Effect of the oligothiophene chain length on the performance of organic photovoltaic cells

We investigate photovoltaic cells based on α-oligothiophene molecules with different chain lengths nT (n=4,6,8) deposited on a sandwich structure between two electrodes: ITO and Al. From the analysis of the absorption and photocurrent spectrum, we have shown that the structure of the oligothiophene affects considerably the absorbing character of the evaporated film, which is increasingly significant from 4T towards 8T, and a significant photocurrent in agreement with strong absorption in the region Al/nT. Moreover, a theoretical model of Ghosh is used to display the photocurrent origin, proved to be sufficient after adjustment with a random term. Then we deduce that the crystalline order of the film improves the transport of charge and the photocurrent. The results are confirmed by electrical characterization under illumination that shows effectively an influence of the expansion of the molecular chains on the increase in the energy efficiencies and on the performances of organic photovoltaic cells.