Optimization of the active-layer morphology with a non-halogenic solvent for bulk-heterojunction polymer solar cells

Polymer solar cells (PSC) based on blended poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61 butyric acid methyl ester (PCBM) were fabricated with a non-halogenic solvent—tetralin—as the casting solvent for the active layer, and the morphology of the active layer was optimized by varying the casting conditions to obtain high efficiency and thermal stability from the PSCs. Tetralin as the casting solvent caused excessive P3HT and PCBM aggregations and hence dismal performance [power conversion efficiency (PCE) = 0.5%] due to its low volatility and high dissolution power, which enhanced the plasticizing effect of residual solvent during the formation of the active layer. Accelerated removal of the residual solvent by a vacuum drying treatment greatly reduced the aggregations and improved the PCE to 2.4%, but the improvement was limited because the rapid drying inhibited full developments of continuous charge-transport pathways and of P3HT crystallinity, both of which are critical for charge transport through the device. The desirable morphology—minimal aggregations but high phase continuity and P3HT crystallinity—was obtained with a low-temperature drying process, which limited P3HT/PCBM aggregations and enhanced nucleation of P3HT crystallites, resulting in a finely interconnected P3HT crystal network that provided both large bulk-heterojunction area and facile hole-transport pathways. The resultant PSCs obtained a high PCE of 3.7%, and because of the morphological rigidity of the P3HT crystal network, showed excellent stability for extended duration at elevated temperature.