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

Abstract

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)/CH₃NH₃PbI_3−xCl_x/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 °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 N₂-filled glove box, with consistently high PCEs (AM 1.5G, 100 mW cm⁻²) of up to 9.1%.