Correlation of absorption profile and fill factor in organic solar cells: The role of mobility imbalance

Journal article

We investigate the role of the spatial absorption profile within bulk heterojunction small molecule solar cells comprising a 50 nm ZnPc:C₆₀ active layer. Exploiting interference effects the absorption profile is varied by both the illumination wavelength and the thickness of an optical spacer layer adjacent to the reflecting electrode. The fill factor under 1 sun illumination is observed to change from 43 to 49% depending on the absorption profile which approximately equals the charge-carrier generation profile. It is shown by varying the mixing ratio between ZnPc and C₆₀ that the importance of the generation profile is correlated with the imbalance of mobilities. Therefore, it is concluded that non-geminate recombination is the dominating loss mechanism in these devices. Numerical drift-diffusion simulations reproduce the experimental observations showing that charge carrier extraction is more efficient if charge carriers are generated close to the contact collecting the less mobile charge carrier type. Furthermore, this effect can explain the dependence of the internal quantum efficiency measured at short circuit on wavelength and implies that the spectral mismatch for a given solar simulator and device depends on the applied voltage. Copyright © 2013 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim.

Authors: Tress, W; Merten, A; Furno, M; Hein, M; Leo, K

• Publication status: Published
• Peer review status: Peer reviewed
• Publisher: Wiley
• Journal: Advanced Energy Materials
• Volume: 3
• Issue: 5
• Pages: 631-638
• Publication date: 2013-05-01
• DOI: 10.1002/aenm.201200835
• EISSN: 1614-6840
• ISSN: 1614-6832
• Language: English
• Keywords: organic solar cell; generation profile; imbalanced mobilities; recombination; bulk heterojunction; fill factor; spectral mismatch; zinc-phthalocyanine; quantum efficiency; C60