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

Journal article

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. © 2006 Elsevier B.V. All rights reserved.

Authors: Glatthaar, M; Riede, M; Keegan, N; Sylvester-Hvid, K; Zimmermann, B

• Journal: Solar Energy Materials and Solar Cells
• Volume: 91
• Issue: 5
• Pages: 390-393
• Publication date: 2007-03-06
• DOI: 10.1016/j.solmat.2006.10.020
• ISSN: 0927-0248
• Language: English
• Keywords: Doping; Model; Kink; Impedance spectroscopy; Organic photovoltaics