Reciprocity between Charge Injection and Extraction and Its Influence on the Interpretation of Electroluminescence Spectra in Organic Solar Cells

Reciprocity relations based on the principle of detailed balance have been frequently used to analyze luminescence intensity and the spectrum of organic solar cells. These reciprocity relations were derived for cases where a linear extrapolation of equilibrium conditions to the nonequilibrium situations present during measurements is possible and therefore requires semiconductors with linear recombination mechanisms. Here, we discuss the impact of nonlinear recombination typically found in organic solar cells on the analysis of luminescence spectra and estimate criteria under which reciprocity relations can still be used to analyze the data. We find that depending on the exact application, only for low mobilities μ <10^-4 cm²/V s or very asymmetric mobilities do substantial disagreements between simulation and analytical equations occur.