This device has a saturation carrier field-effect mobility of 1.2 cm²/V s and on/off current ratio of 〖10〗^8. We found that direct exposure of pentacene to PDLC materials severely degrades TFT performance. To avoid this degradation, an additional layer of PVA was deposited as a protective barrier. Some pentacene TFT degradation was still observed after PDLC assembly; the cause of this degradation is still under investigation. OTFT on currents dropped by a factor of 3-8X after PDLC assembly, due to a reduction in mobility and shift in threshold voltage. Even with this degradation, device performance was sufficient to obtain functioning pixel arrays. The display was completed by depositing a 4:1 mixture of Merck TL205 liquid crystal and PN393 polymer precursor directly onto the passivated OTFT backplane and then laminating a Mylar cover sheet onto the sample. The PDLC cell was then created from the mixture by photoinduced phase separation using illumination from an ultraviolet lamp (10 mW/cm² intensity)for ~20 minute.
PDLC thickness was set by 16μm plastic spacers. The Mylar sheet was coated on one side with ITO to act the common electrode for the display. Figure 2(a) showns a schematic cross section and Fig.2(b) an optical micrograph of a single pentacene TFT pixel on PEN film. Figure 3 shows a photograph of a test chip containing pentacene TFTs, circuits, and pixel arrays on flexible PEN film removed from the glass carrier after PDLC assembly.
Four different types of 250 μm-pitch, 16*16 pixel arrays were fabricated with clear aperture between 38% and 58%, and varying OTFT design. All arrays showed reasonable electro-optic response to 1/4- VAG wave forms with the best performance for array was operated using data voltage = ±20 V, select voltage =-30 V, and deselect voltage
= +25 V with 69 μs line time and 60 Hz refresh rate.Because of the PDLC-degration-related
Reduced OTFT on currents, the pixels were not able to fully converge at the ±20 V data voltage in the 69μs line time. However, even with degraded on currents and their resulting pixel convergence errors, the pixel arrays were able to respond well to changing data and gave good visual performance. Pixel yield was high (≥98%) considering that substrates were not fabricated in a clean room environment. Illumination conditions consisted of a standard iincandescent lamp approximately 45 ◦C off axis with a black absorber placed behind the display. Figure 4 gives reflective electro-optic characteristics for a pixel array showing relative brightness as a function of pixel data voltage for three different cases. For the first case, brightness was measured by averaging the reflected light over a large area containing many pixels. In the other two cases, measurement were taken at the center of the pixel electrode for individual pixels. Each curve is normalized to the brightness at a pixel data voltage of zero volts. The measured contrast ratio is relatively small (˃2:1 for many pixels and ˃3:1 for single pixels) due to reflection from the metal lines and other areas outside areas outside the pixel electrode which is evident from the improve brightness curves and contrast ratio for the individual pixels. The contrast ratio could easily be improved by using a black matrix layer to reduce reflections outside the pixel electrode. Figure 5 shows optical micrographs of a functioning pixel array displaying four different patterns.
Active matrix PDLC display driven by pentacene organic thin-film transistors on flexible polymeric substrates have been fabricated and show good response to ¼-VAG waveforms. Our result confirm that flexible active matrix displays can be made on polymeric substrates using a lowtemperature organic thin-film transistor process. These prototype pixel arrays demonstrate the usefulness of OTFTs for low-cost electronics and are a step toward the realization of cheap, light-weight, and flexible applications such as flat panel displays.
Financial support by the Defense Advanced Research Projects Agency and the Naval Surface Warfare Center Dahlgren Division is gratefully acknowledged.