Constructing continuous acceptor fibrillar networks and achieving uniform phase separation via polymer-assisted morphology control for 20.3% efficiency additive-free organic solar cells

Abstract

Additive-free organic solar cells (OSCs) represent a critical advancement toward scalable, stable photovoltaic devices by eliminating processing complexities associated with solvent additives. However, achieving an optimal active-layer morphology in their absence remains a formidable challenge. Here, we introduce a polymer engineering strategy to enhance morphological control and device performance in additive-free OSCs. By incorporating a small fraction of the polymer acceptor PY-DT into the D18:L8-BO blend system, we demonstrate that PY-DT acts as a nucleating agent during film formation, promoting the ordered molecular stacking of L8-BO. The polymer's rigid backbone further directs epitaxial growth of the L8-BO phase, driving the formation of a continuous fibrillar network within the acceptor domain and enabling uniform phase separation. This hierarchical assembly enhances exciton dissociation and balances charge transport, yielding a champion power conversion efficiency (PCE) of 20.3%, which is one of the highest values for additive-free OSCs. Stability evaluations reveal that devices retain 85.3% of their initial efficiency after 1200 hours of continuous illumination. Our findings establish a novel approach to morphological engineering in additive-free OSCs, offering a pathway toward industrially viable, high-performance devices and advancing the field of organic photovoltaics.