Publications associated with Advanced Functional Materials and Devices Group

Correlation of open-circuit voltage and energy levels in zinc-phthalocyanine: C60 bulk heterojunction solar cells with varied mixing ratio

Physical Review B - Condensed Matter and Materials Physics 88 (2013)

ML Tietze, W Tress, S Pfützner, C Schünemann, L Burtone, M Riede, K Leo, K Vandewal, S Olthof, P Schulz, A Kahn

The maximum open-circuit voltage V of bulk-heterojunction solar cells is limited by the effective HOMO(donor)-LUMO(acceptor) gap of the photoactive absorber blend. We investigate blend layers comprising zinc-phthalocyanine (ZnPc) and the buckminster fullerene C with ultraviolet, x-ray, and inverse photoelectron spectroscopy. By varying the volume mixing ratio ZnPc:C from 6:1 to 1:6, we observe a linear increase of the HOMO(ZnPc)-LUMO(C) gap by 0.25 eV. The trend in this gap correlates with the change in the charge transfer energy measured by Fourier-transform photocurrent spectroscopy as well as with the observed open-circuit voltage of solar cells containing ZnPc:C as the photoactive absorber layer. Furthermore, the morphology of different ZnPc:C blend layers is investigated by grazing-incidence x-ray diffraction. As physical origins for the changed energy levels, a suppressed crystallization of the C phase in the presence of donor molecules as well as concentration-dependent growth modes of the ZnPc phase are suggested. © 2013 American Physical Society.

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