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

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:PC 71BM 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:PC 71BM 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. Lastly, 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 lowermore » band gaps are developed.« less
Authors:
 [1] ; ORCiD logo [2] ;  [2] ;  [3] ; ORCiD logo [4] ;  [4] ;  [3] ;  [1] ; ORCiD logo [2] ; ORCiD logo [5]
  1. Univ. of Washington, Seattle, WA (United States)
  2. Univ. of Southern Mississippi, Hattiesburg, MS (United States)
  3. Univ. of California, Riverside, CA (United States)
  4. Univ. of California San Diego, La Jolla, CA (United States)
  5. Brookhaven National Lab. (BNL), Upton, NY (United States)
Publication Date:
Report Number(s):
BNL-205769-2018-JAAM
Journal ID: ISSN 2195-1071
Grant/Contract Number:
SC0012704; SC0016269
Type:
Accepted Manuscript
Journal Name:
Advanced Optical Materials
Additional Journal Information:
Journal Volume: 6; Journal Issue: 7; Journal ID: ISSN 2195-1071
Publisher:
Wiley
Research Org:
Brookhaven National Laboratory (BNL), Upton, NY (United States)
Sponsoring Org:
USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22)
Country of Publication:
United States
Language:
English
Subject:
36 MATERIALS SCIENCE; organic electronics; ultrafast spectroscopy; delocalization; push-pull; bulk heterojunction
OSTI Identifier:
1454810
Alternate Identifier(s):
OSTI ID: 1420339

Sulas, Dana B., London, Alexander E., Huang, Lifeng, Xu, Lihua, Wu, Zhenghui, Ng, Tse Nga, Wong, Bryan M., Schlenker, Cody W., Azoulay, Jason D., and Sfeir, Matthew Y.. Preferential Charge Generation at Aggregate Sites in Narrow Band Gap Infrared Photoresponsive Polymer Semiconductors. United States: N. p., Web. doi:10.1002/adom.201701138.
Sulas, Dana B., London, Alexander E., Huang, Lifeng, Xu, Lihua, Wu, Zhenghui, Ng, Tse Nga, Wong, Bryan M., Schlenker, Cody W., Azoulay, Jason D., & Sfeir, Matthew Y.. Preferential Charge Generation at Aggregate Sites in Narrow Band Gap Infrared Photoresponsive Polymer Semiconductors. United States. doi:10.1002/adom.201701138.
Sulas, Dana B., London, Alexander E., Huang, Lifeng, Xu, Lihua, Wu, Zhenghui, Ng, Tse Nga, Wong, Bryan M., Schlenker, Cody W., Azoulay, Jason D., and Sfeir, Matthew Y.. 2018. "Preferential Charge Generation at Aggregate Sites in Narrow Band Gap Infrared Photoresponsive Polymer Semiconductors". United States. doi:10.1002/adom.201701138.
@article{osti_1454810,
title = {Preferential Charge Generation at Aggregate Sites in Narrow Band Gap Infrared Photoresponsive Polymer Semiconductors},
author = {Sulas, Dana B. and London, Alexander E. and Huang, Lifeng and Xu, Lihua and Wu, Zhenghui and Ng, Tse Nga and Wong, Bryan M. and Schlenker, Cody W. and Azoulay, Jason D. and Sfeir, Matthew Y.},
abstractNote = {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. Lastly, 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.},
doi = {10.1002/adom.201701138},
journal = {Advanced Optical Materials},
number = 7,
volume = 6,
place = {United States},
year = {2018},
month = {2}
}

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