Solvent Engineering Improves Solar Cells

By devising a multisolvent method for producing highly uniform perovskite thin films, researchers in South Korea have come up with an improved way to make inexpensive high-performance solar cells (Nat. Mater. 2014, DOI: 10.1038/nmat4014). Perovskite-based solar cells, which feature a sunlight-absorbing organometal halide with the perovskite crystal structure, are layered devices typically made via spin-coating, a simple solution-phase film-deposition method. But that method yields perovskite films that don’t perfectly coat the underlying surface, thereby limiting light-to-electricity conversion efficiency. Vacuum deposition is an alternative method that boosts film quality and device performance, but it raises costs. Those challenges led Sang Il Seok and coworkers at Korea Research Institute of Chemical Technology to devise a solvent-processing method that yields more uniform films and leads to conversion efficiencies on par with the best vacuum-deposition perovskite cells reported to date. The team starts by spin-coating a solution of methyl ammonium halides and lead halides in a mixture of γ-butyrolactone and dimethylsulfoxide solvents. During the spinning process, the team adds a third solvent in which the perovskite-forming reagents are insoluble, such as toluene or chloroform, dropwise on the spreading layer. That extra step helps stabilize an intermediate phase, which the researchers heat to yield the desired uniform perovskite film.