Abstract— A 14.1‐in.‐diagonal backplane employing hydrogenated amorphous‐silicon thin‐film transistors (a‐Si:H TFTs) was fabricated on a flexible stainless‐steel substrate. The TFTs exhibited a field‐effect mobility of 0.54 cm2/V‐sec, a threshold voltage of 1.0 V, and an off‐current of 10−13 A. Most of the electrical characteristics were comparable to those of the TFTs fabricated on glass substrates. To increase the stability of a‐Si:H TFTs fabricated on stainless‐steel substrate, the specimens were thermally annealed at 230°C. The field‐effect mobility was reduced to 71% of the initial value because of the strain of the released hydrogen atoms and residual compressive stress in a‐Si:H TFT under thermal annealing at 230°C.
We report the improved AMOLED with a-Si TFT backplane based on our unique structure. Our new structure use the encapsulation glass as a substrate for OLED which enable us to achieve the high aperture ratio, large TFT area, high luminescence uniformity, and low anode resistance. IntroductionFor AMOLED, a-Si TFT backplane is a suitable candidate because it has low manufacturing cost and it is easy to make large-area devices compared to its competitors like LTPS [1]. But it also has the weak point which requires large TFT area to operate OLED, so the top-emitting OLED (TOLED) structure is recommended.Since the conventional TOLED has an anode at bottom and a cathode on top, it is electrically connected to the driving TFT through the anode which is called a cathode common structure. In this structure, the current of the driving TFT is not independent of the operating voltage of the OLED (V OLED ) [2]. The situation can get even worse during the degradation of the display since the V OLED increases affecting the gate-source voltage of the driving TFT.Some groups have proposed an anode common structure to solve the problems generated from the cathode common structure [3], [4]. They employed an inverted-TOLED, i.e. making TOLED that has a cathode at bottom and an anode on top. However, one major challenge in the inverted-TOLED has been to prepare a reflective cathode providing an effective electron injection. Based on such considerations, low work-function metals, such as Mg [4] and Li [5] were introduced to reduce the turn-on voltage. But there remained issues in employing highly reactive metals particularly their handling and operational stability. Another group has demonstrated a more complicated structure using an ultra thin Alq 3 -LiF-Al tri-layer as the electron-injection layer [6].As an alternative anode common structure, we developed the new structure called a dual-plate OLED display (DOD) which use the encapsulation glass as a substrate for OLED and connect to TFT backplane via the contact spacer. For the usual top-emitting type AMOLED, the encapsulation glass is just for the encapsulation or that with the functioning layer for the moisture resistance. We also utilize this as OLED substrate. This enables us to achieve the high aperture ratio, sufficiently large TFT area and another advantage of this structure is highly uniform emission from panel and low anode resistance and low manufacturing cost. DOD structure FabricationThe DOD structure consists of 2 plates which are TFT on bottom substrate and OLED on top substrate connected by the contact spacer as illustrated in Figure 1. Both substrates are separately made and encapsulated afterwards. This way provides the higher yield than the conventional method by screening bad substrates separately. The conventional manufacturing flow is that TFT components are made first on the glass and then OLED component are made on top of TFT. Figure 1. DOD (Dual-plate OLED Display) structure after encapsulation Figure 2. Process flow to fabricate a DOD SID 07 DIGEST • 5 ISSN/0...
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