Wangyikai

Two novel donor‐accepter‐donor (D‐A‐D) structured hole transporting materials based on fluorine substituted benzotriazole (BTA) core building block (2FBTA‐1, 2FBTA‐2) are designed and synthesized through the molecular regulation. Applied these materials into perovskite solar cell (PSC), power conversion efficiencies (PCEs) of 7.55 % and 17.94% are obtained for 2FBTA‐1 and 2FBTA‐2, respectively. The better photovoltaic performance of 2FBTA‐2 could be attributed to its more suitable energy level, more planar molecular configurations and higher hole mobility. Moreover, the devices with 2FBTA‐2 as HTM show good stability under the air condition. The results qualify the BTA promising building block for future HTM design.

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Publication date: February 2020

Source: Solar Energy Materials and Solar Cells, Volume 205

Author(s): Mohsen Ameri, Mashhood Ghaffarkani, Reza Taheri Ghahrizjani, Nasser Safari, Ezeddin Mohajerani

Graphical abstract

We investigate the relevance of grain boundaries location and their electronic activity for optimizing the performance and minimizing the J-V Hysteresis in PSCs.

Publication date: Available online 5 November 2019

Source: Nano Energy

Author(s): Nabonswende Aida Nadege Ouedraogo, Yichuan Chen, Yue Yue Xiao, Qi Meng, Chang Bao Han, Hui Yan, Yongzhe Zhang

Graphical abstract

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.

Perovskite solar cells still have huge room for improvement in photoelectric conversion efficiency. One of the constraints is the defects at the interface between the perovskite and the transport layer. Passivation is considered a key measure to limit defects. This paper systematically categorizes the effective passivation strategies for perovskites in recent years and gives a future outlook.

Abstract

The disorderly distribution of defects in the perovskite or at the grain boundaries, surfaces, and interfaces, which seriously affect carrier transport through the formation of nonradiative recombination centers, hinders the further improvement on the power conversion efficiency (PCE) of perovskite solar cells (PSCs). Several defect passivation strategies have been confirmed as an efficient approach for promoting the performance of PSCs. Herein, recent progress in the defect passivation toward efficient perovskite solar cells are summarized, and a classification of common passivation strategies that elaborate the mechanism according to the location of the defects and the type of passivation agent is presented. Finally, this review offers likely prospects for future trends in the development of passivation strategies.

Organic‐free (all‐inorganic) and lead‐free halide perovskites in combination with dopant‐free hole transport materials (HTMs) are summarized in terms of potential photovoltaic performance, and progress in compositional and morphological design of solution‐processed perovskite absorbers. Strategies to enhance device efficiency are focused on preparation of high quality perovskite and HTM interface.

Abstract

Having demonstrated incredibly fast progress in power conversion efficiency, rising to a level comparable with that of crystalline silicon cells, lead‐based organic–inorganic hybrid perovskite solar cells are now facing the stability tests needed for industrialization. Poor thermal stability (<150 °C) owing to organic constituents and interlayer diffusion of materials (dopants), and environmental incompatibility due to Pb has surged the development of organic‐free, Pb‐free perovskites and dopant‐free hole transport materials (HTMs). The recent rapid increase in efficiency of cells based on inorganic perovskites, crossing 18%, demonstrates the great potential of inorganic perovskites as thermally stable and high‐efficiency cells. Although all kinds of Pb‐free perovskites lag in efficiency in comparison to the hybrid and inorganic perovskites, they also demonstrate better structural and environmental stability. The performance of dopant‐free HTMs matching/surpassing dopant‐containing HTMs makes the former a better choice for stability. Even though the efforts to enhance the stability of Pb‐based hybrid perovskites should continue by different techniques, organic‐free and lead‐free perovskites, and dopant‐free HTMs must be pursued with greater interest for the future. This review describes the present issues and possible strategies to address them, and thus will help to improve the overall performance of robust organic‐free, Pb‐free, and dopant‐free perovskite solar cells.

