Hyunho Lee1 Changhee Lee1

1, Seoul National University, Seoul, , Korea (the Republic of)

The short lifetime of perovskite solar cells (PSCs) has emerged as an urgent issue which needs to be resolved for practical applications. While low reactive energy between lead halide and methyl ammonium halide allows facile formation of perovskite structure, it leads to easy deterioration of perovskite structure under an ambient condition. The structural weakness of perovskite films to humidity or oxygen has been widely studied. It was reported that enhanced lifetime of PSCs can be achieved with compositional variation by introducing cesium or formamidinium to perovskite layer. However, its origin is not well understood. The efficient PSCs are composed of n-type and p-type adjacent buffer layers to enhance charge extraction from perovskite layer. Hence, it is essential to study the interface degradation mechanism between perovskite layer and buffer layer or buffer layer and electrode. Here, we will suggest interface degradation mechanism of inverted PSCs by observing the decay of photovoltaic properties around 50 devices over 1000 hours. At days 10 (240 hours), 20 (480 hours), 30 (720 hours), and 40 (960 hours) after the fabrication of cells, we performed Ag electrode restoration process by peeling-off Ag electrode and re-evaporate Ag electrode. This process results in the power conversion efficiency recovery with interesting variation of photovoltaic parameters. We found direct evidence of perovskite film degradation by time of flight secondary ion mass spectrometry (TOF-SIMS). The iodide ions were diffused from deteriorated perovskite film and accumulated under Ag electrode, resulting in device degradation. The diffusion of iodide ions had thermo-dynamical reaction with fullerenes which induced n-doping effect of [6,6]-phenyl C71 butyric acid methyl ester (PCBM). We analyzed PCBM-halide radical in depth by low temperature measurement (200K ~ 300K). This analysis supports the disorder model for the open circuit voltage increase. Finally, long-term degradation mechanism of inverted PSCs is proposed.