The recent success of organometallic halide perovskites (OHPs) in photovoltaic devices has triggered lots of corresponding research and many perovskite analogues have been developed to look for devices with comparable performance but better stability. Upon the preparation of all inorganic halide perovskite nanocrystals (IHP NCs), research activities have soared due to their better stability, ultrahigh photoluminescence quantum yield (PL QY), and composition dependent luminescence covering the whole visible region with narrow line-width. They are expected to be promising materials for next generation lighting and display, and many other applications. Within two years, a lot of interesting results have been observed. Here, the synthesis of IHPs is reviewed, and their progresses in optoelectronic devices and optical applications, such as light-emitting diodes (LEDs), photodetectors (PDs), solar cells (SCs), and lasing, is presented. Information and recent understanding of their crystal structures and morphology modulations are addressed. Finally, a brief outlook is given, highlighting the presently main problems and their possible solutions and future development directions.

All Inorganic Halide Perovskites Nanosystem: Synthesis, Structural Features, Optical Properties and Optoelectronic Applications.

Despite the exciting progress on power conversion efficiencies, the commercialization of the emerging lead (Pb) halide perovskite solar cell technology still faces significant challenges, one of which is the inclusion of toxic Pb. Searching for Pb-free perovskite solar cell absorbers is currently an attractive research direction. The approaches used for and the consequences of Pb replacement are reviewed herein. Reviews on the theoretical understanding of the electronic, optical, and defect properties of Pb and Pb-free halide perovskites and perovskite derivatives are provided, as well as the experimental results available in the literature. The theoretical understanding explains well why Pb halide perovskites exhibit superior photovoltaic properties, but Pb-free perovskites and perovskite derivatives do not.

From Lead Halide Perovskites to Lead-Free Metal Halide Perovskites and Perovskite Derivatives

Searching for promising nontoxic and air-stable perovskite absorbers for solar cell applications has drawn extensive attention. Here, we show that a promising perovskite absorber should exhibit a high electronic dimensionality. Semiconductors that exhibit a high structural dimensionality, but a low electronic dimensionality have less promise as an absorber, because of barriers to isotropic current flow, enhanced electron/hole effective masses and fundamentally deeper defect states (more effective at causing recombination). Our concept accounts for the device performance of the perovskite-based solar cells reported in literature so far.

Searching for promising new perovskite-based photovoltaic absorbers: the importance of electronic dimensionality

Inorganic halide perovskites (IHPs) have recently attracted huge attention in the field of optoelectronics. IHPs are generally expected to exhibit superior chemical stability over the prevailing hybrid organic–inorganic perovskites that are widely used in optoelectronic devices such as solar cells and light-emitting devices. This is primarily owing to the elimination of weakly-bonded organic components in the IHP crystal structure. Nevertheless, many recent studies have revealed that IHPs still suffer significant issues in chemical instability, and thus, a lot of effort has been made towards the stabilization of IHPs for high-performance devices. In this context, a great deal of interest in the chemistry and perovskite community has been emerging to understand the chemical (in)stability of IHPs and develop engineering strategies for making more robust perovskite devices. This review will summarize the past research progress in this direction, give insights into the IHP (in)stability, and provide perspectives for the future effort in making stable IHP materials and devices.

Chemical stability and instability of inorganic halide perovskites

APbI3−xBrx perovskite solar cells with the bandgap of ∼1.5–1.6 eV, where A represents caesium, methylammonium, formamidinium and mixtures thereof, currently present certified efficiencies very close to those of established thin film technologies (such as CIGS and CdTe) and are thus one of the most important optoelectronics. To restrict the use of lead, as well as to tune the band gap of the material close to the optimum according to the Shockley–Queisser limit (being 1.34 eV), substitution (total or partial) of Pb2+ by Sn2+ should take place. In this review, we present results on single junction solar cells, utilizing CsSnI3−xBrx, CH3NH3SnI3−xBrx or NH2CHNH2SnI3−xBrx perovskites as absorbers, as well as a mixture of Sn2+/Pb2+ being adopted as the metal binary cation, reducing the bandgap to 1.2–1.4 eV. We also highlight very recently recorded efficiencies of perovskite-on-perovskite tandem solar cells, produced by the combination of the above low band gap materials with typical highly performing semi-transparent APbI3−xBrx perovskites of a higher band gap (close to 1.6–1.8 eV). We discuss these fascinating results, focusing on some key points such as, among others, the role of the tin compensator/reducing agent (usually SnF2) during perovskite crystallization. In addition, we present the critical challenges that currently limit the efficiency/stability of these systems and propose prospects for future directions.

A critical review on tin halide perovskite solar cells

From unstable CsSnI3 to air-stable Cs2SnI6: A lead-free perovskite solar cell light absorber with bandgap of 1.48 eV and high absorption coefficient

Lead-free and more air-stable perovskite Cs2SnI6 absorber with a direct bandgap of 1.48 eV is synthesized via a modified solution process. Different nanostructured ZnO nanorod arrays as electron transport layers and hole blocking layers are grown by controlling the seed layer and used to fabricate mesoscopic perovskite solar cells with Cs2SnI6 as light absorber layer. The influences of ZnO seed layers and nanorod morphology on the device photovoltaic performance were also investigated. With careful control of ZnO nanorod length and pore size to ensure high loading of the Cs2SnI6 absorber, we achieved power conversion efficiency of near 1%.

Lead-free mesoscopic Cs2SnI6perovskite solar cells using different nanostructured ZnO nanorods as electron transport layers

Organic–inorganic lead halide perovskite solar cells are potential alternatives to commercial silicon solar cells because of their attractive photon conversion efficiency and general material costs...

Zinc as a New Dopant for NiOx-Based Planar Perovskite Solar Cells with Stable Efficiency near 20%

The air and thermal stabilities of lead-free perovskite variant Cs2SnI6 powder

The performance of organolead halide perovskites based solar cells has been enhanced dramatically due to the morphology control of the perovskite films. In this paper, we present a fast solvent-assisted molecule inserting (S-AMI) strategy to grow high-quality perovskite film, in which the methylammonium iodide/2-propanol (MAI/IPA) solution is spin-coated onto a dimethylformamide (DMF) wetted mixed lead halide (PbX2) precursor film. The DMF can help the inserting of MAI molecules into the PbX2 precursor film and provide a solvent environment to help the grain growth of the perovskite film. The perovskite film grown by the S-AMI approach shows large and well-oriented grains and long carrier lifetime due to the reduced grain boundary. Solar cells constructed with these perovskite films yield an average efficiency over 17% along with a high average fill factor of 80%. Moreover, these unsealed solar cell devices exhibit good stability in an ambient atmosphere.

Yanan Jiang

Papers

From unstable CsSnI3 to air-stable Cs2SnI6: A lead-free perovskite solar cell light absorber with bandgap of 1.48 eV and high absorption coefficient

Lead-free mesoscopic Cs2SnI6perovskite solar cells using different nanostructured ZnO nanorods as electron transport layers

Zinc as a New Dopant for NiOx-Based Planar Perovskite Solar Cells with Stable Efficiency near 20%

The air and thermal stabilities of lead-free perovskite variant Cs2SnI6 powder

High-Quality Perovskite Films Grown with a Fast Solvent-Assisted Molecule Inserting Strategy for Highly Efficient and Stable Solar Cells.