Custom Chemistry Improves Solar Cells

In the ongoing push to advance solar cells based on low-cost materials with a perovskite crystal structure, researchers at South Korea’s Korea Research Institute of Chemical Technology (KRICT) report a strategy for making use of an electronically promising but chemically unstable light-absorbing compound. Most studies in this fast-paced research area have focused on methylammonium lead iodide, (CH₃NH₃)PbI₃. Using a related compound, formamidinium lead iodide, HC(NH₂)₂PbI₃, could increase the range of solar radiation absorbed by a photovoltaic cell and thereby improve its performance. But that compound undergoes a spontaneous, undesirable phase transition from a black perovskite to a yellow nonperovskite that does not absorb as broadly and exhibits inferior charge-transport capabilities. KRICT’s Nam Joong Jeon, Jun Hong Noh, Sang Il Seok, and coworkers now show that the formamidinium compound can be used to make stable solar cells by blending it with small amounts of methylammonium lead bromide (MAPbBr₃) in select molar ratios (Nature 2015, DOI: 10.1038/nature14133). An unoptimized cell made with a MAPbBr₃ mole fraction of 0.15 resulted in an 18.5% efficient solar cell. That value comes close to the 20.1% efficient record-holding perovskite solar cell, which also was made at KRICT.