The photovoltaics of organic−inorganic lead halide perovskite materials have shown rapid improvements in solar cell performance, surpassing the top efficiency of semiconductor compounds such as CdTe and CIGS (copper indium gallium selenide) used in solar cells in just about a decade. Perovskite preparation via simple and inexpensive solution processes demonstrates the immense potential of this thin-film solar cell technology to become a low-cost alternative to the presently commercially available photovoltaic technologies. Significant developments in almost all aspects of perovskite solar cells and discoveries of some fascinating properties of such hybrid perovskites have been made recently. This Review describes the fundamentals, recent research progress, present status, and our views on future prospects of perovskite-based photovoltaics, with discussions focused on strategies to improve both intrinsic and extrinsic (environmental) stabilities of high-efficiency devices. Strategies and challenges regarding compositional engineering of the hybrid perovskite structure are discussed, including potentials for developing all-inorganic and lead-free perovskite materials. Looking at the latest cutting-edge research, the prospects for perovskite-based photovoltaic and optoelectronic devices, including non-photovoltaic applications such as X-ray detectors and image sensing devices in industrialization, are described. In addition to the aforementioned major topics, we also review, as a background, our encounter with perovskite materials for the first solar cell application, which should inspire young researchers in chemistry and physics to identify and work on challenging interdisciplinary research problems through exchanges between academia and industry. CONTENTS 1. Discovery and Background of Perovskite Photovoltaics 3037 1.1.

Methylammonium iodo bismuthate ((CH3NH3)3Bi2I9) (MBI) perovskite is a promising alternative to rapidly progressing hybrid organic-inorganic lead perovskites because of its better stability and low toxicity compared to lead-based perovskites. Solution-processed perovskite fabricated by single-step spin-coating and subsequent heating produced polycrystalline films of hybrid perovskite (CH3NH3)3Bi2I9), whose morphology was influenced drastically by the nature of substrates. The optical measurements showed a strong absorption band around 500 nm. The devices made on anatase TiO2 mesoporous layer showed good performance with current density over 0.8 mA cm(-2) while the devices on brookite TiO2 layer and planar (free of porous layer) was inefficient. However, all the MBI devices were stable to ambient conditions for more than 10 weeks.

Although inorganic perovskite, CsPbI 3 , shows superior thermal stability over organic−inorganic hybrid perovskites, stabilization of the photoactive black phase (α-CsPbI 3 ) of CsPbI 3 perovskite at room temperature and in ambient conditions has remained a challenge. Herein, we present a method of stabilizing the α-CsPbI 3 at lower annealing temperature (85 °C) by incorporation of Eu 3+ (EuCl 3 ) into CsPbI 3 , which prevents the black to the yellow phase (δ-CsPbI 3 ) transformation in ambient air (room temperature) for a reasonably long time (>30 days). Photovoltaic performance of this Eu-stabilized α-CsPbI 3 , as assessed in planar heterojunction solar cells (FTO/TiO 2 /CsPbI 3 :xEu/spiro-OMeTAD/Au), shows a power conversion efficiency above 6% on backward scan (stabilized power output above 4%) for CsPbI 3 :xEu cells with 5−6 mol % of Eu, while CsPbI 3 without Eu, as expected, shows no photovoltaic property at all. However, as the cell stability was found to be affected by composition of organic hole transport material (HTM) (spiro-OMeTAD) and morphology of CsPbI 3 film, it is believed that optimization of cell composition and structure with a more suitable HTM will further improve the cell performance, as well as life.

Having demonstrated incredibly fast progress in power conversion efficiency, rising to a level comparable with that of crystalline silicon cells, lead‐based organic–inorganic hybrid perovskite solar cells are now facing the stability tests needed for industrialization. Poor thermal stability (<150 °C) owing to organic constituents and interlayer diffusion of materials (dopants), and environmental incompatibility due to Pb has surged the development of organic‐free, Pb‐free perovskites and dopant‐free hole transport materials (HTMs). The recent rapid increase in efficiency of cells based on inorganic perovskites, crossing 18%, demonstrates the great potential of inorganic perovskites as thermally stable and high‐efficiency cells. Although all kinds of Pb‐free perovskites lag in efficiency in comparison to the hybrid and inorganic perovskites, they also demonstrate better structural and environmental stability. The performance of dopant‐free HTMs matching/surpassing dopant‐containing HTMs makes the former a better choice for stability. Even though the efforts to enhance the stability of Pb‐based hybrid perovskites should continue by different techniques, organic‐free and lead‐free perovskites, and dopant‐free HTMs must be pursued with greater interest for the future. This review describes the present issues and possible strategies to address them, and thus will help to improve the overall performance of robust organic‐free, Pb‐free, and dopant‐free perovskite solar cells.

