awn

Implementation of nanoparticles (NPs) with plasmonic effects is an effective strategy for photon and charge dynamic management in perovskite solar cells (PSCs). The outstanding effects of plasmonic nanostructures such as Ag NPs decorated on TiO₂ nanowires in electron transport materials as well as localized surface plasmon resonance of Au NPs in hole transport materials enhance the photovoltaic response of PSCs.

Abstract

Perovskite solar cells (PSCs) have emerged recently as promising candidates for next generation photovoltaics and have reached power conversion efficiencies of 25.2%. Among the various methods to advance solar cell technologies, the implementation of nanoparticles with plasmonic effects is an alternative way for photon and charge carrier management. Surface plasmons at the interfaces or surfaces of sophisticated metal nanostructures are able to interact with electromagnetic radiation. The properties of surface plasmons can be tuned specifically by controlling the shape, size, and dielectric environment of the metal nanostructures. Thus, incorporating metallic nanostructures in solar cells is reported as a possible strategy to explore the enhancement of energy conversion efficiency mainly in semi‐transparent solar cells. One particularly interesting option is PSCs with plasmonic structures enable thinner photovoltaic absorber layers without compromising their thickness while maintaining a high light harvest. In this Review, the effects of plasmonic nanostructures in electron transport material, perovskite absorbers, the hole transport material, as well as enhancement of effective refractive index of the medium and the resulting solar cell performance are presented. Aside from providing general considerations and a review of plasmonic nanostructures, the current efforts to introduce these plasmonic structures into semi‐transparent solar cells are outlined.

In this comprehensive review, the main challenges and the current status of perovskite/silicon, perovskite/CIGS, and perovskite/perovskite tandem technologies are presented. A specific focus is set on advanced characterization methods as well as simulations being utilized for perovskite‐based tandem solar cells to overcome the challenges and gain deeper knowledge to further improve device performance. Finally, the efficiency potentials in different experimental and theoretical limits are compared and pathways toward 35% efficiency are outlined.

Abstract

Tandem solar cells are the next step in the photovoltaic (PV) evolution due to their higher power conversion efficiency (PCE) potential than currently dominating, but inherently limited, single‐junction solar cells. With the emergence of metal halide perovskite absorber materials, the fabrication of highly efficient tandem solar cells, at a reasonable cost, can significantly impact the future PV landscape. The perovskite‐based tandem solar cells have already shown that they can convert light more efficiently than their standalone sub‐cells. However, to reach PCEs over 30%, several challenges have to be overcome and the understanding of this fascinating technology has to be broadened. In this review, the main scientific and engineering challenges in the field are presented, alongside a discussion of the current status of three main perovskite tandem technologies: perovskite/silicon, perovskite/CIGS, and perovskite/perovskite tandem solar cells. A summary of the advanced structural, electrical, optical, radiative, and electronic characterization methods as well as simulations being utilized for perovskite‐based tandem solar cells is presented. The main findings are summarized and the strength of the techniques to overcome the challenges and gain deeper knowledge for further performance improvement is assessed. Finally, the PCE potential in different experimental and theoretical limits is compared with an aim to shed light on the path towards overcoming the 30% efficiency threshold for all of the three herein reviewed tandem technologies.

Fluorinated, π‐extended end groups imbue indacenodithienothiophene‐based acceptors (ITN‐F4 and ITzN‐F4) with higher power conversion efficiency, stronger π–π electronic coupling, lower reorganization energies, and highly connected crystal structures. Femtosecond absorption spectroscopy reveals ultrafast hole transfer (<300 fs) in blends with donor polymer poly{[4,8‐bis[5‐(2‐ethylhexyl)‐4‐fluoro‐2‐thienyl]benzo[1,2‐b :4,5‐b ′]‐dithiophene‐2,6‐diyl]‐alt‐[2,5‐thiophenediyl[5,7‐bis(2‐ethylhexyl)‐4,8‐dioxo‐4H ,8H‐benzo[1,2‐c :4,5‐c ′]dithiophene‐1,3‐diyl]]} (PBDB‐TF), despite excimer formation.

