**1. Introduction**

Electronic textiles (e-textiles) are attracting much attention because they are expected to provide new functionality by integrating electronic devices into textiles [1,2]. Especially, as wearable electronics and the Internet of Things are emerging as prominent killer applications of IT technology [3], e-textiles are becoming an increasingly important technology because of their greater convenience and functionality than conventional hand-carried devices. It is expected that e-textiles will be applied in very diverse areas such as healthcare, sports, fashion, and the military [1,2].

Technologies related to e-textiles have evolved from a gadget style, where electronic circuit boards were simply attached onto textile surfaces [4–8], to on-cloth applications, where electronic devices are directly fabricated onto the textiles [1]. In the future, in-cloth textiles will be developed, where active devices such as organic thin-film transistors (OTFTs), organic light-emitting diodes (OLEDs), and organic photovoltaics will be implemented on a single fiber and the active fibers will be woven together to realize electronic functions in textiles [9–14].

A display is a key device needed in order to realize the ubiquitous features of e-textiles; information should be able to be obtained at any place and any time. Among the currently available displays, an OLED is a unique device which can be implemented on textiles because of the attractive properties such as self-emission, the capability of low-cost solution processes, and the possible applications to flexible electronics, enabling to use a variety of substrates [15–19]. However, although articles about OLEDs on fabric substrates [20] and on a single fiber [21] have been published, reports regarding electronic display panels integrating OLEDs and OTFTs on a fabric substrate are rare.

Previously, the authors of this paper published an article regarding an active-mode organic light-emitting diode (AMOLED) panel fabricated on a fabric substrate [22]. In the paper, the AMOLED panel adopted a side-by-side pixel structure having an OLED beside the OTFTs' pixel circuit on the same plane. However, the aperture ratio—the area ratio of the OLED to the whole pixel—could not be larger than 20% due to the large area of the OTFTs required to drive the OLED and the rough surface of the fabric substrate.

In this paper, to improve the aperture ratio of the AMOLED display, a stacked-pixel structure placing the OLED above the OTFTs' circuit layer was developed for the fabric substrate. With the stacked-pixel structure, the aperture ratio was increased by approximately 2.5 times that of the side-by-side pixel structure.

### **2. Design of the AMOLED Panel with a Stacked-Pixel Circuit**

In this study, the AMOLED panel adopted a standard pixel circuit, consisting of two OTFTs, one OLED, and one capacitor, as shown in Figure 1a. The switching OTFT (SW OTFT) was activated by applying a scan voltage (Vscan) to the gate of the SW OTFT. Then, information on the data line (Vdata) was transferred to the storage capacitor (Cst) through the SW OTFT. The key function of the SW OTFT was to supply a su fficiently large current in the on-state to the Cst, and thus, to cause the voltage across the Cst to increase quickly during the scanning period. Having an extremely low o ff-state current in the o ff-state was also an important feature to sustain the voltage on the Cst during the time frame. The driving OTFT (DR OTFT) was activated by the voltage stored in the Cst. The DR OTFT should supply a large current to the OLED to light it up brightly.

**Figure 1.** (**a**) The pixel circuit of the active-mode organic light-emitting diode (AMOLED) panel consisting of 2 organic thin-film transistors (OTFTs), 1 organic light-emitting diode (OLED), and 1 capacitor; and (**b**) the layout and cross-section of the stacked pixel.

Since the mobility of the OTFTs was less than 1 cm<sup>2</sup>/V·s, a large ratio of the channel-width-to-length (W/L) was required to supply a large current, as described above. Therefore, the aperture ratio was generally less than 20% when the OTFTs and OLED were placed on the same plane; this is called a side-by-side structure [22].

In this study, the aperture ratio was enhanced by employing a stacked-pixel structure, where the OLED was placed above the OTFTs' pixel circuit, as shown in Figure 1b. In order to realize the stacked structure in the panel, several technological issues needed to be resolved. First, to supply a su fficiently large on-state current (Ion) to the enlarged OLED in the stacked pixel, the OTFTs' performance needed to be improved, because the OLED area relative to the OTFTs' circuit was larger compared to a side-by-side structure. Next, a protection layer (PL) should be inserted between the OTFTs' circuit layer and the OLED layer in order to protect the OTFTs' circuit from being damaged by the OLED processes performed above it. The PL must not a ffect the OTFTs below and should also have a self-patterning ability. Otherwise, a patterning process, such as photo-lithography, may seriously damage the OTFTs. In the next section, the fabrication processes will be described by focusing on these issues.

Based on a channel length of L = 20 μm as a minimum feature size, the AMOLED panel was designed as having 64 × 64 pixels. The channel width of the DR OTFT and the SW OTFT were designed to be W = 180 × L and 34 × L, respectively. Therefore, the pixel pitch was 1 mm × 0.77 mm, with an aperture ratio of 48%, where the OLED area was 0.76 mm × 0.49 mm, and a panel diagonal length of 3.2 inches. In the stacked pixel, the pixel pitch was reduced by about 70%, meanwhile, the aperture ratio was increased by 2.5 times compared with a side-by-side pixel with the same minimum feature size.
