JIALINGBO

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.

Nature Communications, Published online: 10 January 2020; doi:10.1038/s41467-019-13909-5

Moisture Stability

In article number 1900331, Baomin Xu, Chun Cheng, and co‐workers show that the cascade‐aligned energy levels and the defect‐passivation achieved by combining commercially accessible SnO₂ and home‐made TiO₂ nanoparticles effectively reduce energy loss and inhibit defects in the device. Consequently, the perovskite solar cell delivers a high power conversion efficiency (PCE) of 20.50%, which is superior to that of control devices based on SnO₂ with a PCE of 18.09%.

Herein, a planar n–i–p perovskite solar cell based on a nanocrystalline WO _x electron transfer layer is developed, with a low‐temperature fabrication process (50 °C) and record high efficiency, exceeding 20%.

Low‐temperature, solution‐processed metal oxides are of great interest as alternative materials for electron transport layers in perovskite solar cells. WO _x is a promising candidate that could truly enable low‐temperature (<100 °C) processing. However, its amorphous‐state form typically obtained with the solution process suffers from high defect density. This causes large charge recombination, and consequently significant deterioration of the solar cell efficiency. Herein, an ultra‐low‐temperature processed (50 °C) nanocrystalline WO _x as the electron transport layer, free of this problem, is demonstrated. This material is obtained by the reaction of tungsten chloride with hexanol, which induces transformation of the precursor solution into stable colloidal particles. The best solar cell, with the WO _x electron transport layer, achieved an efficiency of 20.77%, which is a record performance for this class of perovskite solar cells.

Synergistic effects originate from a dual functionalization of the electron transport layer/perovskite interface in solar cells designed on TiO₂ nanorods. TiCl₄ treatment combined with PC₆₁BM monolayer deposition leads to remarkable enhancements in cell efficiency, from 14.2% to 19.5%, and to the suppression of hysteresis.

Abstract

The engineering of the electron transport layer (ETL)/light absorber interface is explored in perovskite solar cells. Single‐crystalline TiO₂ nanorod (NR) arrays are used as ETL and methylammonium lead iodide (MAPI) as light absorber. A dual ETL surface modification is investigated, namely by a TiCl₄ treatment combined with a subsequent PC₆₁BM monolayer deposition, and the effects on the device photovoltaic performance were evaluated with respect to single modifications. Under optimized conditions, for the combined treatment synergistic effects are observed that lead to remarkable enhancements in cell efficiency, from 14.2% to 19.5%, and to suppression of hysteresis. The devices show J _SC, V _OC, and fill factor as high as 23.2 mA cm⁻², 1.1 V, and 77%, respectively. These results are ascribed to a more efficient charge transfer across the ETL/perovskite interface, which originates from the passivation of defects and trap states at the ETL surface. To the best of our knowledge, this is the highest cell performance ever reported for TiO₂ NR‐based solar cells fabricated with conventional MAPI light absorber. Perspective wise, this ETL surface functionalization approach combined with more recently developed and better performing light absorbers, such as mixed cation/anion hybrid perovskite materials, is expected to provide further performance enhancements.

Cubic zinc metatitanate (ZTO) is identified as an excellent electron transport material with interface engineering treatment of dezincification for high efficient perovskite solar cells (PSCs). By integrating an inorganic hole transport layer and rGO protection, the ZTO electron transport layer‐based PSCs exhibit strong resistance to moisture, heat, and ultraviolet light, demonstrating high efficiency and stability toward practical applications.

Perovskite solar cells (PSCs) have experienced considerable development in the past few years. The stability issue has become a focus of research efforts toward their commercial applications. The development and interface engineering of electron transport materials (ETMs) to build up stable interfaces with perovskites has been emerging as a powerful strategy to enhance PSCs' stability. Herein, cubic zinc metatitanate (ZTO) is identified as an excellent ETM with interface engineering treatment of dezincification for fabricating PSCs with much better overall performances than those fabricated from TiO₂, a popularly used ETM. The high electron mobility of ZTO helps minimize the hysteresis. Together with the use of CuSCN as inorganic hole transport material and further protecting the PSCs with reduced graphene oxide, the ZTO‐based PSCs exhibit remarkable enhancement in stability, retaining 95% of initial power conversion efficiency under AM 1.5 G illumination at 85 °C and 85% relative humidity in air for 1000 h at open circuit.

