Design, Synthesis, and Structure–Property Relationships of Isoindigo-Based Conjugated Polymers

Abstract

Conjugated polymers have developed rapidly due to their promising applications in low-cost, lightweight, and flexible electronics. The development of the third-generation donor-acceptor (D-A) polymers greatly improved the device performance in organic solar cells (OSCs) and field-effect transistors (FETs). However, for further improvement of device performance, scientists need to develop new building blocks, in particular electron-deficient aromatics, and gain an in-depth understanding of the structure-property relationships. Recently, isoindigo has been used as a new acceptor of D-A conjugated polymers. An isomer of indigo, isoindigo is a less well-known dye and can be isolated as a by-product from certain biological processes. It has two lactam rings and exhibits strong electron-withdrawing character. This electron deficiency gives isoindigo-based polymers intriguing properties, such as broad absorption and high open circuit voltage in OSCs, as well as high mobility and good ambient stability in FETs. In this Account, we review our recent progress on the design, synthesis, and structure-property relationship study of isoindigo-based polymers for FETs. Starting with some discussion on carrier transport in polymer films, we provide some basic strategies towards high-performance polymer FETs. We discuss the stability issue of devices, the impediment of the alkyl side chains, and the choice of the donor part of conjugated polymers. We demonstrate that introducing the isoindigo core effectively lowers the HOMO levels of polymers and provides FETs with long-time stability. In addition, we have found that when we use inappropriate alkyl side chains or non-centrosymmetric donors, the device performance of isoindigo polymers suffers. To further improve device performance and ambient stability, we propose several design strategies, such as using farther branched alkyl chains, modulating polymer energy levels, and extending π-conjugated backbones. We have found that using farther branched alkyl chains can effectively decrease interchain π-π stacking distance and improve carrier mobility. When we introduce electron-deficient functional groups on the isoindigo core, the LUMO levels of the polymers markedly decrease, which significantly improves the electron mobility and device stability. In addition, we present a new polymer system called BDOPV, which is based on the concept of π-extended isoindigo. By application of some strategies successfully used in isoindigo-based polymers, BDOPV-based polymers exhibit high mobility and good stability both in n-type and in ambipolar FETs. We believe that a synergy of molecular engineering strategies towards the isoindigo core, donor units, and side chains may further improve the performance and broaden the application of isoindigo-based polymers.

Diketopyrrolopyrrole‐Based Conjugated Polymer Entailing Triethylene Glycols as Side Chains with High Thin‐Film Charge Mobility without Post‐Treatments

“…Moreover, a scattering signal at q y = 17.4 nm −1 in the in‐plane direction emerges for thin film of PDPP3T‐1, which is attributed to the interchain π−π stacking with a distance of 0.361 nm. These GIWAXS data indicate that polymer chains of PDPP3T‐1 are predominantly packed in edge‐on mode on the OTS ( n ‐octadecyltrichlorosilane) modified SiO 2 /Si substrate, which is believed to be favorable for charge transport 2, 16. It is noted that the GIWAXS pattern of PDPP3T‐1 is almost unchanged aside from a slight enhancement in scattering intensities after thermal annealing at 100 °C, in good agreement with the observation that thin‐film hole mobility remains almost unchanged following thermal annealing.…”

“…For instance, it was reported that charge mobilities of isoindigo/DPP‐based conjugated polymers were enhanced by moving the branching points of branching alkyl chains away from the conjugated backbone 2, 28. Increased charge mobilities were also reported for conjugated polymers where pure alkyl side chains were replaced with siloxane‐terminated alkyl, fluoroalkyl, or oligo(ethylene glycol) chains 29, 30, 31, 32, 33, 34, 35, 36.…”

“…Conjugated electron donor–acceptor (D‐A) polymers have been intensively explored over the last decades in order to develop higher performing organic semiconductors and photovoltaic materials 1, 2, 3, 4, 5. They have been successfully utilized for fabrication of thin film transistors with high charge mobilities and photovoltaic cells with high power conversion efficiencies,6, 7, 8, 9, 10, 11, 12, 13, 14 clearly demonstrating the promise of conjugated D‐A polymers for use in solution‐processible, large‐area, low‐cost, and flexible electronic devices 15, 16.…”

High‐Mobility Naphthalene Diimide and Selenophene‐Vinylene‐Selenophene‐Based Conjugated Polymer: n‐Channel Organic Field‐Effect Transistors and Structure–Property Relationship

“…[1][2][3][4][5][6][7] Such progress coupled with the high compatibility of solution-processable organic semiconductors with plastic or metal foil substrates makes them ideal candidates for cost-effective, mechanically flexible electronic devices for various applications, such as printed radio frequency identification tags for item-level tagging, drivers for flexible displays, wearable electronics, distributed sensors, and integrated, nonvolatile memory devices. [8][9][10][11][12][13][14][15][16][17][18][19][20] [21][22][23][24][25] Such remarkable p-channel mobilities, obtained in devices with fairly promising shelf-life and operational stabilities, have been demonstrated with conjugated polymers based on new building units, such as diketopyrrolopyrrole, [24,26,27] isoindigo, [28,29] and indacenodithiophene. [30] Similar efforts have been devoted to improving n-channel polymer semiconductors, with the performances of n-type devices still lagging behind their p-channel counterparts.…”

A Facile Approach to Improve Interchain Packing Order and Charge Mobilities by Self‐Assembly of Conjugated Polymers on Water

“…Various conjugated molecules and polymers have been investigated intensively 5, 6, 7, 8, 9, 10, 11. These studies reveal that intermolecular/interchain packing and thin film crystallinity play a vital role in determining the charge transporting properties 12, 13.…”