Electronic and Optoelectronic Studies of Organic Semiconductors

Abstract

Organic semiconductors have made inroad into many area of devices which was formally dominated by inorganic semiconductors because of their wide variety of electronic and optoelectronic properties. They being low cost, light weight and low temperature processing materials provide opportunities to fabricate the variety of devices, such as, solar cells, field effect transistors, lasers, light emitting diodes, sensors, photo detectors, smart windows, large area displays, e-paper, etc. The material manipulation, low cost fabrication techniques and the emerging ideas are bringing about much improved performances in the organic electronic devices. Most of the earlier studies have been reported on the p-type organic semiconductors and little is known about n-types. In the development of future organic electronic industry, all organic complementary circuits are not possible without the availability of both p- and n-type organic semiconductors and data is required on the junction properties and mobility studies of these materials. Plenty of data is available on the junction diodes of p-type organic semiconductors but little is known on the n-type organic semiconductors based junction devices and mobility investigations. In this dissertation, the n-type organic semiconducting materials formyl- TIPPCu(II), N,N ́-di-n-heptyl-2,3:6,7-anthracenetetracarboxydiimide (ADCI7) and N,N ́- di-n-octyl 2,3,6,7 anthracenetetracarboxydiimide (ADCI8) have been investigated as active organic materials for their potential application in organic electronic devices. Using organic semiconductor formyl-TIPPCu(II), junction diode, temperature, light and humidity sensors have been fabricated, while ADCI7 and ADCI8 have been used for the fabrication of n-channel organic thin film transistors. To investigate junction properties of formyl-TIPPCu(II) organic semiconductor, fabrication of Ag/formyl-TIPPCu(II)/p-Si heterojunction diode was undertaken and it was made successfully. Its temperature dependent electrical properties are reported. The values of series resistance, ideality factor, zero bias barrier height are observed strongly dependent on temperature. The series resistance and ideality factor decease while the zero bias barrier height increases with the rise in temperature. viiThe surface type Ag/formyl-TIPPCu(II)/Ag humidity sensors has been fabricated to study the effects of changing relative humidity on the electrical parameters and their frequency dependant responses. The values of capacitance and resistance of the sensors were measured at different humidity levels at frequencies of 1 kHz, 10 kHz and 100 kHz. An increase in capacitance and decrease in resistance were observed during the rise of relative humidity from 45 to 95% RH. The hysteresis response of these humidity sensors was also studied at the frequency of 1 kHz. Effects of temperature and light are studied on the capacitance and resistance of the Au/formyl-TIPPCu(II)/Au device. The relative capacitance of the fabricated sensor increased by 4.3 times by rising temperature from 27 to 187 0 C, while under illumination up to 25000 lx, the capacitance of the Au/formyl-TIPPCu(II)/Au photo capacitive sensor increased by 13.2 times as compared to dark conditions. ADCI7 and ADCI8 were used to fabricate n-channel organic thin film transistors (OTFTs) on oxidized silicon wafers. To get the high performance of the devices and to avoid the trapping of charge carriers, the dielectric surface were modified by developing the buffer layer of PMMA or by self assembly monolayer (SAM) of HMDS. The OTFTs exhibited high charge mobility of the order of 10 -2 cm 2 V -1 S-1 (ADCI7) and 10 -3 cm 2 V -1 S - 1 (ADCI8) with the on/off ratio of the order of 10 4 showing the appreciable enhancement in the field effect properties of these materials as compared to the previously reported researches for the same family of materials. ADCI7 is introduced as new compound for high mobility n-channel OTFTs.