Composition-dependent phase separation of P3HT:PCBM composites for high performance organic solar cells

Phase separation of the poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) active layer was investigated by varying its relative ratio in the bulk heterojunction (BHJ) organic solar cells (OSCs). The UV/visible absorption spectra of P3HT and PCBM were proportional to their compositions in the active layer. However, the crystallinity of P3HT, characterized by X-ray diffraction (XRD), improved with increasing composition up to 50 wt.%, but then degraded upon any further increase in its composition above 50 wt.%. This unique phenomenon corresponded to the microstructure of the layer, which was analyzed using transmission electron microscopy (TEM). The fraction of the highly ordered fiber structure of P3HT increased as the P3HT composition increased up to 50 wt.% and decreased as the composition increased further, which was in accordance with the XRD results. The microstructure of the P3HT:PCBM active layer, determined by the composition-dependent phase separation, supported the optimized performance of the OSCs with the active layer composition of 50% P3HT and 50% PCBM.