Two-step thermal annealing improves the morphology of spin-coated films for highly efficient perovskite hybrid photovoltaics

In this paper, we describe relationships between the morphologies and the power conversion efficiencies (PCE) of perovskite photovoltaics having a conventional p-i-n heterojunction structure, indium tin oxide (ITO)/poly(3,4-ethylenedioxythiophene): polystyrenesulfonate (PEDOT:PSS)/CH3NH3PbI3-xClx/PC61BM/Al. The PCE of such a device is highly dependent on the morphology of the perovskite film, which is governed by the concentrations of its precursors and the annealing conditions. A two-step annealing process allowed sufficient crystallization of the perovskite material, with a high coverage at a high precursor concentration. Relative to the device prepared using a one-step process (90 degrees C for 30 min), we observed a 60% increase in PCE for this optimized device. The corresponding devices exhibited extremely high stability after long-term storage (> 1368 h) in the dark in a N-2-filled glove box, with consistently high PCEs (AM 1.5G, 100 mW cm(-2)) of up to 9.1%.