It is well known that the surface trap states and electronic disorders in the solution-processed CH NH PbI perovskite film affect the solar cell performance significantly and moisture sensitivity of photoactive perovskite material limits its practical applications. Herein, we show the surface modification of a perovskite film with a solution-processable hydrophobic polymer (poly(4-vinylpyridine), PVP), which passivates the undercoordinated lead (Pb) atoms (on the surface of perovskite) by its pyridine Lewis base side chains and thereby eliminates surface-trap states and non-radiative recombination. Moreover, it acts as an electron barrier between the perovskite and hole-transport layer (HTL) to reduce interfacial charge recombination, which led to improvement in open-circuit voltage (V ) by 120 to 160 mV whereas the standard cell fabricated in same conditions showed V as low as 0.9 V owing to dominating interfacial recombination processes. Consequently, the power conversion efficiency (PCE) increased by 3 to 5 % in the polymer-modified devices (PCE=15 %) with V more than 1.05 V and hysteresis-less J-V curves. Advantageously, hydrophobicity of the polymer chain was found to protect the perovskite surface from moisture and improved stability of the non-encapsulated cells, which retained their device performance up to 30 days of exposure to open atmosphere (50 % humidity).

Large datasets are now ubiquitous as technology enables higher-throughput experiments, but rarely can a research field truly benefit from the research data generated due to inconsistent formatting, undocumented storage or improper dissemination. Here we extract all the meaningful device data from peer-reviewed papers on metal-halide perovskite solar cells published so far and make them available in a database. We collect data from over 42,400 photovoltaic devices with up to 100 parameters per device. We then develop open-source and accessible procedures to analyse the data, providing examples of insights that can be gleaned from the analysis of a large dataset. The database, graphics and analysis tools are made available to the community and will continue to evolve as an open-source initiative. This approach of extensively capturing the progress of an entire field, including sorting, interactive exploration and graphical representation of the data, will be applicable to many fields in materials science, engineering and biosciences.

We modulated a solvent-mediated adduct for one-step crystallization of lead-free AgBi2I7 at a lower temperature (90 °C) and to obtain remnant BiI3 by controlling the nature of the substrate and precursor concentration.

Low toxicity and highly stable methylammonium bismuth iodide (MBI) ((CH 3 NH 3 ) 3 Bi 2 I 9 ) as a solutionprocessable photovoltaic absorber produces hexagonal non-uniform morphology leading to poor interfacial contacts with the electron and hole transporting layers. Herein, we tuned the morphology of MBI perovskite by bringing in a small amount of N-methyl-2-pyrrolidone (NMP) as a morphology controller into the MBI-DMF solution. The incorporation of various concentrations of NMP into the precursor solution was found to control the rate of crystallization. An optimal low concentration of 2.5% NMP added to the MBI-DMF precursor solution showed a 50% enhancement in short-circuit current (J sc ). The device showed power conversion efficiencies up to 0.31% with high reproducibility. Moreover, the devices were quite stable when exposed to an ambient atmosphere (relative humidity of 50-60%) for 30 days. † Electronic supplementary information (ESI) available: Experimental procedure, top surface SEM images, UV-vis, J-V curve are provided. See rsc.li/rsc-advances 9456 | RSC Adv., 2017, 7, 9456-9460 This journal isFig. 3 (a) Average J-V curves, (b) IPCE spectra, (c) J sc and (d) PCE histogram plot of devices containing MBI perovskite without and with different concentration of NMP. 9458 | RSC Adv., 2017, 7, 9456-9460 This journal is