Relating Chain Conformation to the Density of States and Charge Transport in Conjugated Polymers: The Role of the β-phase in Poly(9,9-dioctylfluorene)
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Author(s)
Type
Journal Article
Abstract
Charge transport in π-conjugated polymers is characterised by a strong degree of disorder in both the energy of conjugated segments and the electronic coupling between adjacent sites. This disorder arises from variations in the structure and conformation of molecular units, as well as the weak inter-molecular binding interactions. Although disorder in molecular conformation can be expected to influence the density of states (DoS) distribution, and hence optoelectronic properties of the material, until now, there has been no direct study of the relationship between a distinct conformational defect and the charge transport properties of a conjugated polymer. Here, we investigate the impact of introducing an extended, planarised chain geometry, known as the ‘β-phase’, on hole transport through otherwise amorphous films of poly(9,9-dioctylfluorene) (PFO). We show that whilst β-phase introduces a striking ~hundredfold drop in time-of-flight (ToF) hole mobility (μh) at room temperature, it reduces the steady-state μh measured from hole-only devices by a factor of less than ~5. In order to reconcile these observations, we combine high-dynamic-range ToF photocurrent spectroscopy and energy-resolved electrochemical impedance spectroscopy to extract the hole DoS of the conjugated polymer. Both methods show that the effect of the β-phase content is to introduce a sharp sub-bandgap feature into the DoS of glassy PFO lying ~0.3 eV above the highest occupied molecular orbital. The observed energy of the conformational trap is consistent with electronic structure calculations using a tight-binding approach. Using the obtained DoS with a drift-diffusion model capable of resolving charge carriers in both time and energy, we show how the seemingly contradictory transport phenomena obtained via the time-resolved, frequency-resolved, and steady-state methods are reconciled. The results highlight the significance of energetic redistribution of charge carriers in affecting transport behaviour. This work demonstrates how charge-carrier mobility in organic semiconductors can be controlled via molecular conformation and resolves a long-standing debate over how different (equilibrium versus non-equilibrium) transport techniques reveal electronic properties of disordered solids in a unified manner.
Date Issued
2019-05-24
Date Acceptance
2019-03-20
Citation
Physical Review X, 2019, 9 (2)
ISSN
2160-3308
Publisher
American Physical Society
Journal / Book Title
Physical Review X
Volume
9
Issue
2
Copyright Statement
© 2019 The Author(s). Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.
License URL
Sponsor
Engineering & Physical Science Research Council (EPSRC)
Engineering & Physical Science Research Council (EPSRC)
Engineering & Physical Science Research Council (EPSRC)
Commission of the European Communities
Identifier
https://www.webofscience.com/wos/woscc/full-record/WOS:000469032100001
Grant Number
EP/M025020/1
EP/P005543/1
EP/K029843/1
742708
Subjects
Science & Technology
Physical Sciences
Physics, Multidisciplinary
Physics
CARRIER MOBILITY
ELECTRONIC-STRUCTURE
HOLE TRANSPORT
SOLAR-CELLS
TRAP STATES
PHOTOCURRENT MEASUREMENTS
TRANSIENT SPECTROSCOPY
LOCALIZED STATES
PHOTOPHYSICS
AGGREGATION
0206 Quantum Physics
Publication Status
Published
Article Number
ARTN 021038
Date Publish Online
2019-05-24