Ligang Yuan

The lattice of inorganic CsPbBr₃ halide is modulated by doping alkali metal cations to form Cs_1−x R _x PbBr₃ (R = Li, Na, K, Rb, x = 0–1) perovskites. Through tuning R/Cs ratio, a PCE up to 9.86% and improved stability in 80% humidity are achieved for HTL‐free Cs_0.91Rb_0.09PbBr₃ solar cell.

Abstract

The crystal structure of cesium lead halide (CsPbX₃, X = I, Br, Cl) determines its charge‐carrier trap state and solar‐to‐electrical conversion ability in inorganic perovskite solar cells (PSCs). Here, the compositional engineering of inorganic CsPbBr₃ perovskite by means of doping with various alkali metal cations is studied. The lattice dimensions and energy levels of Cs_1‐x R _x PbBr₃ (R = Li, Na, K, Rb, x = 0–1) halides are optimized by tuning Cs/R ratio. Arising from promoting effects of alkali metal cations doped perovskite halides such as lattice shrink, crystallized dynamics, and electrical‐energy distribution, a maximum power conversion efficiency as high as 9.86% is achieved for hole transporting layer‐free Cs_0.91Rb_0.09PbBr₃ tailored solar cell owing to the suppressed non‐radiative losses and radiative recombination. Furthermore, the all‐inorganic Cs_0.91Rb_0.09PbBr₃ solar cell without encapsulation remains 97% of initial efficiency when suffering persistent attack by 80% RH in air atmosphere over 700 h, which is in comparable with state‐of‐the‐art organic–inorganic hybrid and all‐inorganic PSC devices. Employing alkali metal cations to modulate perovskite layers provide new opportunities of making high‐performance inorganic PSC platforms.

Anode contact engineering of p‐i‐n type perovskite solar cells is demonstrated. With the control of the interfacial properties using an ultrathin metallic layer and an insulating layer, a high V _oc of 1.15 V and a power conversion efficiency of 19.3% are obtained. Furthermore, a substrate‐type device with an extremely high V _oc of 1.18 V is also demonstrated.

With photocurrents in perovskite solar cells close to their practical limit, it is imperative to improve their open‐circuit voltage to go beyond a loss‐in‐potential less than 100 mV. However, state‐of‐the‐art p‐i‐n perovskite solar cells are reported with a V _oc of around 1.1–1.5 V, limiting their efficiency improvement. Herein, the authors demonstrate that the V _oc of p‐i‐n perovskite solar cells can be successfully improved via anode contact engineering. First of all, by introducing a partially oxidized nickel layer, the authors are able to remove the potential barrier for hole transport and enhance the crystallinity of the hole transporting layer, both of which are believed to contribute to the V _oc and FF improvement. Furthermore, by separating the inorganic NiMgO_x hole transporting layer from the perovskite absorbing layer with a poly(4‐vinylpyridine) (PVP) insulating layer, the interfacial recombination could be effectively suppressed, the V _oc climbs to an impressive value of 1.15 V along with a power conversion efficiency of 19.3%. Finally, a substrate‐type perovskite solar cell is fabricated with an extremely high V _oc of 1.18 V, representing a very low voltage deficit in the p‐i‐n perovskite solar cells. Our works provide an avenue for further reducing the loss‐in‐potential of perovskite solar cells.

The authors demonstrate use of pristine C₆₀ modification layer for optimizing the PMPSCs performance. Benefiting from an improved electron extraction and an up‐shifted LUMO of the C₆₀ modified TiO₂, the charge transfer at the electron transport layer and perovskite interface is significantly enhanced. As a result, the PCE of PMPSCs is boosted from 13.7% to 15.4%.

Carbon‐based, hole‐conductor‐free, full‐printable, mesoscopic perovskite solar cells (PMPSCs) have attracted much attention due to their superior stability, low cost, and potential scalability. However, TiO₂ generally shows a relatively low electron mobility, which may result in charge accumulation and recombination at the TiO₂/perovskite interface, thus hindering the improvements from the PMPSCs performance. In this study, a simple strategy to deposit a pristine C₆₀ layer using a spray method is reported and the low electrical conductivity of TiO₂ in PMPSCs is modified. The preferable electron extraction and transportation properties of pristine C₆₀ and the up‐shifted lowest unoccupied molecular orbital (LUMO) of the C₆₀ modified TiO₂, that is, TiO₂(C₆₀) layer, efficiently promotes the charge transfer at the interface between the electron transport layer (ETL) and the perovskite. As a result, a significantly improved power conversion efficiency (PCE) of 15.4% is yielded, which is much higher than that of the control device (13.7%). In addition, the TiO₂(C₆₀)‐based device presents a lower hysteresis effect than that of the TiO₂‐based device, which can likely be attributed to the reduced space charge accumulation at the perovskite/ETL interface. The present work provides a simple approach to boost the efficiency of PMPSCs and paves the way for the industrialization of PMPSCs.

Visualization and suppression of interfacial recombination for high-efficiency large-area pin perovskite solar cells

Visualization and suppression of interfacial recombination for high-efficiency large-area pin perovskite solar cells, Published online: 30 July 2018; doi:10.1038/s41560-018-0219-8

Factors that degrade photovoltaic performance of perovskite solar cells (PSCs) include the photo‐thermal effect, electrical bias, moisture, heat, and UV‐light. In most cases, the joint effect of these factors fastens the photovoltaic degradation of PSCs. This article summarizes the most advanced strategies that progress PSCs towards better stability.

