shuhui

An all‐inorganic mixed‐halide perovskite solar cell with a power conversion efficiency of 16.42% is realized by using a Cs₂CO₃‐doped ZnO electron transport layer, which ascribes to the interfacial energy level tuning for reducing ohmic loss at the contact and enlarging the built‐in potential. A high thermostability is simultaneously obtained via surface defect passivation for improving the CsPbI₂Br film against phase transformation.

Abstract

Inorganic mixed‐halide CsPbX₃‐based perovskite solar cells (PeSCs) are emerging as one of the most promising types of PeSCs on account of their thermostability compared to organic–inorganic hybrid counterparts. However, dissatisfactory device performance and high processing temperature impede their development for viable applications. Herein, a facile route is presented for tuning the energy levels and electrical properties of sol–gel‐derived ZnO electron transport material (ETM) via the doping of a classical alkali metal carbonate Cs₂CO₃. Compared to bare ZnO, Cs₂CO₃‐doped ZnO possesses more favorable interface energetics in contact with the CsPbI₂Br perovskite layer, which can reduce the ohmic loss to a negligible level. The optimized PeSCs achieve an improved open‐circuit voltage of 1.28 V, together with an increase in fill factor and short‐circuit current. The optimized power conversion efficiencies of 16.42% and 14.82% are realized on rigid glass substrate and flexible plastic substrate, respectively. A high thermostability can be simultaneously obtained via defect passivation at the Cs₂CO₃‐doped ZnO/CsPbI₂Br interface, and 81% of the initial efficiency is retained after aging for 200 h at 85 °C.

Publication date: March 2020

Source: Nano Energy, Volume 69

Author(s): Guanshui Xie, Zheling Zhang, Zhenying Su, Xiaoling Zhang, Jian Zhang

Graphical abstract

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.

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.

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.

A novel design for all‐inorganic perovskite solar cells with a modified electron transporting layer facilitates excellent thermal‐air stability. This study demonstrates a dynamic‐hot air method with Mn²⁺ incorporated CsPbI₂Br perovskite based on low temperature processed MgZnO which enables higher thermal‐air stability.

Abstract

The high thermal stability and facile synthesis of CsPbI₂Br all‐inorganic perovskite solar cells (AI‐PSCs) have attracted tremendous attention. As far as electron‐transporting layers (ETLs) are concerned, low temperature processing and reduced interfacial recombination centers through tunable energy levels determine the feasibility of the perovskite devices. Although the TiO₂ is the most popular ETL used in PSCs, its processing temperature and moderate electron mobility hamper the performance and feasibility. Herein, the highly stable, low‐temperature processed MgZnO nanocrystal‐based ETLs for dynamic hot‐air processed Mn²⁺ incorporated CsPbI2Br AI‐PSCs are reported. By holding its regular planar “n–i–p” type device architecture, the MgZnO ETL and poly(3‐hexylthiophene‐2,5‐diyl) hole transporting layer, 15.52% power conversion efficiency (PCE) is demonstrated. The thermal‐stability analysis reveals that the conventional ZnO ETL‐based AI‐PSCs show a serious instability and poor efficiency than the Mg²⁺ modified MgZnO ETLs. The photovoltaic and stability analysis of this improved photovoltaic performance is attributed to the suitable wide‐bandgap, low ETL/perovskite interface recombination, and interface stability by Mg²⁺ doping. Interestingly, the thermal stability analysis of the unencapsulated AI‐PSCs maintains >95% of initial PCE more than 400 h at 85 °C for MgZnO ETL, revealing the suitability against thermal degradation than conventional ZnO ETL.

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

Abstract

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

An overview is provided on the recent advances in the fundamental understanding of how the interfaces between the perovskite film and the charge transport layers influence the performance of halide perovskite solar cells. Furthermore, the various design strategies for the improvement of interfacial materials and interfacial phenomena are discussed.

