Preferential Charge Generation at Aggregate Sites in Narrow Band Gap Infrared Photoresponsive Polymer Semiconductors

Authors

Dana B. Sulas, University of Washington
Alexander E. London, University of Southern Mississippi
Lifeng Huang, University of Southern MississippiFollow
Lihua Xu, University of California, Riverside
Zhenghui Wu, University of California, San DiegoFollow
Tse Nga Ng, University of California, San DiegoFollow
Bryan M. Wong, University of California, Riverside
Cody W. Schlenker, University of Washington
Jason D. Azoulay, University of Southern MississippiFollow
Matthew Y. Sfeir, Brookhaven National LaboratoryFollow

Document Type

Article

Publication Date

2-13-2018

Department

Polymers and High Performance Materials

Abstract

Infrared organic photodetector materials are investigated using transient absorption spectroscopy, demonstrating that ultrafast charge generation assisted by polymer aggregation is essential to compensate for the energy gap law, which dictates that excited state lifetimes decrease as the band gap narrows. Short sub‐picosecond singlet exciton lifetimes are measured in a structurally related series of infrared‐absorbing copolymers that consist of alternating cyclopentadithiophene electron‐rich “push” units and strong electron‐deficient “pull” units, including benzothiadiazole, benzoselenadiazole, pyridalselenadiazole, or thiadiazoloquinoxaline. While the ultrafast lifetimes of excitons localized on individual polymer chains suggest that charge carrier generation will be inefficient, high detectivity for polymer:PC₇₁BM infrared photodetectors is measured in the 0.6 < λ < 1.5 µm range. The photophysical processes leading to charge generation are investigated by performing a global analysis on transient absorption data of blended polymer:PC₇₁BM films. In these blends, charge carriers form primarily at polymer aggregate sites on the ultrafast time scale (within our instrument response), leaving quickly decaying single‐chain excitons unquenched. The results have important implications for the further development of organic infrared optoelectronic devices, where targeting processes such as excited state delocalization over aggregates may be necessary to mitigate losses to ultrafast exciton decay as materials with even lower band gaps are developed.

Publication Title

Advanced Optical Materials

Volume

6

Issue

7

Recommended Citation

Sulas, D. B., London, A. E., Huang, L., Xu, L., Wu, Z., Ng, T. N., Wong, B. M., Schlenker, C. W., Azoulay, J. D., Sfeir, M. Y. (2018). Preferential Charge Generation at Aggregate Sites in Narrow Band Gap Infrared Photoresponsive Polymer Semiconductors. Advanced Optical Materials, 6(7).
Available at: https://aquila.usm.edu/fac_pubs/15853