Lead-based organic-inorganic hybrid perovskite (OIHP) solar cells can attain efficiencies over 20%. However, the impact of ion mobility and/or organic depletion, structural changes, and segregation under operating conditions urge for decisive and more accurate investigations. Hence, the development of analytical tools for accessing the grain-to-grain OIHP chemistry is of great relevance. Here, we used synchrotron infrared nanospectroscopy (nano-FTIR) to map individual nanograins in OIHP films. Our results reveal a spatial heterogeneity of the vibrational activity associated to the nanoscale chemical diversity of isolated grains. It was possible to map the chemistry of individual grains in CsFAMA [Cs_0.05FA_0.79MA_0.16Pb(I_0.83Br_0.17)₃] and FAMA [FA_0.83MA_0.17Pb(I_0.83Br_0.17)₃] films, with information on their local composition. Nanograins with stronger nano-FTIR activity in CsFAMA and FAMA films can be assigned to PbI₂ and hexagonal polytype phases, respectively. The analysis herein can be extended to any OIHP films where organic cation depletion/accumulation can be used as a chemical label to study composition.

A new all‐inorganic perovskite material, CsPbI₃:Br:InI₃, is prepared through defect engineering of CsPbI₃. This new perovskite retains the same bandgap as CsPbI₃, but with intrinsic defect concentration largely suppressed. Moreover, it can be prepared in an extremely high humidity atmosphere. By completely eliminating the labile and expensive components in traditional perovskite solar cells (PSCs), these all‐inorganic PSCs exhibit high photovoltaic performances.

Abstract

The emergence of cesium lead iodide (CsPbI₃) perovskite solar cells (PSCs) has generated enormous interest in the photovoltaic research community. However, in general they exhibit low power conversion efficiencies (PCEs) because of the existence of defects. A new all‐inorganic perovskite material, CsPbI₃:Br:InI₃, is prepared by defect engineering of CsPbI₃. This new perovskite retains the same bandgap as CsPbI₃, while the intrinsic defect concentration is largely suppressed. Moreover, it can be prepared in an extremely high humidity atmosphere and thus a glovebox is not required. By completely eliminating the labile and expensive components in traditional PSCs, the all‐inorganic PSCs based on CsPbI₃:Br:InI₃ and carbon electrode exhibit PCE and open‐circuit voltage as high as 12.04% and 1.20 V, respectively. More importantly, they demonstrate excellent stability in air for more than two months, while those based on CsPbI₃ can survive only a few days in air. The progress reported represents a major leap for all‐inorganic PSCs and paves the way for their further exploration in order to achieve higher performance.

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.

A two‐step annealing method is developed for studying the water effect on different kinds of perovskites. It is demonstrated that 60 °C is favorable to the formation of hydrate phase which leads to a reconstruction process in the second annealing stage. The corresponding water effects highly depend on the cations of the perovskite itself.

Water effect on perovskite solar cells has received growing interest in recent years. A widely accepted view is that moderate water content induces the formation of hydrate phase which enhances the recrystallization of the perovskite. However, the underlying factors which influence the formation of hydrate phase are yet to be investigated. Herein, by controlling the annealing temperature, it is demonstrated that 60 °C is the most suitable temperature for the formation of hydrated perovskite. After further annealing at 120 °C, the resulting perovskite film reveals enhanced crystallinity with a more uniform morphology, contributing to device efficiency above 20%. In addition, the water effect on different types of perovskites is studied and it is concluded that the formation of hydrated perovskite is mainly determined by the cations of the perovskite itself. The findings in this work elucidate the conditions for the formation of hydrated perovskite, contributing to the fabrication of highly efficient perovskite solar cells.

The latest progress in exciton–photon coupling of perovskite materials is reviewed. Polaritons in planar and nanowire Fabry–Pérot microcavities are discussed predominantly in terms of materials and photophysics. Large Rabi‐splitting energy (≈656 meV) is achieved in CsPbBr₃. These large values enable polariton condensation and polariton lasers to be realized at high temperature or in low‐Q cavities.

