30.2 Digital PWM-driven AMOLED display on flex reducing static power consumption

Abstract

The efficiency of small-molecule OLED devices increased substantially in recent years, creating opportunities for power-efficient displays, as only light is generated proportional to the subpixel intensity. However, current active matrix OLED (AMOLED) displays on foil do not validate this power-efficient advantage, as too much power is lost in the AM backplane. AMOLED displays use the analog voltage on the gate of a drive transistor (e.g. M1 in Fig. 30.2.1) to control the pixel current and hence the pixel brightness. Accurate and uniform pixel currents can only be obtained when transistor M1 is driven is saturation. In highresolution technologies on foil, transistor parameters W, L and the mobility μ are limited by technology, imposing a minimal V GS -V T to obtain sufficient current, i.e. V GS -V T > 4V for a-IGZO on foil [1]. Subsequently, to obtain saturation, V DS > 4V, which translates in a static backplane power loss surpassing the OLED power consumption (see red stars in Fig 30.2.1). However, when the OLED pixel impedance around a specific reference current can be matched along a display column line, the accurate pixel current control can be imposed by current DACs implemented in external silicon display column drivers. In this work, we operate M1 as a switch and pixel intensity variations are obtained using Pulse Width Modulation (PWM) of a predefined pixel current, i.e. 2μA/pixel [80*80μm 2 ] (which corresponds in our OLED technology to a light output of 2000Cd/m 2 ). When, in a future implementation the external DACs are calibrated at 0.2μA/pixel, the full brightness would correspond to the typical display brightness of a portable PC, i.e. 200Cd/m 2 . This concept enables us to reduce the display power voltage at full brightness from 8.2V in a classical AMOLED display on foil configuration to 5V (measured) and for future implementations even down to 4V (see Fig. 30.2.1). As the OLED current load remains equal, a corresponding static power reduction of the display (and increased battery lifetime) is obtained. Digital driving methods of AMOLED displays have been shown before. However, ΔΣ techniques [2] still integrate charge packets on the gate of M1 and hence do not solve the power issue on foil. Other PWM techniques [3] activate only a single active line in the linedriver yielding difficulties to obtain color depths above 6 bits. When multiple independent linedrivers are implemented and their output is multiplexed to alternately drive a single select line, a higher color depth can be obtained [4]. This leads however to a bulky linedriver, which is hard to get within an e.g. 80μm pitch. The design and implementation of a compact integrated linedriver on foil enabling multiple alternating active signals through a single shift register is demonstrated here.To demonstrate the functionality of the select linedriver, it has been integrated in a small high-resolution red half-QQVGA display on PEN foil (64*160 pixels@80*80μm 2 , 0.54 inch diagonal]. It comprises 22352 a-IGZO transistors, from which 1872 in the...

Metal oxide semiconductor thin-film transistors for flexible electronics

“…Given the recent progresses achieved in the large-area integration of flexible devices, a relevant part of the review has focused on circuits, as well as on systems based on metal oxide semiconductor TFTs. Examples of novel large-area flexible electronic systems include flexible, textile-integrated, rollable and/or foldable optical displays, 39,40,127,158,175,176,203,214,310,316,[346][347][348][349][350][351][352] flexible and/or stretchable systems for temperature, 343 pH, 166,185 and x-ray sensing, 322,355,356 wireless power transmission, 81,315 as well as non-volatile storage and NFC transmission of data. 136,167,204,315 Despite the advances that flexible metal oxide semiconductor TFTs have witnessed in the last decade, there are still some bottlenecks that prevent the commercialization of this technology in new areas of application beyond optical displays.…”

“…316 Even more TFTs have been utilized for an AMOLED line driver based on IGZO capable of 45 frames/s at 11 V on PEN foil, which has also been integrated with an optical display (64 Â 160 pixels) and a 2T1C pixel driver circuit. 309,310 At a supply voltage of 15 V, the flexible line driver consumes a power of %97 lW. 309,310 Similarly, Zhang et al reported a 48 stage scan driver based on IGZO with a output swing of 16 V at 100 kHz.…”

“…309,310 At a supply voltage of 15 V, the flexible line driver consumes a power of %97 lW. 309,310 Similarly, Zhang et al reported a 48 stage scan driver based on IGZO with a output swing of 16 V at 100 kHz. 187 Even larger TFT count has been reported in combination with RFID or near field communication (NFC) applications.…”

“…One year later, Genoe et al proposed the use of a digital pulse width modulation (PWM) to drive a flexible topemitting AMOLED display (0.54-in., 320 ppi). 310 The PWM concept presented by Genoe et al allows reducing the DC power consumption down to 102.4 mW. 310 Recently, Motomura, Nakajima, and Takei proposed the use of airreactive electrode-free inverted OLEDs (iOLEDs) in flexible IGZO TFT-driven AMOLEDs (8-in., 100 ppi) to suppress typical undesired effects like dark spot growth and achieve longer lifetimes.…”

“…310 The PWM concept presented by Genoe et al allows reducing the DC power consumption down to 102.4 mW. 310 Recently, Motomura, Nakajima, and Takei proposed the use of airreactive electrode-free inverted OLEDs (iOLEDs) in flexible IGZO TFT-driven AMOLEDs (8-in., 100 ppi) to suppress typical undesired effects like dark spot growth and achieve longer lifetimes. 127 Although the iOLED characteristics are inferior to those of conventional OLEDs, the flexible display by Motomura, Nakajima, and Takei yields stable and clear moving images even while bent.…”