Transparent and Photo‐stable ZnO Thin‐film Transistors to Drive an Active Matrix Organic‐Light‐ Emitting‐Diode Display Panel

Abstract

Photo and electrically stable transparent ZnO thin-film transistors for an active matrix organic light emitting diode (AM-OLED) panel are reported. Oxide semiconductor-based thin-film transistors (TFTs) have been extensively studied mainly to replace Si-based TFTs in electrical and optical devices. [1][2][3][4][5] Therefore, there have been many reports on oxide-based TFTs: transparent oxide TFTs, [6][7][8][9][10] amorphous oxide TFTs, [11][12][13][14][15] polycrystalline oxide TFTs, [16,17] and even photo-detecting oxide TFTs. [18] In particular, transparent ZnO-TFTs have attracted much attention as display drivers that possess some evident advantages over opaque Si-based TFTs.[19] Transparent TFTs (TTFTs) with a large energy bandgap channel of $3.3 eV would transmit a visible light signal to increase the aperture ratio even in the bottom emission OLED display, which could not be anticipated from any conventional opaque TFTs. Well-designed TTFTs would result in important benefits such as longer life time and lower fabrication price of OLEDs or TFT-LCDs. However, since the ZnO-TFTs usually contain defects in the ZnO channel and deep level defects in the channel/dielectric interface that generate unwanted photo-current during light transmission and device operation, [20,21] the very advantages of the transparent ZnO-TFTs have never been fully utilized or demonstrated for display panels, to the best of our limited knowledge. We recently developed an effective fabrication methodology to greatly reduce the number of defects in the channel and interfacial charge trap defects using a low temperature (200 8C) atomic layer deposition (ALD) for a ZnO semiconductor channel and Al 2 O 3 dielectric. We found that our transparent ZnO-TFTs with defect-controlled channel and channel/dielectric interface maintain good photo-stability during device operation without generating much detectable photocurrent. Our ZnO-TFTs showed a good mobility of $4 cm 2 V À1 s À1 and an excellent high on/off ratio of $10 7. The display back panels with our photo-stable and fully transparent ZnO-TFT array demonstrate a successful operation of a 2.5 inch-sized bottom emission AM-OLED panel under 15 V, which exhibited a high aperture ratio of $60%. Figure 1a and 1b show a microscopic plan view of our 2 TFT/1 storage capacitor (2T/1C) cell structure and a circuit diagram of the cell, respectively. (Please note that the size of a pixel cell was 210 mm 230 mm as measured under an optical microscope.) Based on this back plane composed of a 2T/1C cell matrix we fabricated OLED pixels to operate in a manner of bottom emission. Our TTFT exhibited a higher than 80% transmittance in the visible range as shown in Figure 1c. The schematic but detailed cross section of the cell is shown in Figure 2a, although it only displays our top-gate driving ZnO-TFT and storage capacitor here. In Figure 2a, the driving ZnO-TFT contains an initial 9 nm thin gate dielectric and next a 176 nm thick dielectric. The first thin dielectric was deposited on a ZnO channel at 200 8C...

Ultra-high mobility transparent organic thin film transistors grown by an off-centre spin-coating method

“…Since all the organic materials used here have large bandgaps, the resulting films when cast on ITO substrates showed an excellent transparency of 490% in the visible region (Fig. 1c), which is comparable to the best transparencies reported to date for TFTs, including both organic and metal oxide TFTs 28,29 . Such highly transparent transistors are of great interest for flat-panel display backplane and sensor array applications.…”

An ‘aqueous route’ for the fabrication of low-temperature-processable oxide flexible transparent thin-film transistors on plastic substrates

“…As the coordination number of the indium atoms is fixed at 6 and because a symmetric chemical structure is also indicated by the Raman study, the structure of the IC in an aqueous solution state can be reasonably concluded to be monomeric [In(OH 2 ) 6 ] 3 þ , which indicates no presence of nitrate ions in the complex. The expected chemical structure is In(OH 2 ) 6 , and nitrate ions are not significantly ionically bonded with the In cations but are rather dispersed or coordinated with water molecules. As such, the nitrate ions form HNO 3 molecules (which exhibit a boiling temperature of 83 1C in a dilute nitric acid solution) in a solvated state in water, and the different structure results in different thermal decomposition behavior compared with the chemical structure of pristine In(NO 3 ) 3 Â H 2 O purchased from Aldrich.…”

“…4,5 In the early studies, these materials were primarily prepared using a vacuum process. 6,7 Although the vacuum-based deposition method has advantages, the high fabrication cost and large-area device uniformity restrict its areas of application. We suggest a simple and novel 'aqueous route' for the fabrication of oxide thin-film transistors (TFTs) at low annealing temperatures (that is, o200 1C).…”

Improved Electrical Properties of Indium Gallium Zinc Oxide Thin-Film Transistors by AZO/Ag/AZO Multilayer Electrode

“…Recently, transparent-oxide-semiconductor-based transistors have shown much attention in application such as active-matrix organic light-emitting displays (AMOLED), active-matrix liquid-crystal displays (AMLCDs), and flexible displays [1][2][3]. Thin film transistors (TFTs) using oxide-semiconductor channels have been intensively investigated because oxide semiconductors such as amorphous In-Ga-Zn-O (a-IGZO) can be formed at a low-temperature process and show larger motilities than those of hydrogenated amorphous silicon TFTs [4].…”