Abstract

The synthesis and characterization of new semiconducting materials is essential for developing high‐efficiency organic solar cells. Here, the synthesis, physiochemical properties, thin film morphology, and photovoltaic response of ITN‐F4 and ITzN‐F4, the first indacenodithienothiophene nonfullerene acceptors that combine π‐extension and fluorination, are reported. The neat acceptors and bulk‐heterojunction blend films with fluorinated donor polymer poly{[4,8‐bis[5‐(2‐ethylhexyl)‐4‐fluoro‐2‐thienyl]benzo[1,2‐b:4,5‐b ′]‐dithiophene‐2,6‐diyl]‐alt‐[2,5‐thiophenediyl[5,7‐bis(2‐ethylhexyl)‐4,8‐dioxo‐4H ,8H‐benzo[1,2‐c :4,5‐c ′]dithiophene‐1,3‐diyl]]} (PBDB‐TF, also known as PM6) are investigated using a battery of techniques, including single crystal X‐ray diffraction, fs transient absorption spectroscopy (fsTA), photovoltaic response, space‐charge‐limited current transport, impedance spectroscopy, grazing incidence wide angle X‐ray scattering, and density functional theory level computation. ITN‐F4 and ITzN‐F4 are found to provide power conversion efficiencies greater and internal reorganization energies less than their non‐π‐extended and nonfluorinated counterparts when paired with PBDB‐TF. Additionally, ITN‐F4 and ITzN‐F4 exhibit favorable bulk‐heterojunction relevant single crystal packing architectures. fsTA reveals that both ITN‐F4 and ITzN‐F4 undergo ultrafast hole transfer (<300 fs) in films with PBDB‐TF, despite excimer state formation in both the neat and blend films. Taken together and in comparison to related structures, these results demonstrate that combined fluorination and π‐extension synergistically promote crystallographic π‐face‐to‐face packing, increase crystallinity, reduce internal reorganization energies, increase interplanar π–π electronic coupling, and increase power conversion efficiency.

In article number https://doi.org/10.1002/aenm.2019041961904196, Xiaowei Zhan, Jordi Martorell and co‐workers design a building‐integrated transparent photovoltaic window based on an optically tailored organic solar cell, where enhanced sunlight harvesting at large oblique angles is largely decoupled from visual transmission at normal incidence, setting a new performance standard in the field.

Tremendous efforts have been devoted during the last decade and have achieved remarkable progress in the field of flexible/ultrathin organic solar cells. This essay summarizes how the performance of ultrathin organic solar cells has been developed and discusses future directions of organic solar cells. Improvements in power conversion efficiency and stabilities are summarized, and potential approaches for further improvement are discussed.

Abstract

Extensive efforts have been devoted during the last decade to organic solar cell research that has led to remarkable progress and achieved power conversion efficiencies (PCEs) in excess of 10%. Among the existing flexible organic solar cells, ultrathin organic solar cells with a total thickness <10 µm have important advantages, including good mechanical bending stabilities and good conformability. These advantages have led to power generation solutions for wearable electronics. In this essay, the progress of flexible and ultrathin organic solar cells, and the future research directions pertaining to these cells are discussed based on the potential applications of textile‐compatible solar cells. Both process engineering and development of the material of ultrathin substrate films have improved the PCE of ultrathin organic solar cells, which in turn have led to the small PCE difference between flexible organic solar cells with substrate thickness >10 µm and ultrathin organic solar cells with substrate thickness ≤10 µm. Key technologies for the further improvement of PCE of flexible/ultrathin organic solar cells are discussed. Strategies to improve the stability and some important aspects, which determine the mechanical robustness of flexible organic solar cells, are also presented and discussed.

By involving a methyl ammonium acetate (MAAc) ionic liquid solvent, strong Pb−O interaction and N−H⋅⋅⋅I hydrogen bonds between MAAc and PbI₂ lead to a controllable all‐inorganic perovskite formation in ambient air. The resulting solar cells exhibit high efficiency and excellent stability under continuous light illumination.

