6 research outputs found
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
EUROSENSORS XVII : book of abstracts
Fundação Calouste Gulbenkien (FCG).Fundação para a Ciência e a Tecnologia (FCT)
Technology 2003: The Fourth National Technology Transfer Conference and Exposition, volume 2
Proceedings from symposia of the Technology 2003 Conference and Exposition, Dec. 7-9, 1993, Anaheim, CA, are presented. Volume 2 features papers on artificial intelligence, CAD&E, computer hardware, computer software, information management, photonics, robotics, test and measurement, video and imaging, and virtual reality/simulation