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CPP: Fachverband Chemische Physik und Polymerphysik

CPP 23: Organic Photovoltaics I

CPP 23.10: Vortrag

Dienstag, 13. März 2018, 12:15–12:30, C 243

Apparent Field Dependence of Charge Carrier Generation and Recombination Coefficient in Organic Solar Cells — •Uli Würfel and Moritz Unmüssig — Fraunhofer ISE, Heidenhofstr. 2, 79110 Freiburg, Germany

Charge carrier generation in organic solar cells is often reported to show an electric field dependence. However, it is not measured directly but is derived from transient charge carrier extraction experiments based on the time delayed collection field (TDCF) method. In this contribution, numerical simulations of TDCF experiments are presented which - when analyzed in the same way as reported in literature - result in a field dependence of the charge carrier generation and the recombination coefficient even though field-independent values are used. For the carrier generation this discrepancy is consistently shown to be caused by (unavoidable) bi-molecular recombination in the short time scale prior to and during extraction. The apparent field dependency becomes more pronounced with increasing recombination coefficient and decreasing charge carrier mobilities, well in accordance with TDCF data from literature. The apparent field dependence of the bimolecular recombination coefficient is due to the fact that its derivation from experimental TDCF data is based on spatially averaged values of carrier densities. In contrast, electrons and holes in real devices are distributed inhomogeneously as a consequence of the boundary conditions. Our study shows clearly that in order to explain results from TDCF experiments it is not required to include any field-dependence nor of the photocurrent generation nor of the recombination coefficient.