Issue 23, 2017

On the energetics of bound charge-transfer states in organic photovoltaics

Abstract

A comprehensive understanding of the charge generation mechanism in organic solar cells is critical for further improvement of device performance. Currently, the origin and magnitude of the coulombic binding energy of the charge-transfer state (CTS), an intermediate state which is fundamental for the charge separation process, are still under debate. Here, we propose a new approach for determining the dissociation energy of localised CTSs for a range of devices with different alignments of molecular energy levels (tuned by chemical modifications of fullerene) and disorder (adjusted by the blend composition) using temperature-dependent pump-push photocurrent spectroscopy. We observe that the dissociation of localised CTSs from initial excitation is a temperature-dependent process, and we determined the binding energy of these CTSs by measuring a single activation energy over a wide temperature range. We propose a simple qualitative picture to explain the observation, based on the split between the bound CTSs and free charges. In all the material systems studied here, the activation energy falls within the range of 90 ± 50 meV (corresponding to ∼1 nm separation of an electron–hole pair). Surprisingly, the binding energy does not depend on the material composition or the driving energy (∼150 meV variation) for charge separation. In contrast, the number of formed bound states and their following recombination dynamics are material- and nanomorphology-sensitive. Such observations in the studied benchmark polymer:fullerene systems reveal unexpected similarities in the energetics of CTSs formed in different electronic environments. This makes our results of general importance for understanding the photophysics at the heterojunction interface and for further development of organic photovoltaics.

Graphical abstract: On the energetics of bound charge-transfer states in organic photovoltaics

Supplementary files

Article information

Article type
Paper
Submitted
07 Yan 2017
Accepted
30 Mud 2017
First published
31 Mud 2017

J. Mater. Chem. A, 2017,5, 11949-11959

On the energetics of bound charge-transfer states in organic photovoltaics

J. Zhang, A. C. Jakowetz, G. Li, D. Di, S. M. Menke, A. Rao, R. H. Friend and A. A. Bakulin, J. Mater. Chem. A, 2017, 5, 11949 DOI: 10.1039/C7TA01190C

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