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

Journal article

The maximum open-circuit voltage V_OC 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 C₆₀ with ultraviolet, x-ray, and inverse photoelectron spectroscopy. By varying the volume mixing ratio ZnPc:C₆₀ from 6:1 to 1:6, we observe a linear increase of the HOMO(ZnPc)-LUMO(C₆₀) 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:C₆₀ as the photoactive absorber layer. Furthermore, the morphology of different ZnPc:C₆₀ blend layers is investigated by grazing-incidence x-ray diffraction. As physical origins for the changed energy levels, a suppressed crystallization of the C₆₀ phase in the presence of donor molecules as well as concentration-dependent growth modes of the ZnPc phase are suggested. © 2013 American Physical Society.

Authors: Tietze, M; Tress, W; Pfützner, S; Schünemann, C; Burtone, L

• Publication status: Published
• Peer review status: Peer reviewed
• Publisher: American Physical Society
• Journal: Physical Review B - Condensed Matter and Materials Physics
• Volume: 88
• Issue: 8
• Article number: 085119
• Publication date: 2013-08-20
• DOI: 10.1103/PhysRevB.88.085119
• EISSN: 1550-235X
• ISSN: 1098-0121
• Language: English