Alloy or Blend: Analysis of a Ternary Organic Photovoltaic

The power conversion efficiency of organic solar cells can be enhanced by employing ternary blend bulk heterojunctions, which have been found to have an open-circuit voltage (V_OC) that depends on the blend ratio of acceptors/donors. It has been proposed that the variation in V_OC is due to the formation of molecular alloy between blended acceptors/donors. However, we assert that the formation of a “molecular alloy” should exhibit new electronic states distinguished from those of its constituents. Here, we study the electronic states and energy levels in the P3HT:(ICBA:PC₆₁BM) ternary blend system. We find that the ICBA:PC₆₁BM acceptors shows the same highest occupied molecular orbital levels and exciton energies as that of ICBA, indicating no new exciton state emerges from the blend. In the ternary blend, the charge transfer states are composed of a linear superposition of those in the corresponding binaries. From these results, no evidence of new emergent electronics state is found which would support the existence of a molecular alloy. Indeed, our results point to the conclusion that blends of these non-interacting molecules differ substantially from our understanding of a conventional alloy. In this talk, we will consider the general criteria in identifying a molecular alloy.