Abstract

The semiconductor exciton–polariton, arising from the strong coupling between excitons and confined cavity photon modes, is not only of fundamental importance in macroscopic quantum effects but also has wide application prospects in ultralow‐threshold polariton lasers, slowing‐light devices, and quantum light sources. Very recently, metallic halide perovskites have been considered as a great candidate for exciton–polariton devices owing to their low‐cost fabrication, large exciton oscillator strength, and binding energy. Herein, the latest progress in exciton–polaritons and polariton lasers of perovskites are reviewed. Polaritons in planar and nanowires Fabry–Pérot microcavities are discussed with particular reference to material and photophysics. Finally, a perspective on the remaining challenges in perovskite polaritons research is given.

Herein, three polyfluorene copolymers (TFB, PFB, and PFO) are investigated as hole‐transport materials (HTMs) for the construction of inverted perovskite solar cells. The photovoltaic performance of the device is found to be closely correlated with the electronic properties of HTMs. The TFB‐based device exhibits the highest efficiency of 18.48% due to its high mobility and favored energy‐level alignment.

Inverted perovskite solar cells (PSCs) that can be entirely processed at low temperatures have attracted growing attention due to their cost‐effective production. Hole‐transport materials (HTMs) play an essential role in achieving efficient inverted PSCs, as they determine the effectiveness of charge extraction and recombination at interfaces. Herein, three polyfluorene copolymers (TFB, PFB, and PFO) are investigated as HTMs for construction of inverted PSCs. It is found that the photovoltaic performance of the solar cells is closely correlated with the electronic properties of the HTMs. Due to its high mobility along with the favored energy‐level alignment with perovskite, TFB shows superior charge extraction and suppressed interfacial recombination than PFB‐ and PFO‐based devices, which delivers a high efficiency of 18.48% with an open‐circuit voltage (V _OC) of up to 1.1 V. In contrast, the presence of a large energy barrier in the PFO‐based devices results in substantial losses in both V _OC and photocurrent. These results demonstrate that TFB can serve as a superior HTM for inverted PSCs. Moreover, it is anticipated that the performance of the three HTMs identified here might guide the molecular design of novel HTMs for the manufacture of highly efficient inverted PSCs.

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.

Using fluorine‐substituted benzotriazole (BTA) as the core building block, two novel donor–accepter–donor (D–A–D) structured hole‐transporting materials, 2FBTA‐1 and 2FBTA‐2, are synthesized and applied into perovskite solar cells, achieving a high power conversion efficiency of 17.94%.

Two novel donor–accepter–donor structured hole‐transporting materials based on fluorine‐substituted benzotriazole (BTA) core building blocks (2FBTA‐1 and 2FBTA‐2) are designed and synthesized through molecular regulation. Applying these materials into perovskite solar cells, power conversion efficiencies of 7.55% and 17.94% are obtained for 2FBTA‐1 and 2FBTA‐2, respectively. The better photovoltaic performance of 2FBTA‐2 is attributed to its more suitable energy level, more planar molecular configurations, and higher hole mobility. Moreover, the devices with 2FBTA‐2 as hole transport material (HTM) show good stability in air. The results indicate that BTA is a promising building block for future HTM design.

I/Br/Cl triple‐anion perovskite material with bandgap of 1.8 eV is tailored for indoor light harvesting, which realizes a record high indoor efficiency of 36.2% with increased open circuit voltage (V _oc) and minimal short‐circuit current ( J _sc) loss. The I/Br halide segregation is restrained by Cl‐involvement, realizing a long‐term stability of over 95% after 2000 h.

