Tunable light harvesting properties of a highly crystalline alternating terpolymer for high-performing solar cells

To achieve power conversion efficiency in organic photovoltaic devices, we designed and synthesized a new DPP-based alternating terpolymer, PDPPPyT, containing DPP as an electron accepting unit and pyrene and thiophene as electron donating units. The donor-acceptor alternating copolymers PDPPPy and PDPPT were also prepared as control materials. The optical properties and the highest occupied molecular orbital of PDPPPyT were found to be between those of the PDPPPy and PDPPT copolymers. GI-XRD analysis indicated that the PDPPPy and PDPPT copolymers had a typical edge-on orientation on the substrate, whereas the PDPPPyT terpolymer showed face-on orientation in pristine and annealed films. Although PDPPPyT has two different donating units, pyrene and thiophene, in the repeating unit, the corresponding TFT exhibited a high hole mobility of ∼0.8 cm² V^-1 s^-1. A PSC made of PDPPPyT and PC₇₁BM displayed power conversion efficiency values of around 4.0%, which is higher than those of PDPPPy- and PDPPT-based PSCs. This improved performance was attributed to the more efficient light-harvesting properties of PDPPPyT in the solar spectrum. These results unambiguously demonstrated that this new synthetic strategy for regular conjugated terpolymers could be used to tune the molecular energy levels and optical properties to obtain high-performance PSCs.