Abstract

Organic–inorganic hybrid perovskite solar cells (HPSCs) have achieved an impressive power conversion efficiency (PCE) of 25.2% in 2019. At this stage, it is of paramount importance to understand in detail the working mechanism of these devices and which physical and chemical processes govern not only their power conversion efficiency but also their long‐term stability. The interfaces between the perovskite film and the charge transport layers are among the most important factors in determining both the PCE and stability of HPSCs. Herein, an overview is provided on the recent advances in the fundamental understanding of how these interfaces influence the performance of HPSCs. Firstly, it is discussed how the surface energy of the charge transport layer, the energy level alignment at the interfaces, the charge transport in interfacial layers, defects and mobile ions in the perovskite film, and interfacial layers or at the interfaces affect the charge recombination as well as hysteresis and light soaking phenomenon. Then it is discussed how the interfaces and interfacial materials influence the stability of HPSCs. At the same time, an overview is also provided on the various design strategies for the interfaces and the interfacial materials. At the end, the outlook for the development of highly efficient and stable HPSCs is provided.

The efficiency and stability of 2D perovskite solar cells are synergistically improved through metal ion doping. The hole extraction and transport abilities are significantly enhanced by Cu ion doping in the NiO _x layers, while the optoelectronic properties of the BA₂MA₃Pb₄I₁₃ (BA = butylamine; MA = methylammonium) layers are effectively improved with Cs ion doping.

Abstract

2D perovskites hold a great prospective to create highly efficient and stable solar cell devices. In order to explore their full potential, every component of 2D perovskite solar cells (PSCs) has to be carefully designed and engineered. Herein, the metal ion doping strategy is taken to optimize both the hole transport layers (HTLs) and the light absorbing layers of the BA₂MA₃Pb₄I₁₃ (BA = butylamine; MA = methylammonium) based 2D PSC devices. The hole extraction and transport abilities are significantly enhanced by Cu ion doping in the nickel oxide layers, while the optoelectronic properties of the BA₂MA₃Pb₄I₁₃ layers are effectively improved with Cs ion doping. The synergistic incorporations of Cu and Cs ions have boosted the device power conversion efficiency to 13.92%, the highest for 2D PSCs based on inorganic HTLs. In addition, the inorganic nature of the Cu doped nickel oxide film and the high quality of the Cs doped 2D perovskite film also endow the PSC device with extraordinary humidity and thermal stabilities.

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.

Recent progress in first‐principles simulations of carrier recombination in halide perovskites is reviewed. Misunderstandings relating to the impact of the Rashba effect on radiative recombination are clarified. The origin of exceptionally strong Auger recombination and avenues for improved materials design are discussed. Critical analysis of the recombination mechanisms reveals fruitful directions for improving the performance of halide perovskites.

Abstract

In recent years, there have been remarkable developments in halide perovskites, which are used in highly efficient optoelectronic devices and exhibit intriguing materials physics. Detailed knowledge of carrier recombination mechanisms is essential for understanding their excellent performance and to further increase their efficiency. Obtaining such knowledge is challenging however, and different studies have reached divergent conclusions in some cases. This progress report outlines the critical developments in understanding the carrier recombination mechanisms in halide perovskites from a computational perspective. The primary focus is radiative and Auger recombination, since they have not been systematically assessed and discussed before, and a number of important issues have been actively debated. This comprehensive discussion of the carrier recombination mechanisms is aimed at establishing physically justified insights that can form the basis for better materials and devices design.

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.

Low‐cost and efficient organic small molecules are desired as hole transporting materials for high‐performance perovskite solar cells. Two new molecules containing a benzodithiophene core and triphenylamine side chains are synthesized from cheap starting materials by a simple and low‐cost method. Lead‐free, tin‐based perovskite solar cells employing these new benzodithiophene‐based hole transporting materials achieve good efficiencies.

Abstract

Developing efficient interfacial hole transporting materials (HTMs) is crucial for achieving high‐performance Pb‐free Sn‐based halide perovskite solar cells (PSCs). Here, a new series of benzodithiophene (BDT)‐based organic small molecules containing tetra‐ and di‐triphenyl amine donors prepared via a straightforward and scalable synthetic route is reported. The thermal, optical, and electrochemical properties of two BDT‐based molecules are shown to be structurally and energetically suitable to serve as HTMs for Sn‐based PSCs. It is reported here that ethylenediammonium/formamidinium tin iodide solar cells using BDT‐based HTMs deliver a champion power conversion efficiency up to 7.59%, outperforming analogous reference solar cells using traditional and expensive HTMs. Thus, these BDT‐based molecules are promising candidates as HTMs for the fabrication of high‐performance Sn‐based PSCs.

