Experimental and Theoretical Investigation into the Polaron Structure of K-Doped Polyfluorene Films

Yongzhen Chen,Sarbani Ghosh,Slawomir Braun,Mats Fahlman,Igor V Zozoulenko,Xianjie Liu

doi:10.1021/acs.jpcc.0c08442

Abstract

The evolution of the electronic structure and optical transition upon n-doping of poly(9,9-dioctylfluorene) (PFO) films is elucidated with photoelectron spectroscopy, optical absorption, density functional theory (DFT), and time-dependent DFT (TD-DFT) calculations. Optical absorption measurements extending into near infrared show two low-energy absorption features at low doping ratios and an additional peak at a higher energy of ∼2.2 eV that disappears with increasing doping ratios. A gap state (i.e., polaronic state) close to the Fermi level and a significantly destabilized highest valence band appear in the experimentally measured ultraviolet photoelectron spectra. These experimental results are interpreted by the TD-DFT calculations, which show that the lower energy peaks originate from the excitation from polaronic states to the conduction band, while the higher energy peak mainly originates from the destabilized valence band to conduction band transitions and only appears at low doping ratios (cred ≤ 50%, 0.5 potassium atom per fluorene monomer). The DFT calculations further indicate that polaron pairs rather than bipolarons are preferentially formed at high doping ratios. Comparing the results of doped glassy and β-phase films, we find that the ordered segments in the β-phase film disappear due to the dopant (potassium) insertion, resulting in a similar polaronic structure.

Highlights

I.e., quasi-particles formed upon electron−phonon coupling, are of the essence in conjugated polymers and are involved in charge transport and exciton dissociation.[1−4] Polarons are localized charge carriers whose energy levels reside in the forbidden energy gap, and they are affected by the extent of the charge localization and structure reorganization of the polymer (“stiff” vs “soft” conjugated backbone), which is critical to the design of materials for devices.[5−8] Chemical doping creates polarons through charge transfer from oxidants (p-doping) or reductants (n-doping),[9−11] and this process is utilized in electrochemical devices such as light-emitting electrochemical cells and organic electrochemical transistors.[12−15] Chemically doped organic semiconductors are used as transparent electrode materials, PEDOT:PSS being a widely used example thereof.[16,17]
By comparing the experimental and theoretical results, we provide the assignments of the absorption peaks appearing upon doping
We investigate the electronic structures of polarons in K-doped glassy and β-phase PFO films by UV− vis−NIR absorption and photoelectron spectroscopy

Summary

Introduction

I.e., quasi-particles formed upon electron−phonon coupling, are of the essence in conjugated polymers and are involved in charge transport and exciton dissociation.[1−4] Polarons are localized charge carriers whose energy levels reside in the forbidden energy gap, and they are affected by the extent of the charge localization and structure reorganization of the polymer (“stiff” vs “soft” conjugated backbone), which is critical to the design of materials for devices.[5−8] Chemical doping creates polarons through charge transfer from oxidants (p-doping) or reductants (n-doping),[9−11] and this process is utilized in electrochemical devices such as light-emitting electrochemical cells and organic electrochemical transistors.[12−15] Chemically doped organic semiconductors are used as transparent electrode materials, PEDOT:PSS being a widely used example thereof.[16,17]To better understand their formation mechanism and optical properties, polarons were widely investigated both by experiment and theory in the past decades.[1,18−26] Occupied polaron states can be directly detected using ultraviolet photoelectron spectroscopy (UPS),[26−28] and transitions involving polaron states can be probed by UV−vis−NIR optical absorption measurements.[20−24] The theoretical description of polarons was initially limited by the semi-empirical approaches used,[18,29,30] but recent work using density functional theory (DFT) has shed new light on the nature of polaron states in organic semiconductors and the doping-induced optical transitions, largely altering the pre-DFT description of both the electronic structure and optical transitions resulting from polaron formation.[31−33] The p-doping of organic semiconductors has been extensively studied, as the p-doped organic semiconductors typically are air-stable, which enables easy experimental correlation between UPS data on the (occupied) electronic structure and UV−vis−NIR absorption measurements that provide optical transition energies and indirect data on the unoccupied states.[9,33−35] Comprehensive studies on the electronic structure and optical properties of ndoped organic semiconductors are rare because they require a capability for in situ UPS and vis−NIR spectroscopy to keep the samples in an ultra-high vacuum, water- and oxygen-free environment during sequential doping and measurements.In our present work, we investigate the polaronic structures of K-doped poly(9,9-dioctylfluorene) (PFO) films by UV−Received: September 16, 2020 Revised: November 24, 2020 Published: December 22, 2020The Journal of Physical Chemistry C vis−NIR spectroscopy, X-ray photoelectron spectroscopy (XPS), and UPS. The two conformations enable us to probe the effects of molecular order, if any, on the doping process and its resulting polaronic structure

Methods

Results

Discussion

Conclusion