Organic thin-film transistors (OTFTs) have received great attention due to their low-cost and large-area array application. In 1960, using small-molecule organic material, condensed hydro-carbons and dyes, successfully fabricated the field effect transistor. However, their semiconductor characteristics were poor and their
reproducibility very low, which caused large difficult to develop any real devices. A relatively minor fundamental interest was maintained in organic materials until the 1970s, when the first world energy crisis launched a renewed interest in organic-based semiconductor, aimed at the development of cheap, flexible, large-area organic-based solar cells. However, after a decade of intense research, both in universities and in private enterprise, results showed that organic semiconductors suffer from severe limitations, linked to the existence of a very high density of defects and traps, as well as to very low carrier mobility. Although significant experimental effort was invested during the 1980s, the inability to increase significantly the carrier mobility above this low value of about 10-4 cm2/Vs led many research groups to question the real potential of organic-based semiconductors for use as active layers in electronic devices. Until 1990s, short conjugated oligmoer and sexithiophene showed the mobility of the order of 10-1 cm2/Vs, almost matching that of a-Si:H TFTs.
To date, the performance of organic thin-film transistors (OTFTs) has been obviously improved by the application of new organic channel compounds and advanced fabrication process. In 2000, field-effect mobility and Ion/Ioff of pentacene-based TFTs had reached 3.2 cm2V−1s−1 ratio and >109 were demonstrated.
Because the large barrier between Au and pentacene blocks electron injection from contact into the channel, the channel of OTFT is usually P-type. Fig. 1-2 (a) Pentacene is made up of five benzene rings (b) a single pentacene molecule (C22H14) consists of 5 benzene rings. In each molecule 14 of the 22 C atoms are bonded to two other C atoms and to one hydrogen atom. The four inequivalent H atoms are numbered 1–4. Other H atoms are related by symmetry. (c) A single C22H16 molecule (dihydropentacene) is shown here. The two C-H2 units are positioned on sites 1 and 8 [4].
More recently, displays have been made on OTFT backplanes on flexible polymeric substrates. OTFT arrays to drive liquid crystal (LC) [2] [5] or organic light emitting diode (OLED) [6] which showed full-color moving pictures had been demonstrated. In these reports, OTFTs are encapsulated by passivation layer to avoid exposing to oxygen or moisture in air, and to avoid damage from the subsequent LC or OLED process. When flexible substrates are substituted for conventional glass substrate, the general products can have new application and for different usage as rollable light-weight displays or environmental sensor integrated into clothing or irregular surface of consumer electronics. In particular, organic materials can be deposited at room temperature by spin coating, or by roll-to-roll technology compatible with ink-jet printing. Compared with conventional instruments for fabricating inorganic electric devices, spin coating and roll-to-roll technology can provide low-cost fabrication process due to the lower requirements in vacuum.
Because the roll-to-roll technology has high-throughput, the fabrication cost of organic electronic devices on flexible substrates can be further reduced. Here, a roll of thin plastic or metal foil used as the substrate can be kilometers long and meters compared to the glass size of 10 generation about 3 m × 3 m in a flat-panel display (FPD) manufacturing process.
Although the field-effect mobility of organic electric device is already comparable to that of hydrogenated amorphous silicon (a-Si:H), the carrier transport in organic semiconductors, such as pentacene, is sensitive to contamination and is strongly interface-dependent. However, there is difficult to fabricate the very smooth morphology of gate dielectric surface and the uniform of surface state for the large substrate. In the other hand, these organic materials are still sensitive to moisture in ambient air and need superior passivation to cover the organic active layer. Thus, inorganic and organic passivation thin-film technology still has to be further studied.
In previous reports, OTFTs are encapsulated by passivation layer to avoid exposing to oxygen or moisture in air, and to avoid damage from the subsequent LC or OLED process. However, even when devices are encapsulated or operated in an inert environment, OTFTs are known to suffer from bias stress effect (BSE) that causes significant threshold voltage shift.
The bias-stress effect in OTFTs had been studied by using different organic active materials or different gate insulators on different device structures [7]. It was found that, for p-type OTFTs under steady-state bias stress, positive gate bias stress caused a positively-shifted threshold voltage (Vth) and negative gate bias stress caused a negatively-shifted Vth. The BSE was reversible by removing gate bias or by applying opposite polarity gate bias. Light irradiation also enhanced the reversal process.
However, the prolong illumination causes a positively-shifted Vth. due to trapping light-induced electrons.
Charge trapping, ion migration, charged-state creation and the formation of bound hole pairs (bipolaron) are several proposed mechanisms to explain the BSE [8].
Charge trapping and ion migration were found to be dominant mechanisms in OTFTs with an organic dielectric [9]. When using thermally-grown SiO2 as the gate dielectric to study OTFTs reliability, charged-state creation is usually believed to be responsible for the threshold voltage shift (ΔVth). John E. Northrup and Michael L. Chabinyc used density functional calculation to simulate defect states generation in pentacene film and found that it was due to the formation of oxygen- and hydrogen-related defects such as C-H2, OH, and C-HOH in organic semiconductors [10]. Gu et al. also studied the response time of the defect states in pentacene. Long-lifetime deep electron traps were proposed to explain the hysteresis effect in pentacene-based OTFTs.
In this thesis, the reliability issues of pentacene-based OTFTs are studied and discussed in the following separated chapters.