Unpinning the Open-Circuit Voltage in Organic Solar Cells through Tuning Ternary Blend Active Layer Morphology

The use of ternary, as opposed to binary, blends having complementary absorption in active layers of organic bulk heterojunction solar cells is a simple approach to increase overall light absorption. While the open-circuit voltage ($V_{oc})$ of such solar cells have generally been shown to be pinned by the smallest energy level difference between the donor and acceptor constituents, there have been materials systems, that when incorporated into active layers of solar cells, exhibit composition dependent and tunable $V_{oc}$. Herein, we demonstrate that this $V_{oc}$ tunability in ternary blend solar cells is correlated with the morphology of the active layer. Chemical compatibility between the constituents in the blend, as probed by grazing-incidence X-ray diffraction (GIXD) measurements, affords $V_{oc}$ tuning. The constituents need not ``co-crystallize''; limited miscibility between the constituents in the active layers of solar cells affords $V_{oc}$ tunability. Poor physical interactions between the constituent domains within the active layers, on the other hand, result in devices that exhibit an invariant $V_{oc}$ that is pinned by the smallest energy level difference between the donor(s) and the acceptor(s). Our morphological studies thus support the proposed alloying model that was put forth originally.