The vast majority of perovskite solar cell research has focused on organic–inorganic lead trihalide perovskites. Herein, we present working inorganic CsPbI3 perovskite solar cells for the first time. CsPbI3 normally resides in a yellow non-perovskite phase at room temperature, but by careful processing control and development of a low-temperature phase transition route we have stabilised the material in the black perovskite phase at room temperature. As such, we have fabricated solar cell devices in a variety of architectures, with current–voltage curve measured efficiency up to 2.9% for a planar heterojunction architecture, and stabilised power conversion efficiency of 1.7%. The well-functioning planar junction devices demonstrate long-range electron and hole transport in this material. Importantly, this work identifies that the organic cation is not essential, but simply a convenience for forming lead triiodide perovskites with good photovoltaic properties. We additionally observe significant rate-dependent current–voltage hysteresis in CsPbI3 devices, despite the absence of the organic polar molecule previously thought to be a candidate for inducing hysteresis via ferroelectric polarisation. Due to its space group, CsPbI3 cannot be a ferroelectric material, and thus we can conclude that ferroelectricity is not required to explain current–voltage hysteresis in perovskite solar cells. Our report of working inorganic perovskite solar cells paves the way for further developments likely to lead to much more thermally stable perovskite solar cells and other optoelectronic devices.

/pdf/inorganic-caesium-lead-iodide-perovskite-solar-cells-32k4we6hl7.pdf

Inorganic caesium lead iodide perovskite solar cells

Organic-inorganic hybrid perovskites have cemented their position as an exceptional class of optoelectronic materials thanks to record photovoltaic efficiencies of 22.1%, as well as promising demonstrations of light-emitting diodes, lasers, and light-emitting transistors. Perovskite materials with photoluminescence quantum yields close to 100% and perovskite light-emitting diodes with external quantum efficiencies of 8% and current efficiencies of 43 cd A(-1) have been achieved. Although perovskite light-emitting devices are yet to become industrially relevant, in merely two years these devices have achieved the brightness and efficiencies that organic light-emitting diodes accomplished in two decades. Further advances will rely decisively on the multitude of compositional, structural variants that enable the formation of lower-dimensionality layered and three-dimensional perovskites, nanostructures, charge-transport materials, and device processing with architectural innovations. Here, the rapid advancements in perovskite light-emitting devices and lasers are reviewed. The key challenges in materials development, device fabrication, operational stability are addressed, and an outlook is presented that will address market viability of perovskite light-emitting devices.

/pdf/perovskite-materials-for-light-emitting-diodes-and-lasers-4it32b32ij.pdf

Perovskite Materials for Light-Emitting Diodes and Lasers.

In recent years, there has been an unprecedented rise in the performance of metal halide perovskite solar cells. They are now in a position to compete on performance with traditional crystalline solar cells, and as such the most pressing questions concern the long term operational stability of this class of solar cell. Here, recent developments in understanding and overcoming stability concerns of metal halide perovskite solar cells are highlighted. An overview of possible instability issues due to electrical, atmospheric, heat, and light stresses is provided and the different implications to the most commonly used device architectures are discussed.

Stability of Metal Halide Perovskite Solar Cells

High-performance perovskite solar cells (PSCs) based on organometal halide perovskite have emerged in the past five years as excellent devices for harvesting solar energy. Some remaining challenges should be resolved to continue the momentum in their development. The photocurrent density–voltage (J−V) responses of the PSCs demonstrate anomalous dependence on the voltage scan direction/rate/range, voltage conditioning history, and device configuration. The hysteretic J–V behavior presents a challenge for determining the accurate power conversion efficiency of the PSCs. Here, we review the recent progress on the investigation of the origin(s) of J–V hysteresis behavior in PSCs. We discuss the impact of slow transient capacitive current, trapping and detrapping process, ion migrations, and ferroelectric polarization on the hysteresis behavior. The remaining issues and future research required toward the understanding of J–V hysteresis in PSCs will also be discussed.

