Enhanced Photovoltaic Performance of Diketopyrrolopyrrole (DPP)-Based Polymers with Extended π Conjugation

To extend the pi conjugation along backbone units is an effective way to tune the optoelectronic properties of conjugated polymers. In this work, to investigate the influence of the extended conjugation on photovoltaic properties of the polymer, we employed a two-step molecular optimization process, and a series of diketopyrrolopyrrole (DPP)-based conjugated polymers, PBDPP-1, PBDPP-2, and PBDPP-3, was prepared and applied in polymer solar cells. After this two-step optimization of molecular structure, the compatibility between the polymer and PC71BM becomes much better; as a result, bicontinuous phase separation with appropriate domain size can be formed in the blend of PBDPP-3/PC71BM. Therefore, the PBDPP-3/PC71BM-based device shows a short circuit current density (J(sc)) of 16.25 mA/cm(2), which is more than two times of that of the PBDPP-1/PC71BM-based device. In addition, the blend films processed by DIO/CB showed smoother surfaces as well as smaller domain size compared with the blend films processed by pure CB. The results in this work indicate that when conjugation of the DPP-polymer's backbone is extended in two dimensions the morphology of the active layers in photovoltaic device can be optimized effectively, and hence the overall efficiency can be improved from 2.83 to 6.18%.