Abstract
We fabricated an array consisting of organic TFTs(OTFT) and organic LEDs (OLED) in order todemonstrate the possible application of OTFTs toflexible active matrix OLED (AMOLED). The panelwas composed of 64 x 64 pixels on 4 inch size polyethylene-terephehalate (PET) substrate in which eachpixel had one OTFT integrated with one green OLED.The panel successfully displayed some letters andpictures by emitting green light with a luminance of1.5 cd/m2 at 6 V, which was controlled by the gatevoltage of OTFT
.1. Introduction
The flexible AMOLED is attracting much attentionbecause it enables a lighter and thinner display [1, 2].In addition, large area displays can be made cheaplybecause of the low temperature process used and theirpossible roll-to-roll manufacturing. However, theflexible AMOLED display is at the proof-of-conceptstage for conformable and rollable displays.For the flexible AMOLED, the driving transistors,which should be able to supply sufficient current toOLED, should have high compatibility with plasticsubstrate. At present, three types of transistors such aslow-temperature poly-silicon TFT and amorphoussilicon TFT and organic thin film transistor (OTFT)are available for flexible AMOLED. Among themOTFTs exhibited the highest compatibility to plasticsubstrate because of their low temperature (~ 100oC)process [3]. OTFTs have recently been welldeveloped to exhibit mobility of ~ 5 cm2/V.sec at thediscrete-device level[4].Several groups are working on flexible AMOLED.DuPont and Honeywell demonstrated flexibleAMOLED using a-Si on glass as well as plasticsubstrate [5]. Lehigh University also fabricatedflexible AMOLED on stainless substrate using poly-Si [6] and Philips chose polymer TFT to drive E-ink[7]. A meaningful result about AMOLED panel usingpentacene TFTs was reported by Pioneer group butthe substrate was not plastic but glass, and the gatewas also an inorganic material such as Ta2O5 [8]. Andwe have presented the preliminary results aboutintegration of pentacene TFTs with OLEDs on PETsubstrate [9]. To our knowledge, flexible AMOLEDpanels driven by organic TFTs with a large scale suchas the panel in this paper have not yet been reported.In the work reported here, we fabricated an arrayof 64 x 64 pixels on 4 inch size poly-ethyleneterephehalate(PET) substrate in which each pixel wascomposed of one OTFT and one OLED. The purposeof this panel was to investigate the current drivingcapability of OTFTs for OLEDs and to develop thefabrication process for flexible AMOLED. TheOTFTs used poly(4-vinylphenol) (PVP) for gateinsulator and pentacene for organic semiconductor.For OLED, two layers of Alq3 and TPD wereemployed to generate green light. In this letter we willdiscuss the fabrication process and operation resultsof the array considering the possible application ofOTFTs to flexible AMOLED.
2. ExperimentalA. Pentacene TFTs with PVP Gate Insulator
PVP has been used for a gate of OTFT on a Sisubstrate [10]. However, in our work, PVP processwas developed for PET substrate. The PVP organicgate material consisted of PVP polymer and cross-linkagent (CLA), and propylene glycol monomethyl etheracetate (PGMEA) as a solvent. We found that theoptimum ratio of components was 10wt% of PVPmixed with 5wt% of CLA in 100wt% of PGMEA interms of mobility and on/off current ratio. The CLAwas activated by thermal heating and the optimumtemperature was found by considering the chemicalresistance against acetone used for photoresiststripping. The thermally cross-linked PVP provided ahydrophobic surface with the contact angle of 61.9ocorresponding to a surface energy of 25 dyne/cm,compared with the non-cross-linked PVP polymerfilm which had the contact angle of 52.1o and surfaceenergy of 35 dyne/cm.


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