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The role of additives in spiro-OMeTAD as a hole transport material (HTM) has been found to be vitally important for achieving decent photovoltaic performance of perovskite solar cells (PSCs). Four spiro-OMeTAD recipes with controlled concentrations of additives are studied in the context of PSCs. It is found that a small change in the concentration of additives results in a huge difference in the overall power conversion efficiency, which stems from the distinct charge extraction process in PSCs. Furthermore, the effect of the third additive, a cobalt complex (FK102, Co(III)TFSI), on the carrier density and charge recombination in PSCs is extensively investigated. It is found that the presence of the cobalt additive can significantly improve the photovoltaic performance of PSCs via the suppression of charge recombination at the interface of the perovskite layer and the HTM layer, which leads to a more than doubled power conversion efficiency.
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