Proceedings of Materials for Sustainable Development Conference (MAT-SUS) (NFM22)
DOI: https://doi.org/10.29363/nanoge.nfm.2022.144
Publication date: 11th July 2022
Probing the photoluminescence of free charge carriers in organic solar cells: derivation of the separation of the quasi Fermi levels
Uli Würfel1,2
1 Fraunhofer Institute for Solar Energy Systems ISE, Heidenhofstr. 2, 79110 Freiburg, Germany
2 Freiburg Materials Research Center FMF, University of Freiburg, Stefan-Meier-Str. 21, 79104 Freiburg
Photoluminescence (spectroscopy) is an important characterization method in photovoltaics and it has proven to be very valuable for all types of crystalline inorganic solar cells. This is because in these types of devices, the luminescence signal originates from radiative recombination of free electrons and holes and is therefore a direct measure for the product of their concentrations within the photoactive layer. However, in the case of organic solar cells, the interpretation of photoluminescence is much more complex as the absorption of a photon generates a rather strongly bound exciton in the donor or the acceptor phase. Although most photogenerated excitons dissociate into free charge carriers at donor/acceptor interfaces some of them will decay before this happens. In addition, the probability for their decay to be radiative is much larger than for free charge carriers that recombine via charge transfer (CT) states at the donor/acceptor interface. As a consequence, the PL signal of an organic solar cell is dominated by the contribution from photogenerated singlet excitons in the respective material phases and is therefore not directly correlated to the separation of the quasi Fermi levels.
We have developed a new method of how to separate the PL signals of the photogenerated singlet excitons from the one of the free charge carriers in organic solar cells. This paves the way for gathering deeper insight into the working principles of these devices. Moreover, this particular PL method has the advantage of being applicable not only to complete solar cells but also to half cells and even pure absorber films.
Our results from highly efficient organic solar cells show excellent agreement between the quasi Fermi level separation as derived from the PL intensity of free charge carriers and the electrical voltage as measured between the terminals of the devices.
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