Abstract

Several strategies are proposed and implemented to improve the stability of perovskite solar cells (PSCs). The authors separate strategies toward stable PSCs according to the long‐term, hundreds of hours, and short‐term, minutes to hours, stability issue encountered in the operation of PSCs under standard working condition—light and load. To enhance the long‐term stability, the authors can rely on technological solutions such as encapsulation of the full device, application of hydrophobic hole selective materials, insertion of a polymer layer on top of perovskite film and enhancement of the intrinsic stability of perovskite material itself via tuning the composition. For the short‐term stability, which is mainly linked to formation and migration of ionic defects within the perovskite, bulk and interface passivation of perovskite film are demonstrated to be critically important. This article summarizes and analyzes the most recent updated strategies that are progressing PSCs toward commercialization.

Reduced graphene oxide has a multidimensional utility in perovskite solar cells, and it is of particular interest in improving their long‐term stability as an alternative hole‐transporting material, passivating agent, and a spacer layer. A brief overview of the present functions and future challenges for the applications of reduced graphene oxide in perovskite solar cells is provided.

Abstract

One of the most challenging obstacles to commercialization of perovskite solar cells (PSCs) is their instability toward environmental conditions. An emerging solution to overcome these challenges relies on graphene‐based composites, in particular, solution‐processable reduced graphene oxide (rGO), whose high electrical and thermal conductivity, along with beneficial passivation effects, display multifaceted utility in PSCs. This review provides a brief summary of functions of rGO in different PSC architectures, from a potential replacement for transparent conductive oxides, through usage in electron‐transporting and perovskite layers, with the particular emphasis on its application as a hole‐transporting material and a spacer layer. Despite ongoing research challenges, the synergy between rGO and PSCs promises to facilitate the future applications of efficient and stable PSCs.

In this review article, the authors analyze the possible origins of the defects (both in the bulk material and at the interfaces) in perovskite solar cells and summarize various approaches being utilized to reduce them. The authors demonstrate that defect engineering is an essential way to further boost the device performance and enhance their long‐term stability.

Abstract

Metal halide perovskite solar cells are emerging candidates amongst the next‐generation thin‐film photovoltaic devices with extremely low fabrication cost and high power conversion efficiency. Defects (both in the bulk material and at the interfaces) are recognized as one of the most fundamental reasons for the compromised device performance and long‐term stability of perovskite solar cells. In this review article, the authors analyze the possible origins of the defects formation in metal halide perovskites, followed by the rationalization of various approaches being utilized to reduce the density of defects. The authors demonstrate that defect engineering, including adding dopants in the precursor solutions, interface passivation, or other physical treatments (thermal or light stress), is an essential way to further boost the device performance and enhance their long‐term stability. The authors note that although the exact mechanisms of defect elimination in some approaches are yet to be elucidated, the research on defect engineering is expected to have enormous impact on next wave of device performance optimization of metal halide perovskite solar cells toward Shockley–Queisser limit.

Charge extraction and transport in perovskite solar cells (PSCs) are strongly influenced by the interfaces and in particular the energy level alignment (ELA). The recent advances of the research regarding energy level alignment in PSCs are reviewed. Perspective and outlook for precisely determining ELA, designing the device architecture, and fabricating high performance PSCs are discussed.

Abstract

The rapid progress of organic–inorganic metal halide perovskite solar cells (PSCs) has attracted broad interest in photovoltaic community. A typical PSC consists of anode/cathode, a perovskite layer as absorber, and carrier transport layer(s) (electron/hole transport layer(s)), which are stacked together, resulting in multi‐interfaces between these layers. Charge extraction and transport in these solar cell devices are strongly influenced by the interfaces and in particular the energy level alignment (ELA). It is the synergy of multiple interfaces and bulk films embedded in the cell architecture that has led to the extraordinary success of PSCs. Here, the authors review the progress of the studies on energy level alignment in PSCs, including several sections: methods for deriving ELA, semiconductor type of perovskite, bottom layer–dependent energy level shift of perovskite, density of states–governed ELA, ELA for specific interfaces, instability‐induced ELA variation, and defects and ion migration–induced ELA variation. Perspective and outlook for precisely determining ELA, designing the device architecture, and fabricating high performance PSCs are discussed.

The impact of transition metal dichalcogendies and black phosphorus 2D layered materials in the performance, stability, and scalability of solution processable perovskite solar cells is summarized in this focused Research News article. These type materials are mainly utilized as HTLs that address very successfully the trade‐off between high conductivity and high stability that their counterparts (organics and GRMs) fail to address.

Abstract

In this Research News, the authors summarize the work that has been done on the utilization of 2D materials beyond graphene in the electron transport layer (ETL) and hole transport layer (HTL) selected of metal halide perovskite solar cells (PSCs). On the one hand, the impact of such materials in the stability of PSCs under operational conditions is impressive. Stabilities for encapsulated PSCs cells under illumination and ambient conditions at maximum power point that retain 80% of their initial performance (T₈₀) have been reported. On the other hand, the extraction of photogenerated holes has been reduced to few microsecond when a 2D material–based HTL or ETL is introduced. The faster photogenerated charge extraction lowered the recombination rates and resulted in higher power conversion efficiencies. The most interesting thing is that the 2D materials have been successfully applied for large active area cells (≈1.05 cm²) as well. The exploitation of 2D materials, beyond graphene, is a very interesting and promising technology that shows a high potential pathway towards the envisioned future commercialization of PSCs through addressing their performance, stability, and scalability priorities.