Dithienothiophene based polymer as electron donor in plastic solar cells

In this work we have tested in bulk heterojunction solar cells a polymer with a high oxidation potential, Poly-(dithieno[3,2-b:2′,3′-d]thiophene)-3-decylthiophene (P1), as electron donor together with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) as electron acceptor. We have studied the photovoltaic characteristics of solar cells made with different P1:PCBM blend composition. The incident photon-to-current efficiency spectra reaches EQEs of 40%, evidencing good charge photogeneration. The best power conversion efficiency, η = 0.83 under AM1.5 illumination, is found for solar cells containing 67% in weight of PCBM.As expected for polymers with high oxidation potentials, the open circuit voltage Voc attains high values (Voc ≥ 0.8 V), independently from the photoactive blend composition, but the fill factors FF of the cells are low (FF: 0.3–0.38), evidencing poor charge transport properties. The data here reported suggest that transport features, probably arising from poor polymer hole mobility, are the main limitation for cell performances.