Abstract

All‐inorganic lead halide perovskites are promising candidates for optoelectronic applications. However, fundamental questions remain over the component interaction in the perovskite precursor solution due to the limitation of the most commonly used solvents of N,N‐dimethylformamide (DMF) and dimethyl sulfoxide (DMSO). Here, we report an interaction tailoring strategy for all‐inorganic CsPbI_3−x Br _x perovskites by involving the ionic liquid solvent methylammonium acetate (MAAc). C=O shows strong interaction with lead (Pb²⁺) and N−H⋅⋅⋅I hydrogen bond formation is observed. The interactions stabilize the perovskite precursor solution and allow production of the high‐quality perovskite films by retarding the crystallization. Without the necessity for antisolvent treatment, the one‐step air‐processing approach delivers photovoltaic cells regardless of humidity, with a high efficiency of 17.10 % along with long operation stability over 1500 h under continuous light illumination.

How fluorine atoms substitute the aromatic ring of spacer cations has subtle influence in the final device performance based on 2D/3D hybrid perovskites. Hydrophobic ortho‐ , meta‐ , and para‐ fluorophenyl groups act as the protective umbrella to prevent the recombination and invasion of water. The resulting devices exhibit high V _OC and good air stability.

The notoriously poor stability of organic–inorganic hybrid perovskite solar cells is a crucial issue restricting the commercial application of such burgeoning technology. Passivation of bulk perovskite absorber by fluorinated aromatic ammonium salt via low‐dimensional perovskites has been proved to be an effective way of improving stability and efficiency. Herein, the influence of fluorination position (ortho‐ , meso‐ , and para‐ ) on the aromatic moiety is studied in terms of their dipole moments and the ability to reduce defect density, extend carrier lifetimes, and assist charge transfer. In addition to the improved power conversion efficiency (PCE) from 19.17% to above 20%, the device treated with 2‐(o‐fluorophenyl)ethylamine iodide exhibits a remarkable open‐circuit voltage (V _OC) of 1.21 V. While the 2‐(p‐fluorophenyl)ethylamine iodide‐treated device shows only 1% loss of its initial value under ambient atmosphere (with RH of 10–30%) without encapsulation for 1440 h storage. The molecular structure of fluorinated aromatic cations plays multiple roles in passivating the interface of the perovskite device.

A facile immersing and washing strategy (I‐method) is reported to prepare effective organic monomolecular layers (MLs) as hole transport layers (ML‐HTLs). The I‐method can largely reduce the process cost as well as realize batch preparation of ML‐HTLs. Perovskite solar cells based on ML‐HTLs show improved power conversion efficiency and stability.

Hole transport materials and their processing occupy at least one‐third of the cost of perovskite solar cells (PSCs), which leaves plenty of room to improve the process of device fabrication. Herein, a facile immersing and washing strategy (I‐method) is reported to prepare effective organic monomolecular layers (MLs) of poly[N ,N ′‐bis(4‐butylphenyl)‐N ,N ′‐bis(phenyl) benzidine] (polyTPD) as hole transport layers (ML‐HTLs) to construct cost‐effective planar inverted PSCs. The ML enables an enhanced wettability to perovskite precursors and thus results in the growth of compact and uniform perovskite films. In addition, the ML exhibits better energy‐level alignment with perovskite. Consequently, the ML‐polyTPD‐based PSCs deliver significantly enhanced power conversion efficiency (PCE) and reproducibility, as compared to that of pristine polyTPD based devices. The practical consumption of polyTPD during the I‐method is cut to the bone, with the cost of $0.8 for 1 m² substrate being achieved, which is 0.15% of that by S‐method. The developed I‐method is facile, and time‐ and cost‐saving with low requirement for facilities as well as with low temperature and solution processability. This strategy is cost‐effective to prepare ML‐HTLs for large‐area and flexible PSCs with competitive photovoltaic performance and enhanced reproducibility.

Discotic Liquid Crystals

In article number 2000047, Jian‐Qiang Liu, Chao‐Chao Qin, Xiao‐Tao Hao, and co‐workers introduce 2,3,6,7,10,11‐hexaacetoxytriphenylene (HATP) as an interlayer between poly(3,4‐ethylenedioxythiophene): poly(styrene sulfonate) and the active layer in organic solar cells. This method achieves 3D charge transportation and higher mobility. Organic solar cells with HATP show weak triplet exciton generation and charge recombination. Thus, the short‐circuit current density and the resulting power conversion efficiency are increased.