Origin of J–V Hysteresis in Perovskite Solar Cells

During the past 6 years, perovskite solar cells have experienced a rapid development and shown great potential as the next-generation photovoltaics. For the perovskite solar cells with regular structure (n-i-p structure), device efficiency has reached over 20% after the intense efforts of researchers from all over the world. Recently, perovskite solar cells with the inverted structure (p-i-n structure) have been becoming more and more attractive, owing to their easy-fabrication, cost-effectiveness, and suppressed hysteresis characteristics. Some recent progress in their device performance and stability has indicated their promising future. Here, recent developments and future perspectives about inverted perovskite solar cells are reviewed. Interface engineering, film morphology control, device stability, hysteresis phenomena and other research hotspots are discussed to present the roadmap for the development of inverted perovskite solar cells.

Inverted Perovskite Solar Cells: Progresses and Perspectives

Low-temperature solution-processed organic–inorganic halide perovskites have drawn worldwide attention due to their remarkable performance in photovoltaic devices. [ 1–11 ] The high energy conversion effi ciency is attributed to their long carrier lifetimes [ 12–15 ] and high photoluminescence effi ciencies [ 15 ] of the hybrid perovskites. Recently, the materials also demonstrate interesting light-emitting properties. They exhibit amplifi ed spontaneous light emission with low thresholds, [ 16 ] optically pump lasing, [ 17 ] and bright electroluminescence (EL) in perovskite light-emitting diodes (LEDs). [ 18,19 ]

/pdf/organic-inorganic-perovskite-light-emitting-electrochemical-4t4epgsqgo.pdf

Organic–Inorganic Perovskite Light‐Emitting Electrochemical Cells with a Large Capacitance

The accumulation of mobile ions causes space charge at interfaces in perovskite solar cells. There is a slow dynamic process of ion redistribution when the bias is changed. The interface charge affects band bending and thus the photocurrent of the solar cells. Consequently the dynamic process of the interface charge governs the current-voltage hysteresis. Very low interface charge density leads to hysteresis-free devices.

Dynamic interface charge governing the current–voltage hysteresis in perovskite solar cells

The accumulated ionic interface charge (IIC) affects the performance of perovskite solar cells significantly. In this study, we establish a model to describe the formation of capacitance. Mobile ions in perovskite accumulate at the interface and form a plate capacitor with screen charge on the electrodes. Reducing the roughness of perovskite or covering with a thick electron transporting layer significantly regulates the ionic capacitance. The results indicate that mobile ions exist in the perovskite layer, the accumulated IIC density is determined by the thickness of the carrier transport layer, and the capacitor area is affected by interface roughness.

http://iopscience.iop.org/article/10.7567/JJAP.56.090305/pdf

Effects of surface morphology on the ionic capacitance and performance of perovskite solar cells

Organic–inorganic halide perovskites are currently generating extensive interest for applications in solar cells. The perovskite morphology and composition have significant roles in solar cells. Impure phases, which will influence the performance of solar cells, are inevitably present in the film of perovskite. We found that another MAI deposition on the previous perovskite could ameliorate the film. The post-deposited MAI participates in the reconstruction of the perovskite, leading to reduced amount of impure phase, increased grain size, increased absorption and significantly improved power conversion efficiency. The results demonstrate a treatment approach to fabricate efficient planar heterojunction perovskite solar cells.

Hong Liu

Papers

Organic–Inorganic Perovskite Light‐Emitting Electrochemical Cells with a Large Capacitance

Dynamic interface charge governing the current–voltage hysteresis in perovskite solar cells

Effects of surface morphology on the ionic capacitance and performance of perovskite solar cells

CH3NH3I post-treatment improves the performance of perovskite solar cells via eliminating the impure phases

Solid Electrolytes: Organic–Inorganic Perovskite Light‐Emitting Electrochemical Cells with a Large Capacitance (Adv. Funct. Mater. 46/2015)