Modulation of charge transport and rectification behavior in CsSnI3 thin films through A-site cation engineering

CsSnI3 perovskite is a thin-film semiconductor with high intrinsic conductivity for various device applications (thermoelectric, photovoltaics, etc.). Stoichiometric CsSnI3 has high-density defects and structural imperfections affecting device performance. In this work, we made an investigation on A-site cation engineering to evaluate the correlation between structural and transport parameters for effective operation in rectifying devices. Here, we analyzed CsSnI3 thin films modified with methylamine (MA), formamidine (FA), guanidine (GuA), and 5-ammonium valeric acid (AVA) cations, correlating structural parameters obtained by Rietveld refinement with their optoelectronic and diode characteristics. MA-, FA-, and GuA-substituted films exhibited low sheet resistance (∼450–2200 Ω/sq); however, strain-induced lattice distortions and accumulated defects in GuA-substituted films significantly hindered effective charge collection and increased recombination losses. AVA substitution formed low-conductivity 2D interlayers, increasing resistance (>105 Ω/sq) and altering transient response characteristics, yet provided minimal reverse switching losses (∼100 μW/cm2), beneficial for high-frequency applications. FA substitution emerged as optimal, balancing structural stability, conductivity, minimal defects, and relevant diode properties. The obtained results highlight that targeted lattice modifications strongly influence the practical performance of rectifying p–i–n diodes based on CsSnI3.