A Spray-Processable, Low Bandgap, and Ambipolar Donor−Acceptor Conjugated Polymer

Abstract

Combining a strong donor, tris(dodecyloxy)phenyl)-dithieno[3,2-b:2',3'-d]pyrrole, with a strong acceptor, 4,8-dithien-2-yl-2lambda(4)delta(2)-benzo[1,2-c;4,5-c']bis[1,2,5]thiadiazole, has yielded the lowest bandgap, soluble, spray-processable polymer to date. The polymer has access to four different redox states and shows ambipolar behavior in OFETs. Multiple techniques, including transmission/absorption spectroscopy on SWCNTs and reflectance spectroscopy on gold were used to accurately estimate the optical bandgap at 0.5-0.6 eV, which correlates well to theoretical calculations.

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

“…41 Previous density functional theory (DFT) calculations for BBT-based DA polymers predict a lowest lying excited state energy of 0.5 eV, providing a low onset of optical absorption. 7 Molecular orbital distributions show a HOMO that is delocalized across the entire DA molecule, while the LUMO is relatively localized on the donor, indicating some degree of intramolecular charge transfer during copolymerization. 13,41 The lowest energy absorption band in absorbance spectra, however, has a larger oscillator strength than subsequent excited state transitions, indicating that it is likely dominated by π − π * character.…”

“…The emergence of high-mobility DA polymers with strong electron and hole transport is a result of both HOMO and LUMO electron orbitals that are often well delocalized over the entire DA molecules. 6,7 We can infer from the difference in peak energies that the electron polaron states are more weakly bound, and lie slightly closer in energy to the extended states than the hole polarons. This is also reflected in the higher electronic mobility we observe in transistor characteristics in Fig.…”

“…The difference in energy between the polaronic states and the HOMO and LUMO orbitals indicates the energy gained by the system from the geometrical relaxation associated with the lattice distortion. 19, 34,35 IRAV and polaron spectral features are a hallmark of conducting polymers, and are very well understood in the context of both electrochemical and electrostatic doping 7,10,19,[34][35][36][37][38][39] . The decrease of the gate-induced change in absorption below zero at energies just above the polaronic response is indicative of polymer bleaching.…”

“…As different molecules are substituted into the copolymer structure, the HOMO and LUMO energy levels can shift significantly, affecting the bandgap, environmental stability, and band offsets determining the charge injection properties for various metal contacts. [2][3][4][5][6][7]11 Electronic transport, especially, is extremely sensitive to the choice of donor. The emergence of high-mobility DA polymers with strong electron and hole transport is a result of both HOMO and LUMO electron orbitals that are often well delocalized over the entire DA molecules.…”

“…[2][3][4] Specifically, the donor-acceptor (DA) approach to synthesizing polymers has led to a new generation of high-mobility ambipolar systems, a necessary precondition for many transistor, photovoltaic, and light-emitting device applications. [5][6][7][8] Recently, DA polymers based on acceptor benzobisthiadiazole (BBT) 7,9,10 and donor diketopyrrolopyrrole (DPP) [10][11][12][13][14][15] have demonstrated strong ambipolarity as well as exceptionally low optical energy badgaps as small as 0.5 eV. 11 The highest field-effect mobilities in these systems have surpassed 1 cm 2 V −1 s −1 for both electrons and holes.…”