Perovskite Solar Cells Shine in the "Valley of the Sun"
"Perovskite Based Photovoltaics and Optoelectronic Devices" (Symposium EP3) was one of the most active symposiums in the Material Research Society (MRS) Spring Meeting 2016 (March 28 April 1) held in Phoenix, Arizonathe "Valley of the Sun" A large number of scientists and engineers from academia and industry met together to discuss the progress of perovskite solar cells (PSCs) and the understanding of the critical science related to PSCs. This symposium included 18 invited talks, 67 contributing talks, 130 posters, and more than 300 participants. These numbers clearly indicate that the surge in research interest continues in this emerging solar cell technology. Just a couple of months before this symposium, a new power conversion efficiency (PCE) record of 22.1% was released by Sang Il Seok's group (KRICT/UNIST, South Korea) Such a swift rise of PSC efficiency within only 6 years has been unprecedented in the history of photovoltaic (PV) technology. Of course, several challenges remain (for example, scaled up fabrication, stability, toxicity, and hysteresis) before PSCs can successfully enter the PV market. However, the perceived huge potential of this future lab to market transition has stimulated myriad efforts from both the scientific community and industry. This symposium reported some of the recent achievements in the context of addressing these challenges. Is it the time to consider scaling up PSCs? The answer is a resounding "Yes" as evinced by many exciting talks from different groups worldwide. Trystan M. Watson (SPECIFIC, Swansea University, United Kingdom) discussed a roadmap for commercial scale PSC production from laboratory hero devices, to print trials, to continuous module fabrication, and finally, into large scale production. A photograph of a flexible printed PSC fabricated based on the low cost metal foil from Watson's group is shown in Figure 1A. Watson noted the following potential bottleneck in the process of continuous (roll to roll) perovskite thin film fabrication: in the conventional "one step" or "two step" processing of halide perovskites, a prolonged duration of heat treatment is usually required to form highpurity perovskite films from the precursor phases. Considering a roll to roll process rate of 10 m/min, a 90 min heating step will require an oven that is 900 m long. This heat treatment issue was also discussed by Doojin Vak (CSIRO, Australia) who has been working on the protocol and instrumental development of roll to roll fabrication of perovskite thin films on flexible plastic substrates. Instead of using one step annealing, Vak and co workers engineered the microstructure of the first printed PbI2 precursor film and subsequently printed the organic halide precursor film to form the CH3NH3PbI3 (MAPbI3) or HC(NH2)2PbI3 (FAPbI3) perovskite rapidly, as shown in Figure 1B. It is apparent that lessons learned from research on spin coating, the most popular film processing method in the laboratory, cannot be easily transferred to slotdie coating trials. This is because rapid solvent evaporation simultaneously occurs during the precursor solution spincoating process, which is different from the case of the slotdie coating. This gap needs to be closed by researchers in order to advance the processing science that is suitable for large scale continuous PSC fabrication. Instead of focusing on the roll toroll fabrication of PSCs, talks by Bin Fan (Weihua Solar Company, China) and Hongwei Han (Huazhong University of Science and Technology, China) showed that carbon electrodebased hole conductor free PSC modules on rigid glass substrates based on batch processing have more promise in the nearest term for PSC commercialization. Both Han and Fan demonstrated large PSC panels (Figure 1C and D) with encouraging power conversion capability and outdoor stability. In particular, PSCs based on carbon based triple layer mesoscopic architecture invented by Han's group have already demonstrated outstanding stability in outdoor tests in a hot desert climate and in indoor long term light soaking as well as heat exposure for 3 months at 80 85 degC in an earlier report. Although the scale up of solution fabrication of perovskite thin films has been shown, it remains a challenge to form continuous, smooth, high purity perovskite thin films at large scale, which is associated with difficulty in controlling the crystallization behavior of perovskites at different scales. In this context, Nitin P. Padture (Brown University, United States) discussed the solution evolution of hybrid perovskites from a fundamental material science perspective, including classical nucleation and growth theory as well as the coarsening mechanism. Padture and his co workers showed that using (anti)solvent solvent extraction to induce the nucleation of perovskites at room temperature is a feasible way to form highly smooth perovskite thin films rapidly. This can be attributed to perovskite nucleation and growth occurring in the red band region in Figure 2, where the growth rate is suppressed and the nucleation rate approaches the maxima. In contrast, using conventional annealing with elevated temperature, the growth dominates the crystallization process, as shown in the greenband region in Figure 2, and the resultant perovskite film usually contains large crystal domains with pinholes. Interestingly, Padture, Yabing Qi (Okinawa Institute of Science and Technology, Japan) and Yixin Zhao (Shanghai Jiao Tong University, China) all reported use of methylamine (CH3NH2) gas for rapid (in seconds) processing of ultrasmooth perovskite thin films at room temperature, where the aforementioned nucleation and growth theory may be in play. It is worth noting that the kinetics of the perovskite crystallization process in Figure 2 are based on the simplest assumption that the perovskite evolves directly from its precursor solution, which may not be the exact case in many real situations. For example,