An effective polar molecule of (2‐aminothiazole‐4‐yl)acetic acid (ATAA) is incorporated onto a ZnO electron transport layer to simultaneously achieve defect passivation and work function modulation by forming permanent interface dipoles. It minimizes the charge recombination loss in perovskite solar cells, and the ZnO–ATAA‐based device ultimately achieves an enhanced efficiency of 19.74% while suppressing the device hysteresis.

The intrinsic characteristics of a ZnO electron transport layer (ETL) lead to severe charge loss in perovskite solar cells (PSCs), such as photogenerated charge accumulation recombination in the perovskite layer due to the low electron extraction capacity, and defect‐induced charge recombination at the interface due to the unfavorable defects, causing efficiency loss and device hysteresis. Here, the polar molecule of (2‐aminothiazole‐4‐yl)acetic acid (ATAA) is self‐assembled onto a ZnO layer with the help of oxygen vacancy defects, combining the advantages of lowering the work function by forming the permanent interface dipole and simultaneously passivating defect states. It effectively strengthens the electron extraction capacity and reduces the density of defect states. Therefore, the resulting PSCs with a ZnO–ATAA ETL yield an enhanced efficiency of 19.74% with evidently reduced device hysteresis.

A simple low‐temperature‐processed In₂O₃/SnO₂ bilayer electron‐transport layer (ETL) is used for fabricating efficient perovskite solar cells (PSCs). The bilayer ETL with appropriate energy alignment is beneficial for charge transfer, thus minimizing open‐circuit voltage (V _OC) loss. An optimized planar PSC with a power conversion efficiency (PCE) of 23.24% is obtained. In contrast, devices based on single SnO₂ only achieve efficiency of 21.42%.

Abstract

An electron‐transport layer (ETL) with appropriate energy alignment and enhanced charge transfer is critical for perovskite solar cells (PSCs). However, interfacial energy level mismatch limits the electrical performance of PSCs, particularly the open‐circuit voltage (V _OC). Herein, a simple low‐temperature‐processed In₂O₃/SnO₂ bilayer ETL is developed and used for fabricating a new PSC device. The presence of In₂O₃ results in uniform, compact, and low‐trap‐density perovskite films. Moreover, the conduction band of In₂O₃ is shallower than that of Sn‐doped In₂O₃ (ITO), enhancing the charge transfer from perovskite to ETL, thus minimizing V _OC loss at the perovskite and ETL interface. A planar PSC with a power conversion efficiency of 23.24% (certified efficiency of 22.54%) is obtained. A high V _OC of 1.17 V is achieved with the potential loss at only 0.36 V. In contrast, devices based on single SnO₂ layers achieve 21.42% efficiency with a V _OC of 1.13 V. In addition, the new device maintains 97.5% initial efficiency after 80 d in N₂ without encapsulation and retains 91% of its initial efficiency after 180 h under 1 sun continuous illumination. The results demonstrate and pave the way for the development of efficient photovoltaic devices.

Novel cathode interlayers (CILs) are developed by tailoring an organic electron acceptor, viz. ITIC. A high efficiency of 16.6% is achieved in an organic solar cell with S‐3 as the CIL. It is demonstrated that the difference of electrostatic surface potential between the CIL molecule and the polymer donor can promote exciton dissociation, contributing to additional charge generation.

Abstract

With the rapid advance of organic photovoltaic materials, the energy level structure, active layer morphology, and fabrication procedure of organic solar cells (OSCs) are changed significantly. Thus, the photoelectronic properties of many traditional electrode interlayers have become unsuitable for modifying new active layers; this limits the further enhancement in OSC efficiencies. Herein, a new design strategy of tailoring the end‐capping unit, ITIC, to develop a cathode interlayer (CIL) material for achieving high power conversion efficiency (PCE) in OSCs is demonstrated. The excellent electron accepting capacity, suitable energy level, and good film‐forming ability endow the S‐3 molecule with an outstanding electron extraction property. A device with S‐3 shows a PCE of 16.6%, which is among the top values in the field of OSCs. More importantly, it is demonstrated that the electrostatic potential difference between the CIL molecule and the polymer donor plays a crucial role in promoting exciton dissociation at the CIL/active layer interface, contributing to additional charge generation; this is crucial for enhancement of the current density. The results of this work not only develop a new design strategy for high‐performance CIL, but also demonstrate a reliable approach of density functional theory (DFT) calculation to predict the effect of the CIL chemical structure on exciton dissociation in OSCs.

