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 and ac ceptor materials, and, on the other hand, on bulk heterojunction morphology optimization [1]. Post-de position treatment of the bulk-heterojunctions such as thermal annealing is commonly applied to improve the OSC performance [2]. This calls for a quick an d uninvasive method to control the morphology during annealing of the already manufactured OSCs for every perspective combination 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 exampl e. 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 amorphous and quasi-crystalline p hases of the polymer (Fig. 1a,b). We denote the relative contribution of the quasi-crystalline phas e component as an index of molecular order (IMO). I MO dynamics during the annealing process (Fig.1c,d) re flect s evolution of the morphology: the IMO increases from 0.3 in the as-cast sample to the asimptotic va lue of ~0.9 in the annealed samples. For the polyme r- fullerene devices processed at different annealing temperatures, there exists a clear correlation betw een the IMO and energy conversion efficiency (Fig. 1e). We envision that the proposed technique, complemented by the recently-developed method of pr edicting OSC efficiency from fluorescence quenching [3] to become a valuable tool in optimization of th e OSCs.