IEEE ELECTRON DEVICE LETTERS, VOL. 18, NO. 12, DECEMBER 1997 609 Integration of Organic LED’s and Amorphous Si TFT’s onto Flexible and Lightweight Metal Foil Substrates C. C. Wu, S. D. Theiss, G. Gu, M. H. Lu, J. C. Sturm, S. Wagner, and S. R. Forrest Abstract—We report the integration of organic light emitting devices (OLED’s) and amorphous Si (a-Si) thin-film transistors (TFT’s) on both glass, and unbreakable and lightweight thin stainless steel foil substrates. The doped-polymer OLED’s were built following fabrication of driver TFT’s in a stacked structure. Due to the opacity of the steel substrate, top-emitting OLED structures were developed. It is shown that the a-Si TFT’s provide adequate current levels to drive the OLED’s at video brightness ( 100 cd/m ). This work demonstrates that lightweight and rugged TFT backplanes with integrated OLED’s are essential elements for robust and highly portable active-matrix emissive flat-panel displays. I. INTRODUCTION A long-sought-after goal has been a flat-panel display that is unbreakable, lightweight, flexible, and low cost. Organic light emitting devices (OLED’s) based on organic thin films have this potential because of their demonstrated performance, their versatility of colors, their lack of a need for a crystalline substrate, and their potential low cost [1]–[3]. Meanwhile, amorphous Si (a-Si:H) thin-film transistors (TFT’s) have been in widespread production for a number of years as the switch- ing elements in high-resolution, active matrix liquid crystal displays (AMLCD’s). Both of these thin-film technologies are typically fabricated on fragile glass substrates. In this letter, we report the integration of a-Si TFT’s and OLED’s onto glass substrates as well as onto rugged and lightweight stainless steel thin foils. Although there have been successful demonstrations of flexible OLED’s fabricated on plastic substrates [3]–[5], the fabrication of a-Si TFT’s on plastics has proven difficult due to mechanical and chemical instabilities of such substrates at the processing temperatures typically needed for a-Si TFT’s ( 300 C) [6]. Therefore, integration of both OLED’s and TFT’s on a flexible plastic substrate to make an unbreakable display is still problematic. Since OLED’s can be made to emit light from the top surface [7]–[8], transparency of substrates is not required for OLED/TFT integration, lending more freedom to our choice of substrates. Thin steel foils, which have Manuscript received May 5, 1997; revised July 29, 1997. This work was supported by the New Jersey Commission on Science and Technology through POEM, and by grants from NSF, DARPA/Wright-Patterson AFB, and Universal Display Corporation. The authors are with the Department of Electrical Engineering, Center for Photonics and Optoelectronic Materials (POEM), Princeton University, Princeton, NJ 08544 USA. Publisher Item Identifier S 0741-3106(97)08907-6. high mechanical strength, flexibility, light weight and thermal stability, have been previously demonstrated by Theiss et al. to be compatible with TFT processing [6], [9]. In this letter, we demonstrate the integration of a-Si TFT’s with OLED’s on steel foil substrates which may find uses in large-area active matrix displays. II. EXPERIMENT OLED’s are carrier-injection devices conventionally built on glass substrates precoated with indium tin oxide (ITO) used as the bottom, hole-injecting anode contact. In this configuration, light emits through the transparent ITO layer and the glass substrate. To make the top-emitting OLED’s on the opaque steel substrate, the transparent bottom anode contact has been replaced by the high work function metal Pt, which is found to have a hole injection efficiency comparable to ITO. On the other hand, semitransparent cathode contacts needed for top surface emission can be formed by using thin ( 20 nm) layers of low work function metals such as Ag [10]. These thin single-layer metal films, however, cannot provide efficient electron injection and have high sheet resistance, leading to rather low quantum efficiency ( 0.01%) and high drive voltage. In this device, we therefore employ double-layer cathode contacts [8], in which a thin (100–170 ˚ A) semitrans- parent Mg:Ag layer provides for electron injection, while a transparent and conducting ITO cap layer provides for high conductivity and environmental robustness. A transmittance of 70% can be achieved using this cathode composition [8]. A schematic cross section of the integrated TFT/OLED is shown in Fig. 1(a), with the circuit shown in Fig. 1(b). The steel foil substrates are 200- m thick, one-side polished, grade 430 stainless steel, and possess an rms surface roughness of 70 nm. An insulating barrier layer, a-SiN :H, was first deposited to electrically isolate the active devices. a-Si TFT’s with an inverted-staggered strucrture and a W/L ratio of 776 m/42 m( 18) were then fabricated on top of the barrier layer. All a-Si:H and a-SiN :H layers were deposited at a pressure of 500 mtorr in a three-chamber plasma enhanced chemical vapor deposition (PECVD) system, in which un- doped a-Si:H (at 250 C), n a-Si:H (at 260 C) and a-SiN :H (at 310 C) are deposited in separate chambers. Chromium gate and drain/source contacts were deposited by a separate thermal evaporating system. The details of material growth and TFT device fabrication have been previously described 0741–3106/97$10.00  1997 IEEE