Fabrication of high photovoltaic performance tin-perovskite film using greener material

Tin-perovskite solar cells (TPSCs) are among the most promising third-generation photovoltaic technologies. However, highly toxic solvents such as N,N-dimethylformamide (DMF) are generally required for fabricating high-quality tin-perovskite (TPsk) films, hindering their large-scale commercialization. In this work, we investigate less-toxic solvent systems for tin-perovskite precursors. Several solvent systems were examined, and only dimethylacetamide (DMAc)/dimethyl sulfoxide (DMSO) and ethanol (EtOH)/DMSO mixtures were found to enable the formation of continuous TPsk films, denoted as DMAc-TPsk and EtOH-TPsk, respectively. Inverted TPSCs based on DMAc-TPsk, EtOH-TPsk achieved maximum power conversion efficiencies (PCEs) of 9.34%, and 3.53%, respectively, compared with 8.56% for devices based on DMF-TPsk films prepared from a conventionally used DMF/DMSO precursor solution. The superior photovoltaic performance of DMAc-TPsk is attributed from its high crystallinity, large average grain size, fast carrier mobility, and favorable energy alignment with the PEDOT:PSS (Poly(3,4-ethylenedioxythiophene) polystyrene sulfonate) hole-transport layer. Several physicochemical parameters relevant to solvent dissolving ability-including density, dielectric constant, Kamlet-Taft solvent parameters, dipole moment, and donor number-of DMAc were found to be very similar to those of DMF. These similarities enable DMAc to serve as a viable replacement for highly toxic DMF in tin-perovskite precursor solutions. The poor photovoltaic performance of EtOH-TPsk may be due to the significantly different dissolving power of EtOH compared with commonly used solvents such as DMF or DMSO as well as its poorer affinity toward PEDOT:PSS film. Furthermore, dynamic light scattering (DLS) measurements revealed that the strong interaction between DMAc and perovskite precursors provides a "buffering effect" that retards the rapid desolvation process, resulting in a higher nucleation density and reduced grain-growth rate. This effect facilitates the formation of dense, high-quality polycrystalline films. This work provides useful guidelines for identifying environmentally benign solvent system and clarifies the requirement for fabricating TPsk film with high photovoltaic performance.