Lisong Zhou

Organic thin-film transistor-driven polymer-dispersed liquid crystal displays on flexible polymeric substrates

We have fabricated organic thin-film transistor (OTFT)-driven active matrix liquid crystal displays on flexible polymeric substrates. These small displays have 16×16 pixel polymer-dispersed liquid crystal arrays addressed by pentacene active layer OTFTs. The displays were fabricated using a low-temperature process (<110 °C) on flexible polyethylene naphthalate film and are operated as reflective active matrix displays.

All-organic active matrix flexible display

We have fabricated pentacene organic thin-film transistor (OTFT) driven active matrix organic light-emitting diode (OLED) displays on flexible polyethylene terephthalete substrates. These displays have 48×48 bottom-emission OLED pixels with two pentacene OTFTs used per pixel. Parylene is used to isolate the OTFTs and OLEDs with good OTFT yield and uniformity.

An experimental study of contact effects in organic thin film transistors

We report on parasitic contact effects in organic thin film transistors (OTFTs) fabricated with pentacene films. The influence on the OTFT performance of the source and drain contact metal and the device design was investigated. Top contact (TC) and bottom contact (BC) gated transmission line model (gated-TLM) test structures were used to extract the combined parasitic contact resistance as a function of gate voltage swing and drain-source voltage for OTFTs with gold source and drain contacts. For comparison BC test structures with palladium contacts were studied. Differences in the bias dependence of the contact resistance for TC and BC OTFTs indicate that charge injection and device performance are strongly affected by the device design and processing. The results from this investigation show that TC and BC device performances may be contact limited for high mobility OTFTs with channel lengths less than 10μm.

Thin-film transistors based on well-ordered thermally evaporated naphthacene films

We report on organic thin-film transistors fabricated using the small-molecule organic semiconductor naphthacene as the active layer material with device performance suitable for several large-area or low-cost electronics applications. We investigated naphthacene thin films deposited by thermal evaporation onto amorphous substrates held near room temperature. Using atomic-force microscopy and x-ray diffraction we find naphthacene films consist of a high density of submicron-sized grains with a surprisingly high degree of molecular order. Thin-film transistors fabricated using evaporated naphthacene films on thermally oxidized silicon substrates have field-effect mobility larger than 0.1 cm2/V s, current on/off ratio greater than 106, negative threshold voltage, and subthreshold slope of 1 V/decade.

Pentacene TFT driven AM OLED displays

Pentacene organic thin-film transistors (TFTs)-driven active matrix organic light-emitting diode (OLED) displays has been investigated. This letter addresses several process issues unique to this type of display which are important in achieving bright and uniform displays. A bottom contact structure was used to fabricate the pentacene TFT backplane. Polyvinyl alcohol and parylene were used to isolate the pentacene active layer and passivate the backplane. The low processing temperature may allow the use of polymeric substrates and lower cost processing. Uniform TFT performance is achieved with reasonably good mobility and on/off ratio on the backplane. The initial OLED display performance is also presented.

Flexible substrate micro-crystalline silicon and gated amorphous silicon strain sensors

We present two different kinds of semiconductor strain sensors: ungated n+ micro-crystalline silicon (n+ /spl mu/C-Si), and gated hydrogenated amorphous silicon (a-Si:H). Both sensor types are fabricated on flexible polyimide substrates. The sensors were characterized with bending perpendicular, parallel, and at 45/spl deg/ with respect to the sensor bias direction, and for several bending diameters. Sensor size and power consumption are significantly reduced compared to metallic foil strain sensors. Small sensor size and ease of integration with a-Si:H thin-film transistors also allows arrays of strain sensors or combinations of strain sensors with varying geometric orientation to allow strain direction as well as magnitude to be unambiguously determined.

Flexible electronics for space applications

NASA is currently developing a host of deployable structures for the exploration of space. These include balloons, solar sails, space-borne telescopes and membrane-based synthetic aperture radar. Each of these applications is driven by the need for a thin, low mass, large area structure (i.e., polymer-based) which could not be implemented using conventional engineering materials such as metals and alloys. In each case, there is also the need to integrate sensing and control electronics within the structure. However, conventional silicon-based electronics are difficult to integrate with such large, thin structures, due to a variety of concerns including mass, reliability and manufacturing issues. Flexible electronics, particularly thin film transistors (TFTs), are a potentially key enabling technology that may allow the integration of a wide range of sensors and actuators into these types of structures. There are numerous challenges, however, regarding the survivability of such devices during stowage and deployment of the structure, as well as during operation in the harsh environments of space. We have fabricated TFTs on polyimide substrates, and are investigating the durability of these devices with respect to relevant space environments. We are also developing flexible sensor technologies for the integration of distributed sensor networks on large area structures.

Flexible substrate a-Si:H TFTs for space applications

We have fabricated hydrogenated amorphous silicon (a-Si:H) TFTs on Kapton/sup (R)/ polyimide flexible substrates and characterized their response to deployment-like mechanical stresses and to radiation exposure. To maintain substrate flatness and provide improved thermal transfer during fabrication, we used a pressure-sensitive silicone gel adhesive layer to mount Kapton/sup (R)/ substrates onto glass carriers. The test results, presented in this paper, are encouraging for space use of a-Si:H TFTs on polymeric substrates. Device function was retained even after 1 Mrad fast electron irradiation, and irradiation-induced device changes were removed by low-temperature thermal annealing. Although some TFTs were destroyed by substrate stressing, the majority survived with only small changes, suggesting that care in device design and placement may reduce or eliminate this problem.

All-organic Active Matrix OLED Flexible Display

We have fabricated pentacene organic thin-film transistor (OTFT) driven active matrix organic light-emitting diode (OLED) displays on both glass and flexible polyethylene terephthalete (PET) substrates. These displays have 48 × 48 bottom-emission OLED pixels with two pentacene OTFTs used per pixel. Polyvinyl alcohol (PVA) and parylene were used to photolithographically pattern the pentacene active layer and isolate the OTFT backplane from the OLEDs. Pentacene OTFTs are able to easily supply the current required for OLED operation, but improvements in device uniformity and stability are of interest.

Polymeric substrate microcrystalline-silicon strain sensor

Metallic foil and semiconductor piezoresistors are frequently used as strain sensors in shape or strain monitoring applications. The sensors are typically connected in a Wheatstone bridge configuration and mounted on the surface or body to be tested. Semiconductor sensors, for example crystalline silicon, can provide good strain sensitivity with significantly reduced sensor area and also reduced bridge power compared to metal resistor bridges. a-Si:H strain sensors fabricated on glass substrates have recently been demonstrated (G. de Cesare et al, Thin Solid Films, vol. 427, p. 191, 2003). We report here the first microcrystalline-silicon (/spl mu/C-Si) strain sensors fabricated directly on flexible polyimide substrates with similar gage factor but very low power and higher yield compared to metallic strain sensor.