hydrogen-bond formation between N-H and I (iodine) assisted the primary C=O binding with the antisite Pb (lead) defect to maximize surface-defect binding. A stabilized power conversion efficiency of 22.6% of photovoltaic device was demonstrated with theophylline treatment.

Propyl ammonium (C3A) is introduced into phenethylammonium (PEA)‐based 2D perovskites with <n> = 3. It is found that tuning the CH···π interaction between organic cations can remove undesirable n = 1 phase, lower the density of trap states, and achieve larger crystalline grains to improve the perovskite solar cell efficiency to ≈10%. C3A with other aromatic cations shows similar improvement.

Phenethylammonium (PEA)‐based 2D perovskite is an interesting example of 2D perovskites, serving as the gateway for further introduction of functional conjugated organic cations into 2D perovskites for a variety of applications, for example, photovoltaics. However, the efficiency of photovoltaic devices based on PEA 2D perovskites only achieved ≈7% for <n> = 3, which was significantly lower than that achieved for other cation‐based 2D perovskites. Here, by introducing propyl ammonium (C3A) into the PEA‐based 2D perovskites, the device efficiency is improved to ≈10% for 1:1 C3A:PEA‐based 2D perovskites (<n> = 3). Further investigation reveals that tuning the CH···π interaction (between C3A and PEA or between two PEA molecules) can have multiple beneficial impacts on such modified 2D perovskites, including a) removal of undesirable n = 1 phase, b) lowering the density of trap states, and c) achieving larger crystalline grains. Additionally, after substitution with 50% C3A, other aromatic ammonium cation‐based 2D perovskites (<n> = 3) also show similar efficiency enhancement in their photovoltaic devices, thus exhibiting the general applicability of this method. The results of this study highlight that the strategic tuning of non‐covalent interactions (such as CH···π interaction) is a viable and important method to further develop 2D perovskites for photovoltaics.

Band‐Gap Engineering

In article number 1900304, Jingjing Chang and co‐workers summarize the various reported bandgap engineering strategies. The two most widely used strategies including impurity and pressure as well as their underlying mechanisms are reviewed comprehensively. In addition, intermediate band and external electric field for bandgap structure tuning are also discussed. Moreover, future research directions are outlined to guide further investigation.

Self‐Recrystallization

In article number 1900302, Keiko Waki and co‐workers discover the ability of functionalized carbon nanotubes to make use of the self‐recrystallization nature of perovskites. As a result, junctions between perovskite layers and electrodes are reconstructed, leading to a reduction of charge transfer resistance and a significant improvement in the conversion efficiency, for instance from 3.21% to 11.19%, after 77 days of storage in ambient air.

Organic amine cation, GA⁺ is intentionally incorporated in MA_0.7FA_0.3PbI₃ perovskite to stiffen the inorganic Pb–I lattice, restrain the formation of iodine vacancies defects, and reduce ion diffusion. Solar cells based on this component engineering and PFN‐Br interfacial strategy demonstrate an enhanced power conversion efficiency value over 21% for SnO₂‐based planar perovskite solar cells and excellent thermal stability.

Tin oxide (SnO₂) offers its advantages in widespread applications that require efficient carrier transport. However, the usages of SnO₂ in organic solar cells are hindered because of dangling bonds on the surface of SnO₂. Herein, PFN‐Br as an interfacial layer to tailor the work function of SnO₂ is adopted, making it an ideal candidate for interfacial material in organic electronics. Meanwhile, such an efficient SnO₂/PFN‐Br electron transport layer (ETL) can also be applied to perovskite devices and achieve competitive efficiency. To eliminate current–voltage hysteresis and improve poor thermodynamic stability of perovskite solar cells (PSCs), 5 mol% of guanidinium iodide (GAI) into the (MA) _x (FA)_1 − x PbI₃ precursor solution is incorporated, enabling the formation of triple‐cation perovskite films with excellent optoelectronic quality and stability. The combination of an SnO₂/PFN‐Br ETL and GAI doping strategy finally delivers power conversion efficiencies over 21% and negligible current–voltage hysteresis in planar PSCs. These improvements arise from the strong hydrogen bonding caused by the incorporation of GA⁺. It can stiffen the inorganic Pb–I lattice of the unit cell and restrain the formation of iodine vacancies defects. Moreover, the strong hydrogen bonding can immobilize iodide ion and thus enhance the thermal stability of the corresponding device.

