Elucidating Diiodomethane-Induced Improvement in Photonically Cured MAPbI(3) Solar Cells

Diiodomethane (CH2I2) has been reported to improve the photonically cured perovskite solar cell (PSC) device performance by reducing pinholes. Here, we show that even for pinhole-free methylammonium lead iodide (MAPbI(3)) made by photonic curing, adding CH2I2 promotes significant performance improvement. We elucidate the mechanisms behind the observed improvement. In addition to current density versus voltage measurements, we perform a wide variety of characterizations to compare the crystallinity, grain structure, optical, chemical, and electrical properties of the photonically cured samples with and without CH2I2 in the MAPbI(3) layer to those of thermally annealed devices. The addition of CH2I2 promotes grain growth in the vertical direction, increases the I/Pb ratio in the final film, and removes the I- ionic diffusion and defect signature associated with iodine interstitials. As a result, we achieve a champion efficiency of 15.04% with a MAPbI(3) conversion time of 20 ms, comparable to PSCs that have the MAPbI(3) layer thermally annealed for 10 min. Understanding the mechanisms behind additive-induced improvements for nonthermal annealing processes is critical to enabling high-speed PSC manufacturing.