Perovskite solar cells (PSCs) are the rising star in the photovoltaic (PV) industry. However, the best performing PSCs contain Pb in a toxic level that may restrict its universal application. Currently, tin-based perovskite solar cells (Sn-PSCs) are the most promising alternatives to lead-based derivatives. Nonetheless, easy oxidation of Sn (II) to Sn (IV) and fast crystallization are the most notorious properties that have limited their performance. Various additives have been applied to address the challenge of tin oxidation and to control film growth dynamics. To date, most of the high performing Sn-PSCs have PEDOT:PSS as the hole transporting layer (HTL). Although PEDOT:PSS is the most common HTL in Sn-PSCs, it is also known to be harmful for the stability of PV devices. It is not well understood why PEDOT:PSS is so successful as a HTL in Sn-PSCs, particularly considering the highest performing Pb-based PSCs use HTLs other than PEDOT:PSS. We hypothesize that the Lewis acid-base interactions between the sulfonate groups from PSS with SnI<sub>2</sub> in the perovskite precursor solution controls the morphology and helps to reduce the concentration of defect states. We find that a thin layer of PSS on different HTL polymers helps the PV performance. We also find that the sulfonate group facilitates oxidation of Sn in the precursor solution. These results suggest that we should reconsider using PEDOT:PSS as a HTL for Sn-PSCs because of these counteractive properties.

Mixed roles of poly(styrene sulfonate) in tin-perovskite solar cells

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