Defect Passivation

In article number 1900529, Qinye Bao and co‐workers propose a strategy using abundant and color organic dyes as an additive to passivate defect states and to produce more n‐type perovskite films, which remarkably increases the power conversion efficiency of perovskite solar cells. The rich hydrogen bonds and carbonyl structures in the organic dye can significantly enhance the device stability both in terms of humidity and thermal stress.

MABr induces the remarkably oriented growth of tin halide perovskite films (MA _x FA_{1− x} SnI_{3− x} Br _x ) by alloying, which results in an optimal device conversion efficiency of 9.31% enhanced from 5.02% of the pristine FASnI₃ device and maintained above 80% of the initial efficiency after 300 h light soaking while the control device fails within 120 h.

Abstract

As the most promising lead‐free branch, tin halide perovskites suffer from the severe oxidation from Sn²⁺ to Sn⁴⁺, which results in the unsatisfactory conversion efficiency far from what they deserve. In this work, by facile incorporation of methylammonium bromide in composition engineering, formamidinium and methylammonium mixed cations tin halide perovskite films with ultrahighly oriented crystallization are synthesized with the preferential facet of (001), and that oxidation is suppressed with obviously declined trap density. MA⁺ ions are responsible for that impressive orientation while Br^‐ ions account for their bandgap modulation. Depending on high quality of the optimal MA_0.25FA_0.75SnI_2.75Br_0.25 perovskite films, their device conversion efficiency surges to 9.31% in contrast to 5.02% of the control formamidinium tin triiodide perovskite (FASnI₃) device, along with almost eliminated hysteresis. That also results in the outstanding device stability, maintaining above 80% of the initial efficiency after 300 h of light soaking while the control FASnI₃ device fails within 120 h. This paper definitely paves a facile and effective way to develop high‐efficiency tin halide perovskites solar cells, optoelectronic devices, and beyond.

The interfaces determine the overall device performance and stability. These interfaces include the intralayer grain boundaries inside the perovskites, the interface between perovskites with electron/hole transport layer (ETL/HTL), and the interface of ETL/HTL with electrodes. With the aim of minimizing traps, promoting carrier extraction, and improving stability, herein, an overview of recent interfacial engineering strategies is provided.

Abstract

Rapid progress in the domain of perovskite solar cells (PSCs) has boosted the power conversion efficiency (PCE) of such cells to 25.2%. However, the long‐term stability of a high‐performance PSCs is still the foremost concern that hinders its practical application. The interfaces are considered as the key part that determines the overall device performance and longevity. These interfaces include the intralayer grain boundaries (GBs) inside the perovskites, the interface between perovskites with electron/hole transport layer (ETL/HTL), and the interface of ETL/HTL with top/down contacts. To acquire a deep and detailed understanding of the impacts of interfacial properties, herein, a concise overview of recent interfacial engineering strategies with the aim of minimizing traps, promoting carrier extraction, and improving stability are summarized.

High‐efficiency and stable dopant‐free poly(3‐hexylthiophene) (P3HT)‐based CsPbI₂Br solar cells are achieved by introducing an optimized preannealing process to engineer the nucleation and crystallization of CsPbI₂Br films. Further incorporation of an ultrathin wide‐bandgap diphenylamine derivative layer (poly[(9,9‐dioctylfluorenyl‐2,7‐diyl)‐co ‐(4,4′‐(N ‐(4‐sec‐butylphenyl)diphenylamine)]) to regulate the band alignment of CsPbI₂Br and P3HT delivers a record‐high efficiency of 15.50% for dopant‐free P3HT‐based CsPbI₂Br solar cells.

