3,7-Bis((E)-2-oxoindolin-3-ylidene)-3,7-dihydrobenzo[1,2-b:4,5-b′]dithiophene-2,6-dione (IBDT) based polymer with balanced ambipolar charge transport performance

Highlights

• A novel acceptor building block, IBDT, is developed.

• A IBDT-based polymer has favorable energy levels for ambipolar charge transport.

• This polymer shows balanced ambipolar characteristics with electron mobility of 0.14 cm² V⁻¹ s⁻¹ and hole mobility of 0.10 cm² V⁻¹ s⁻¹.

Abstract

A novel acceptor building block, 3,7-bis((E)-2-oxoindolin-3-ylidene)-3,7-dihydrobenzo[1,2-b:4,5-b′]dithiophene-2,6-dione (IBDT), is developed to construct a donor-acceptor polymer PIBDTBT-40. This polymer has favorable highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels for balanced ambipolar charge transport. Organic thin film transistors (OTFTs) based on this polymer shows well-balanced ambipolar characteristics with electron mobility of 0.14 cm² V⁻¹ s⁻¹ and hole mobility of 0.10 cm² V⁻¹ s⁻¹ in bottom-gate bottom-contact devices. This polymer is a promising semiconductor for solution processable organic electronics such as CMOS-like logic circuits.

Introduction

Polymer semiconductors have been extensively studied as channel materials in organic thin film transistors (OTFTs) due to their good solution processability and mechanical robustness [1], [2], [3]. Different from unipolar p-type or n-type semiconductors, ambipolar semiconductors are able to transport both holes and electrons, allowing for certain new device designs such as single-component complementary metal oxide semiconductor (CMOS)-like circuits and ambipolar light emitting transistors [4], [5], [6]. The use of a single-component ambipolar polymer can significantly simplify the complicated patterning and fabrication processes for CMOS-like circuits [7]. Ambipolar light emitting transistors can integrate both the light-emission capability and the electrical current modulation in one single device architecture, which may enable the next generation of light-emitting devices [8]. For both single-component CMOS-like circuits and ambipolar light emitting transistors the semiconductor with balanced hole and electron charge transport characteristics is highly preferable [9]. To achieve balanced ambipolar charge transport characteristics, the frontier molecular orbital energy levels, i.e., the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the polymer semiconductor are crucial. First, the HOMO and LUMO levels of the polymer semiconductor should have small and similar differences against the Fermi energy of the source contact to minimize and balance the injection barriers for electrons and holes, respectively [10]. In this regard, it is desired that the polymer has a small band gap (the difference between the HOMO and LUMO levels), while the Fermi level of the source electrode sits in between the LUMO and HOMO levels. Second, the energy levels of the polymer need to be lower than ca. −5.0 eV [11], [12] for the HOMO and ca. −3.7∼−4.0 eV [12], [13], [14], [15] for the LUMO in order to achieve stable hole and electron transport, respectively. The common approach to delicately tune the energy levels of polymer semiconductors is to incorporate alternating donor–acceptor (D–A) structure in the polymer backbone, whereas the HOMO and LUMO levels of the polymers are primarily governed by an appropriate combination of the donor and acceptor building blocks, respectively [16], [17]. However, available electron acceptor building blocks that are able to bring the LUMO level of polymers below ca. −3.7∼−4.0 eV for stable and efficient electron transport are rare [18], [19], [20], [21]. Hence, extensive effort has been made to discover new electron acceptor building blocks [22], [23], [24].

Recently we developed a new strong electron acceptor building block, (3E,7E)-3,7-bis(2-oxoindolin-3-ylidene)-benzo[1,2-b:4,5-b′]difuran-2,6(3H,7H)-dione (IBDF, Fig. 1), for D-A polymers for OTFTs [15], [18], [25], [26], [27], [28], [29]. Due to their deep-lying LUMO levels, IBDF-based polymers showed n-type dominant charge transport behavior with high electron mobility up to 1.74 cm² V⁻¹ s⁻¹ [26]. By replacing the benzodifurandione (BDF) core in IBDF with a less electron-withdrawing benzodipyrroledione (BDP), we developed another electron acceptor, IBDP (Fig. 1). Opposed to the IBDF-based polymers, the IBDP-based polymers showed p-type dominant charge transport behaviors with hole mobility as high as 1.92 cm² V⁻¹ s⁻¹ [30], [31]. In this work, we changed the core unit to benzodithiophenedione (BDT) to successfully develop another new acceptor building block in this series, 3,7-bis((E)-1-2-oxoindolin-3-ylidene)-3,7-dihydrobenzo[1,2-b:4,5-b′]dithiophene-2,6-dione (IBDT, Fig. 1) and demonstrated that IBDT is a proper electron acceptor building block for making ambipolar polymers with well-balanced hole and electron mobilities.