Enhancing the performance of indoor organic photovoltaics through precise modulation of chlorine density in wide bandgap random copolymers

We present highly efficient indoor organic photovoltaic (IOPV) devices based on a series of four wide-bandgap random copolymers, denoted as B30T70-XCl (X = 0, 2, 4 and 6). The absorption range of these copolymers efficiently covers the spectral range of indoor light sources, with a systematic decrease in the HOMO levels based on the number of chlorine atoms (0 > 2 > 4 > 6Cl). The introduction of Cl is an effective and cost-efficient strategy because of the simplicity of the synthesis. We use PC71BM as the electron acceptor, which not only effectively absorbs indoor light spectra, but also significantly reduces production costs compared with state-of-the-art non-fullerene acceptors (NFAs). Among the B30T70-XCl:PC71BM blends, the B30T70-2Cl-based devices exhibit optimized power conversion efficiencies (PCEs) with a high V-OC, achieving a record-breaking PCE of 25.0% under fluorescent lamp (FL) illumination, compared with reported fullerene-based IOPVs. Through a comprehensive analysis of the energy levels, transient absorption dynamics, and blend morphology, we reveal that increasing the Cl density decreases the HOMO offset between the polymer donors and the PC71BM acceptor and induces a phase-separated blend morphology, critically impacting the performance of IOPVs by influencing the population of charge-separated states and charge transport behavior, respectively. The performance of these IOPVs based on wide-bandgap random copolymers and the PC71BM acceptor suggests that the development of such classical, low-cost photoactive layer blends holds promise for integration into low-power portable electronics and Internet-of-Things (IoT) sensors.