We synthesized three semicrystalline polymers (PTTBTBO, PDTBTBO, and P2FDTBTBO) by modulating the intra- and intermolecular noncovalent Coulombic interactions and investigated their photovoltaic characteristics under various light intensities. Low series (Rs) and high shunt (Rsh) resistances are essential prerequisites for good device properties under standard illumination (100 mW cm-2). Considering these factors, among three polymers, PDTBTBO 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. P2FDTBTBO PSCs exhibited a low PCE of 3.69% under standard light due to significant charge recombination with high Rs (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 Rsh of over 1000 kΩ cm2. Rs is less significant under dim light because the generated current is too small to cause noticeable Rs-induced voltage losses. Instead, high Rsh 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.
Keywords: dim light illumination; indoor applications; organic photovoltaics; photovoltaic polymers; polymer solar cells; shunt resistance.