Influence of Environmental Conditions on Electrical Stability of Pentacene Thin-film Transistors with Cross-linked Poly(4-vinylphenol-co-methyl methacrylate) Gate Dielectric Layer (original) (raw)
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Electrical characterization of pentacene thin-film transistors with polymeric gate dielectric
Synthetic Metals, 2004
Pentacene thin films obtained by thermal evaporation at room temperature have been incorporated as the active layer in bottom-gate thin-film transistors (TFTs). The dielectric was spin-cast polymethyl methacrylate (PMMA) baked at only 170 • C. Crystalline silicon wafers and polyethylenenaphtalate (PEN) polymer foils were used as substrates. These devices were electrically characterised by measuring the output and transfer characteristics at different temperatures. Both the channel conductance and field-effect mobility evidenced similar thermal activation energies around 0.15 eV. These results could indicate that electrical transport is mainly controlled by trapping and thermal release of carriers from localised states.
Journal of Non-crystalline Solids, 2004
Pentacene thin-film transistors using polymethyl methacrylate as a gate dielectric have been fabricated. A bottom gate, inverted staggered structure was selected to study the influence of the dielectric on the device performance. Crystalline silicon wafers and polyethylenenaphtalate polymer foils were used as substrates. Pentacene thin-films were deposited by thermal evaporation in a highvacuum system. The maximum process temperature was 170°C, corresponding to the baking of polymethyl methacrylate. These devices showed satisfactory p-type electrical characteristics with on/off ratios exceeding 10 3 for V GS ranging from )30 to 30 V. The field-effect mobility and threshold voltage were around 0.01 cm 2 V À1 s À1 and )14 V, respectively. The polymethyl methacrylate dielectric also seems to provide some advantages of the so-called self-assembling monolayers.
Electrical stability of pentacene thin film transistors
Organic Electronics, 2007
The influence of environmental conditions on the device operation and the stability of polycrystalline pentacene thin film transistors (TFTs) were investigated. Electrical in-situ and ex-situ measurements of staggered pentacene TFTs were carried out to study the influence of dry oxygen and moisture on the device stability. The transistors were fabricated by organic molecular beam deposition on thermal oxide dielectrics. Oxygen exposure of the pentacene films lead to the creation of acceptor-like states in the bandgap. The acceptor-like states cause a shift of the onset of the drain current towards positive gate voltages. The charge carrier mobility and the on/off ratio of the transistor are not affected by the acceptor-like states. Furthermore, the acceptor-like states have an influence on the stability of the TFTs. Devices exposed to oxygen exhibit a shift of the threshold voltage upon prolonged biasing. Transistors characterized under vacuum conditions (no oxygen exposure) do not exhibit a shift of the threshold voltage (bias stress effect) as a consequence of prolonged biasing. The experimental results show a clear correlation between the device behavior upon oxygen exposure and the stability of the devices. The shift of the onset voltage upon oxygen exposure correlates with the shift of the threshold voltage upon prolonged bias. The influence of dry oxygen on the onset voltage, the threshold voltage, and the electrical stability will be described. Furthermore, the influence of bias stress on the operation of organic circuits like an active matrix addressed OLED displays will be discussed.
Journal of Applied Physics, 2007
The influence of environmental conditions on the electronic transport and the device stability of polycrystalline pentacene transistors were investigated. Electrical in situ and ex situ measurements of pentacene thin film transistors were carried out to study the influence of dry oxygen and moisture on the device operation. The staggered thin film transistors were fabricated by organic molecular beam deposition on thermal oxide dielectrics. Exposing the pentacene films to oxygen leads to the creation of acceptorlike states in the band gap. The acceptorlike states cause a shift of the onset of the drain current towards positive gate voltages. A simple analytical model will be presented which directly correlates the onset voltage of the transistors with the acceptor concentration in the pentacene film. Exposing the pentacene film to moisture causes a drop of the charge carrier mobility, a reduction of the threshold voltage, and a shift of the onset voltage. Besides the creation of acceptorlike states in the pentacene film the interface between the drain and source electrodes and the pentacene film is affected by moisture. The injection of holes in the highest occupied molecular orbital level of the pentacene film is inhibited, which causes an apparent drop of the charge carrier mobility and a reduction of the threshold voltage.
Influence of structural properties on environmental stability of pentacene thin film transistors
Organic Electronics, 2011
We studied environmental stability of top contact pentacene TFTs with active layer evaporated at different growth rates. We measured the transfer characteristics in vacuum and in air and after storing the devices in oxygen for several days. Different pentacene growth rates result in different grain size of active layer. This morphology difference influences the hysteresis of transfer characteristics induced by water absorption. On the contrary, aging effects on the transfer characteristics of pentacene O-TFTs, induced by oxygen diffusion into the active layer, are not related to structural characteristics of pentacene film.
