Abstract
The power conversion efficiency of perovskite solar cells (PSCs) has risen steadily in recent years; however, one important aspect of the puzzle remains to be solved-the long-term stability of the devices. We believe that understanding the underlying reasons for the observed instability and finding means to circumvent it is crucial for the future of this technology. Not only the perovskite itself but also other device components are susceptible to thermal degradation, including the materials comprising the hole-transporting layer. In particular, the performance-enhancing oxidized hole-transporting materials have attracted our attention as a potential weak component in the system. Therefore, we performed a series of experiments with oxidized spiro-OMeTAD to determine the stability of the material interfaced with five most popular perovskite compositions under thermal stress. It was found that oxidized spiro-OMeTAD is readily reduced to the neutral molecule upon interaction with all five perovskite compositions. Diffusion of iodide ions from the perovskite layer is the main cause for the reduction reaction which is greatly enhanced at elevated temperatures. The observed sensitivity of the oxidized spiro-OMeTAD to ion diffusion, especially at elevated temperatures, causes a decrease in the conductivity observed in the doped films of spiro-OMeTAD, and it also contributes significantly to a drop in the performance of PSCs operated under prolonged thermal stress.