Charge Dynamics in Organic Photovoltaic Materials: Interplay between Quantum Diffusion and Quantum Relaxation

Abstract

Original citation:Lee, Myeong H., Aragó, Juan and Troisi, Alessandro. (2015) Charge dynamics in organic photovoltaic materials : interplay between quantum diffusion and quantum relaxation. The Journal of Physical Chemistry Part C, 119 (27). pp. 14989-14998. Permanent WRAP url:http://wrap.warwick.ac.uk/73402 Copyright and reuse:The Warwick Research Archive Portal (WRAP) makes this work by researchers of the University of Warwick available open access under the following conditions. Copyright © and all moral rights to the version of the paper presented here belong to the individual author(s) and/or other copyright owners. To the extent reasonable and practicable the material made available in WRAP has been checked for eligibility before being made available.Copies of full items can be used for personal research or study, educational, or not-for profit purposes without prior permission or charge. Provided that the authors, title and full bibliographic details are credited, a hyperlink and/or URL is given for the original metadata page and the content is not changed in any way. Publisher's statement:This document is the Accepted Manuscript version of a Published Work that appeared in final form in The Journal of Physical Chemistry Part C, copyright © American Chemical Society after peer review and technical editing by the publisher. To access the final edited and published work, see http://dx.doi.org/10.1021/acs.jpcc.5b03989 A note on versions:The version presented here may differ from the published version or, version of record, if you wish to cite this item you are advised to consult the publisher's version. Please see the 'permanent WRAP url' above for details on accessing the published version and note that access may require a subscription. For more information, please contact the WRAP This paper discusses the mechanism of generation of free charges in organic photovoltaic cells (OPV) from electrostatically bound electron-hole pairs. The efficiency of this process is explained when interfacial charge-transfer (CT) states are generated by direct optical excitation. We used semiclassical quantum dynamics at short timescale (∼100 fs) and Redfield theory at relatively long timescale (∼10-100 ps) to cover both the process of dissociation and the relaxation to the lowest energy state. Our calculations suggest that a CT state with an intermediate electron-hole separation can evolve into a charge-separated (CS) state on ultrafast timescales (∼100 fs) as a result of quantum diffusion. On long timescales, however, the CS states ultimately relax to the low-energy CT states due to the interaction with the thermal bath, indicating that the yield of free charge carrier generation is determined by the interplay between ultrafast charge separation, due to quantum diffusion, and the much slower quantum relaxation process.

Quantum dynamics of a vibronically coupled linear chain using a surrogate Hamiltonian approach

“…Such a system represents a good minimal model [90][91][92][93] for the process of charge generation/recombination in OPV cells. These states can be thought as those of an electron acceptor under the influence of a positive hole in the electron donor separated by a distance D from the nearest electron.…”

Modeling organic electronic materials: bridging length and time scales

“…A tight-binding Hamiltonian is a matrix of interactions between neighbouring states (commonly referred to as coupling between states). An example of a model Hamiltonian is provided in Equations (4)- (7) [55]. This set of equations modifies the typical tight-binding Hamiltonian (Equation (5)) to include coupling to a bath of phonons.…”

“…To approximate disorder, the on-site energies and coupling constants have been randomised around a mean value [47,[57][58][59]. Recently, coupling to vibrational modes and a thermal bath have been added to model Hamiltonian approaches to test the effects of dynamic and static disorder on electronic dynamics [55].…”

“…al. provide an example of a model Hamiltonian approach applied to OSCs to explain the charge separation through quantum diffusion [55]. Figure 3 shows a schematic of what the model Hamiltonian represents, which is accompanied by some relevant results.…”

“…At experimental temperatures, we expect multiple instances of each configuration present in Figure 2(1), which demonstrates the importance of sampling multiple configurations when relating DFT simulation results to OSC properties. In addition to charge-transfer properties, Gaussian based DFT can be used to parameterise classical molecular dynamics models [54] and model Hamiltonian based schemes [55]. Molecular dynamics simulations require an estimation of partial atomic charges, and the potential arising from the nuclei and the electron density (determined from DFT simulations of a particular conformer) is fit to an effective electrostatic potential originating from non-integer charges centred at the nuclear positions [56].…”