Zone-Refinement Effect in Small Molecule−Polymer Blend Semiconductors for Organic Thin-Film Transistors

Abstract

The blend films of small-molecule semiconductors with insulating polymers exhibit not only excellent solution processability but also superior performance characteristics in organic thin-film transistors (OTFTs) over those of neat small-molecule semiconductors. To understand the underlying mechanism, we studied triethylsilylethynyl anthradithiophene (TESADT) with small amounts of impurity formed by weak UV exposure. OTFTs with neat impure TESADT had drastically reduced field-effect mobility (<10(-5) cm(2)/(V s)), and a disappearance of the high-temperature crystal phase was observed for neat impure TESADT. However, the mobility of the blend films of the UV-exposed TESADT with poly(α-methylstyrene) (PαMS) is recovered to that of a fresh TESADT-PαMS blend (0.040 cm(2)/(V s)), and the phase transition characteristics partly return to those of fresh TESADT films. These results are corroborated by OTFT results on "aged" TIPS-pentacene. These observations, coupled with the results of neutron reflectivity study, indicate that the formation of a vertically phase-separated layer of crystalline small-molecule semiconductors allows the impurity species to remain preferentially in the adjacent polymer-rich layer. Such a "zone-refinement effect" in blend semiconductors effectively removes the impurity species that are detrimental to organic electronic devices from the critical charge-transporting interface region.

Self-assembly of semiconductor/insulator interfaces in one-step spin-coating: a versatile approach for organic field-effect transistors

“…58,65,79,97,104 Improved environmental stability is firstly shown in the work by Arias et al 79 Spin-coating a blend of PQT-12 and PMMA resulted in a PMMA-PQT-12 bilayer film on the OTS surface, due to the selective adsorption of PQT-12 on the OTS surface as mentioned before. In this case, the electronically inert insulator layer separates organic semiconductors from ambient air, which will serve as a barrier to water and oxygen and alleviate the problem of oxidation under illumination.…”

“…58,[93][94][95] Both the order in which each component crystallizes, as well as the flow of solution, amorphous, or crystalline states influence the final structure. 81,85,96,97 This interesting structure can be explained by the following process that occurs during spincoating: firstly TIPS-PEN form crystallites, which are expelled to the top and bottom by the spinning liquid and trap the PaMS below it; and then subsequently the remaining solvent evaporates from the PaMS. For example, spin-coating a blend of crystalline semiconductor 6,13-bis-(triisopropyl-silylethynyl)-pentacene (TIPS-PEN) and polymer dielectric poly(a-methylstyrene) (PaMS) resulted in a triplelayer film: TIPS-PEN/PaMS/TIPS-PEN.…”

Ultra-high mobility transparent organic thin film transistors grown by an off-centre spin-coating method

“…Previously, the higher mobility in small molecule/polymer blends was also attributed to the 'zone refinement effect' during the vertical phase separation process, and this purification effect may also be present in this case 43,44 . Furthermore, the lower dielectric constant of PS (e r ¼ 2.6) than PVP (e r ¼ 4.2) provides a less polar environment at the dielectric/C8-BTBT interface, leading to a less energetic disorder in the dielectric 45 .…”

Vertical Phase Separation in Small Molecule:Polymer Blend Organic Thin Film Transistors Can Be Dynamically Controlled

“…[8][9][10][11] Achieving continuous thin film formation via solution processing can be challenging due to molecular shape and packing anisotropies and a propensity to dewet. [12][13] Forming films with excellent substrate coverage and lamellar texture also requires crystallization to be initiated from the solid-liquid interface, [14] while slight fluctuations in drying conditions and extended solvent vapor exposure can strongly impact the film formation and degree of crystallinity. [15,16] conditions and the resulting morphology in terms of phase separation and crystallization of the OSC within the blends, as well as the corresponding device performance is still in its infancy for small molecule-polymer blends, but some of the desirable features are clearly identified.…”