Correlation between Distribution of Polymer Orientation and Cell Structure in Organic Photovoltaics

The backbone orientation of semiconducting polymers is one of the important structural factors that determines the charge transport and thus the performance of optoelectronic devices. Here, we study two sets of thiophene-thiazolothiazole polymers, which primarily form edge-on and face-on orientations, termed "edge-on-polymers" and "face-on-polymers", respectively; both orientation types co-exist in their polymer/fullerene blend films. Interestingly, we find that the dependence of the photoactive layer thickness on the fill factor in the photovoltaic cells, with the inverted and conventional structures, is quite distinct in the edge-on-polymer; however, this is not evident in the face-on-polymers. An in-depth study by grazing incidence X-ray diffraction analysis reveals that the face-on/edge-on ratio is unevenly distributed through the film thickness in the edge-on-polymers, while it is evenly distributed in the face-on-polymers. The difference in the distribution of the backbone orientation correlates with the difference in the thickness dependence on the fill factor. We thus propose that the distribution of the backbone orientation is an important factor to understand the performance of polymer-based photovoltaic cells and that the cell structure should be carefully selected by considering the distribution for maximizing the performance.