Effect of drying time on morphology and photovoltaic characteristics of polymer solar cells of Bis-PCBM/P3HT composites

Slowdrying has been proposed as away to improve the performance of polymer solar cells (PSCs) where the drying time for the coated films is increased using a condensed solvent vapor atmosphere. This work investigated the slow drying mechanism by using PSCs consisting of poly(3-hexylthiophene-2,5-diyl) (P3HT) and bis-[6,6]-phenyl-C61-butyric acid methyl ester (bis-PCBM) and focusing on the phase-separated morphology of bulk heterojunction films. With conventional spin casting, the power conversion efficiency (PCE) of the cells was very low (0.83%). The application of slow drying resulted in notable improvements in the short-circuit current density (5.85→8.07 mA/cm2) and fill factor (0.12→0.55). Optimization of the thermal annealing and cathode interfacial conditions resulted in a high PCE of 4.05%. As the drying time was increased, the P3HT domains changed from connected band-like shapes to small separated ball-like shapes. The mechanism for this effect of slow drying on the bis-PCBM/P3HT cell characteristics is understood as follows: The slow drying causes the P3HT domains to separate into small pieces, which causes the area of the p-n interfaces to increase, resulting in an increase in free carrier generation and increased the short-circuit current density of PSCs.