Department of Physics; Institute of Advanced Material
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.
bulk heterojunction, carrier transport, photothermal deflection spectroscopy, photovoltaics, traps
Source Publication Title
Advanced Energy Materials
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.
Research Grant Council of Hong Kong. Grant Numbers: HKBU211913, T23-713/11 ; Research Committee of HKBU. Grant Number: RC-ICRS/15-16/4A-SSK
Link to Publisher's Edition
Ho, Carr Hoi Yi, Sin Hang Cheung, Ho-Wa Li, Ka Lok Chiu, Yuanhang Cheng, Hang Yin, Mau Hing Chan, Franky So, Sai-Wing Tsang, and Shu Kong So. "Using ultralow dosages of electron acceptor to reveal the early stage donor–acceptor electronic interactions in bulk heterojunction blends." Advanced Energy Materials 7.12 (2017): 1602360.
Additional FilesUsing Ultralow Dosages of Electron Acceptor_suppl.pdf (635 kB)