Organic molecular solids feature various properties considered advantageous for next-generation photovoltaic devices such as mechanical flexibility and ease of fabrication by, e.g., large-scale and large volume printing. Additionally, Singlet-Exciton Fission may allow surpassing the Shockley-Queisser limit. Here, one photoexcited singlet-type exciton decays into two triplet-type excitons, effectively doubling the number of excited charge carriers. Hence, above-unity quantum efficiencies may be achieved in photovoltaics and have been reported in for example, pentacene (PEN) –C60 heterojunctions.
Here, we study the carrier dynamics at well-defined PEN-C60 interface model systems by time-resolved photoluminescence spectroscopy experiments for different excitation photon energies. Thereby, we disentangle charge transfer and excitation dynamics, i.e., injection, transport, dissociation, and extraction.
The photoluminescence spectra reveal two distinct transition energies associated with charge-transfer (CT) states expected from photoelectron spectroscopy experiments. These long-lived transitions show a clear dependence on excitation energy, corroborating the proposed CT transitions and revealing the fact that carriers need to be created in both individual constituents for CT transitions to be observable.
Additionally, the C60 photoluminescence efficiency strongly quenches for increasing PEN coverage while the lifetime is drastically enhanced yielding strong evidence for an electron transfer between the PEN ground state and C60 when only the latter is photoexcited.
Robin Carl Döring, Andrea Karthäuser, Tobias Breuer, Gregor Witte, and Sangam Chatterjee, "Charge carrier dynamics at the pentacene-C60 interface (Conference Presentation)," Proc. SPIE 10101, Organic Photonic Materials and Devices XIX, 101010Q (Presented at SPIE OPTO: February 01, 2017; Published: 19 April 2017); https://doi.org/10.1117/12.2254336.5390916571001.
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