An integrated scan-line driver, driving half a QQVGA flexible AMOLED display using amorphous-IGZO backplane technology on foil, has been designed and measured. A pulse-width modulation technique has been implemented, enabling to drive the OLEDs with a duty cycle up to almost 100%. The digital driving method also results in a 40% static power reduction of the display. Dynamic logic and bootstrapping techniques enabled the use of clock frequencies up to 300 kHz in unipolar amorphous-IGZO technologies on foil.

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 VGS-VT to obtain sufficient current, i.e. VGS-VT > 4V for a-IGZO on foil [1]. Subsequently, to obtain saturation, VDS > 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*80μm2] (which corresponds in our OLED technology to a light output of 2000Cd/m2). 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/m2. 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 - own 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.

AbstractWe present a QQVGA (160times120 with 3 sub‐pixel) top‐emitting AMOLED display with 85ppi resolution using a self‐aligned (SA) IGZO TFT backplane on polyimide‐foil. The back plane process flow is based on a 5 layer photolithography process. The aperture ratio of the top‐emitting OLEDs is approx. 25%. For operation at 6 V supply voltage (VDD), the brightness of the display exceeds 150cd/m2. On the same substrate a 160 stage gate‐driver was measured at FHD rate.

A process to make self-aligned top-gate amorphous indium-gallium-zinc-oxide (a-IGZO) thin-film transistors (TFTs) on polyimide foil is presented. The source/drain (S/D) region's parasitic resistance reduced during the SiN interlayer deposition step. The sheet resistivity of S/D region after exposure to SiN interlayer deposition decreased to 1.5k/. TFTs show field-effect mobility of 12.0cm(2)/(V.s), sub-threshold slope of 0.5V/decade, and current ratio (I-ON/OFF) of >10(7). The threshold voltage shifts of the TFTs were 0.5V in positive (+1.0MV/cm) bias direction and 1.5V in negative (-1.0MV/cm) bias direction after extended stressing time of 10(4)s. We achieve a stage-delay of similar to 19.6ns at V-DD=20V measured in a 41-stage ring oscillator. A top-emitting quarter-quarter-video-graphics-array active-matrix organic light-emitting diode display with 85ppi (pixels per inch) resolution has been realized using only five lithographic mask steps. For operation at 6V supply voltage (V-DD), the brightness of the display exceeds 150cd/m(2).

AbstractAMOLED backplanes were fabricated on both rigid glass and flexible plastic substrates using a solution processed oxide semiconductor processed at temperatures <250C with mobilities greater than 2 cm2/Vs. The backplanes were integrated onto a thin film moisture barrier and QQVGA AMOLED displays were successfully fabricated.

In this paper, we present some of the technology challenges and process temperature trade-offs when realizing AM OLED displays on thin flexible plastic films that can be mechanically bent to a roll radius of ~1 cm. We furthermore present complementary approaches to realize low-power, high resolution OLED displays using self-aligned IGZO TFT architecture; a novel driving method using a compact 2T-1C pixel engine.

AbstractAMOLED displays using oxide TFTs and high‐quality moisture barrier were fabricated on ultrathin, flexible plastic substrates to give maximum mechanical flexibility. Total display thickness is below 150μm, and repeated rollability at 1 cm roll radius has been demonstrated. Electrical/Mechanical characteristics and reliability of the flexible displays will be presented.

The availability of a high performance thin‐film barrier is the most critical challenge in upscaling and commercializing flexible OLED products. We report a flexible thin‐film‐barrier technology that meets lifetime specifications for OLED lighting, and demonstrate it in rollable QVGA a‐IGZO AMOLED displays. Roll‐to‐roll manufacturing of this high performance barrier film will be presented.

Paper presented during the International Display Workshops 2013, Sapporo, Japan, 4-6 december.