Abstract
Blending organic semiconductors with insulating polymers has been known to be an effective way to overcome the disadvantages of single-component organic semiconductors for high-performance organic field-effect transistors (OFETs). We show that when a solution processable organic semiconductor (6,13-bis(triisopropylsilylethynyl)pentacene, TIPS-pentacene) is blended with an insulating polymer (PS), morphological and structural characteristics of the blend films could be significantly influenced by the processing conditions like the spin coating time. Although vertical phase-separated structures (TIPS-pentacene-top/PS-bottom) were formed on the substrate regardless of the spin coating time, the spin time governed the growth mode of the TIPS-pentacene molecules that phase-separated and crystallized on the insulating polymer. Excess residual solvent in samples spun for a short duration induces a convective flow in the drying droplet, thereby leading to one-dimensional (1D) growth mode of TIPS-pentacene crystals. In contrast, after an appropriate spin-coating time, an optimum amount of the residual solvent in the film led to two-dimensional (2D) growth mode of TIPS-pentacene crystals. The 2D spherulites of TIPS-pentacene are extremely advantageous for improving the field-effect mobility of FETs compared to needle-like 1D structures, because of the high surface coverage of crystals with a unique continuous film structure. In addition, the porous structure observed in the 2D crystalline film allows gas molecules to easily penetrate into the channel region, thereby improving the gas sensing properties.
Highlights
Provides blend films with no loss in the inherent charge carrier mobility due to the small molecule semiconductors through vertical phase-separation[9,10,11]
We deliberately stopped the spin-coating at a given spin time and examined the effects of the residual solvent on the phase-separation and crystallization of TIPS-pentacene/ insulating polymer blends, as discussed
Spun-coat films of a TIPS-pentacene/insulating polymer blend were used as active layers in organic field-effect transistors (OFETs) gas sensors
Summary
Provides blend films with no loss in the inherent charge carrier mobility due to the small molecule semiconductors through vertical phase-separation[9,10,11]. Field-effect mobilities of semi-crystalline polymer semiconductors could be improved by controlling the spin-casting time because the molecular orientation and π-π stacking interaction of the conjugated molecules could be enhanced by controlling the crystallization speed resulting from the residual solvent after spin-coating. In small-molecule semiconductor/insulating polymer blends, controlling the residual solvent in spin-cast blend films is important for inducing vertical phase-separation, while achieving control over the crystallization of small molecule semiconductor for high-performance OFETs10,16,18. Porous organic semiconductor films are recommended as active layers because the porous microstructure allows the analytes to reach the active layer more efficiently and reduce the time for the adsorption/desorption process with the active layer of the device[27,28]. The electrical characteristics and gas sensing properties of the FET devices were correlated with the microstructure of the blend films, which is governed by two different crystallization modes
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