Study On The Pentacene-Based Organic Thin-Film Transistors

In the last twenty years, organic electronics based on organicsemiconductor materials and its applications in the information industry havemade rapid progress. The studies on the organic semiconductor devicesinclude organic light-emitting diodes (OLEDs), organic thin-film transistors(OTFTs), organic solar cell and organic sensors, etc. And now, OLEDs andOTFTs are on the way to industrialization.The first example of an OTFTs based on polyacetylene was reported inthe literature in 1983. And in 1990, the OTFTs based on short conjugatedoligomer, e.g., sexithiophene, showed a mobility of the order of 10^-1 cm²/Vs,almost matching that of a-Si:H. And now, it has two important applicationsthat OTFTs will be showed in the market, one is the TFT unit device using inthe active matrix driving circuit, especially OTFTs are compatible with theflexible backbone circuit. The other one is the OTFTs can be used in thelow-cost Radio Freqency Identification (RFID).Most of switch devices in the active matrix circuit driving fiat planedisplay, such as AMLCD, are based on hydrogenated amorphous silicon(a-Si:H) with typical field-effect mobility of 0.5 - 1.0 cm²/Vs and on/offcurrent ratio > 10⁷. However, high performance a-Si:H TFTs require processtemperature > 200℃and therefore it limited compatibility with polymeric substrates.Organic TFTs on the other hand offer an attractive low-temperaturealternative. TFTs based on the organic seminconductor pentacene currentlyshow the best performance for OTFTs with typical field-effect mobility of 1.However, there are some problems should be solved such as stability, highthreshold voltage and high energy consuming etc.Pentacene is a p-type organic seminconductor, and in the vacuumdeposited polycrystalline pentacene film bulk, the free carrier density is verylow and the film is abundant of hole traps. At low gate voltage, most of theholes injected in the semiconductor are trapped into these localized states. Atan appropriately high gate voltage, all trap states are filled and subsequentlyinjected carriers move with the microscopic mobility associated with carriersin the delocalized band. So, the interface characteristic between Au andpentacene play an important role in the OTFT performance. Researchers havestudied the interface property. And they found the gold atom will diffusion intothe pentacene film surface when gold is deposited onto pentacene, there willform an metallic interface which result in the gold surface work function lower0.8eV, and then the hole injection barrier between Au and pentacene increasedto 1 eV which will confine the hole injection and lead to high contact resistance.This thesis reported relative high performance pentacene OTFTs by modifyingthe interface between Au electrodes and pentacene.We introduced the excellent organic hole injection material (HIM)m-MTDATA into the OTFTs, which has been widly used in OLEDs for itsexcellent uniformity, high dark conductance and stability in air. In the interfacelayer contact with Au, we doped HIM in it with ratio of 1:1. the performancesof the OTFTs with HIM are improved, the field-effect mobility increased from0.16 cm²/Vs to 0.51 cm²/Vs,and the absolute threshold voltage valuedecreased from 11V to 2.8V. That is say that with the HIM, the hole injection ability is enhanced and the contact resistance is reduced.Second, we insert a MoO3 buffer layer between Au and pentacene. Whenthe thickness of buffer layer varied from 0 to 10 nm, the performance ofOTFTs increased. The field-effect mobility in the linear region increased to0.68 cm²/Vs and threshold value down to 5.5V, and when the buffer layerthickness increased to 20nm, the performance of the OTFTs is almost samewith 10nm. So, the OTFTs with the buffer layer of 10nm MoO₃ have relativehigh mobility and low operation voltage, this method can be used in theindustry process.Otherwise, we deduced the conclusion that the field-effect mobility of thepentacene-based organic thin-film transistors dependent on the gate voltage.For the high density of hole traps linked to grain boundaries in thepolycrystalline pentacene film, theμ_FET is expected to be much lower thanmicrocosmic mobility. Base on the relation of the mobility and gate voltage,we estimate the parameter of the experience expressions.The last, we introduced the manufacture process of our OTFTs and thetest system based on the IEEE standard test methods for the characterization ofOTFTs, in 2004.