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

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


Advanced energy materials 3 (2013), No.5, pp.631-638
ISSN: 1614-6840
ISSN: 1614-6832
Journal Article
Fraunhofer IPMS ()

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 t he 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. The spatial absorption profile in the bulk heterojunction of an organic solar cell is varied by employing different thicknesses of an optical spacer layer. The fill factor correlates with the profile and the imbalance in charge carrier mobilities. These results indicate that extraction of holes in competition with bimolecular recombination limits the device performance. Therefore, the spectral shape of the external quantum efficiency and, in turn, the spectral mismatch change with applied voltage.