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Efficiency limiting factors of organic bulk heterojunction solar cells identified by electrical impedance spectroscopy

: Glatthaar, M.; Riede, M.; Keegan, N.; Sylvester-Hvid, K.; Zimmermann, B.; Niggemann, M.; Hinsch, A.; Gombert, A.


Krebs, F.C.:
Selected papers from the European Conference on Hybrid and Organic Solar Cells, ECHOS 2006 : Paris, France, 28 - 30 June 200
Amsterdam: Elsevier, 2007 (Solar energy materials and solar cells 91.2007, Nr.5)
European Conference on Hybrid and Organic Solar Cells (ECHOS) <2007, Paris>
Conference Paper, Journal Article
Fraunhofer ISE ()

The current–voltage characteristic and the performance of organic bulk-heterojunction solar cells are very sensitive to small variations in the production steps or environmental influences. In our experiments, we found a large variation of the short-circuit current, which does not correspond to the device thickness as one might expect. The fill factor of some devices is below 25% under illumination, while the best devices have a fill factor of about 70%. Electrical impedance spectroscopy can provide information about the conductivity of different regions within the device. In earlier measurements, it was observed that devices with a thick absorber layer might consist of a conductive bulk region and a very poorly conductive depletion region at the metal contact. Using a standard semiconductor device model, it is shown in this paper that this reduces the charge collection efficiency under short-circuit conditions, as there is no electrical field in the bulk region, supporting the charge separation. For devices with the low fill factor, a thin-current limiting layer under forward bias can be identified by electrical impedance spectroscopy and is suggestive of a corroded metal contact.