Roles of Blend Morphology and Energy Level Alignment in Determining Open‐Circuit Voltage and Fill Factor of Ternary All‐Polymer Solar Cells

Abstract Tuning the open‐circuit voltage (V_OC) and fill factor (FF) without sacrificing the short‐circuit current density is crucial for achieving performance advantages in ternary blend organic solar cells as an alternative to the binary ones. This study investigates ternary all‐polymer solar cells comprised of two polymer donors (D1 and D2) and one polymer acceptor (A), using two types of D2 polymers with different compatibilities with D1. As demonstrated by photoelectron yield spectroscopy, the ionization energy (IE) of the ternary blends continuously shifts between the IEs of D1:A and D2:A binary blends when D2 forms a single indistinguishable domain with D1. In contrast, the IE of the blends does not exhibit any noticeable change when D2 induces distinct phase separation with D1. Models describing the blend composition dependence of the V_OC and FF are constructed by correlating the effects of the blend morphology and the shift in the energy level of the hole‐transporting state in the ternary blends. This study demonstrates that a pseudo‐binary blend morphology with a mixed D1 and D2 donor domain and an A acceptor domain is necessary to obtain a broad tunability of V_OC and tolerance of FF to the blend composition in ternary all‐polymer solar cells.