Statistical and block conjugated polymers for bulk heterojunction solar cells: Molecular orientation, charge transfer dynamics and device performance

Abstract Two terpolymers based on fluorene, benzothiadiazole, and carbazole monomer units were investigated as electron donor materials in organic solar cells. Variation of the microstructure, either statistical or in block, significantly affected the polymers' optoelectronic properties. Both terpolymers presented a high open-circuit voltage in photovoltaic devices (ca. 0.8–0.95 V). Polymer solar cells assembled with the active layer composed of the statistical polymer combined with PCBM exhibited good photovoltaic performance, with power conversion efficiency (PCE) of 1.5% using chlorobenzene as the solvent and an annealing treatment of the active layer. In its turn, the block terpolymer showed the best result when the same solvent and the additive 1,8-diiodooctane (DIO) were used in the mixture. After introducing the additive in the preparation of the active layer, changing the solvent to dichlorobenzene, and varying the ratio of donor to acceptor, i.e., block polymer:PC60BM, the device showed PCE of 1.2% and 2.2%, with ratio 1:2 and 1:1 w/w, respectively. To better understand the different devices' performance, spectroscopic techniques were employed to gain information on the electronic structure, morphology, and dynamic processes of the polymeric films' charge transfer.