Pramana, 2008
Organic thin film transistors (OTFTs) were fabricated using pentacene as the active layer with two different gate dielectrics, namely SiO2 and poly(methyl methacrylate) (PMMA), in top contact geometry for comparative studies. OTFTs with SiO 2 as dielectric and gold deposited on the rough side of highly doped silicon (n + -Si) as gate electrode exhibited reasonable field effect mobilities. To deal with poor stability and large leakage currents between source/drain and gate electrodes in these devices, isolated OTFTs with reduced source/drain contact area were fabricated by selective deposition of pentacene on SiO 2/PMMA through shadow mask. This led to almost negligible leakage currents and no degradation in electrical performance even after 14 days of storage under ambient conditions. But, the field effect mobilities obtained were lower than 10 −3 cm 2 V −1 s −1 , whereas by using PMMA as gate dielectric with chromium deposited on the polished side of n + -Si as gate electrode, improved field effect mobilities (>0.02 cm 2 V −1 s −1 ) were obtained. PMMA-based OTFTs also exhibited lower leakage currents and reproducible output characteristics even after 30 days of storage under ambient conditions.
Materials, 2010
We investigated the electrical stabilities of two types of pentacene-based organic thin-film transistors (OTFTs) with two different polymeric dielectrics: polystyrene (PS) and poly(4-vinyl phenol) (PVP), in terms of the interfacial charge depletion. Under a short-term bias stress condition, the OTFT with the PVP layer showed a substantial increase in the drain current and a positive shift of the threshold voltage, while the PS layer case exhibited no change. Furthermore, a significant increase in the off-state current was observed in the OTFT with the PVP layer which has a hydroxyl group. In the presence of the interfacial hydroxyl group in PVP, the holes are not fully depleted during repetitive operation of the OTFT with the PVP layer and a large positive gate voltage in the off-state regime is needed to effectively refresh the electrical characteristics. It is suggested that the depletion-limited holes at the interface, i.e., interfacial charge depletion, between the PVP layer and the pentacene layer play a critical role on the electrical stability during operation of the OTFT.
Environmental stability and electronic transport of pentacene thin film transistors
Journal of Non-crystalline Solids, 2006
The influence of environmental conditions on the electronic structure and the device stability of polycrystalline pentacene transistors were investigated. Electrical in situ measurements of pentacene TFTs were carried out to study the influence of dry oxygen on the device operation. The polycrystalline pentacene thin film transistors were fabricated by organic molecular beam deposition (OMBD) on thermal oxide dielectrics. Exposing the pentacene film to oxygen leads to the creation of acceptor-like states deep in the bandgap. The acceptorlike states cause a reduction of the subthreshold slope and a shift of the onset of the drain current to positive gate voltages. The threshold voltage and the mobility of the transistors are not affected by the dry oxygen. A simple model will be described which directly correlates the onset voltage of the transistors with the acceptor concentration in the pentacene film.
Pentacene thin-film transistors with polymeric gate dielectric
Organic Electronics, 2004
Pentacene thin-film transistors have been obtained using polymethyl methacrylate as a gate dielectric. The maximum process temperature was 170°C, which corresponds to the baking of the polymeric gate dielectric. These devices presented good electrical performances with field-effect mobilities of 0.01 cm 2 V À1 s À1 and low threshold voltages ()15 V). Atomic force microscopy studies reveal that the microstructure of pentacene layers is strongly conditioned by the surface morphology of the dielectric.
The European Physical Journal Applied Physics, 2012
The organic thin-film transistors (OTFTs) incorporating pentacene/SU-8 interface were fabricated and characterized. SU-8, a reliable epoxy-based photoresist, is tested as a potential highly-stable polymeric gate dielectric for OTFTs. The fabricated devices showed promising electrical performance with on-off ratio up to 10 7 and field-effect mobility up to 0.56 cm 2 /V s. Several device characteristics are further analyzed. There existed a leakage current path due to the uncontrolled pentacene coverage and we revealed that precise alignment of the evaporation mask of pentacene is critical for eliminating this problem. Pentacene grain formation largely depended on the growth condition on the SU-8 surface and small-grain films offered outstanding performance possibly owing to enhanced inter-domain connections. Natural degradation of the OTFTs is also discussed in terms of environmental stability and the pentacene/SU-8 transistor operated with noticeable air stability under ambient conditions.