What is the Binding Energy of a Charge Transfer State in an Organic Solar Cell?

The high efficiencies reported for organic solar cells and an almost negligible thermal activation measured for the photogeneration of charge carriers have called into question whether photoinduced interfacial charge transfer states are bound by a significant coulomb attraction, and how this can be...

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Bibliographic Details
Published in:	Advanced energy materials Vol. 9; no. 24
Main Authors:	Athanasopoulos, Stavros, Schauer, Franz, Nádaždy, Vojtech, Weiß, Mareike, Kahle, Frank‐Julian, Scherf, Ullrich, Bässler, Heinz, Köhler, Anna
Format:	Journal Article
Language:	English
Published:	Weinheim Wiley Subscription Services, Inc 01-06-2019
Subjects:	Activation energy Binding energy Charge transfer Computer simulation Current carriers Electrochemical impedance spectroscopy Energy Entropy Organic chemistry organic photovoltaic Photodissociation Photoelectric effect Photoelectric emission Photovoltaic cells Questions Solar cells Temperature dependence Topology
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Summary:	The high efficiencies reported for organic solar cells and an almost negligible thermal activation measured for the photogeneration of charge carriers have called into question whether photoinduced interfacial charge transfer states are bound by a significant coulomb attraction, and how this can be reconciled with very low activation energies. Here, this question is addressed in a combined experimental and theoretical approach. The interfacial binding energy of a charge‐transfer state in a blend of MeLPPP:PCBM is determined by using energy resolved electrochemical impedance spectroscopy and is found to be about 0.5 eV. Temperature‐dependent photocurrent measurements on the same films, however, give an activation energy that is about one order of magnitude lower. Using analytical calculations and Monte Carlo simulation the authors illustrate how i) interfacial energetics and ii) transport topology reduce the activation energy required to separate the interfacial electron–hole pair, with about equal contributions from both effects. The activation energy, however, is not reduced by entropy, although entropy increases the overall photodissociation yield. The coulomb binding energy of an electron and a hole on adjacent chromophores is in the order of 0.5 eV, yet for efficient solar cells, very little activation energy is required for the photodissociation of excitations. It is shown here how the combined effects of interfacial electrostatics, wave function delocalization, and disorder can account for this.
ISSN:	1614-6832 1614-6840
DOI:	10.1002/aenm.201900814