Benzo[1,2‐b:4,5‐b′]Dithiophene–6,7‐Difluoroquinoxaline Small Molecule Donors with >8% BHJ Solar Cell Efficiency

Solution-processable small molecule (SM) donors are promising alternatives to their polymer counterparts in bulk-heterojunction (BHJ) solar cells. While SM donors with favorable spectral absorption, self-assembly patterns, optimum thin-film morphologies, and high carrier mobilities in optimized donor-acceptor blends are required to further BHJ device efficiencies, material structure governs each one of those attributes. As a result, the rational design of SM donors with gradually improved BHJ solar cell efficiencies must concurrently address: (i) bandgap tuning and optimization of spectral absorption (inherent to the SM main chain) and (ii) pendant-group substitution promoting structural order and mediating morphological effects. In this paper, the rational pendant-group substitution in benzo[1,2-b:4,5-b']dithiophene-6,7-difluoroquinoxaline SMs is shown to be an effective approach to narrowing the optical gap (E-opt) of the SM donors (SM1 and SM2), without altering their propensity to order and form favorable thin-film BHJ morphologies with PC71BM. Systematic device examinations show that power conversion efficiencies >8% and open-circuit voltages (V-OC) nearing 1 V can be achieved with the narrow-gap SM donor analog (SM2, E-opt = 1.6 eV) and that charge transport in optimized BHJ solar cells proceeds with minimal, nearly trap-free recombination. Detailed device simulations, light intensity dependence, and transient photocurrent analyses emphasize how carrier recombination impacts BHJ device performance upon optimization of active layer thickness and morphology.