Effects of hydrazine compounds as additives on the characteristics of organic-inorganic hybrid lead-tin perovskite photovoltaic device

Lead-tin perovskite solar cells (Pb-Sn PSCs) are a more environmentally friendly alternative to the mainstream lead-based PSCs, but their development has been hampered by their instability due to the strong tendency of the Sn ²⁺ constituent to oxidize. This study examines the effects of three types of hydrazine compound—phenylhydrazine hydrochloride (PH•HCl), 4-fluorophenylhydrazine hydrochloride (FPH•HCl), and 1-[2,5-bis(trifluoromethyl)phenyl]hydrazine hydrochloride (BTFMPH•HCl)—as additives to Pb-Sn perovskite precursor solutions on the stability and performance of Pb-Sn PSC devices, taking advantages of the hydrazine functional group’s ability to suppress oxidation of Sn ²⁺, and of the hydrophobicity provided by the phenyl structure and/or fluorine (F) contents of the selected compounds. All of the three additives significantly improved the stability of the resultant Pb-Sn PSC devices, with the additives containing more F contents (BTFMPH•HCl > FPH•HCl > PH•HCl) showing slightly greater improvements. The PH•HCl additive markedly enhanced the power conversion efficiency (PCE) while reducing the hysteresis ratio of the PSC devices thanks to its multifaceted effects including improving morphology, reducing non-radiative losses, alleviating recombination defects, and enhancing charge-carrier mobility of the Pb-Sn perovskite layer, as elucidated by a series of photo-luminescence and electrical characterizations. Both the BTFMPH•HCl and FPH•HCl additives slightly degraded the device PCE due to their poorer resultant perovskite morphology, which was attributed to their lower affinity with the perovskite as a result of their F contents. Our results provide useful insights in selecting and designing hydrazine-based additives for PSC.