Issue 26, 2016

The lowest-energy charge-transfer state and its role in charge separation in organic photovoltaics

Abstract

Energy independent, yet higher than 90% internal quantum efficiency (IQE), has been observed in many organic photovoltaics (OPVs). However, its physical origin remains largely unknown and controversial. The hypothesis that the lowest charge-transfer (CT) state may be weakly bound at the interface has been proposed to rationalize the experimental observations. In this paper, we study the nature of the lowest-energy CT (CT1) state, and show conclusively that the CT1 state is localized in typical OPVs. The electronic couplings in the donor and acceptor are found to determine the localization of the CT1 state. We examine the geminate recombination of the CT1 state and estimate its lifetime from first principles. We identify the vibrational modes that contribute to the geminate recombination. Using material parameters determined from first principles and experiments, we carry out kinetic Monte Carlo simulations to examine the charge separation of the localized CT1 state. We find that the localized CT1 state can indeed yield efficient charge separation with IQE higher than 90%. Dynamic disorder and configuration entropy can provide the energetic and entropy driving force for charge separation. Charge separation efficiency depends more sensitively on the dimension and crystallinity of the acceptor parallel to the interface than that normal to the interface. Reorganization energy is found to be the most important material parameter for charge separation, and lowering the reorganization energy of the donor should be pursued in the materials design.

Graphical abstract: The lowest-energy charge-transfer state and its role in charge separation in organic photovoltaics

Article information

Article type
Paper
Submitted
09 Mar 2016
Accepted
03 Jun 2016
First published
03 Jun 2016

Phys. Chem. Chem. Phys., 2016,18, 17546-17556

The lowest-energy charge-transfer state and its role in charge separation in organic photovoltaics

G. Nan, X. Zhang and G. Lu, Phys. Chem. Chem. Phys., 2016, 18, 17546 DOI: 10.1039/C6CP01622G

To request permission to reproduce material from this article, please go to the Copyright Clearance Center request page.

If you are an author contributing to an RSC publication, you do not need to request permission provided correct acknowledgement is given.

If you are the author of this article, you do not need to request permission to reproduce figures and diagrams provided correct acknowledgement is given. If you want to reproduce the whole article in a third-party publication (excluding your thesis/dissertation for which permission is not required) please go to the Copyright Clearance Center request page.

Read more about how to correctly acknowledge RSC content.

Social activity

Spotlight

Advertisements