Dissolution swelling effect-assisted interfacial morphology refinement enables high efficiency all-polymer solar cells

All-polymer solar cells (all-PSCs), while having the merits of materials robustness, high mechanical flexibility, and low sensitivity of photovoltaic efficiencies to thickness variation, still suffer from non-satisfactory photovoltaic performance. This is largely caused by the difficulty of morphology control at the polymeric donor-acceptor interface, subject to the strong intermolecular interaction and entanglement effects. Here, by in-situ optical and structural analyses we unveiled the evolution of interface morphology in all-PSCs prepared with solution-based sequential deposition (SSD). We show that through incorporating a band-gap resembling small molecule BTA3 as expander into the donor host, the favorable dissolution/swelling effect is afforded, which modifies the interpolation within the blends, the crystallization quality of polymer donor and the infiltration of acceptor, eventually leading to optimized pseudo-planar heterojunction morphology. We clarify that during the morphology establishment, the BTA3 expander plays a vital role in boosting the long-range molecular ordering in the photoactive layer, which improves carrier transport and reduces recombination losses. The PM6:BTA3/PY-IT based all-PSCs yielded impressive photovoltaic efficiencies of 19.39% and 17.71% in small-area (0.04 cm2) and large-area (1 cm2) devices. Across a range of representative acceptor molecules as expanders, we established a universal correlation between the polymer host-small molecule expander interaction and device efficiency enhancements, which provides useful guidelines for receiving further efficiency boosts in polymer solar cells.