Understanding Mott-Schottky measurements under illumination in organic bulk heterojunction solar cells
Irene Zonno, Forschungszentrum Jülich, Jülich, GermanyAlberto Martinez-Otero, Forschungszentrum Jülich, Jülich, GermanyJan-Christoph Hebig, Forschungszentrum Jülich, Jülich, GermanyThomas Kirchartz, Forschungszentrum Jülich, Jülich, Germany
Impedance spectroscopy is a powerful, non-destructive characterisation tool to study many of the electrical properties of materials and their interfaces with electrodes. Impedance based techniques have been frequently used to study doping concentration , charge recombination  and density of states  in various thin-film solar cells including organic solar cells. The most basic impedance based technique is the capacitance voltage (CV) measurement in the dark that allows one to derive the doping concentration for sufficiently thick absorber layers and gives some information about the amount of band bending in the device. While the CV measurement in the dark is a well understood method , CV measurements under illumination have so far been much more difficult to interpret and have shown features  that were not straightforward to explain with analytical equations.
Here, we present experiments and simulations to show which physical mechanisms affect the Mott-Schottky analysis on bulk-heterojunction solar cells under illumination. Provided that the device is sufficiently thin and not affected by a high doping concentration, we show that the CV curves can be approximately described by a theory of photocapacitance developed by Crandall  for amorphous silicon solar cells. However, for practical cases we observe differences between the theory of Crandall and the experimental results which we attribute to the asymmetry of the system considered and the consequent non-uniform electric field. Using numerical simulations and analytical estimates of the space charge limited photocurrent, we show that the presence of a non-uniform electric field plays a major role in determining the shape of CV curves under illumination. In addition, we show that the apparent shift of the built-in voltage can be explained by a shift of the onset of space charge limited collection with illumination intensity.
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