Alkoxybenzothiadiazole-Based Fullerene and Nonfullerene Polymer Solar Cells with High Shunt Resistance for Indoor Photovoltaic Applications

We synthesized three semicrystalline polymers (PTTBT_BO, PDTBT_BO, and P2FDTBT_BO) by modulating the intra- and intermolecular noncovalent Coulombic interactions and investigated their photovoltaic characteristics under various light intensities. Low series (R_s) and high shunt (R_sh) resistances are essential prerequisites for good device properties under standard illumination (100 mW cm^-2). Considering these factors, among three polymers, PDTBT_BO polymer solar cells (PSCs) exhibited the most desirable characteristics, with peak power conversion efficiencies (PCE) of 7.52 and 9.60% by being blended with PC₇₁BM under standard and dim light (2.5 mW cm^-2), respectively. P2FDTBT_BO PSCs exhibited a low PCE of 3.69% under standard light due to significant charge recombination with high R_s (9.42 ω cm²). However, the PCE was remarkably improved by 2.3 times (8.33% PCE) under dim light, showing negligible decrease in open-circuit voltage and remarkable increase in fill factor, which is due to an exceptionally high R_sh of over 1000 kω cm². R_s is less significant under dim light because the generated current is too small to cause noticeable R_s-induced voltage losses. Instead, high R_sh becomes more important to avoid leakage currents. This work provides important tips to further optimize PSCs for indoor applications with low-power electronic devices such as Internet of things sensors.