3 research outputs found
Compensating for the Threshold Voltages of Both the Driving Thin-Film Transistor and the Organic Light-Emitting Diode for Active-Matrix Organic Light-Emitting Diode Displays
This paper proposes a novel pixel circuit design and driving method for active-matrix organic light-emitting diode (AM-OLED) displays that use low-temperature polycrystalline-silicon thin-film transistors (LTPS-TFTs) as driving element. The automatic integrated circuit modeling simulation program with integrated circuit emphasis (AIM-SPICE) simulator was used to verify that the proposed pixel circuit, which comprises five transistors and one capacitor, can supply uniform output current. The voltage programming method of the proposed pixel circuit comprises three periods: reset, compensation with data input, and emission periods. The simulated results reflected excellent performance. For instance, when Δ TH = ±0.33 V, the average error rate of the OLED current variation was low (< 0.8%), and when Δ TH OLED = +0.33 V, the error rate of the OLED current variation was 4.7%. Moreover, when the × (current × resistance) drop voltage of a power line was 0.3 V, the error rate of the OLED current variation was 5.8%. The simulated results indicated that the proposed pixel circuit exhibits high immunity to the threshold voltage deviation of both the driving poly-Si TFTs and OLEDs, and simultaneously compensates for the × drop voltage of a power line
Pixel design and characterization of high-performance tandem OLED microdisplays
Organic Light-Emitting Diode (OLED) microdisplays - miniature Electronic Displays comprising a
sandwich of organic light emitting diode over a substrate containing CMOS circuits designed to function
as an active matrix backplane – were first reported in the 1990s and, since then, have advanced to the
mainstream. The smaller dimensions and higher performance of CMOS circuit elements compared to
that of equivalent thin film transistors implemented in technologies for large OLED display panels offer
a distinct advantage for ultra-miniature display screens. Conventional OLED has suffered from lifetime
degradation at high brightness and high current density. Recently, tandem-structure OLED devices have
been developed using charge generation layers to implement two or more OLED units in a single stack.
They can achieve higher brightness at a given current density. The combination of emissive-nature, fast
response, medium to high luminance, low power consumption and appropriate lifetime makes OLED a
favoured candidate for near-to-eye systems. However, it is also challenging to evaluate the pixel level
optical response of OLED microdisplays as the pixel pitch is extremely small and relative low light
output per pixel. Advanced CMOS Single Photon Avalanche Diode (SPAD) technology is progressing
rapidly and is being deployed in a wide range of applications. It is also suggested as a replacement for
photomultiplier tube (PMT) for photonic experiments that require high sensitivity. CMOS SPAD is a
potential tool for better and cheaper display optical characterizations.
In order to incorporate the novel tandem structure OLED within the computer aided design (CAD) flow
of microdisplays, we have developed an equivalent circuit model that accurately describes the tandem
OLED electrical characteristics. Specifically, new analogue pulse width modulation (PWM) pixel
circuit designs have been implemented and fabricated in small arrays for test and characterization
purposes. We report on the design and characterization of these novel pixel drive circuits for OLED
microdisplays. Our drive circuits are designed to allow a state-of-the-art sub-pixel pitch of around 5 μm
and implemented in 130 nm CMOS. A performance comparison with a previous published analogue
PWM pixel is reported. Moreover, we have employed CMOS SPAD sensors to perform detailed optical
measurements on the OLED microdisplay pixels at very high sampling rate (50 kHz, 10 μs exposure),
very low light level (2×10-4 cd/m2) and over a very wide dynamic range (83 dB) of luminance. This
offers a clear demonstration of the potential of the CMOS SPAD technology to reveal hitherto obscure
details of the optical characteristics of individual and groups of OLED pixels and thereby in display
metrology in general.
In summary, there are three key contributions to knowledge reported in this thesis. The first is a new
equivalent circuit model specifically for tandem structure OLED. The model is verified to provide
accurately illustrate the electrical response of the tandem OLED with different materials. The second is
the novel analogue PWM pixel achieve a 5μm sub-pixel pitch with 2.4 % pixel-to-pixel variation. The
third is the new application and successful characterization experiment of OLED microdisplay pixels
with SPAD sensors. It revealed the OLED pixel overshoot behaviour with a QIS SPAD sensor