Electron and hole polaron accumulation in low-bandgap ambipolar donor-acceptor polymer transistors imaged by infrared microscopy

Abstract

A resurgence in the use of the donor-acceptor (DA) approach in synthesizing conjugated polymers has resulted in a family of high-mobility ambipolar systems with exceptionally narrow energy bandgaps below 1 eV. The ability to transport both electrons and holes is critical for device applications such as organic light-emitting diodes (OLEDs) and transistors (OLETs). Infrared spectroscopy offers direct access to the low-energy excitations associated with injected charge carriers. Here we use a diffraction-limited IR microscope to probe the spectroscopic signatures of electron and hole injection in the conduction channel of an organic field-effect transistor (OFET) based on an ambipolar DA polymer polydiketopyrrolopyrrole-benzobisthiadiazole (PDPPBBT). We observe distinct polaronic absorptions for both electrons and holes, and spatially map the carrier distribution from the source to drain electrodes for both unipolar and ambipolar biasing regimes. For ambipolar device configurations, we observe the spatial evolution of hole-induced to electron-induced polaron absorptions throughout the transport path. Our work provides a platform for combined transport and infrared studies of organic semiconductors on micron length scales relevant to functional devices.

Intrinsically distinct hole and electron transport in conjugated polymers controlled by intra and intermolecular interactions

“…Here, through Charge Modulation Spectroscopy and Microscopy (CMS and CMM) studies on operating organic field-effect transistors (OFETs), 26 as well as polarized UV-Vis and Transient Absorption (TA) spectroscopy acquired on thin DPP polymer films, we highlight for the first time the preferential ground state molecular interactions which can selectively favor either hole or electron transport in a donoracceptor copolymer semiconductor. A first evidence of the different spectral signature of holes and electron polarons found in the charge absorption region was presented by Kathib et al 27,28 However, the two polarons signature appeared as a broad band in the near-infrared (NIR) region making it difficult to provide information on their inter-or intra-chain nature. Here, by comparing polarization dependent ground state absorption spectra of anisotropically aligned polymer films with CMS bleaching spectra, we observe that holes and electrons selectively bleach different features of the ground state absorption.…”

Fast Holes, Slow Electrons, and Medium Control of Polaron Size and Mobility in the DA Polymer F8BT

“…10 More recently, electron accumulation has been used to study negative polarons in N2200 11 and both holes and electrons in homopolymer polyselenophene, 12 and in donor−acceptor (DA) polymers using both CMS 7 and a DC bias method. 13 An important artifact to be aware of in these types of measurements arises if the detector picks up fluorescence from the, often highly luminescent, polymer which can be modulated by a very efficient exciton-polaron quenching mechanism; however, this can be corrected with the use of an appropriate filter. 7 Pulse radiolysis uses a high energy, short (<50 ps) electron pulse to rapidly inject charges, of the desired polarity and a known concentration, onto polymer chains in many different solvents.…”

First-Principles Study of the Nuclear Dynamics of Doped Conjugated Polymers

“…30 In nondegenerate ground state polymers, IRAV modes are directly related to charged polarons. 23,31 Thanks to their specificity and sensitivity, over the past two decades IRAV modes have been largely employed as a direct probe of polaron density in pristine polymers, 26,28,32 chargetransfer processes and polaron dynamics in donor−acceptor blends, 23,31,33,34 structure−charge relationships in polymers, 26,35,36 polaron spatial distribution within the active region of working devices, 37,38 and, most recently, coupling of polarons to surface plasmons in infrared nanoantennas. 39 From the theoretical standpoint, substantial work was done to understand and rationalize the description of IRAV modes.…”