Publication date: February 2020

Source: Nano Energy, Volume 68

Author(s): Cong Chen, Xinmeng Zhuang, Wenbo Bi, Yanjie Wu, Yanbo Gao, Gencai Pan, Dali Liu, Qilin Dai, Hongwei Song

Graphical abstract

The chemical decomposition inhibition strategy was introduced in perovskite films through iodine bromide to modify the crystal defects, leading to PSCs with suppressed hysteresis effects, attractive PCE of 21.5% and superior durability of 5000 h.

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.

Metal halide perovskite solar cells (PSCs), with their exceptional properties, hold potential as photoelectric converters. However, defects in the perovskite layer, particularly at the grain boundaries (GBs), seriously restrict the performance and stability of PSCs. Herein, we present a simple post‐treatment procedure by applying 2‐aminoterephthalic acid to the perovskite to produce efficient and stable PSCs. By optimizing the post‐treatment conditions, we created a device that achieved a remarkable power conversion efficiency (PCE) of 21.09% and demonstrated improved stability. This improvement was attributed to the fact that the 2‐aminoterephthalic acid acted as a cross‐linking agent that inhibited the migration of ions and passivated the trap states at GBs. These findings provide a potential strategy for designing efficient and stable PSCs regarding the aspects of defect passivation and crystal growth.

Gadolinium fluoride (GdF₃) and aminobutanol are introduced for Ostwald ripening in the crystal growth of perovskite to overcome the double dilemma of internal defects and external humidity. The GdF₃‐ and aminobutanol‐treated perovskite solar cell achieves a power conversion efficiency of 21.21% with good stability and small hysteresis, while the pristine device only shows an efficiency of 18.10%.

Abstract

As one kind of promising next‐generation photovoltaic devices, perovskite solar cells (PVSCs) have experienced unprecedented rapid growth in device performance over the past few years. However, the practical applications of PVSCs require much improved device long‐term stability and performance, and internal defects and external humidity sensitivity are two key limitation need to be overcome. Here, gadolinium fluoride (GdF₃) is added into perovskite precursor as a redox shuttle and growth‐assist; meanwhile, aminobutanol vapor is used for Ostwald ripening in the formation of the perovskite layer. Consequently, a high‐quality perovskite film with large grain size and few grain boundaries is obtained, resulting in the reduction of trap state density and carrier recombination. As a result, a power conversion efficiency of 21.21% is achieved with superior stability and negligible hysteresis.

This work demonstrates inkjet‐printed triple‐cation perovskite solar cells with high efficiencies of >21% and stabilized power conversion efficiencies up to 18.5%. The printed thin films are micrometer thick and show large columnar grains over the entire layer. Material properties are linked to device performance and a detailed report on the fabrication process is given.

Abstract

Transferring the high power conversion efficiencies (PCEs) of spin‐coated perovskite solar cells (PSCs) on the laboratory scale to large‐area photovoltaic modules requires a significant advance in scalable fabrication methods. Digital inkjet printing promises scalable, material, and cost‐efficient deposition of perovskite thin films on a wide range of substrates and in arbitrary shapes. In this work, high‐quality inkjet‐printed triple‐cation (methylammonium, formamidinium, and cesium) perovskite layers with exceptional thicknesses of >1 µm are demonstrated, enabling unprecedentedly high PCEs > 21% and stabilized power output efficiencies > 18% for inkjet‐printed PSCs. In‐depth characterization shows that the thick inkjet‐printed perovskite thin films deposited using the process developed herein exhibit a columnar crystal structure, free of horizontal grain boundaries, which extend over the entire thickness. A thin film thickness of around 1.5 µm is determined as optimal for PSC for this process. Up to this layer thickness X‐ray photoemission spectroscopy analysis confirms the expected stoichiometric perovskite composition at the surface and shows strong deviations and inhomogeneities for thicker thin films. The micrometer‐thick perovskite thin films exhibit remarkably long charge carrier lifetimes, highlighting their excellent optoelectronic characteristics. They are particularly promising for next‐generation inkjet‐printed perovskite solar cells, photodetectors, and X‐ray detectors.

The present study reveals a strong influence of sputtered NiO _x on the perovskite crystallization and the appearance of residual PbI₂ grains resulting in low photovoltaic device performance. Among different methylammonium (MA⁺) halide additives and vapor treatment (to improve the perovskite crystallization) only MA⁺ halide vapor‐treated perovskite shows suppressed recombination, enhanced carrier lifetime, and device efficiency.

