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 material robustness, high mechanical flexibility, and low sensitivity of photovoltaic efficiencies to thickness variation, still suffer from non-satisfactory photovoltaic performance. This is largely due to 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 by incorporating a band-gap resembling small molecule BTA3 as an expander into the donor host, a favorable dissolution/ swelling effect is achieved, which modifies the interpolation within the blends, the crystallization quality of the polymer donor and the infiltration of the acceptor, eventually leading to optimized pseudo-planar heterojunction morphology. We clarify that during 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% for 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 achieving further efficiency boosts in polymer solar cells.