A research team from Los Alamos National Laboratory (LANL) has created large grain perovskite photovoltaics that have power-conversion efficiencies of up to 18%. The films are defect-free, have high crystallinity and, significantly, show hysteresis-free device operation.
Perovskite photovoltaics are organic–inorganic hybrids such as CH3NH3PbX3—where X is a halide—that are in the perovskite crystal phase. They are more attractive than the conventional silicon-based solar cells due to their cost-effective fabrication and high power-conversion efficiency. However, they have issues of stability and reproducibility and exhibit persistent hysteresis during device operation. Therefore, the article published in the January 30 issue of Science (DOI: 10.1126/science.aaa0472; p. 522), by lead authors Aditya Mohite and Hsing-Lin Wang and colleagues at LANL, represents a significant development in moving toward cheap, commercially viable perovskite photovoltaics.
To create a uniform perovskite film, an equimolar solution of PbI2 and methylamine hydrochloride were dissolved in a high-boiling-point solvent such as N,N-dimethylformamide (DMF), heated to 70°C and cast onto a substrate maintained at 180°C. This was then spin-coated for 15 s. Using this “hot casting” process, the team was able to attain grain sizes as large as 1–2 mm with the required composition and morphology. (The average conventional grain size is 1–2 µm.) A high-boiling-point solvent such as DMF allows excess solvent to be present during substrate heating, allowing for the growth of larger grain sizes. This method may be industrially scalable for the production of large-area crystalline thin films. The solution composition, choice of solvent, and substrate temperature were optimized for best performance.
The highest power-conversion efficiency was observed for grain sizes of 180 µm. Large grain sizes reduce the interfacial area between grains, reducing charge trapping and hysteresis during device performance. They further have low defect concentrations allowing for unimpeded movement of charge carriers for longer durations. “The optical and transport properties of these perovskites are comparable to that of the state-of-the-art high-quality direct bandgap semiconducting materials like GaAs,” says Mohite. The researchers further tested these arguments with optoelectronic simulations that confirmed the high crystalline quality of the large grain perovskites. The device modeling used drift and diffusion equations to fit the experimentally observed current–voltage curves under solar cell operation to extract charge carrier mobility.
(a) Optical image of large grain-size organometallic perovskite film; (b) current–voltage sweep of a typical large grain perovskite solar cell showing no hysteresis. Credit: Wanyi Nie, Hshinhan Tsai, and Aditya Mohite.
The short-circuit current density, the open-circuit voltage, and the fill factor measured for the device are close to the upper limit predicted by Shockley-Queisser, demonstrating reduced recombination of photogenerated charge carriers. Henry Snaith of the University of Oxford, who was not associated with this work, said that “this is a very interesting article which demonstrates that perovskite solar cells could evolve toward a multi-crystalline morphology typically seen in silicon solar cells. This approach could ultimately lead to very high efficiencies due to elimination of losses at grain boundaries.” Due to innovative fabrication, approaches such as these perovskite solar cells may one day lead to high-efficiency, low-cost solar modules.
