Document Type

Journal Article

Department/Unit

Department of Physics; Institute of Advanced Material

Language

English

Abstract

Tuning the donor–acceptor (D–A) weight ratio is an essential step to optimize the performance of a bulk heterojunction (BHJ) solar cell. The unoptimized regime with a low acceptor concentration is generally unexplored despite it may reveal the early stage electronic D–A interactions. In this study, PTB7:PC71BM is used to examine factors that limit the device performance in unoptimized regime. The key limiting factor is the creation of traps and localized states originated from fullerene molecules. Photothermal deflection spectroscopy is used to quantify the trap density. Starting with pristine PTB7, addition of small concentration of fullerene increases the electron trap density and lowers the electron mobility. When the D–A weight ratio reaches 1:0.1, fullerene percolation occurs. There is an abrupt drop in trap density and simultaneously a six orders of magnitude increase in the electron mobility. Furthermore, the fill factors of the corresponding photovoltaic devices are found to anticorrelate with the trap density. This study reveals that electron trapping is the key limiting factor for unoptimized BHJ solar cells in low fullerene regime.

Keywords

bulk heterojunction, carrier transport, photothermal deflection spectroscopy, photovoltaics, traps

Publication Date

6-21-2017

Source Publication Title

Advanced Energy Materials

Volume

7

Issue

12

Start Page

1602360

Publisher

Wiley

Peer Reviewed

1

Copyright

This is the peer reviewed version of the following article: C. H. Y. Ho, S. H. Cheung, H.-W. Li, K. L. Chiu, Y. Cheng, H. Yin, M. H. Chan, F. So, S.-W. Tsang, S. K. So, Adv. Energy Mater. 2017, 7, . https://doi.org/10.1002/aenm.201602360, which has been published in final form at http://dx.doi.org/10.1002/aenm.201602360. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Self-Archiving.

Funder

Research Grant Council of Hong Kong. Grant Numbers: HKBU211913, T23-713/11 ; Research Committee of HKBU. Grant Number: RC-ICRS/15-16/4A-SSK

DOI

10.1002/aenm.201602360

Link to Publisher's Edition

http://dx.doi.org/10.1002/aenm.201602360

ISSN (print)

16146832

ISSN (electronic)

16146840

Additional Files

Using Ultralow Dosages of Electron Acceptor_suppl.pdf (635 kB)
Supporting Information

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