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Schwarz, Christian ; Milan, Felix ; Hahn, Tobias ; Reichenberger, Markus ; Kümmel, Stephan ; Köhler, Anna:
Ground State Bleaching at Donor–Acceptor Interfaces.
In: Advanced Functional Materials.
Bd. 24
(2014)
Heft 41
.
- S. 6439-6448.
ISSN 1616-3028
DOI: https://doi.org/10.1002/adfm.201400297
Abstract
Charge separation at the donor–acceptor interface is a key step for high efficiency in organic solar cells. If interfacial hybrid states exist already in the dark it is plausible that they can have a major impact on the dissociation of optically generated excitations. In this work we probe such interfacial states via steady state absorption spectroscopy. A substantial bleaching of the absorption spectrum is found near the absorption edge when an electron-accepting layer of either trinitrofluorenone (TNF), C60, or a perylene-diimide derivative is deposited on top of a layer of electron-donating conjugated polymers, such as MEH-PPV or various poly-phenylene. This is in part attributed to the formation of ground state complexes with low oscillator strength. The experiments bear out a correlation between the reduction of the absorbance with the energy gap between the donor-HOMO and acceptor-LUMO, the effective conjugation length of the donor, and the efficiency of exciton dissociation in the solar cell. The effect originates from mixing of the donor-HOMO and the acceptor LUMO. Calculations using density functional theory support this reasoning. Implications for efficiency of organic solar cells will be discussed.