Novel crystal growth technique developed for high efficiency perovskite solar cells

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Aditya Hohite and Wanyi Nie (Materials Synthesis and Integrated Devices, MPA-­‐11) perfect a crystal production technique to improve perovskite crystal production for solar cells.

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State-of-the-art photovoltaics – using high-purity, large-area, wafer-scale single-crystalline semiconductors grown by sophisticated, high temperature crystal-growth processes – offer promising routes for developing solar-based clean energy for the future. Solar cells composed of the recently discovered material organic-inorganic perovskites offer the efficiency of silicon, yet suffer from a variety of deficiencies limiting the commercial viability of perovskite photovoltaic technology. The deficiencies are instability, non-reproducibility, and hysteresis during device operation (possibly due to defect-assisted trapping). Los Alamos researchers and collaborators have developed a process to grow perovskite films that overcomes the technical challenges blocking commercial viability. The journal Science published their findings.

The team revealed a new solution-based hot-casting technique to grow continuous, pinhole-free thin films of organometallic perovskites that eliminates these limitations. The method enables the preparation of high-quality, large-area, millimeter-scale perovskite crystals and demonstrates that highly efficient and reproducible solar cells with reduced trap assisted recombination can be realized. The hot-casting technique enables the prolonged growth of the pervoskite crystal grain, yielding large crystalline grains. The team suggests that there are two primary benefits of growing crystals with large grain size: 1) The reduced interfacial area associated with large grains suppresses charge trapping and eliminates hysteresis, and 2) larger grains have lower bulk defects and higher mobility, allowing for the photogenerated carriers to propagate through the device without frequent encounters with defects and impurities. The crystalline quality of the pervoskite films is comparable with that of high-quality silicon and gallium arsenide semiconductors

The researchers fabricated planar solar cells with efficiencies approaching 18%, among the highest reported in the field of perovskite-based light-to-energy conversion devices. The cells demonstrate little cell-to-cell variability, resulting in devices showing hysteresis-free photovoltaic response. Photovoltaic hysteresis had been a fundamental bottleneck for stable operation of previous perovskite devices. The team’s characterization and modeling results attribute the improved performance to reduced bulk defects and improved cha.rge carrier mobility in large-grain devices.

The method offers promising routes to develop low-cost, solar-based clean global energy solutions for the future. The benefit is a solution-processible technology that can form the active material for solar cells using a relatively low-temperature process (100-200C). In contrast, silicon requires high temperature, expensive, sophisticated crystal growth techniques to produce the quality required to make efficient solar cells. The researchers anticipate that their solution-based crystal growth technique could lead to synthesis of wafer-scale crystalline perovskites necessary for the fabrication of high-efficiency solar cells. The method is applicable to several other material systems plagued by polydispersity, defects, and grain boundary recombination in solution-processed thin-films. The casting method is appropriate for both pure and mixed halide perovskite combinations and might lead to the production of industrially scalable large-area crystalline thin films made from other materials for solution-processed, large-area crystal growth.

Reference: “High-efficiency Solution-processed Perovskite Solar Cells with Millimeter-scale Grains,” Science 347, 522 (2015); doi: 10.1126/science.aaa0472. Authors include Wanyi Nie, Gautam Gupta, and Aditya D. Mohite (Materials Synthesis and Integrated Devices, MPA-11); Hsinhan Tsai, Jean-Christophe Blancon, Jared J. Crochet, and Hsing-Lin Wang (Physical Chemistry and Applied Spectroscopy, C-PCS); Amanda J. Neukirch and Sergei Tretiak (Physics and Chemistry of Materials, T-1); Reza Asadpour and Muhammad A. Alam (Purdue University); and Manish Chhowalla (Rutgers University).

The DOE Office of Basic Energy Sciences and a Laboratory Directed Research and Development (LDRD) project funded different aspects of the LANL work. This research was performed in part at the Center for Integrated Nanotechnologies (CINT), a DOE Office of Science User Facility. The work supports the Laboratory’s Energy Security mission area and Materials for the Future science pillar through the development of materials for high efficiency solar cells to generate electricity. Technical contact: Aditya Mohite

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Processing scheme for perovskite thin film using hot-­‐casting methods and observations for large-­‐area millimeter-­‐scale crystal grain formation for a perovskite (PbCH3NH3I3-­‐xClx) based thin film. (A) Hot-­‐casting scheme for large-­‐area crystal growth [ITO, indium tin oxide; FTO, fluorine-­‐doped tin oxide; PEDOT, poly(3,4-­‐   ethylenedioxythiophene) polystyrene sulfonate]. (B) Optical micrographs illustrating grain formation as a function of substrate temperature with the casting solution maintained at 70°C. (C) Large area grain formation using casting solvents with high boiling points (DMF, N,N-­‐dimethlformamide; NMP, N-­‐methyl-­‐2-­‐pyrrolidone). (D) Comparison of grain size as a function of processing temperature obtained for the hot-­‐casting and conventional post-­‐annealing methods.