Effect of alkylsulphanyl substituent on the performance of polythiophene as donor material for bulk heterojunction solar cells

Polymer solar cells have evolved as a promising cost-effective alternative to siliconbasedsolar cells. However, low efficiency of these plastic solar cells limits their feasibility forcommercial use. The efficiencies of polymer photovoltaic cells got a major boost with theintroduction of the bulk heterojunction (BHJ) concept consisting of an interpenetrating network ofelectron donor and acceptor materials. In this work we describe the preparation of BHJ organic solarcells using blend films of poly[4,4’-bis(alkylsulfanyl)-2,2’-bithiophene] (PST) and 1-(3-methoxycarbonyl)-propyl-1-phenyl-(6,6)C61 (PCBM). Photovoltaic cells were prepared with a layersandwiched between electrodes with different work functions. We made a comparative study of theelectrical behaviour of different devices to investigate the influence of different fabricationconditions (donor/acceptor ratio, solvent, layer thickness, device annealing, etc.). The electricalperformance of the devices were compared with the corresponding one prepared using a blend ofcommercial poly(3-hexylthiophene) (P3HT) and PCBM. The quantum efficiency of the devicefabricated with PST/PCBM blend shows a better response in the visible region compared to the onewith P3HT/PCBM. Theoretical calculations on PST and P3HT were also carried out to explain theeffect of the alkylsulphanyl substituent (compared to hexyl one) on the geometry, band gap andHOMO/LUMO levels of polythiophene.