Using unsorted single-wall carbon nanotubes to enhance mobility of diketopyrrolopyrrole-quarterthiophene copolymer in thin-film transistors

Highlights

• SWCNTs and diketopyrrolopyrrole-quarterthiophene (DPP-QT) copolymer composite.

• 2 Times improvement of field-effect mobility of DPP-QT copolymer.

• The maximum mobility reaches 1.3 cm² V⁻¹ s⁻¹.

• Understood the effects of SWCNT properties on the composite and the devices.

Abstract

Organic thin film transistors (OTFTs) were fabricated for the first time using a semiconductor copolymer of diketopyrrolopyrrole-quarterthiophene (DPP-QT) and unsorted single walled carbon nanotubes (SWCNTs). Three different SWCNTs having different tube diameters, length, and shape were used to investigate the effects of carbon nanotubes’ properties on dispersion of the SWCNTs in DPP-QT polymer, as well as the mobility and current on/off ratio of the OTFTs. The DPP-QT polymer was able to selectively disperse two types of SWCNTs. An optimal SWCNT loading was found to be 1.5–2.5 wt% for these SWCNTs, before the on/off ratio fell below 10⁵ due to increased metallic tube content of the film. At this optimal loading, the field effect mobility was improved by a factor of two, with the maximum mobility reaching 1.3 cm² V⁻¹ s⁻¹, when the SWCNTs with a short length and small tube diameter were used.

Introduction

A major goal for printable electronic devices is the development of organic thin-film transistors (OTFTs) [1], [2], [3]. By replacing the brittle silicon based semiconductor with an organic semiconductor material, new opportunities for low-cost, flexible, and lightweight electronics exist, since an organic semiconductor could be integrated into large area electronic device via printing techniques in a roll to roll manner [4]. One of the most challenging aspects to this approach is generating a cost effective semiconductor materials with comparable electrical performance to amorphous silicon which has a mobility of 0.5–1 cm² V⁻¹ s⁻¹. Over the past 10 years, a wealth of research has been published using single walled carbon nanotubes (SWCNTs) to generate high mobility films due to their high field effect mobility measured to be 79,000 cm² V⁻¹ s⁻¹ and an intrinsic mobility estimated at 100,000 cm² V⁻¹ s⁻¹ [5]. SWCNTs can be incorporated into the semiconductor layer of existing OTFT designs making them an ideal candidate for technology development [6]. Research into individual SWCNT channels grown by chemical vapour deposition (CVD) have been reported [7], [8], [9], [10], but these methods suffer from their inability to be scaled to production demands. Success has been made depositing filtered films of SWCNTs, but the process still requires multiple steps, making it labour intensive [11], [12]. Due to their tendency to aggregate in solution, a film of pure CNTs cannot be solution cast without the aid of a dispersing agent, which can affect film preparation and SWCNT performance [13], [14], [15]. It has been shown that oxidized or chemically functionalized CNT’s are more easily dispersed in solvents than defect free CNT’s [16]. However, these methods are not useful for semiconductor electronics, because altering the chemical structure of a CNT diminishes its semiconductor properties.

An alternative approach to generating a pure film of SWCNTs is to use SWCNTs to enhance the performance of organic semiconductors. Due to their high aspect ratio, a very small amount of SWCNTs can be added to a semiconductor polymer film increasing the films overall mobility [17]. This approach requires the semiconductor to be soluble and able to stabilize a dispersed SWCNT solution. Fortunately most semiconductor polymers consist of a conjugated backbone, allowing for a π–π interaction to help prevent the aggregation of CNTs in solution. One of the most studied semiconductors polymers employing this approach is poly(3-alkythiophene) [18], [19], [20]. Although mobility improvements up to 10 times have been reported [21], due to the low mobility of poly(3-alkythiophene), the absolute mobility values are still very low, for example less than 0.1 cm² V⁻¹ s⁻¹. Recently a new class of semiconductor, diketopyrrolopyrrole-quarterthiophene copolymer with mobility up to 1.0 cm² V⁻¹ s⁻¹, has been reported. It would be interesting to study if the mobility can be further enhanced by using SWCNTs as additive.

A major challenge with SWCNTs is that when synthesized, they are generated as a mixture of approximately 1/3 metallic carbon nanotubes (m-CNT) and 2/3 semiconducting carbon nanotubes (sc-CNT) [9]. The presence of metallic tubes is detrimental for OTFTs because beyond the percolation threshold they can create a conductive pathway essentially short circuiting the device. Additionally they have been found to reduce the on/off ratio of OTFTs [12]. To avoid the detrimental effects caused by m-CNTs, SWCNTs are semiconductor enriched. Three major techniques are employed for semiconductor enrichment to separate m-CNTs from sc-CNTs. Density gradient ultracentrifugation (DGU) has achieved the best purity, but has very small throughput ∼1 mg every 48 h [22], [23], [24], [25], [26], [27], [28]. Other methods such as column chromatography [29], [30], selective oxidation [31] or separation employing SWCNT selective polymers [32], [33] offer higher throughput but are still well below commercial requirements. Most literature employing SWCNT films utilizes these semiconductor enriched SWCNTs [14], [34], [35], [36]. The challenge with this approach is that the cost to purify these tubes drives the cost of materials so high that an amorphous silicon film remains a cheaper option.

Instead of using high-cost purified SWCNTs we looked into the cheaper alternative of using unsorted SWCNTs. The focus of this research was to determine if this mixture of metallic and semiconducting tubes could be used as a viable option for mobility enhancement of the new semiconductor copolymer of diketopyrrolopyrrole-quarterthiophene, and if so, what physical properties of the unsorted SWCNTs are best for this application.