The Influence of the TiO2 Compact Layer on the Performance of Carbon-Based Ambient-Synthesized CH3NH3PbI3 Solar Cells

Since their discovery in 2009, perovskite solar cells (PSCs) have demonstrated rapid progress. Ambient-processed, carbon-based PSCs utilizing a pre-heating step offer a cost-effective fabrication route. Nevertheless, the role of the compact titanium dioxide (TiO2-c) layer in ambient conditions has remained under-explored and inconsistently reported in the literature. This study then investigated the impact of TiO2-c layer thickness, ranging from 70 nm to 155 nm, on the performance of PSCs fabricated entirely in ambient air with high relative humidity (RH > 70%). The layers were deposited via the sol-gel spin-coating method. Experimental results then revealed that the thinnest layer (70 nm) yielded the lowest average power conversion efficiency (PCE) of 2.05% due to diminished Jsc and Voc values. The optimized TiO2-c thickness was also identified at 95 nm, achieving an average PCE of 2.95% and a peak efficiency of 4.5%. Structural analysis via XRD confirmed the presence of both anatase and brookite phases. Notably, increasing the thickness from 70 nm to 155 nm resulted in a slight reduction in the anatase peak and a corresponding increase in the brookite peak. The superior performance at 95 nm could be attributed to a balanced crystal intensity between these two phases. Furthermore, TiO2-c thickness was found to correlate with larger aggregate formation, better uniform shape grains, and reduced surface roughness, significantly influencing the morphology of the subsequent mesoporous TiO2-m layer. These findings then provided critical insights into how thickness variation in the TiO2-c layer could influence the performance of ambient-processed carbon-based PSCs.