Direct Determination of the Steady State and Time-Resolved Quasi-Fermi Level Separation in Organic Solar Cells from Electroluminescence Measurements
The detection of photoluminescence (PL) is an important characterization method for many photovoltaic technologies providing direct information about the separation of the quasi-Fermi levels (QFL), ΔEF. However, for organic solar cells, the PL is dominated by excitons, which decay radiatively before they form free charge carriers via dissociation at donor/acceptor interfaces. This (major) part of the PL signal does therefore not correlate with ΔEF. In contrast, electroluminescence (EL) stems from injected electrons and holes, which recombine via charge transfer (CT) states. This work evaluates whether ΔEF can be derived directly from EL emission and whether this also holds true for transient measurements. To do so, ΔEF data derived from steady-state EL measurements of highly efficient organic solar cells are compared with the electrical voltage, both in the dark and under illumination at equal recombination currents. Furthermore, ΔEF is also, to the best of the authors’ knowledge for the first time, derived from transient EL experiments, and in all cases, an excellent agreement is found. This means that the occupation of CT states is in equilibrium with the free charge carrier densities and can, thus, be described by the same QFL, even in the transient case.