Organic Thin‐film Transistors Based on Polythiophene Nanowires Embedded in Insulating Polymer

Abstract

Organic thin-film transistors (OTFTs) have attracted considerable attention because of their potential applications in large-area, flexible, and printed electronics. To achieve OTFT devices with desirable properties, recent research has primarily focused on molecular design, [1,2] dielectric-semiconductor interfacial engineering, [3][4][5] and device optimization. [6][7][8][9][10][11] The use of conjugated polymer blends as active materials has brought a new way to tune and optimize the electronic properties of devices; for example, ambipolar field-effect charge transport has been reported in binary blends of p-and n-type conjugated polymers or oligomers. [12,13] Semiconducting and insulating polymer blends have also attracted increasing interest, because they can combine the electronic properties of semiconducting polymers with the low cost and excellent mechanical characteristics of insulating polymers. However, the presence of the insulating component tends to degrade the device performance by diluting the current density of the film. [14,15] To the best of our knowledge, the only effective approach to overcome this drawback is controlling the blended films to form vertically phase-separated structures, to keep the connectivity of the semiconducting layer in the presence of insulating components. In recent works, the composites with this structures have been used in OTFTs to fabricate low-voltage-driven devices, to improve environmental stability or reduce semiconductor cost. [16][17][18][19][20][21] However, the phase-separation process in polymer blends is very complicated. The final morphology in the blend films is highly sensitive to many factors, including the solvent evaporation rate, solubility parameters, film-substrate interactions, the surface tension of the components, and the film thickness. Vertical phase separation can only take place under extreme conditions. [22,23] Therefore, to develop a more facile and general method for realizing high-performance, low-semiconductor-cost devices is of great technological and academic significance.In this paper, we show that the percolation threshold of semiconducting/insulating polymer blends can be drastically decreased by depositing them from a marginal solvent with temperature-dependent solubility. Morphology and crystallinestructure studies reveal that the excellent electronic performance of the devices derives from the efficient charge transport and the good connectivity observed in highly crystalline, interconnected nanofibrillar networks of semiconductors embedded in an insulator matrix.Semiconductor/insulator-blend mother solutions were prepared by blending poly(3-hexylthiophene) (P3HT) and amorphous polystyrene (PS) in dichloromethane (CH 2 Cl 2 ), which is a marginal solvent for P3HT.[24] To completely dissolve P3HT, the CH 2 Cl 2 solution was kept at approximately 40 8C. For comparison, chloroform (CHCl 3 ), which is a good solvent for P3HT, was used as a reference. Thin films with different P3HT and PS ratios were fabricated on a silicon substrat...

Experimental evidence that short-range intermolecular aggregation is sufficient for efficient charge transport in conjugated polymers

“…27) prefers the SiO 2 surface. A similar surface-induced phase separation is typically reported for other semiconductor/insulator polymer blends (28)(29)(30). A characteristic single-phase morphology with randomly oriented crystalline grains on the order of 10-30 nm is observed in neat films of P(NDI2OD-T2) (Fig.…”

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

“…In the phase separation process of P3HT in polymer blends, there has been no report of nanofiber formation when only chloroform is used as a solvent, 9 which is consistent with the present work. Furthermore, the electrical conductance was less than 10 À11 S and the thin film was almost insulative.…”

“…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).…”

Structure and Morphology Control in Thin Films of Conjugated Polymers for an Improved Charge Transport

“…As a result, the branched and hyperbranched nanostructures were tuned by the ratio of oDCB and TEA (Figure 21). A third nucleation-growth scheme is dispersed nucleation and assisted growth in phase separation room [160][161][162][163][164]. Through mixing the conjugated polymers with insulting flexible polymers with a small content of conjugated polymers, i.e., polystyrene, the nucleates are dispersed in the matrix of flexible polymers due to phase separation and grow further to form percolation pathways for charge transport at a low percolation threshold (1.7 vol % [162]).…”