Trend of Perovskite Solar Cells: Dig Deeper to Build Higher

Trend of Perovskite Solar Cells: Dig Deeper to Build Higher U sing solar energy efficiently to produce electricity has been the subject of intensive research because of the rapid increase of global energy demand and the need to reduce the emission of greenhouse gases from electricity generation by fossil fuels.Breakthroughs are needed to produce low-cost, high-efficiency solar cells with good durability.Organicinorganic hybrid halide perovskites (e.g., CH 3 NH 3 PbI 3 or MAPbI 3 ) have rapidly become a focal point of the photovoltaic (PV) community as a promising next-generation PV technology.The certified efficiency of a single-junction perovskite solar cell (PSC) has reached 20.1% after only a few years of active research.Despite this remarkable progress, many fundamental questions still remain that need to be addressed at both the material and device levels.During the 2015 Material Research Society (MRS) Spring Meeting (April 6-10, 2015, San Francisco, CA), hundreds of scientists and engineers gathered at Symposium C-Perovskite Solar Cellsto discuss recent progress, challenges, and future directions for PSCs. 1 This symposium, which lasted for 4 days (April 7-10), hosted about 75 talks and 75 posters.Four posters were selected for the Symposium's Best Poster Awards (Ahn C3.04, Kaltenbrumer C3.25, Yu C10.10, and Bokdam C10.33).The enormous success of this symposium is reflecting the growing number of research groups interested in PSCs, clearly highlighting the consolidation of this new technology.In this Guest Commentary, we summarizebased on the presentations and discussions at the symposiumour views on the recent trend and various issues that researchers have been studying to continue the successful development of PSCs.Materials Preparation and Characterization.The majority of the talks and posters focused on the standard perovskite MAPbI 3 .The superior PV properties of MAPbI 3 , summarized in a theory-based talk (Yan C5.06), are mainly attributed to the combination of direct-band-gap p-p transitions enabled by the Pb lone pair s orbitals and perovskite symmetry, high ionicity, large lattice constant, and strong antibonding coupling between Pb lone pair s and I p orbitals.Significant effort was devoted to improving synthetic controls (for both solution and vaporphase deposition) for growing high-quality perovskite films, leading to >17% efficiencies from many groups.Despite the rapid progress in making high-quality perovskite films, the perovskite growth mechanisms, especially using solution deposition, are still not fully understood.Cheng (C7.01),Zhou (C7.05), and others have suggested that controlling the nucleation and grain-growth processes is critical for preparing a uniform, compact perovskite thin film.Specifically, accelerating the nucleation rate with respect to the growth rate via adjusting processing conditions (e.g., second-solvent dripping or bathing) represents a good approach for making continuous pinhole-free perovskite thin films.Talks by Hillhouse (C11.04) and Xiao (C11.07)demonstrated that proper selection of postgrowth atmospheres could lead to much improved perovskite film quality (e.g., grain size and carrier lifetime).In particular, the vapor-equilibrated regrowth treatment (Hillhouse C11.04) is shown to dramatically improve the photoluminescence (PL)