Abstract

Indoor photovoltaics are promising to enable self‐powered electronic devices for the Internet of Things. Here, reported is a triple‐anion CH₃NH₃PbI_{2− x} BrCl _x perovskite film, of which the bandgap is specially designed for indoor light harvesting to achieve a record high efficiency of 36.2% with distinctive high open circuit voltage (V _oc) of 1.028 V under standard 1000 lux fluorescent light. The involvement of both bromide and chloride suppresses the trap‐states and nonradiative recombination loss, exhibiting a remarkable ideality factor of 1.097. The introduction of chloride successfully restrains the halide segregation of iodide and bromide, stabilizing the triple‐anion perovskite film. The devices show an excellent long‐term performance, sustaining over 95% of original efficiency under continuous light soaking over 2000 h. These findings show the importance and potential of I/Br/Cl triple‐anion perovskite with tailored bandgap and suppressed trap‐states in stable and efficient indoor light recycling.

A UV‐inert ZnTiO₃ is demonstrated to be an electron selective layer in perovskite solar cells. ZnTiO₃ is a perovskite‐structured semiconductor with excellent chemical stability and poor photocatalysis. Planar perovskite solar cells based on ZnTiO₃ exhibit power conversion efficiency of 20.1% with improved photostability. The best device holds 90% of its initial efficiency after 100 h of ultraviolet soaking.

Abstract

Although planar‐structured perovskite solar cells (PSCs) have power conversion efficiencies exceeding 24%, the poor photostability, especially with ultraviolet irradiance (UV) severely limits commercial application. The most commonly‐used TiO₂ electron selective layer has a strong photocatalytic effect on perovskite/TiO₂ interface when TiO₂ is excited by UV light. Here a UV‐inert ZnTiO₃ is reported as the electron selective layer in planar PSCs. ZnTiO₃ is a perovskite‐structured semiconductor with excellent chemical stability and poor photocatalysis. Solar cells are fabricated with a structure of indium doped tin oxide (ITO)/ZnTiO₃/Cs_0.05FA_0.81MA_0.14PbI_2.55Br_0.45/Sprio‐MeOTAD/Au. The champion device exhibits a stabilized power conversion efficiency of 19.8% with improved photostability. The device holds 90% of its initial efficiency after 100 h of UV soaking (365 nm, 8 mW cm⁻²), compared with 55% for TiO₂‐based devices. This work provides a new class of electron selective materials with excellent UV stability in perovskite solar cell applications.

An all‐layer‐inorganic perovskite solar cell (PSC) based on inorganic CsPbI₂Br perovskite absorber layer and tailored NiO hole‐transporting layer (HTL) is fabricated. The tailored NiO nanocrystalline films exhibit uniform, pinhole‐free morphologies, efficient charge‐extraction capabilities, and intrinsic chemical stability, which gives the whole photovoltaic device a high efficiency and much improved stability compared with PSCs based on the organic HTLs.

Cesium‐based inorganic perovskite solar cells (PSCs) have attracted great attention due to the superior thermal stability of the light absorbers. However, the reported devices normally contain organic charge‐transporting layers (CTLs), such as spiro‐OMeTAD, which is expensive and highly sensitive to ambient atmosphere and temperature. It is of great significance to develop inorganic CTLs with low cost and robust stability. To date, it is still a big challenge to achieve high‐quality inorganic CTL films via the solution process, especially for the hole‐transporting layer (HTL) in conventional n‐i‐p structures. Herein, tailored NiO nanocrystalline films as HTLs in an all‐layer‐inorganic CsPbI₂Br‐based PSCs are developed, which exhibit uniform, pinhole‐free morphologies and efficient charge‐extraction capabilities. Consequently, the as‐constructed all‐layer‐inorganic PSCs, with an optimal power conversion efficiency (PCE) of 15.14% and a stabilized power output of 14.82%, present robust long‐term thermal stability: retained 85% of their initial PCEs after a thermal treatment at 85 °C in the dark in a nitrogen atmosphere with encapsulation for 1000 h, greatly surpassing the performance of the PSCs based on the organic HTLs.