Dispersive Charge Transfer State Electroluminescence in Organic Solar Cells

Abstract

The notion of quasi-equilibrium is central to most solar cells; however, it has been questioned in organic photovoltaics (OPVs) owing to strong energetic disorder that frustrates efficient relaxation of electrons and holes within their respective density of states (DOS). Here, modulation electroluminescence (EL) spectroscopy is applied to OPVs and it is found that the frequency response of charge transfer (CT) state EL on the high energy side of the spectrum differs from that of the low energy side. This observation confirms that static disorder contributes substantially to the linewidth of the steady-state EL spectrum and is unambiguous proof that the distribution of CT states formed by electrical injection in the dark is not in quasi-equilibrium. These results emphasize the need for caution when analyzing OPV cells on the basis of reciprocity models that assume quasi-equilibrium holds, and highlight a new method to study this unusual aspect of OPV operation.

Solution treatment controls charge-transfer states and energy-level alignment at hybrid CuSCN-organic interfaces

“…Moreover, compared with the flatter and ordered surface of the CuSCN #HF film, the relatively disordered and uneven surface of CuSCN #xF has a larger contact interface area with C 60 , boosting the generation of CT states. Considering the CT state as a fundamental property of the donor and acceptor heterojunction mixture, 79 while in devices for electroluminescent characterization, the recombination region of excitons will inevitably manifest itself inside a single functional layer, 80 which is the reason for the appearance of emission peaks of C 60 in Fig. 3.…”