Three symmetrically di‐fluorinated organic semiconductors (namely D5T2F‐P, D5T2F‐S, and D5T2F‐T) containing rhodanine‐flanked pentathiophene structures are synthesized and used as donors in all‐small‐molecule organic solar cells (ASM‐OSCs) prepared with the small‐molecule acceptor 2,2'‐((2Z,2'Z)‐((4,4,9,9‐tetrahexyl‐4,9‐dihydro‐s‐indaceno[1,2‐b:5,6‐b']dithiophene‐2,7‐diyl)bis(methanylylidene))bis(5,6‐difluoro‐3‐oxo‐2,3‐dihydro‐1H‐indene‐2,1‐diylidene))dimalononitrile (IDIC‐4F). The different substitutional positions of the fluorine atoms (‐F) in the conjugated backbone of the donor molecule leads to various material and photovoltaic properties being exhibited. Among the three isomers, the centrally‐fluorinated D5T2F‐P exhibits a redshifted absorption spectrum, downshifted highest occupied molecular orbital (HOMO) energy level, and improved miscibility with IDIC‐4F in the blend films, all of which result in superior device performance. The power conversion efficiency (PCE) of the ASM‐OSCs based on D5T2F‐P:IDIC‐4F reaches the impressive value of 9.36% with an open‐circuit voltage (V _OC ) value of 0.86 V and a short‐circuit current density (J _SC ) value of 16.94 mA/cm², while those of D5T2F‐S (6.11%) and D5T2F‐T (5.42%) are much lower. In comparison, an ASM‐OSC based on the non‐fluorinated analogue DRCN5T fabricated under the same conditions exhibits poorer performance (8.03% with IDIC‐4F), revealing a 16% enhancement in PCE achieved through backbone fluorination. To the best of our knowledge, the PCE of 9.36% is one of the highest efficiencies of oligothiophene‐based ASM‐OSCs reported in the literature to date.

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An effective composite electron transport layer (ETL) is fabricated using carboxylic‐acid‐ and hydroxyl‐rich red‐carbon quantum dots to dope low‐temperature solution‐processed SnO₂. The electron mobility of SnO₂ is dramatically increased by ≈20 times from 9.32 × 10⁻⁴ to 1.73 × 10⁻² cm² V⁻¹ s⁻¹. A planar perovskite solar cell based on this novel SnO₂ ETL demonstrates an outstanding improvement in efficiency up to 22.77%.

Abstract

An efficient electron transport layer (ETL) plays a key role in promoting carrier separation and electron extraction in planar perovskite solar cells (PSCs). An effective composite ETL is fabricated using carboxylic‐acid‐ and hydroxyl‐rich red‐carbon quantum dots (RCQs) to dope low‐temperature solution‐processed SnO₂, which dramatically increases its electron mobility by ≈20 times from 9.32 × 10⁻⁴ to 1.73 × 10⁻² cm² V⁻¹ s⁻¹. The mobility achieved is one of the highest reported electron mobilities for modified SnO₂. Fabricated planar PSCs based on this novel SnO₂ ETL demonstrate an outstanding improvement in efficiency from 19.15% for PSCs without RCQs up to 22.77% and have enhanced long‐term stability against humidity, preserving over 95% of the initial efficiency after 1000 h under 40–60% humidity at 25 °C. These significant achievements are solely attributed to the excellent electron mobility of the novel ETL, which is also proven to help the passivation of traps/defects at the ETL/perovskite interface and to promote the formation of highly crystallized perovskite, with an enhanced phase purity and uniformity over a large area. These results demonstrate that inexpensive RCQs are simple but excellent additives for producing efficient ETLs in stable high‐performance PSCs as well as other perovskite‐based optoelectronics.

Perovskites are versatile ABX₃ crystals, hosting many intriguing physical properties. While most are inorganic compounds with cationic A‐ and B‐, and anionic X‐sites, recently, the introduction of organic ions (hybrid perovskites) and structures with inverted ionic charges (inverse (hybrid) perovskites) have been explored. Thus, the combinatorial space for design with optimized properties has new dimensions.

