Spectroscopic investigation of the effect of microstructure and energetic offset on the nature of interfacial charge transfer states in polymer: fullerene blends
File(s)
Published version
Author(s)
Type
Journal Article
Abstract
Despite performance improvements of organic photovoltaics, the mechanism of photoinduced electron-hole separation at organic donor-acceptor interfaces remains poorly understood. Inconclusive experimental and theoretical results have produced contradictory models for electron-hole separation in which the role of interfacial charge-transfer (CT) states is unclear, with one model identifying them as limiting separation and another as readily dissociating. Here, polymer-fullerene blends with contrasting photocurrent properties and enthalpic offsets driving separation were studied. By modifying composition, film structures were varied from consisting of molecularly mixed polymer-fullerene domains to consisting of both molecularly mixed and fullerene domains. Transient absorption spectroscopy revealed that CT state dissociation generating separated electron-hole pairs is only efficient in the high energy offset blend with fullerene domains. In all other blends (with low offset or predominantly molecularly mixed domains), nanosecond geminate electron-hole recombination is observed revealing the importance of spatially localized electron-hole pairs (bound CT states) in the electron-hole dynamics. A two-dimensional lattice exciton model was used to simulate the excited state spectrum of a model system as a function of microstructure and energy offset. The results could reproduce the main features of experimental electroluminescence spectra indicating that electron-hole pairs become less bound and more spatially separated upon increasing energy offset and fullerene domain density. Differences between electroluminescence and photoluminescence spectra could be explained by CT photoluminescence being dominated by more-bound states, reflecting geminate recombination processes, while CT electroluminescence preferentially probes less-bound CT states that escape geminate recombination. These results suggest that apparently contradictory studies on electron-hole separation can be explained by the presence of both bound and unbound CT states in the same film, as a result of a range of interface structures.
Date Issued
2019-03-20
Date Acceptance
2019-02-01
Citation
Journal of the American Chemical Society, 2019, 141 (11), pp.4634-4643
ISSN
0002-7863
Publisher
American Chemical Society
Start Page
4634
End Page
4643
Journal / Book Title
Journal of the American Chemical Society
Volume
141
Issue
11
Copyright Statement
© 2019 American Chemical Society. This is an open access article published under a Creative Commons Attribution (CC-BY) License, which permits unrestricted use, distribution and reproduction in any medium,provided the author and source are cited. https://pubs.acs.org/page/policy/authorchoice_ccby_termsofuse.html
License URL
Sponsor
Engineering and Physical Sciences Research Council
Identifier
https://www.ncbi.nlm.nih.gov/pubmed/30807130
Subjects
Science & Technology
Physical Sciences
Chemistry, Multidisciplinary
Chemistry
ORGANIC SOLAR-CELLS
INTERNAL QUANTUM EFFICIENCY
TRANSFER EXCITONS
FIELD-DEPENDENCE
SEPARATION
DISSOCIATION
RECOMBINATION
MISCIBILITY
DYNAMICS
ACCEPTOR
General Chemistry
03 Chemical Sciences
Publication Status
Published
Coverage Spatial
United States
Date Publish Online
2019-02-26