The blend films of small molecule semiconductors with insulating polymers exhibit not only an excellent solution processability, but also superior performance characteristics (field-effect mobility, on/off ratio, threshold voltage and stability) over those of neat small molecule semiconductors. To understand the underlying mechanism, we studied triethylsilylethynyl anthradithiophene (TESADT), as a model system for adding a small amount of impurity by weak ultraviolet (UV)light exposure that results in drastically reduced field-effect mobility (< 10-5 cm2/Vs) and a disappearance of the high-temperature crystal phase. However, the mobility of the blend films of the UV-exposed TESADT with poly(α-methylstyrene) (PαMS) is recovered to that of fresh TESADT-PαMS blend (0.040 cm2/Vs) and the phase transition characteristics return to those of fresh TESADT films. Moreover, these results are corroborated by the OTFT results on blend films of aged 6,13-bis(tri-isopropylsilylethynyl) pentacene (TIPS-pentacene) and PαMS. Coupled with the neutron reflectivity study on thin films of small molecule-polymer blends, they indicate that the formation of vertically separated zones of pure crystalline small molecule semiconductors at the gate interface allows the impurity species to remain preferentially in adjacent polymer-rich amorphous layer. Such zone-refinement effect would effectively remove not only the organic impurity but also ionic impurity charges, which are detrimental to the organic electronic devices such as organic thin film transistors and organic photovoltaic solar cells but are very difficult to remove completely from the solution-processable samples.
Citation: Journal of American Chemical Society
Pub Type: Journals
organic electronics, polymer, semiconductor, zone-refinement, thin film transistor, vertical segregation, neutron , reflectivity