Evaporation-controlled spray deposition process achieving ultrasmooth photoactive layer for efficient organic solar cells

Spin-coating is a deposition method widely used in lab-scale experiments with organic solar cells. This method is, however, not adequate for large-area coatings since it produces excessive material waste. For this reason, a growing community of researchers is investigating alternative scalable deposition techniques to deposit photovoltaic devices from polymer solution. In this study, we introduce a facile spray coating method to fabricate a smooth and uniform bulk heterojunction morphology of organic photovoltaic devices based on a mixture of conjugated polymer and fullerene. The high surface roughness of spray-coated films is significantly reduced by controlling the spray deposition parameters closely related to the solvent evaporation rate. The drying kinetics of solvents affect surface roughness of the spray-deposited film and polymer crystallinity, correlating with the photodiode characteristics, such as the leakage current and rectifying ratio. This spray coating method is optimized for device fabrication to achieve a high-power conversion efficiency (PCE) of 3.55% for P3HT and a PCE of 7.53% when applied to PTB7-Th, a low band-gap conjugated polymer.