Highest reported efficiency cesium lead halide perovskite solar cells are realized by tuning the bandgap and stabilizing the black perovskite phase at lower temperatures. CsPbI2Br is employed in a planar architecture device resulting in 9.8% power conversion efficiency and over 5% stabilized power output. Offering substantially enhanced thermal stability over their organic based counterparts, these results show that all-inorganic perovskites can represent a promising next step for photovoltaic materials.

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Bandgap-tunable cesium lead halide perovskites with high thermal stability for efficient solar cells

Solution-processed hybrid organic–inorganic perovskites (HOIPs) exhibit long electronic carrier diffusion lengths, high optical absorption coefficients and impressive photovoltaic device performance. Recent results allow us to compare and contrast HOIP charge-transport characteristics to those of III–V semiconductors — benchmarks of photovoltaic (and light-emitting and laser diode) performance. In this Review, we summarize what is known and unknown about charge transport in HOIPs, with particular emphasis on their advantages as photovoltaic materials. Experimental and theoretical findings are integrated into one narrative, in which we highlight the fundamental questions that need to be addressed regarding the charge-transport properties of these materials and suggest future research directions. The charge transport properties of hybrid organic—inorganic perovskites, which can explain their excellent photovoltaic performance, are reviewed through an integrated summary of experimental and theoretical findings. The potential origins of these properties are discussed and future research directions are indicated.

Hybrid organic—inorganic perovskites: low-cost semiconductors with intriguing charge-transport properties

Even though the mesoporous-type perovskite solar cell (PSC) is known for high efficiency, its planar-type counterpart exhibits lower efficiency and hysteretic response. Herein, we report success in suppressing hysteresis and record efficiency for planar-type devices using EDTA-complexed tin oxide (SnO2) electron-transport layer. The Fermi level of EDTA-complexed SnO2 is better matched with the conduction band of perovskite, leading to high open-circuit voltage. Its electron mobility is about three times larger than that of the SnO2. The record power conversion efficiency of planar-type PSCs with EDTA-complexed SnO2 increases to 21.60% (certified at 21.52% by Newport) with negligible hysteresis. Meanwhile, the low-temperature processed EDTA-complexed SnO2 enables 18.28% efficiency for a flexible device. Moreover, the unsealed PSCs with EDTA-complexed SnO2 degrade only by 8% exposed in an ambient atmosphere after 2880 h, and only by 14% after 120 h under irradiation at 100 mW cm−2. The development of high efficiency planar-type perovskite solar cell has been lagging behind the mesoporous-type counterpart. Here Yang et al. modify the oxide based electron transporting layer with organic acid and obtain planar-type cells with high certified efficiency of 21.5% and decent stability.

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High efficiency planar-type perovskite solar cells with negligible hysteresis using EDTA-complexed SnO2

Perovskite solar cells (PSCs) have attracted much attention because of their rapid rise to 22% efficiencies. Here, we review the rapid evolution of PSCs as they enter a new phase that could revolutionize the photovoltaic industry. In particular, we describe the properties that make perovskites so remarkable, and the current understanding of the PSC device physics, including the operation of state-of-the-art solar cells with efficiencies above 20%. The extraordinary progress of long-term stability is discussed and we provide an outlook on what the future of PSCs might soon bring the photovoltaic community. Some challenges remain in terms of reducing non-radiative recombination and increasing conductivity of the different device layers, and these will be discussed in depth in this review.

The rapid evolution of highly efficient perovskite solar cells

Metal halide perovskites represent a flourishing area of research, driven by both their potential application in photo-voltaics and optoelectronics, and for the fundamental science underpinning their unique optoelectronic properties. The advent of colloidal methods for the synthesis of halide perovskite nanocrystals has brought to the attention inter-esting aspects of this new type of materials, above all their defect-tolerance. This review aims to provide an updated survey of this fast-moving field, with a main focus on their colloidal synthesis. We examine the chemistry and the ca-pability of different colloidal synthetic routes to control the shape, size and optical properties of the resulting nano-crystals. We also provide an up to date overview of their post-synthesis transformations, and summarize the various so-lution processes aimed at fabricating halide perovskite-based nanocomposites. We then review the fundamental optical properties of halide perovskite nanocrystals, by focusing on their linear optical properties, on the effects of quantum confinement and, then, on the current knowledge of their exciton binding energies. We also discuss the emergence of non-linear phenomena such as multiphoton absorption, biexcitons and carrier multiplication. At last, we provide an outlook in the field, with the most cogent open questions and possible future directions.