Abstract

CsPbI₂Br is emerging as a promising all‐inorganic material for perovskite solar cells (PSCs) due to its more stable lattice structure and moisture resistance compared to CsPbI₃, although its device performance is still much behind this counterpart. Herein, a preannealing process is developed and systematically investigated to achieve high‐quality CsPbI₂Br films by regulating the nucleation and crystallization of perovskite. The preannealing temperature and time are specifically optimized for a dopant‐free poly(3‐hexylthiophene) (P3HT)‐based device to target dopant‐induced drastic performance degradation for spiro‐OMeTAD‐based devices. The resulting P3HT‐based device exhibits comparable power conversion efficiency (PCE) to spiro‐OMeTAD‐based devices but much enhanced ambient stability with over 95% PCE after 1300 h. A diphenylamine derivative is introduced as a buffer layer to improve the energy‐level mismatch between CsPbI₂Br and P3HT. A record‐high PCE of 15.50% for dopant‐free P3HT‐based CsPbI₂Br PSCs is achieved by alleviating the open‐circuit voltage loss with the buffer layer. These results demonstrate that the preannealing processing together with a suitable buffer layer are applicable strategies for developing dopant‐free P3HT PSCs with high efficiency and stability.

Herein, a novel precursor (HCOOCs and HPbX₃) for deposition of high‐quality CsPbI₂Br films, irrespective of humidity is presented. CsPbI₂Br cells prepared in an atmosphere with 30% and 91% relative humidity exhibit efficiencies of 16.1% and 15.1%, respectively, which are the highest among all inorganic CsPbX₃ (X: I, Br, or mixed halides) PSCs prepared in a medium or high humid atmosphere.

Abstract

High temperature stable inorganic CsPbX₃ (X: I, Br, or mixed halides) perovskites with their bandgap tailored by tuning the halide composition offer promising opportunities in the design of ideal top cells for high‐efficiency tandem solar cells. Unfortunately, the current high‐efficiency CsPbX₃ perovskite solar cells (PSCs) are prepared in vacuum, a moisture‐free glovebox or other low‐humidity conditions due to their poor moisture stability. Herein, a new precursor system (HCOOCs, HPbI₃, and HPbBr₃) is developed to replace the traditional precursors (CsI, PbI₂, and PbBr₂) commonly used for solar cells of this type. Both the experiments and calculations reveal that a new complex (HCOOH•Cs⁺) is generated in this precursor system. The new complex is not only stable against aging in humid air ambient at 91% relative humidity, but also effectively slows the perovskite crystallization, making it possible to eliminate the popular antisolvent used in the perovskite CsPbI₂Br film deposition. The CsPbI₂Br PSCs based on the new precursor system achieve a champion efficiency of 16.14%, the highest for inorganic PSCs prepared in ambient air conditions. Meanwhile, high air stability is demonstrated for an unencapsulated CsPbI₂Br PSC with 92% of the original efficiency remaining after more than 800 h aging in ambient air.

Recent progress in inorganic lead‐based and lead‐free CsBX₃ perovskite solar cells using various strategies is reviewed and their prospects and challenges in the future are discussed in detail.

Abstract

All‐inorganic perovskite semiconductors have recently drawn increasing attention owing to their outstanding thermal stability. Although all‐inorganic perovskite solar cells (PSCs) have achieved significant progress in recent years, they still fall behind their prototype organic–inorganic counterparts owing to severe energy losses. Therefore, there is considerable interest in further improving the performance of all‐inorganic PSCs by synergic optimization of perovskite films and device interfaces. This review article provides an overview of recent progress in inorganic PSCs in terms of lead‐based and lead‐free composition. The physical properties of all‐inorganic perovskite semiconductors as well as the hole/electron transporting materials are discussed to unveil the important role of composition engineering and interface modification. Finally, a discussion of the prospects and challenges for all‐inorganic PSCs in the near future is presented.

The amorphous–crystalline heterophase SnO₂ electron transport bilayer (Bi‐SnO₂) exhibits improved surface morphology, fewer oxygen defects, and better energy band alignment with the perovskite, which enables more efficient electron extraction. The use of Bi‐SnO₂ boosts the efficiency of small‐area (0.09 cm²) and large‐area (3.55 cm²) perovskite solar cells up to 20.39% and 14.93%, respectively.

