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.
Perylene-3,4,9,10-tetracarboxylic acid (PTCDA) and respective derivatives (e.g. perylene diimide - PDI) are widely used as dyes but also for device applications such as organic field effect transistors or in organic photovoltaics. Due to their intrinsically high quantum efficiencies they are also used as spectroscopic standards. One major drawback of these materials is their low solubility in organic solvents which can be addressed by long alkyl substitutions.
When introducing a tertiary amine into the molecule a mechanism known as photoinduced electron transfer (PET) can occur. Here, following an optically excited HOMO-LUMO transition of the core, an electron from the electron lone pair of the amine is transferred to the HOMO of the perylene core. Hence, radiative recombination is disallowed and photoluminescence effectively quenched.
Here, we perform a systematic study of the distance dependence of the PET by introducing alkyle groups as spacer units between PDI core and the tertiary amine. Dynamics of the PET are extracted from ultrafast time-resolved photoluminescence measurement data.
A rate equation model, simulating a three level system, reveals rate constant of the back electron transfer, otherwise not accessible with our experimental methods. Assuming a Marcus model of electron transfer, electronic coupling strength between the electronic states involved in the respective transitions can be calculated. In addition to the distance dependence, the effects of protonation and methylation of the the tertiary amine units are studied.
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