In particular, in recent work, we showed that the development of BHJ morphologies between donor and acceptor components, [20] material processing, and optimization of thin-film morphologies via postprocessing treatments [21] is not independent of the substitution pattern and functional groups appended to SM donors. 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 contribution, we report on the rational pendant-group substitution and concurrent bandgap reduction in benzo[1,2-b:4,5-b′]dithiophene-6,7-difluoroquinoxaline SM donors, raising BHJ solar cell efficiencies from ≈6 to >8% with PC 71 BM, and we provide a detailed understanding of the role of the selective substitutions on material and BHJ device performance improvements. Taking advantage of the synthetic modularity of electrondeficient quinoxaline motifs, we show that the introduction of acrylate functions on the pyrazine ring is an effective approach to significantly increasing the electron affinity of the acceptor unit, in turn narrowing the optical gap of the SM donor. Importantly, our device examinations show that this discrete synthetic modification does not alter the propensity of benzo[1,2-b:4,5-b′]dithiophene-6,7-difluoroquinoxaline SM donors to order and form favorable thin-film BHJ morphologies with PCBM; carrier extraction in optimized BHJ active layers proceeds with notably suppressed geminate and bimolecular recombination, while the high open-circuit voltage (V OC ) of the BHJ solar cells is maintained to ≈1 V.In the design of symmetrical SM donors alternating donor and acceptor motifs (DADAD) setting their optical gap, two avenues have prevailed thus far: (i) swapping the donor [acceptor] units for more electron-rich [-deficient] motifs-albeit with distinctly different molecular structure [14,22] -and (ii) increasing [reducing] their conjugation length by replicating [removing] DA motifs [14] or, more commonly, by varying the number of rings involved in the main chain. [1,17] In either cases, the synthetic modifications generally impact SM self-assembly in thin films, solution-processing and device optimization procedures, [18,23] 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...