Fabrication and characterization of organic field effect transistors

Organic semiconductors are being developed for use in electronic and optoelectronic devices such as organic light-emitting diodes, organic field-effect transistors (OFETs), and organic solar cells. One of the most important advantages of using organic materials in electronics instead of inorganic materials is the possibility of using low-cost, low temperature, fine tuning of their properties by precise synthesis, solution-based processes and good compatibility with flexible substrates. An OFET is essentially a resistor (with the two terminals called ''source'' and ''drain'' and a semiconductor material) in combination with the metal–insulator–semiconductor. Control over the source–drain current in OFETs via a third terminal has resulted in their widespread applications in switches, radio frequency identification (RFID) tags, smart cards, active matrix liquid crystal displays, electronic paper and chemical sensors. In such applications, one of the critical parameters is the field-effect carrier mobility, which remains smaller than that of inorganic counterparts. The overall performance of OFET devices is determined primarily by the charge carrier mobility. A remarkable progress in organic based electronics, especially in the field of organic field-effect transistors has been achieved during the last two years, e.g., mobility as high as 3–5 cm2/V s and on/off current ratio (Ion/Ioff) larger than 106. Although molecular materials with the field-effect carrier mobility as high as 1 cm2/V s have been reported, they present processing difficulties due to their limited solubility, high leakage current density and stability issues such as hysteresis still exist as serious obstacles to the commercial use of OFETs with organic layers. The search for high dielectric constant (high-k) gate dielectric and high carrier mobility semiconductor materials for OFET applications has stimulated important research activities in both conventional and unconventional electronics. In a OFET, the gate voltage, VG, required to switch the transistor "on" scales with the insulator thickness, d, and inversely with the insulator dielectric constant. It means that high k at low d is desirable. However, both minimising d, and maximising k, leads to practical problems: a thin, fragile dielectric may lead to increased leakage currents through the insulation layer, whereas polar (high-k) insulators introduce undesirable effects at the organic semiconductor-insulator interface generally leading to increased trapping. Therefore, the choice of a proper insulator is an important issue and not easy and usually depends on the intended applications. Thermally grown SiO2 has often been used as OFET insulator, but more for the reason of commercial availability rather than performance, as SiO2 does not display much higher gate capacitance than polymers. Moreover, thermally grown SiO2 is not flexible and too expensive for disposable electronics. Polymeric OFET insulators (e.g. polietilen (PE), and polyvinyalcohol (PVA) having the advantage to be processed from solution, usually have rather low surface polarities but have to be fairly thick to avoid formation of pin-holes and their dielectric constants k are rather small. This results in insulators with low gate capacitance and consequently, operational voltages exceeding 30–50V. Another important concept in the development of an OFET is the choice of a suitable semiconductor material. In a number of applications for molecular semiconductor thin films, a sufficiently high mobility of charge carriers can be regarded a necessary condition. Metallophthalocyanines (MPcs) and metalloporphyrins (MPPs) represent a large family of functional π macrocycle materials with high chemical and thermal stability. Among the different classes of materials studied for organic electronics, semiconducting phthalocyanines (Pcs) and their peripherally substituted derivatives is among the most common and promising material for application in OFETs. In this thesis, top-contact bottom-gate organic field effect transistors OFET devices having a top source-drain contact geometry based on solution processed phthalocyanine semiconductor layer, polymethylmethacrylate (PMMA) and CoO2 dielectric layer have been fabricated. Tin oxide coated glass served as back gate electrode. The gold drain and source electrodes were deposited by thermal evaporation in vacuum using a shadow mask. These electrodes have a width of 4 mm. For comparison, the OFET devices were fabricated with various channel width between 30-100 m. The electrical properties of the produced devices were investigated and different parameters such as charge carrier mobility, on/off current and threshold voltage were determined. It was found that the device parameters depend on the channel width and the choice of the dielectric layer. Maximum mobility was obtained with the structure of P-Si/PMMA/Alq3/Au.