A study on the material characteristics of low temperature cured SnO₂ films for perovskite solar cells under high humidity

Electron transport layer (ETL) plays a crucial role on the fabrication of perovskite solar cells (PSCs) by separating and transporting the charge carriers. Titanium dioxide (TiO₂) has been extensively used as an ETL in PSCs; however, high temperature thermal annealing requirement impedes its integration with flexible polymer substrates for roll to roll fabrication. Herein, we have demonstrated that SnO₂ is a potential ETL candidate when fabricated at low temperature (180 °C) using spin coating technique. XRD and XPS analysis revealed synthesis of rutile SnO₂ tetragonal phase. TEM micrographs with SAED pattern proved formation of nanosized (3 to 4 nm) crystals of SnO₂ with polycrystalline phase. FESEM analysis revealed the SnO₂ nanocrystals fully covered the FTO surface and elemental mapping confirmed the uniformly distribution tin (Sn) and (O) elements throughout the surface. In addition to this, transmission analysis confirmed that SnO₂ film exhibited good transmission property. PSCs were fabricated in ambient air (relative humidity ranges from 55% to 65%) with concentrated SnO₂ colloidal solution and diluted SnO₂ with different concentrations (1:1 v/v, 1:2 v/v, 1:4 v/v and 1:6 v/v). It was found that 1:4 v/v based diluted colloidal solution of SnO₂ in DI water film exhibited the highest PSC performance of 8.51% in ambient conditions. Thus, low temperature solution processed SnO₂ is an efficient ETL and well-suited for low cost automated fabrication of PSCs at large scale.