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Proceedings Paper

A predictive theory of charge separation in organic photovoltaics interfaces
Author(s): Alessandro Troisi; Tao Liu; Domenico Caruso; David L. Cheung; David P. McMahon
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Paper Abstract

The key process in organic photovoltaics cells is the separation of an exciton, close to the donor/acceptor interface into a free hole (in the donor) and a free electron (in the acceptor). In an efficient solar cell, the majority of absorbed photons generate such hole-electron pairs but it is not clear why such a charge separation process is so efficient in some blends (for example in the blend formed by poly(3- hexylthiophene) (P3HT) and a C60 derivative (PCBM)) and how can one design better OPV materials. The electronic and geometric structure of the prototypical polymer:fullerene interface (P3HT:PCBM) is investigated theoretically using a combination of classical and quantum simulation methods. It is shown that the electronic structure of P3HT in contact with PCBM is significantly altered compared to bulk P3HT. Due to the additional free volume of the interface, P3HT chains close to PCBM are more disordered and, consequently, they are characterized by an increased band gap. Excitons and holes are therefore repelled by the interface. This provides a possible explanation of the low recombination efficiency and supports the direct formation of “quasi-free” charge separated species at the interface. This idea is further explored here by using a more general system-independent model Hamiltonian. The long range exciton dissociation rate is computed as a function of the exciton distance from the interface and the average dissociation distance is evaluated by comparing this rate with the exciton migration rate with a kinetic model. The phenomenological model shows that also in a generic interface the direct formation if quasi-free charges is extremely likely.

Paper Details

Date Published: 13 September 2012
PDF: 5 pages
Proc. SPIE 8477, Organic Photovoltaics XIII, 84770Q (13 September 2012); doi: 10.1117/12.931662
Show Author Affiliations
Alessandro Troisi, The Univ. of Warwick (United Kingdom)
Tao Liu, The Univ. of Warwick (United Kingdom)
Domenico Caruso, The Univ. of Warwick (United Kingdom)
David L. Cheung, The Univ. of Warwick (United Kingdom)
David P. McMahon, The Univ. of Warwick (United Kingdom)

Published in SPIE Proceedings Vol. 8477:
Organic Photovoltaics XIII
Zakya H. Kafafi; Christoph J. Brabec; Paul A. Lane, Editor(s)

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