Influence of donor–acceptor materials on the photovoltaic parameters of conjugated polymer/fullerene solar cells

Organic solar cells based on poly(3-hexylthiophene-2,5-diyl): [6, 6]-phenyl-C61-butyric acid methyl ester are the most researched solar cells, but still there is a need to achieve higher efficiency and better stability. The active layer blends of poly [N-9′′-heptadeca-nyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzo-thiadiazole)]: [6, 6]-phenyl-C71-butyric acid methyl ester (PCDTBT:PC₇₁BM) show a promise in this regard. In this work, PCDTBT:PC₇₁BM thin films were prepared with different blend ratios and concentrations, and were characterized for optical and morphological properties. UV-visible spectral studies showed that the absorption decreases as the acceptor concentration in the blend increases. Atomic Force Microscopy studies showed that surface roughness increases with the increase in acceptor concentration. Optimization of the active layer was carried out by using different blend ratios and concentration to achieve better performance, and it was observed that the highest efficiency was obtained at the optimized blend ratio of 1:4 and a concentration of 12 mg/ml. Further, the variation observed in the open circuit voltage with blend ratio has been explained by the disorder induced tail states in the energy levels of donor and acceptor materials.