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Correlation of Absorption Profile and Fill Factor in Organic Solar Cells: The Role of Mobility Imbalance
Author(s) -
Tress Wolfgang,
Merten André,
Furno Mauro,
Hein Moritz,
Leo Karl,
Riede Moritz
Publication year - 2013
Publication title -
advanced energy materials
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 10.08
H-Index - 220
eISSN - 1614-6840
pISSN - 1614-6832
DOI - 10.1002/aenm.201200835
Subject(s) - materials science , absorption (acoustics) , heterojunction , charge carrier , optoelectronics , diffusion , wavelength , active layer , molecular physics , solar cell , organic solar cell , electrode , quantum efficiency , charge (physics) , layer (electronics) , nanotechnology , chemistry , physics , polymer , quantum mechanics , composite material , thermodynamics , thin film transistor
We investigate the role of the spatial absorption profile within bulk heterojunction small molecule solar cells comprising a 50 nm ZnPc:C 60 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 C 60 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.