Effect of crystallinity in small molecular weight organic heterojunction solar cells†
Planar-heterojunction (PHJ; pn) solar cells were fabricated using a thermally convertible well soluble benzoporphycene (BPc) precursor and [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM). The conversion temperature was controlled in order to investigate the influence of the crystallinity of the organic semiconducting layer in the PHJ solar cell on the cell performance. The crystallinity of BPc increased with annealing temperature, but the surface roughness did not change. The cell with BPc converted at 250 °C had a power conversion efficiency that was 2.5 times better (1.49%) than that of the cell converted at 150 °C, and 1.5 times better than the cell with a vacuum-deposited BPc layer. The mobility of BPc, measured from the space charge limited current characteristics, supports a clear correlation between crystallinity and cell performance. Thus, the importance of increasing the crystallinity and mobility in organic solar cells was experimentally demonstrated.