Abstract
In this thesis, we report on an environmental stability study of mixed tin/ lead perovskite (PSK) solar cell materials. By systematically varying the cation ratios of Sn/Pb and formamidinium (FA)/ methylammonium (MA) in the PSK for the highest efficiency of solar cell based on p-i-n configuration of FTO/ PEDOT:PSS/ PSK/ C60/ BCP/Ag, an optimal composition of MA0.5FA0.5Sn0.25Pb0.75I3 was obtained with a power conversion efficiency of 11.7%. This MA0.5FA0.5Sn0.25Pb0.75I3 and the other two sole-cation PSK’s, MASn0.25Pb0.75I3 and FASn0.25Pb0.75I3, were then irradiated with AM1.5 light while exposed to three types of gaseous ambient for 6 h: dry O2, moist N2, and moist O2, with relative humidity controlled either below 10% or above 70% for dry and moist conditions, respectively. XPS results show that the light illumination in dry O2 condition leads to a large I 3d signal decrease and the formation of I2, believed to be due to superoxide formed from electron attachment reaction of O2 with electrons provided by the photosensitized PSK. Moreover, FA is found to be less reactive toward superoxide than MA, as judged from XPS N 1s signal. In contrast, under moist N2 condition, FA is found to degrade further than MA mostly through deprotonation reactions, presumably due to its higher affinity with water molecules via H-bonding. Sn+2 in PSK is easily oxidized into Sn+4 to yield SnO2 and other PSK of +4 oxidation state like MA2SnI6, whereas almost no chemical state change is found for Pb. In moist O2 environment, the oxidation of Sn seems to resemble that occurred in moist N2, suggesting that PSK is prone to the attack by H2O than O2. Taken together, the degradation of mixed Sn and Pb PSK proceeds through the loss of both types of cations as dictated by their respective chemistry, and the oxidation of metal cations.