Fabrication and Study of Organic Electronic and Optoelectronic Devices

Abstract

From the last few decades, organic semiconductors based electronic and optoelectronic devices have been the area of intense research. Organic semiconductors have gained optoelectronic industry due to remarkable importance in electronic and conjugated structure, low cost production, mechanical flexibility, ease of device fabrication and environmental friendly processing. In the research work reported in this dissertation, electronic, optoelectronic and amplified spontaneous emission (ASE) studies are carried out on (i) junction diodes using n-type organic semiconductors, (ii) sensors fabricated using organic semiconductors and organic-inorganic nanocomposites, and (iii) newly synthesized single crystal oligothiophene, respectively. The organic n-type semiconductors N-Butyl-N′-(6-hydroxy-hexyl) perylene- 3,4,9,10- tetracarboxylic-acid-diimide (N-BuHHPDI) and perylene- 3,4,9,10- tetra-carboxylic acid di-anhydride (PTCDA) have been synthesized and employed for their potential applications in organic devices such as Schottky barrier junction (SBJ), heterojunction (HJ) and organic multifunctional sensors. The fabricated rectifying junctions, i.e. Schottky junction and heterojunction, are characterized by conventional current-voltage (I-V) method at ambient atmospheric condition at in dark. Different junction factors i.e. turn-on voltage (V t ), ideality factor/quality factor (n), rectification ratio (RR), barrier height (φ b ), reverse saturation current (I 0 ), series resistance and shunt resistance are determined from the I-V curves. Other two well known characterization techniques, Norde’s technique and Cheungs’ functions, are applied to measure the aforementioned parameters of the diode. The parameters extracted by these different characterization methods are compared and found in good agreement with each other. To understand the temperature dependence of the heterojunction device parameters, the I-V properties of the heterojunction are studied at various temperatures from 300 K to 330 K. This has been observed that with the viiiincrease in temperature the ideality factor, turn-on voltage and series resistance of the junction are significantly decreased. While the reverse saturation current and rectification ratios are increased. The morphological study of the thin films of N-BuHHPDI and PTCDA deposited on quartz glass has been carried out by scanning electron microscope (SEM) and atomic force microscope (AFM). Size of the grain, shape, orientation and average surface roughness of the thermally deposited films has been measured from the microscopy. Energy dispersive spectroscopy (EDX) has been done for these samples to confirm the composition of these newly synthesized organic materials. To measure the optical band gap of these materials, ultra-violet and visible (UV-Vis) spectroscopy has been performed. The N-BuHHPDI is insoluble in water and possesses excellent hydrophobic properties. This hydrophobic property makes these materials more distinctive candidates for humidity sensing applications as compared to other water soluble organic materials. The high sublimation temperature (700 K) is another convincing characteristic of this class of materials for their potential application as a temperature sensor to monitor temperature at elevated levels. At the same time, the broad UV-Vis spectrum and good absorption of light recommend perylenes equally useful for light sensing applications. Thus, taking advantages of these motivating properties, the potential of perylene has been explored for humidity, temperature and light sensing applications due to the possession of such interesting properties all together. To enhance the performance of the organic semiconductor based sensors, one way is to make the sensor at nanoscale and the other is to blend organic semiconductor with some functional nanoparticles/nanofibers. For this reason, an organic azo dye, methyl orange (MO), has been used as an active semiconducting matrix in which the TiO 2 nanoparticles have been disapersed. The fabricated sensor response has been observed at different humidity and ixtemperature values. Fast recovery time and response time of the sensor has been achieved due to the introduction of TiO 2 nanoparticles by taking advantage of large surface-to-volume ratio of the nanoparticles. Concerning photonic characteristics of organic polymers and small molecules, conjugated polymers based optically pumped lasers have been produced both from solution and thin films. However, significant hard work is being carried out on these conjugated molecules to enhance their carriers’ mobility and to minimize the threshold energy required for lasing to develop electrically pumped polymer laser diodes. Such types of materials for lasers may ultimately compete with their inorganic counterparts in many ways such as inexpensive, low-temperature and high throughput fabrication. One special type of these materials is single crystal oligomers which, in the last decade, have got incredible attention due to their interesting properties such as high charge carriers’ mobility, chemically adjustable wavelengths, large photoluminescence quantum efficiency (PLQE), large stimulated emission cross-sections and as a high optical gain media for their prospective uses in solid state lasers and broadband amplifiers. The amplified spontaneous emission (ASE), optical gain and PLQE properties of a newly synthesized single crystal oligothiophene 5, 5’’’’’-diphenyl-2, 2’:5’, 2’’:5’’, 2’’’:5’’’, 2’’’’:5’’’’, 2’’’’’-sexithiophene (P6T) are reported. This oligomer crystal has been grown on glass substrate in a dimension of 5mm x 2mm with 10 μm thickness, by Prof. Shu Hotta, Kyoto Institute of Technology, Japan and its photonic properties have been studied at Cavendish Lab, Cambridge, UK.