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.