Versatile Use of Vertical‐Phase‐Separation‐Induced Bilayer Structures in Organic Thin‐Film Transistors

Abstract

Recently, the use of solution-processable conjugated polymer semiconductors in thin-film transistors (TFTs) has been extensively studied because of their suitability for fabricating large-area devices using established solution-deposition techniques (e.g., spin-coating, screen printing, or inkjet printing). [1][2][3][4][5][6][7][8] An attractive feature of the solution process is that different materials can be easily blended to optimize the electronic and optoelectronic properties for device applications. [9][10][11] Blends of semiconducting and dielectric polymers can combine the optical and electrical properties of semiconductors with the characteristics of dielectric polymers. However, the use of these blends as active layers in TFTs always causes a decrease in the device performance because the dielectric polymer ''dilutes'' the current density. [12,13] Controlling the phase separation in the direction perpendicular to the substrate to form bilayer structures should be an effective way to diminish this effect because it allows retention of the connectivity of the semiconducting layer in the channel region. To this end, organic-semiconductor/dielectric-polymer blends with vertical phase separation have been used to fabricate high-performance TFTs with low operating voltage [14] or with improved environmental stability [15] at high semiconductor concentration (!40%). In a recent publication, [16] Goffri et al. reported that the concentration of semiconductor in crystalline/crystalline bicomponent semiconductor/dielectricpolymer systems can be reduced to a value as low as 3 wt % without any degradation in device performance. However, all structures reported in previous studies are dielectric-top and semiconductor-bottom structures. It would be very interesting to investigate a semiconductor-top and dielectric-bottom bilayer structure, because this structure is identical to the configuration of the semiconductor and dielectric layers in the bottom-gate TFT device. For this reason formation of a semiconductor-top and dielectric-bottom bilayer in a one-step process may provide a simple route for the fabrication of TFT devices.In the present Communication, we report for the first time the fabrication of a semiconductor-top and dielectric-bottom bilayer structure by means of surface-induced vertical phase separation of poly(3-hexylthiophene) (P3HT) and poly(methyl methacrylate) (PMMA) blends. Because the ultrathin and defect-free PMMA dielectric layer can act as blind material, a modifier at the semiconductor/dielectric interface, or a dielectric layer, these bilayer blends have versatile uses in TFTs.Films composed of P3HT/PMMA blends were fabricated by spin-casting chlorobenzene solutions of the polymers onto bare silicon substrates (see Fig. 1a, inset). Since the hydrophilicities of P3HT and PMMA are very different, water contact-angle measurements were carried out to determine qualitatively the changes in composition taking place on the surface of the blended films. The variation of the water contact angle as a ...

Organic Semiconductor/Insulator Polymer Blends for High-Performance Organic Transistors

“…Figure 3b shows the electrical properties of OFETs based on a poly(3-hexylthiophene) (P3HT): PMMA blended film where phase-separated P3HT at the air-film interface serves as a semiconducting layer and phase-separated PMMA at the film-substrate interface serves as a dielectric layer (Figure 2b) [29]. Remarkably, P3HT content in a P3HT:PMMA blend was reduced down to 2% without compromising the electrical properties.…”

Moderate doping leads to high performance of semiconductor/insulator polymer blend transistors

“…This property triggered intense research on the structural and electrical properties of the active blend layer. Improved performance of the blends with mobility up to 0.1 cm 2 V À 1 s À 1 has been attributed to an enhanced crystallinity of the conjugated polymer in the blend 24 and/or enrichment of the conjugated polymer at the polymer film/dielectric interface 24,25 or the film/vacuum surface 26 . In addition, some of these blend OFETs exhibit superior stability in air, and this stability was mainly ascribed to the encapsulation of the active component 25 .…”

“…However, for a widespread application of these devices, air stability and material costs are important features that need to be improved. Blends of semiconducting and insulating polymers have recently been shown to exhibit reasonably high mobilities and on/off ratios [21][22][23][24][25][26][27][28] , often exceeding the corresponding values measured for pure layers of the semiconducting polymers. As high performance could be realized even for a low concentration of the active semiconductor component mixed into an inert polymer matrix, these blends allow a fine tuning of the active layer's rheological and optical properties without compromising charge transport.…”

Microscopic conduction pathways of poly(3-hexylthiophene) nanofibers embedded in polymer film

“…A 1 wt% solution of the polymer blend with only chloroform was also prepared as a reference. 8 Pt electrodes (0.6 mm long) with a gap space of 40 mm were fabricated with a bottom-contact configuration by sputter deposition, using a shadow mask on a piece of Si wafer with a 255-nm thermally grown SiO 2 layer on top (SiO 2 /Si; KST World Corp., Fukui, Japan). Thin films of P3HT/PMMA were prepared by spin-coating at 3000 r.p.m.…”

“…These reports suggest that control of phase separation of the conjugated conducting polymer is quite important to form a continuous nanofiber network and exhibit the OFET effect. [8][9][10] The electrical percolation behavior in the bulk polymer and on the surfaces of organic electronics containing conducting nanofibers is strongly influenced by the condition of the nanofiber network. Therefore, we have directed our attention to investigation of the microscopic conduction pathways in a nanofiber network of regioregular P3HT (rr-P3HT).…”