Improvement of electrical conductivity for high-performance organic solar cells by multi-temperature solvent annealing

The crystallinity of the photoactive layer in organic solar cells is a key parameter for improving charge carrier transport. To form a highly crystalline photoactive layer of poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b?]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl] thieno[3,4-b]thiophenediyl]] (PTB7:PCBM), we developed a multi-temperature solvent annealing method with a two-step annealing process with the first annealing process performed at a relatively low temperature of 0 ?C to induce the stable crystal growth of the conductive PTB7 polymer without interruption by the active diffusion of the PCBM molecules. The residual solvent was then removed by the second annealing process at a high temperature of 70 ?C to prevent further diffusion of PCBM. The solar cells fabricated by this method showed a 36% improvement in crystallinity and a significantly higher electrical conductivity than those fabricated by the conventional annealing method. These solar cells resulted in the improvement of power conversion efficiency up to 5.36% owing to the enhancement of only the fill factor as indicated by the high electrical conductivity of the photoactive layer.