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.

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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.

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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 ]

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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

Synergistic Enhancement of Electrocatalytic Nitrogen Reduction over Few-Layer MoSe2-Decorated Ti3C2Tx MXene

The grain boundaries of perovskite polycrystalline are regarded as a defect region that not only provides carrier recombination sites but also introduces device degradation pathways. Efforts to enlarging the grain size of a perovskite film and reducing its grain boundary are crucial for highly efficient and stable perovskite solar cells (PSCs). Some effective methods that facilitate grain growth are postdeposition thermal annealing and solvent vapor annealing. However, a detailed understanding of grain growth mechanisms in perovskite films is lacking. In this study, perovskite films were prepared by adding ethylamine hydrochloride (EACl) to the precursor solution. This additive strategy promotes a new grain growth mode, secondary grain growth, in perovskite films. Secondary grain growth leads to much larger grains with a high crystallographic orientation. These excellent properties lead to reduced grain boundaries and the densities of boundary defects. The improved film quality results in a prolonged charge-carrier lifetime and a significantly enhanced power conversion efficiency (PCE). Compared with the 18.42% PCE of the control device, the PCE of the device with EACl additives reaches 21.07%.

Secondary Grain Growth in Organic–Inorganic Perovskite Films with Ethylamine Hydrochloride Additives for Highly Efficient Solar Cells

In this work, organic–inorganic perovskite thin films with a mixed FA0.85MA0.15Pb(Br0.07I0.93)3 composition were prepared using a two-step sequential procedure. The preparation was manipulated by the addition of a small fraction of DMSO to the precursor of the second step. By adjusting the fraction of DMSO, the crystallisation and the PbI2 residue of the perovskite films can be finely controlled. The change in dissolution due to the addition of DMSO altered nucleation dynamics, and molecular intercalation occurred in the initial stage of the reaction, followed by a dissolution–crystallisation process of growth for the crystals. The approach used in this work can be easily controlled and has high reproducibility. The optimised perovskite solar cells prepared here displayed high performance, with the best device achieving a power conversion efficiency of 19.29% without hysteresis.

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Mengjie Sun

Papers

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

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

Synergistic Enhancement of Electrocatalytic Nitrogen Reduction over Few-Layer MoSe2-Decorated Ti3C2Tx MXene

Secondary Grain Growth in Organic–Inorganic Perovskite Films with Ethylamine Hydrochloride Additives for Highly Efficient Solar Cells

Tailoring a dynamic crystalline process during the conversion of lead-halide perovskite layer to achieve high performance solar cells