Effects of spin-coating speed and precursor concentration in Formamidinium-based layered perovskite films

The effects of precursor solution concentration, spin coating speed and substrate temperature on the crystallinity and optical properties of (BA)2FA2Pb3I10 films are comparatively investigated. On substrates at room temperature or at 110 degrees C, two-dimensional (2D) perovskite films and solar cells using (BA)2(FA)n- 1PbnI3n+1 (n = 3 nominally), with the same molar ratio of components, are fabricated with adding thiourea in the precursor solution. The film fabrication procedure includes two groups: one is with same concentration of 0.25 mol/L (M) based on PbI2 but various spin-casting speeds from 1000 rpm to 6000 rpm; the other is with same spin-coating speed of 4000 rpm but with various concentrations from 0.1 M to 1 M. We find that optical properties and film morphologies (including domain size and crystal orientation) are not sensitive to the spin-casting speed; while the concentration of precursor solution but not substrate temperature had significant effects on the crystalline orientation and domain size, namely, higher concentration of solution results in more preferential out of plane alignment with respect to the substrate and larger crystalline domain size. Device from the 0.25 M solution spincasted at 4000 rpm on room temperature substrate achieves the best power conversion efficiency (PCE) of 7.5% without J-V hysteresis. Current work demonstrates the highest PCE reported for mixed BA and FA based 2D perovskite solar cells with n < 5, as well as suggests the robustness of (BA)2FA2Pb3I10 as an active material in preparing solar cells.