Top-emitting 230dots∕in. active-matrix polymer light-emitting diode displays on flexible metal foil substrates

Abstract

A top-emitting 230 dots/ in. monochrome active-matrix polymer light-emitting diode ͑PLED͒ display having the VGA format and fabricated on a flexible steel foil utilizing the polycrystalline silicon thin-film transistor ͑TFT͒ technology is reported. The pixel circuitry architecture consists of the conventional two TFT circuitries made of two p-channel metal-oxide-semiconductor ͑PMOS͒ transistors and one storage capacitor. The average field-effect hole mobility and threshold voltage of the PMOS polysilicon TFTs fabricated on the metal foil are 37͑±4͒ cm 2 / V s and −1.9͑±0.6͒ V, respectively. The light turn-on voltage of the PLED is 4.0 V.Active-matrix organic light-emitting diode ͑AMOLED͒ displays based on either small molecules 1 or polymer LED 2,3 ͑PLED͒ are envisioned as candidates for the next generation displays because of their light weight, low power, selfemission, wide viewing angle, and capability of being conformal or foldable. Demonstrations of OLEDs and PLEDs onto flexible substrates 4,5 as well as of amorphous 6 and polysilicon thin-film transistors 7-9 ͑TFTs͒ onto flexible metal foils have propelled the development of flexible AMOLED or AMPLED displays onto metal foils. 10-12 Thin metal foils are particularly attractive for flexible AMOLED or AMPLED displays because they are excellent barrier to water and oxygen that degrade the lifetime of these displays. Polysilicon TFTs offer a number of advantages for activematrix displays compared to amorphous silicon TFTs, such as higher carrier mobility, immunity to threshold-voltage shift, and ability to integrate complementary metal-oxidesemiconductor ͑CMOS͒ display driver circuits onto the display panel. Stainless steel foils, in particular, have superior chemical resistance in a number of chemical solutions used during the device fabrication, and they are stable during high-temperature thermal processing steps such as those used during the preparation of gate dielectric, thermal activation of dopants, and silicidation. In addition, the dimensional stability of the steel foil substrates enables the implementation of circuit designs with high resolution. Recently, we have demonstrated n-channel TFTs onto flexible steel foil substrates having both channel width and channel length as small as 1 m and with an effective electron mobility of 358 cm 2 / V s as well as high performance CMOS circuits suitable for integrated display driver applications. 9,13 In this letter we report the demonstration of a flexible AMPLED display onto flexible metal foil substrate. In contrast to previously reported flexible displays on metal foils that integrate either amorphous TFTs with PLEDs ͑Ref. 10͒ or polysilicon TFTs with small-molecule OLEDs, 11,12 this work reports the integration of polysilicon TFTs with PLEDs. Polymers, used in AMPLED displays, are readily compatible with spin-cast or ink-jet printing process 14 that offers reduced process complexity compared with the evaporation of the small-molecule materials used in AMOLED displays. The opaque nature of the flexible met...

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