A new small molecule IBC‐F as the third component to improve efficiency and stability of ternary polymer solar cells is developed. The ternary device with 20% IBC‐F exhibits a higher efficiency of 15.06% compared with the host binary PBDB‐T:IE4F‐S‐based device with an efficiency of 13.70%. Furthermore, the ternary devices show better thermal and photoinduced stability compared the binary devices.

Abstract

The use of a ternary active layer offers a promising approach to enhance the power conversion efficiency (PCE) of polymer solar cells (PSCs) via simply incorporating a third component. Here, a ternary PSC with improved efficiency and stability facilitated by a new small molecule IBC‐F is demonstrated. Even though the PBDB‐T:IBC‐F‐based device gives an extremely low PCE of only 0.21%, a remarkable PCE of 15.06% can be realized in the ternary device based on PBDB‐T:IE4F‐S:IBC‐F with 20% IBC‐F, which is ≈10% greater than that (PCE = 13.70%) of the control binary device based on PBDB‐T:IE4F‐S. The improvement in the device performance of the ternary PSC is mainly attributed to the enhancement of fill factor, which is due to the improved charge dissociation and extraction, suppressed bimolecular and trap‐assisted recombination, longer charge‐carrier lifetime, and enhanced intermolecular interactions for preferential face‐on orientation. Additionally, the ternary device with 20% IBC‐F shows better thermal and photoinduced stability over the control binary device. This work provides a new angle to develop the third components for building ternary PSCs with enhanced photovoltaic performance and stability for practical applications.

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

Abstract

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

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

Abstract

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

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

Abstract

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

A simple perovskite solar cell architecture, which is based on dopant‐free electron and hole conductors and carbon back contact deposited at room temperature, is demonstrated. The resulting architecture leads to the fabrication of cheap and highly efficient perovskite solar cells exhibiting unprecedented long‐term operational and UV stability thus hold immense potential for large‐scale deployment.

Abstract

Today's perovskite solar cells (PSCs) mostly use components, such as organic hole conductors or noble metal back contacts, that are very expensive or cause degradation of their photovoltaic performance. For future large‐scale deployment of PSCs, these components need to be replaced with cost‐effective and robust ones that maintain high efficiency while ascertaining long‐term operational stability. Here, a simple and low‐cost PSC architecture employing dopant‐free TiO₂ and CuSCN as the electron and hole conductor, respectively, is introduced while a graphitic carbon layer deposited at room temperature serves as the back electrical contact. The resulting PSCs show efficiencies exceeding 18% under standard AM 1.5 solar illumination and retain ≈95% of their initial efficiencies for >2000 h at the maximum power point under full‐sun illumination at 60 °C. In addition, the CuSCN/carbon‐based PSCs exhibit remarkable stability under ultraviolet irradiance for >1000 h while under similar conditions, the standard spiro‐MeOTAD/Au based devices degrade severely.

An ultrathin functional polymer layer is used to enhance the nucleation of atomic layer deposited (ALD) SnO₂ contacts in metal‐halide perovskite solar cells. These nucleation‐enhanced ALD layers act as “built‐in” barriers to both internal and external degradation pathways, significantly improving the long‐term operational stability of high efficiency unencapsulated devices (>18%) in air.

Abstract

Metal‐halide perovskites show promise as highly efficient solar cells, light‐emitting diodes, and other optoelectronic devices. Ensuring long‐term stability is now a major priority. In this study, an ultrathin (2 nm) layer of polyethylenimine ethoxylated (PEIE) is used to functionalize the surface of C₆₀ for the subsequent deposition of atomic layer deposition (ALD) SnO₂, a commonly used electron contact bilayer for p–i–n devices. The enhanced nucleation results in a more continuous initial ALD SnO₂ layer that exhibits superior barrier properties, protecting Cs_0.25FA_0.75Pb(Br_0.20I_0.80)₃ films upon direct exposure to high temperatures (200 °C) and water. This surface modification with PEIE translates to more stable solar cells under aggressive testing conditions in air at 60 °C under illumination. This type of “built‐in” barrier layer mitigates degradation pathways not addressed by external encapsulation, such as internal halide or metal diffusion, while maintaining high device efficiency up to 18.5%. This nucleation strategy is also extended to ALD VO _x films, demonstrating its potential to be broadly applied to other metal oxide contacts and device architectures.