ИСТИНА

Impressive increase in efficiency of organic solar cells (OSCs) in the recent years relies, on the one hand, on continuous development of new donor an d acceptor materials, and, on the other hand, on bulk heterojunction morphology optimization [1]. Post-deposition treatment of the bulk- heterojunctions such as thermal annealing is common ly applied to improve the OSC performance [2]. This calls for a quick and uninvasive method t o control the morphology during annealing of the already manufactured OSCs for every perspective com bination of the materials. Here we present a technique for in situ probing of molecular order in the OSCs by resonant Raman spectroscopy, with the P3HT-based OSCs as an examle. The method is based on decomposition of the Raman spectrum of the blend in the region of carbon-carbon stretching modes onto two components that correspond to the amorphou s and quasi-crystalline phases of the polymer (Fig. 1a,b). We denote the relative contribution of the quasi-crystalline phase component as an index of molecular order (IMO). IMO dynamics during the annealing process (Fig.1c,d) reflects evolution of the morphology: the IMO increases from 0.3 in the as-cast sample to the asimptotic value of ~0.9 in the annealed samples. For the poly mer-fullerene devices processed at different annealing temperatures, there exists a clear correl ation between the IMO and energy conversion efficiency (Fig. 1e). We envision that the proposed technique, complemented by the recently- developed method of predicting OSC efficiency from fluorescence quenching [3] to become a valuable tool in optimization of the OSCs.