Interplay among electronic characteristics, morphology and device efficiency in three fluorene alternated copolymers

Abstract Aiming to evaluate the effect of chain length on the performance of photovoltaic devices, three alternated copolymers were tested. Fluorene, chosen as a common base for comparison was linked to thiophene (Th), ethylene dioxythiophene (EDOT), and benzothiadiazole (Bz). The three comonomers were selected from the most common units in photovoltaic polymers, with different electro-affinities. The electronic properties such as frontier levels and band gap were very similar, but the chain length varied considerably, exerting a strong effect on the cell morphology. That in turn determined the charge transport efficiency and surface contact between the polymer layer and the fullerene, resulting in better performance for the polymer with the highest molecular mass, which was not the result that would be anticipated, based on the difference of electronic affinity towards to the common unit, the fluorene.