Contents I. Introduction
Organic light-emitting diode (OLED) display technology has been widely studied recently. The OLED display technology has several properties such as it is light weight, has fast response time, a wide viewing angle, high efficiency and flexibility [1]–[3]. Although a passive matrix OLED (PMOLED) is easier to realize than active-matrix OLEDs (AMOLEDs), PMOLEDs are only utilized for low-level products, simply because of its generation of high power consumption when applied to large areas or high-resolution displays, and high instantaneous luminance degrades OLED devices. Therefore, large high-resolution displays typically use AMOLED structures [1], [4]. An AMOLED pixel circuit can be fabricated using amorphous silicon (a-Si) or low temperature poly-silicon (LTPS). The a-Si technology generates excellent uniformity over large areas and it is frequently utilized in active matrix liquid crystal displays. Although this technology is popular for industry for its economical manufacturing costs, however, a-Si technology provides low mobility and cannot be used in p-type devices [5]. Moreover, the stability of a-Si thin-film transistors (TFTs) is poor, with threshold voltage shifts due to electrical stress over time [6]. Conversely, the LTPS provides complementary TFTs (n-type and p-type TFTs) and high current capability due to the higher mobility than that of a-Si. However, the mismatch in different TFT parameters, including mobility and , are inevitable problems due to the uncontrollable gate oxide trap density and random distribution of grain boundaries in the material [7]. The effect of temporal shift in a-Si TFTs on the display uniformity is the same as the spatial -mismatch in poly-Si TFTs. Consequently, several compensation methods, such as voltage driving [8]–[17], current-driving [18]–[20], digital-driving [21], [22], and AC-driving methods have been developed [23]. However, these compensation schemes only resolve the problem associated with threshold voltage shift, and the degradation of emission efficiency due to OLED device decay remains a primary concern, resulting in image burn-in and short product lifetime. Hence, several methods, such as optical feedback compensation [24], [25], LUT-based compensation [26], and circuit compensation [12], [16], [17], have been utilized to minimize OLED degradation. The optical feedback scheme can detect OLED degradation, thereby facilitating correction of both TFT drift and OLED degradation simultaneously. However, strong wavelength dependence on photon efficiency and sensitivity to ambient light are disadvantages of optical feedback. The LUT-based technique effectively reduces differential aging using the embedded compensation model; however, establishing exactly the OLED degradation profile is complex. Lee et al. [12] recently proposed the 6T1C circuit compensation pixel circuit that successfully minimizes any decrease in OLED current caused by threshold voltage degradation of a-Si TFTs and OLEDs. Although the 6T1C circuit, which is independent of threshold voltage of TFTs and OLEDs, provides highly stable OLED current, luminance drops are still caused by OLED degradation [12], [27]. This study presents a novel pixel circuit with a feedback structure that detects the OLED aging and generates additional current to reduce the effect of the decline in luminance. This circuit can be implemented in LTPS and a-Si TFT technologies as well as all-p-type and all-n-type TFTs. Compared to the 6T1C pixel circuit proposed by Lee et al. [12], the proposed pixel circuit decreased one TFT and is beneficial to aperture ratio.