Abstract

Investigating the low efficiency issue of radio frequency‐sputtered nickel oxide (sp‐NiO _x )‐based perovskite solar cells (PSCs) due to a limited understanding of the correlation between perovskite growth and sp‐NiO _x on the optoelectronic properties and photovoltaic device performance is critical. Herein, the crystallization of methylammonium (MA) lead iodide (MAPbI₃) thin film (obtained from stoichiometric precursor ratio) on sp‐NiO _x is shown, resulting in appearance of residual PbI₂ grains. This is in contrast to perovskite growth on solution‐processed NiO _x . The amount of residual PbI₂ is suppressed by 1) adding excess MACl/MAI additives and 2) annealing the perovskite film in MACl/MAI vapor atmosphere. Structural and morphological results reveal significant reduction in the amount of residual PbI₂ and enhanced grain size for all the cases while photophysical measurements reveal mitigation of trap/defect sites (within the bulk and at the interfaces) only for MACl/MAI vapor annealing case. As a result, photovoltaic devices exhibit improved performance only for the vapor annealing case. These results elucidate the critical role of maintaining stoichiometric ratio in perovskite and its crystallization on sp‐NiO _x by eliminating the associated defects (influenced by sp‐NiO _x ) in rendering improved performance, which can be insightful to further enhance the performance of PSCs.

All held together: A simple post‐treatment procedure uses 2‐aminoterephthalic acid as a cross‐linking agent to modify the exposed grain boundary of a perovskite film, which is directly observed with conductive atomic force microscopy (C‐AFM). Under the optimized cross‐linking agent concentration, a solar cell achieves a power conversion efficiency (PCE) of 21.09 % and improved stability.

Abstract

Metal halide perovskite solar cells (PSCs), with their exceptional properties, show promise as photoelectric converters. However, defects in the perovskite layer, particularly at the grain boundaries (GBs), seriously restrict the performance and stability of PSCs. Now, a simple post‐treatment procedure involves applying 2‐aminoterephthalic acid to the perovskite to produce efficient and stable PSCs. By optimizing the post‐treatment conditions, we created a device that achieved a remarkable power conversion efficiency (PCE) of 21.09 % and demonstrated improved stability. This improvement was attributed to the fact that the 2‐aminoterephthalic acid acted as a cross‐linking agent that inhibited the migration of ions and passivated the trap states at GBs. These findings provide a potential strategy for designing efficient and stable PSCs regarding the aspects of defect passivation and crystal growth.

High‐efficiency stable perovskite/gallium arsenide two‐terminal and four‐terminal tandem cells are demonstrated for the first time. For this purpose, high‐performance photostable wide‐bandgap perovskite photovoltaics (PVs) (1.8–1.9 eV) are developed by a solvent‐controlled process. Tandem architectures are shown to be feasible for thin‐film flexible devices with superior bendability, essential to commercialization. This approach is expected to improve the usability of GaAs PV with enhanced efficiency and lower cost.

Abstract

Gallium arsenide (GaAs) photovoltaic (PV) cells have been widely investigated due to their merits such as thin‐film feasibility, flexibility, and high efficiency. To further increase their performance, a wider bandgap PV structure such as indium gallium phosphide (InGaP) has been integrated in two‐terminal (2T) tandem configuration. However, it increases the overall fabrication cost, complicated tunnel‐junction diode connecting subcells are inevitable, and materials are limited by lattice matching. Here, high‐efficiency and stable wide‐bandgap perovskite PVs having comparable bandgap to InGaP (1.8–1.9 eV) are developed, which can be stable low‐cost add‐on layers to further enhance the performance of GaAs PVs as tandem configurations by showing an efficiency improvement from 21.68% to 24.27% (2T configuration) and 25.19% (4T configuration). This approach is also feasible for thin‐film GaAs PV, essential to reduce its fabrication cost for commercialization, with performance increasing from 21.85% to 24.32% and superior flexibility (1000 times bending) in a tandem configuration. Additionally, potential routes to over 30% stable perovskite/GaAs tandems, comparable to InGaP/GaAs with lower cost, are considered. This work can be an initial step to reach the objective of improving the usability of GaAs PV technology with enhanced performance for applications for which lightness and flexibility are crucial, without a significant additional cost increase.