12-05-2011, 03:01 PM
Abstract
This paper presents a novel parallel addressingscheme for voltage-programmed active-matrix organic lightemittingdiode (OLED) displays which provides high precisionrecovery of the threshold voltage shift. As a result, the uniformityover the panel is significantly improved. In addition, a new pixelcircuit is presented that is capable of providing a predictablyhigher current as the pixel ages, so as to compensate for the OLEDluminance degradation.Index Terms—Active-matrix organic light-emitting diode(AMOLED), hydrogenated amorphous silicon (a-Si:H), thin-filmtransistor (TFT), threshold voltage shift (VT shift), voltageprogrammedpixel circuit (VPPC).
I. INTRODUCTION
ACTIVE-MATRIX organic light-emitting diode(AMOLED) displays have been valued as a promisingtechnology for low-cost displays on glass and plastic substratessince they benefit from the low-cost and low-temperaturefabrication of OLEDs and hydrogenated amorphous silicon(a-Si:H) thin-film transistors (TFTs) [1]. The simplestAMOLED pixel circuit is the 2-TFT pixel shown in Fig. 1(a),where the OLED current is controlled by a drive TFT, i.e.,T1 [2], [3]. The VT of the drive TFT in the pixel circuitincreases over time (ΔT ), which results in pixel luminancedegradation. Results of lifetime measurements of a 2-TFTpixel circuit, with parameters listed in Table I, is depictedin Fig. 1(b). The initial pixel current of 2 μA drops to 1 μAafter 15 days of operation. Thus, there is a critical need forcompensation techniques to stabilize the pixel luminance.Current-programmed pixel circuits have demonstrated highimmunity against variation in pixel parameters, including VTshift. However, they experience a long settling time that limitstheir application to small-/medium-sized displays [2]. On theother hand, voltage-programmed pixel circuits (VPPCs) arefast, but they are prone to imperfect compensation due to thelimited programming time available, coupled with the lowmobility of a-Si:H TFTs [2]. Therefore, a new addressingscheme is required to extend the programming time withoutaffecting the frame rate. In the next sections, we review the effect of the limited timeassigned to the programming cycles in a typical VPPC andintroduce a new parallel addressing scheme along with a newpixel circuit based on this scheme.
II. IMPERFECT COMPENSATION
In VPPCs, the major operating cycles pertain to the compensationvoltage (VCOMP) generation, VT generation, currentregulation, and driving [3]–[5]. Fig. 2 shows a typical VPPCduring the different operating cycles. The VT of the drive TFT(T1) is generated during the VT generation cycle, and theprogramming voltage (VP ) is added to it during the currentregulation cycle. Thus, the OLED current becomes independentof the VT of T1
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