Influence of Alkyl Side-Chain Length on the Performance of Poly(3-alkylthiophene)/Polyfluorene All-Polymer Solar Cells

We have investigated the influence of the alkyl side-chain length of poly(3-alkylthiophene) (P3AT) on the performance of all-polymer blend photovoltaic devices with the polyfluorene copolymer F8TBT (poly((9,9-dioctylfluorene)-2,7-diyl-alt-[4,7-bis(3-hexylthien-5-yl)-2,1,3-benzothiadiazole]-2',2 ''-diyl)) used as the electron acceptor. Butyl, hexyl, octyl, and decyl side-chains have been studied, with clear trends in the device performance observed. The properties of the unblended polythiophene are first reported, namely their thermal, photophysical, electrochemical, and morphological characteristics. The observed trends in photovoltaic device performance are interpreted in terms of the complex interplay between energy level offsets, blend morphology and charge carrier mobility. Device performance is especially sensitive to the annealing temperature employed that influences both the recrystallization of P3AT and the coarsening of phase separation. Poly(3-hexylthiophene) (P3HT) is found to be the optimum P3AT studied, because of the appropriate energy level offsets with F8TBT and the fact that P3HT recrystallization occurs before significant phase separation.