Abstract

Improving the ohmic contact and interfacial morphology between an electron transport layer (ETL) and perovskite film is the key to boost the efficiency of planar perovskite solar cells (PSCs). In the current work, an amorphous–crystalline heterophase tin oxide bilayer (Bi‐SnO₂) ETL is prepared via a low‐temperature solution process. Compared with the amorphous SnO₂ sol–gel film (SG‐SnO₂) or the crystalline SnO₂ nanoparticle (NP‐SnO₂) counterparts, the heterophase Bi‐SnO₂ ETL exhibits improved surface morphology, considerably fewer oxygen defects, and better energy band alignment with the perovskite without sacrificing the optical transmittance. The best PSC device (active area ≈ 0.09 cm²) based on a Bi‐SnO₂ ETL is hysteresis‐less and achieves an outstanding power conversion efficiency of ≈20.39%, which is one of the highest efficiencies reported for SnO₂‐triple cation perovskite system based on green antisolvent. More fascinatingly, large‐area PSCs (active areas of ≈3.55 cm²) based on the Bi‐SnO₂ ETL also achieves an extraordinarily high efficiency of ≈14.93% with negligible hysteresis. The improved device performance of the Bi‐SnO₂‐based PSC arises predominantly from the improved ohmic contact and suppressed bimolecular recombination at the ETL/perovskite interface. The tailored morphology and energy band structure of the Bi‐SnO₂ has enabled the scalable fabrication of highly efficient, hysteresis‐less PSCs.

Three asymmetric small‐molecule acceptors are developed by changing the fluorine atoms on the terminal group of Y6 to chlorine atoms, namely SY1, SY2, and SY3, with Y6, and Y6‐4Cl are utilized as the reference. Organic solar cells based on the PM6:SY1 blend demonstrate a champion power conversion efficiency of 16.83%. This work can provide a deeper and more comprehensive understanding of applying the asymmetric molecule design method.

Abstract

Small‐molecule acceptors (SMAs)‐based organic solar cells (OSCs) have exhibited great potential for achieving high power conversion efficiencies (PCEs). Meanwhile, developing asymmetric SMAs to improve photovoltaic performance by modulating energy level distribution and morphology has drawn lots of attention. In this work, based on the high‐performance SMA (Y6), three asymmetric SMAs are developed by substituting the fluorine atoms on the terminal group with chlorine atoms, namely SY1 (two F atoms and one Cl atom), SY2 (two F atoms and two Cl atoms), and SY3 (three Cl atoms). Y6 (four F atoms) and Y6‐4Cl (four Cl atoms) are synthesized as control molecules. As a result, SY1 exhibits the shallowest lowest unoccupied molecular orbital energy level and the best molecular packing among these five acceptors. Consequently, OSCs based on PM6:SY1 yield a champion PCE of 16.83% with an open‐circuit voltage (V _OC) of 0.871 V, and a fill factor (FF) of 0.760, which is the best result among the five devices. The highest FF for the PM6:SY1‐based device is mainly ascribed to the most balanced charge transport and optimal morphology. This contribution provides deeper understanding of applying asymmetric molecule design method to further promote PCEs of OSCs.

The doping capability of conjugated polymer is governed by the degree of electronic coupling with the dopant. The high doping capability of conjugated polymers can be utilized to form an electric dipole layer at the surface of perovskite with a hole transporting layer, which facilitates charge extraction and enhances the performance of perovskite solar cells.

Abstract

Developing electrical organic conductors is challenging because of the difficulties involved in generating free charge carriers through chemical doping. To devise a novel doping platform, the doping capabilities of four designed conjugated polymers (CPs) are quantitatively characterized using an AC Hall‐effect device. The resulting carrier density is related to the degree of electronic coupling between the CP repeating unit and 2,3,5,6‐tetrafluoro‐7,7,8,8‐tetracyanoquinodimethane (F4‐TCNQ), and doped PIDF‐BT provides an outstanding electrical conductivity, exceeding 210 S cm⁻¹, mainly due to the doping‐assisted facile carrier generation and relatively fast carrier mobility. In addition, it is noted that a slight increment in the electron‐withdrawing ability of the repeating unit in each CP diminishes electronic coupling with F4‐TCNQ, and severely deteriorates the doping efficiency including the alteration of operating doping mechanism for the CPs. Furthermore, when PIDF‐BT with high doping capability is applied to the hole transporting layer, with F4‐TCNQ as the interfacial doping layer at the interface with perovskite, the power conversion efficiency of the perovskite solar cell improves significantly, from 17.4% to over 20%, owing to the ameliorated charge‐collection efficiency. X‐ray photoelectron spectroscopy and Kelvin probe analyses verify that the improved solar cell performance originates from the increase in the built‐in potential because of the generation of electric dipole layer.