Metal Halide Perovskite Nanocrystals: Synthesis, Post-Synthesis Modifications and Their Optical Properties

Two-inch-sized perovskite crystals, CH3 NH3 PbX3 (X=I, Br, Cl), with high crystalline quality are prepared by a solution-grown strategy. The availability of large perovskite crystals is expected to transform its broad applications in photovoltaics, optoelectronics, lasers, photodetectors, LEDs, etc., just as crystalline silicon has done in revolutionizing the modern electronics and photovoltaic industries.

Two-Inch-Sized Perovskite CH3NH3PbX3 (X = Cl, Br, I) Crystals: Growth and Characterization

Recently, lead halide-based perovskites have become one of the hottest topics in photovoltaic research because of their excellent optoelectronic properties. Among them, organic-inorganic hybrid perovskite solar cells (PSCs) have made very rapid progress with their power conversion efficiency (PCE) now at 23.7 %. However, the intrinsically unstable nature of these materials, particularly to moisture and heat, may be a problem for their long-term stability. Replacing the fragile organic group with more robust inorganic Cs+ cations forms the cesium lead halide system (CsPbX3 , X is halide) as all-inorganic perovskites which are much more thermally stable and often more stable to other factors. From the first report in 2015 to now, the PCE of CsPbX3 -based PSCs has abruptly increased from 2.9 % to 17.1 % with much enhanced stability. In this Review, we summarize the field up to now, propose solutions in terms of development bottlenecks, and attempt to boost further research in CsPbX3 PSCs.

All‐Inorganic CsPbX3 Perovskite Solar Cells: Progress and Prospects

As the black cesium lead iodide (CsPbI3) tends to transit into a yellow δ-phase at ambient, it is imperative to develop a stabilized black phase for photovoltaic applications. Herein, we report a distorted black CsPbI3 film by exploiting the synergistic effect of hydroiodic acid (HI) and phenylethylammonium iodide (PEAI) additives. It is found that the HI induces formation of hydrogen lead iodide (HPbI3+x), an intermediate to the distorted black phase with appropriate band gap of 1.69 eV; while PEAI provides nucleation for optimized crystallization. More importantly, it stabilizes the distorted black phase by hindering phase transition via its steric effects. Upon optimization, we have attained solar cell efficiency as high as 15.07%. Specifically, the bare cell without any encapsulation shows negligible efficiency loss after 300 h of light soaking. The device keeps 92% of its initial cell efficiency after being stored for 2 months under ambient conditions.

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All-inorganic cesium lead iodide perovskite solar cells with stabilized efficiency beyond 15

It is imperative to develop a large-aspect-ratio grain-based thin film with low trap density for high-performance inorganic perovskite CsPbI2Br solar cells. Herein, by using Mn2+ ion doping to modulate film growth, we achieved CsPbI2Br grains with aspect ratios as high as 8. It is found that Mn2+ ions insert into the interstices of the CsPbI2Br lattice during the growth process, leading to suppressed nucleation and a decreased growth rate. The combination aids in the achievement of larger CsPbI2Br crystalline grains for increased JSC values as high as 14.37 mA/cm2 and FFs as large as 80.0%. Moreover, excess Mn2+ ions passivate the grain boundary and surface defects, resulting in effectively decreased recombination loss with improved hole extraction efficiency, which enhances the built-in electric field and hence increases VOC to 1.172 V. As a result, the champion device achieves stabilized efficiency as high as 13.47%, improved by 13% compared with only 11.88% for the reference device.

Jingru Zhang

Papers

Two-Inch-Sized Perovskite CH3NH3PbX3 (X = Cl, Br, I) Crystals: Growth and Characterization

All‐Inorganic CsPbX3 Perovskite Solar Cells: Progress and Prospects

All-inorganic cesium lead iodide perovskite solar cells with stabilized efficiency beyond 15

Interstitial Mn2+-Driven High-Aspect-Ratio Grain Growth for Low-Trap-Density Microcrystalline Films for Record Efficiency CsPbI2Br Solar Cells

3D–2D–0D Interface Profiling for Record Efficiency All-Inorganic CsPbBrI2 Perovskite Solar Cells with Superior Stability