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Identifying the impact of surface recombination at electrodes in organic solar cells by means of electroluminescence and modeling

: Reinhardt, J.; Grein, M.; Bühler, C.; Schubert, M.; Würfel, U.


Advanced energy materials 4 (2014), No.11, Art. 1400081, 9 pp.
ISSN: 1614-6840
ISSN: 1614-6832
Journal Article
Fraunhofer ISE ()
Solarzellen - Entwicklung und Charakterisierung; Farbstoff-; Organische und Neuartige Solarzellen; Organische Solarzellen; Electroluminescence; Solar Cells; Selectivity; Recombination

This work reports on combining current-voltage characteristics, electroluminescence (EL) measurements, and modeling to identify the selectivity of the electrodes in bulk-heterojunction organic solar cells. Devices with the same photoactive layer but different contact materials are compared and the impact of surface recombination at the contacts on their performance is determined. The open-circuit voltage, V OC, depends strongly on the selectivity of the electrodes and it is observed that the EL signal of cells with lower V OC is dramatically reduced. This is ascribed to an enhanced rate of surface recombination, which is a non-radiative recombination pathway and does therefore not contribute to the EL yield. In addition, these cells have a lower current in forward direction despite the fact that the surface recombination occurs in addition to the recombination in the bulk. A theoretical model was set up and in the corresponding numerical simulations all three findings (lower VOC, strongly reduced EL signal and lower forward current) could be clearly reproduced by varying just one single parameter which determines the selectivity of the electrode.