Study On Organic Thin Film Transistor

Because of low cost, simple process, enabling a large area processing and compatiblewith flexible substrates, organic semiconductor materials were commercially used byLight-Emitting Diodes、Organic Thin Film Transistors and Organic photovoltaic cell. In theorganic semiconductor materials, organic polymer, which is readily soluble in organic solvent,are able to used to fabricate thin films by using low cost solution process, but have lowmobility, meanwhile, organic small molecule material have high mobility, but need to befabricated using vacuum evaporation process with high cost. Because it is a small moleculematerial but also readily soluble in organic solvent，Tips-Pentacene have advantages of lowcost and high mobility. For these reasons, Tips-Pentacene has attracted widely attention.In this paper, based on space-charge-limited current method, we studied the effect ofprocess parameters on the electrical properties of Tips-Pentacene thin films. The main work isas follow:(1) Ohmic contact between the electrode and Tips-Pentacene film was formed bymodifying electrode with PEDOT.(2) The thin films were prepared with the solution of1wt%Tips-Pentacene. Then theywere annealed in373K,383K and393K. It was found that the thin film annealed in383Khas a maximum current density.(3) At the best temperature of383K, the thin films were annealed for different time. Theresults of current transport properties show that with the increasing bias voltage, the J-Vcurves of the devices annealed for2and5hours in double-logarithmic scale exist differentregions, where slopes are2, larger than3and2successively, but the curve of the deviceannealed for more than8hours doesn’t have initial region with the slope equal to2. UsingSpace Charge Limited Current model, charge transport mechanisms in different regions wereanalyzed and the hole trap densities and mobilities were extracted. Annealed for more than10hours, material has the lowest trap density of5.70×1018/cm3and the maximum hole mobilityof1.68×10-4cm2/(V·s). Meanwhile, the transport mechanism of the material annealed formore than8hours changing shows that the single trap density, relevant to solvent residues,deduces considerably.(4) The thin films annealed for more than10hours, which was prepared using thesolution of2wt%Tips-Pentacene, can achieve the best performance. By comparing themaximum mobility and the minimum trap state density of the same thickness films whichwere prepared by the solutions of1wt%and2wt%Tips-Pentacene, it was found that theperformance improvement of the latter is not obvious. The reason is that the highconcentration solution prepared thin films require faster spin speed, which can lead to fastevaporation of the solvent, so the film doesn’t have sufficient time to crystallize.(5) The thin films of different thicknesses were prepared with different spin-coatingspeed of the solutions of2wt%Tips-Pentacene, The mobilities of the thicker films changewith the electric field. We used Poole-Frenkel formula to describe this change and extractedthe formula factor. (6) Admittance spectroscopy based on Space-Charge-Limited Current theory was used tostudy of the carrier transport of Tips-Pentacene thin films preliminarily.Finally, the all-solution process bottom-gate top-contact OTFT was fabricated using theTips-Pentacene film prepared by the optimal process, PVP insulator prepared using spincoating method and inkjet print electrodes. The field effect mobility is0.0093cm2/(V·s), thethreshold voltage is-6.7V and switch ratio is341.