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Preferential Charge Generation at Aggregate Sites in Narrow Band Gap Infrared Photoresponsive Polymer Semiconductors
Author(s) -
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.
Publication year - 2018
Publication title -
advanced optical materials
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.89
H-Index - 91
ISSN - 2195-1071
DOI - 10.1002/adom.201701138
Subject(s) - materials science , exciton , optoelectronics , ultrafast laser spectroscopy , band gap , charge carrier , infrared , picosecond , ultrashort pulse , excited state , delocalized electron , polymer , absorption (acoustics) , spectroscopy , optics , atomic physics , chemistry , condensed matter physics , physics , organic chemistry , laser , quantum mechanics , composite material
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 71 BM 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 71 BM 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.