Trend of Perovskite Solar Cells: Dig Deeper to Build Higher.

U 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. Organic− inorganic hybrid halide perovskites (e.g., CH3NH3PbI3 or MAPbI3) 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. 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 MAPbI3. The superior PV properties of MAPbI3, 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) lifetime of the MAPbI3 film, so that it is substantially longer than that of the mixed halide MAPbI3−xClx. This result further confirms the importance of controlling the perovskite morphology. Cl has been extensively used to make high-quality MAPbI3 films, although where and how much (x) Cl exists still lacks sufficient evidence. Snaith (C4.05) showed that Cl not only improves the quality of MAPbI3, by an increase of grain size, but also exists in the lattice of the perovskite by demonstrating the blue-shifted luminescence and band gap assigned to MAPbI3−xClx. AR-XPS analysis demonstrated that Cl is preferentially located near the TiO2/MAPbI3 interface (Pellegrino, C7.03). One interesting area related to material preparation is to understand the role of perovskite grain size (or grain boundaries). Many groups are able to produce >17% PSCs using polycrystalline perovskite films based on micrometeror submicrometer-sized grains; however, a clear trend by various groups is to increase grain size, as mentioned above. The general assumption is that grain boundaries in polycrystalline perovskite films could host deep-level defects, leading to nonradiative recombination (NRR) loss in a solar cell. Along this direction, Nie (C1.07) presented an interesting solution approach (hot casting) to make millimeter-sized crystal grains, which is further shown to be necessary for achieving >17% efficiency in their study. However, more characterizations were requested by the audience to verify the single-crystalline nature of the apparent grains, as well as to understand the relatively low photovoltage, lower than what is reported by others using smaller perovskite grains. Regarding the role of grain boundaries, Snaith (C4.05) presented the use of confocal fluorescence microscopy coupled with scanning electron microscopy to spatially resolve the PL decay dynamics in the mixed halide perovskite MAPbI3−xClx. The results show that grain boundaries display faster NRR. Similarly, Xiao (C11.07) pointed out NRR at grain boundaries in MAPbI3 films. In addition, electron-beam-induced current and electron energy loss spectroscopy mapping were also shown to be useful to examine grain boundaries. Clearly, more studies with high spatial resolution are necessary to understand the role of grain boundaries (Xiao C11.07). In this regard, the focused ion beam can be a viable method for preparing specimens for highresolution transmission electron microscopy studies, as demonstrated by several groups (e.g., Padture C5.03, Park C6.01, Cheng C7.01, Xiao C11.07). Perovskite Composition Control with Band Gap Tuning. In addition to the standard MAPbI3 perovskite composition, researchers are showing increased interest in perovskites based on HC(NH2)2 (or FA) or the mixture of FA and MA (e.g., Graẗzel C1.01, Seok C1.05, Etgar C1.06, Dalal C2.01, Snaith C4.05, Park C6.01, Zhao C11.03, Bein C11.08, Ruiz C11.05, Wozny C12.05). The advantages of using the FA-based perovskite include higher photocurrent density (due mainly to the smaller optical band gap of FA-based perovskites) and