Abstract

Materials science evolves to a state where the composition and structure of a crystal can be controlled almost at will. Given that a composition meets basic requirements of stoichiometry, steric demands, and charge neutrality, researchers are now able to investigate a wide range of compounds theoretically and, under various experimental conditions, select the constituting fragments of a crystal. One intriguing playground for such materials design is the perovskite structure. While a game of mixing and matching ions has been played successfully for about 150 years within the limits of inorganic compounds, the recent advances in organic–inorganic hybrid perovskite photovoltaics have triggered the inclusion of organic ions. Organic ions can be incorporated on all sites of the perovskite structure, leading to hybrid (double, triple, etc.) perovskites and inverse (hybrid) perovskites. Examples for each of these cases are known, even with all three sites occupied by organic molecules. While this change from monatomic ions to molecular species is accompanied with increased complexity, it shows that concepts from traditional inorganic perovskites are transferable to the novel hybrid materials. The increased compositional space holds promising new possibilities and applications for the universe of perovskite materials.

Although high power conversion efficiency of up to 23.3% is certified for perovskite solar cells (PSCs), it is still far from the theoretical Shockley–Queisser limit efficiency (30.5%). Nonradiative recombination and charge back transfer at interfaces are mainly responsible for conversion loss. Interface engineering is the most important approach toward the theoretical efficiency in PSCs.

Abstract

Organic–inorganic hybrid perovskite materials are receiving increasing attention and becoming star materials on account of their unique and intriguing optical and electrical properties, such as high molar extinction coefficient, wide absorption spectrum, low excitonic binding energy, ambipolar carrier transport property, long carrier diffusion length, and high defects tolerance. Although a high power conversion efficiency (PCE) of up to 22.7% is certified for perovskite solar cells (PSCs), it is still far from the theoretical Shockley–Queisser limit efficiency (30.5%). Obviously, trap‐assisted nonradiative (also called Shockley–Read–Hall, SRH) recombination in perovskite films and interface recombination should be mainly responsible for the above efficiency distance. Here, recent research advancements in suppressing bulk SRH recombination and interface recombination are systematically investigated. For reducing SRH recombination in the films, engineering perovskite composition, additives, dimensionality, grain orientation, nonstoichiometric approach, precursor solution, and post‐treatment are explored. The focus herein is on the recombination at perovskite/electron‐transporting material and perovskite/hole‐transporting material interfaces in normal or inverted PSCs. Strategies for suppressing bulk and interface recombination are described. Additionally, the effect of trap‐assisted nonradiative recombination on hysteresis and stability of PSCs is discussed. Finally, possible solutions and reasonable prospects for suppressing recombination losses are presented.

Research on compositional engineering can realize power conversion efficiency (PCE) over 25%. Interfacial engineering along with optimal perovskite solar cell device structure is expected to lead to stable and theoretical PCE over 30%.

Abstract

Discovery of the 9.7% efficiency, 500 h stable solid‐state perovskite solar cell (PSC) in 2012 triggered off a wave of perovskite photovoltaics. As a result, a certified power conversion efficiency (PCE) of 25.2% was recorded in 2019. Publications on PSCs have increased exponentially since 2012 and the total number of publications reached over 13 200 as of August 2019. PCE has improved by developing device structures from mesoscopic sensitization to planar p‐i‐n (or n‐i‐p) junction and by changing composition from MAPbI₃ to FAPbI₃‐based mixed cations and/or mixed anion perovskites. Long‐term stability has been significantly improved by interfacial engineering with hydrophobic materials or the 2D/3D concept. Although small area cells exhibit superb efficiency, scale‐up technology is required toward commercialization. In this review, research direction toward large‐area, stable, high efficiency PSCs is emphasized. For large‐area perovskite coating, a precursor solution is equally important as coating methods. Precursor engineering and formulation of the precursor solution are described. For hysteresis‐less, stable, and higher efficiency PSCs, interfacial engineering is one of the best ways as defects can be effectively passivated and thereby nonradiative recombination is efficiently reduced. Methodologies are introduced to minimize interfacial and grain boundary recombination.

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.