MABr induces the remarkably oriented growth of tin halide perovskite films (MA _x FA_{1− x} SnI_{3− x} Br _x ) by alloying, which results in an optimal device conversion efficiency of 9.31% enhanced from 5.02% of the pristine FASnI₃ device and maintained above 80% of the initial efficiency after 300 h light soaking while the control device fails within 120 h.

Abstract

As the most promising lead‐free branch, tin halide perovskites suffer from the severe oxidation from Sn²⁺ to Sn⁴⁺, which results in the unsatisfactory conversion efficiency far from what they deserve. In this work, by facile incorporation of methylammonium bromide in composition engineering, formamidinium and methylammonium mixed cations tin halide perovskite films with ultrahighly oriented crystallization are synthesized with the preferential facet of (001), and that oxidation is suppressed with obviously declined trap density. MA⁺ ions are responsible for that impressive orientation while Br^‐ ions account for their bandgap modulation. Depending on high quality of the optimal MA_0.25FA_0.75SnI_2.75Br_0.25 perovskite films, their device conversion efficiency surges to 9.31% in contrast to 5.02% of the control formamidinium tin triiodide perovskite (FASnI₃) device, along with almost eliminated hysteresis. That also results in the outstanding device stability, maintaining above 80% of the initial efficiency after 300 h of light soaking while the control FASnI₃ device fails within 120 h. This paper definitely paves a facile and effective way to develop high‐efficiency tin halide perovskites solar cells, optoelectronic devices, and beyond.

CsPbI₃ inorganic perovskite exhibits some special unique properties including crystal‐structure distortion and quantum confinement effect, yet the poor phase stability severely hinders its application. The nature of the photoactive CsPbI₃ phase transition from the perspective of PbI₆ octahedral rotation is revealed and a facile method to simultaneously stabilize the photo‐active phase and reduce the defect density of CsPbI₃ is developed.

Abstract

CsPbI₃ inorganic perovskite has exhibited some special properties particularly crystal structure distortion and quantum confinement effect, yet the poor phase stability of CsPbI₃ severely hinders its applications. Herein, the nature of the photoactive CsPbI₃ phase transition from the perspective of PbI₆ octahedra is revealed. A facile method is also developed to stabilize the photoactive phase and to reduce the defect density of CsPbI₃. CsPbI₃ is decorated with multifunctional 4‐aminobenzoic acid (ABA), and steric neostigmine bromide (NGBr) is subsequently used to further mediate the thin films' surface (NGBr‐CsPbI₃(ABA)). The ABA or NG cation adsorbed onto the grain boundaries/surface of CsPbI₃ anchors the PbI₆ octahedra via increasing the energy barriers of octahedral rotation, which maintains the continuous array of corner‐sharing PbI₆ octahedra and kinetically stabilizes the photoactive phase CsPbI₃. Moreover, the added ABA and NGBr not only interact with shallow‐ or deep‐level defects in CsPbI₃ to significantly reduce defect density, but also lead to improved energy‐level alignment at the interfaces between the CsPbI₃ and the charge transport layers. Finally, the champion NGBr‐CsPbI₃(ABA)‐based inorganic perovskite solar cell delivers 18.27% efficiency with excellent stability. Overall, this work demonstrates a promising concept to achieve highly phase‐stabilized inorganic perovskite with suppressed defect density for promoting its optoelectronic applications.

Publication date: 17 June 2020

Source: Joule, Volume 4, Issue 6

Author(s): Shengfan Wu, Jie Zhang, Zhen Li, Danjun Liu, Minchao Qin, Sin Hang Cheung, Xinhui Lu, Dangyuan Lei, Shu Kong So, Zonglong Zhu, Alex.K.-Y. Jen