Polymer bulk-heterojunction (BHJ) solar cells are typically composed of binary blends of conjugated polymers and fullerene derivatives that act as electron donors and acceptors, respectively. Ternary blend BHJ devices have an active layer that is composed of three components, either two donors and one acceptor or two acceptors and one donor. Ternary BHJs offer a possible method to increase the absorption of a solar cell and thus the short circuit current. Although one might expect the open-circuit voltage (Voc) of ternary blend solar cells to be pinned to the smaller of the related binary blends, several groups have found that the Voc in ternary blends changes continuously with composition without detriment to the fill factor of the solar cell. The implication is that the energy levels of the band gap change continuously forming an organic alloy. This leaves open many fundamental questions about the nature of this ‘alloying’ in organic ternary systems. This work is aimed at addressing some of these basic questions. We focus on two ternary acceptor systems, one classically-behaving blend (PSEHTT:ICBA:PCBM) and one blend with more complex behavior (PTB7:ICBA:PCBM). Our goal is to combine photoluminescence, electroluminescence, sub-gap external quantum efficiency and other measurements to probe the nature of different electronically alloyed states. Our measurements allow us to quantify the composition at which charge transport becomes significantly hindered, the precise effective band gaps of the ternary blends, and the dominant forms of recombination, all as a function of composition. These experiments explain why the high-performance polymer PTB7 has traditionally failed with ICBA as the acceptor, as well as provide a better understanding of the electronic structure and processes in organic ternary blends for photovoltaic applications.

Device physics of the organic alloying effect in high-efficiency ternary blend polymer/fullerene bulk-heterojunction solar cells

251st National Meeting (2016)

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