In case of thin-film solar cells it is often rather difficult to determine what the dominant recombination mechanism is. In particular it is difficult to distinguish recombination at the interface between the absorber layer and the electrodes (typically called surface recombination) from recombination in the bulk of the absorber -or in case of organic solar cells at the internal donor-acceptor interfaces. Here, we suggest a method to distinguish surface and bulk recombination in thin-film solar cells based on the thickness dependence of the saturation current density, which we derive from the open-circuit voltage and the photocurrent at short circuit or reverse bias. By means of numerical simulations, we show that surface and bulk recombination currents scale differently with thickness assuming the material properties to be unchanged. We test our predictions on a range of organic solar cell data from our laboratory and from literature and show that in the field of organic photovoltaics the whole range of cases, from mostly surface limited to purely bulk limited, is observed.
The Mott-Schottky analysis in the dark is a frequently used method to determine the doping concentration of semiconductors from capacitance-voltage measurements, even for such complex systems as polymer:fullerene blends used for organic solar cells. While the analysis of capacitance-voltage measurements in the dark is relatively well established, the analysis of data taken under illumination is currently not fully understood. Here, we present experiments and simulations to show which physical mechanisms affect the Mott-Schottky analysis under illumination. We show that the mobility of the blend has a major influence on the shape of the capacitance-voltage curve and can be obtained from data taken under reverse bias. In addition, we show that the apparent shift of the built-in voltage observed previously can be explained by a shift of the onset of space-charge-limited collection with illumination intensity.
In recent years, efficiencies of bulk heterojunction solar cells have risen substantially mostly due to the development of well-absorbing small molecules that replace fullerenes as the acceptor molecule. The improved light absorption due to the combination of two strongly absorbing molecules raises the question, how to best combine the absorption onsets of the donor and acceptor molecule to maximize efficiency. By using numerical simulations, we explain under which circumstances complementary absorption or overlapping absorption bands of the two molecules will be more beneficial for efficiency. Only when mobility and lifetime of charge carriers are sufficiently high to allow sufficient charge collection for layer thicknesses around the second interference maximum, a combination of complementary absorbing molecules is more efficient. For smaller thicknesses, a blend of molecules with the same absorption onset achieves higher efficiencies.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.