Organic photodetectors (OPDs) are attracting interest as various sensing platforms such as photo/chemical sensors, healthcare sensors, x-ray scanner, and image sensors. In particular, a distinct advantage of organic materials, i.e., orthogonal photosensitivity to the specific wavelength such as blue (B), green (G), red (R), and even infrared has recently facilitated promising applications to organic full colour image sensors. For instance, vertical stacks of G-wavelength selective organic photoconversion layers on conventional Si CMOS imagers with B/R color filters have been made to realize highly sensitive image sensors by doubling the light detecting area compared to the planar R/G/B pixel structure.
Our recent investigations on small molecule OPDs with bulk heterojunction structure have shown high peak external quantum efficiencies over 60% and extremely low dark current densities below 0.1 nA/cm2 at reverse bias of 3V, which are comparable to the typical performance of Si-based PDs. On the other hand, their photoresponse characteristics have not been systematically studied. For example, the Si PD exhibited the rising time of photoresponse speed as fast as 10 us at 99.9% of the peak photocurrent, whereas the OPD showed 20 times slower response time plausibly due to the reduced charge carrier mobility. Thus, in order to investigate the practical use of OPDs as image sensor applications, we will present the current status of dynamic characteristics of OPDs in terms of photoresponse speed, frequency response, and transient photocurrent. Further, the possible origin of photoresponse characteristics of OPDs will be described.
Typical CMOS colour image sensors consist of Si-based photodetectors (PDs) attached with colour filter arrays (i.e., the Bayer pattern). Recent trends on the development of high resolution image sensors, however, require downsizing the pixel dimension, which inevitably results in the loss of sensitivity due to the reduction in the photon acquisition. Very recently, hybrid stacks of organic photodetectors (OPDs) on conventional CMOS technologies have been proposed as one of the promising approaches to realise highly sensitive image sensors by doubling the light detecting area in the limited pixel size. Specifically, OPDs with orthogonal photosensitivity to green light and Si-based PDs with red and blue colour filters serve as the top and bottom photo-conversion layers, respectively. In this presentation, we will introduce the recent development of high performance green light sensitive OPDs and the demonstration of colour images from hybrid CMOS image sensors proposed.
OPDs consisting of small molecule organic bulk heterto-junction structures, hole/electron buffer layers, and transparent top/bottom ITO electrodes exhibited peak external quantum efficiencies of 60-65% at 550-560 nm wavelengths and full width at half maximum of ~120 nm at reverse bias of 3 V. Extremely low dark current densities in the range of 0.2-0.5 nA/cm2 at reverse bias of 3V and consequently high specific detectivities over 2×10^13 Jones were obtained from the developed OPD system. Further investigations in terms of the molecular structures of organic light absorbing materials, buffer materials, layer sequences, and even integration issues of the OPD on the CMOS will be described in detail.
An arrayed beam steering device enables much simplified system architectures for high quality multiview 3D displays by adapting time multiplexing and eye tracking scheme. An array device consisting of microscale liquid prisms is presented, where the prism surface between two immiscible liquids is electrically controlled to steer light beams by the principle of electrowetting. An array prototype with 280×280μm pixels was fabricated and demonstrated of its full optical performances. The maximum tilting angle of each prism was measured to be 22.5° in average, with a tracking resolution of less than 0.04°. In this paper, we report a design and fabrication of eletrowetting based prism array, opto-fluidic simulations, optical characterizations, as well as applications to achieve low fatigue 3D displays.
Light-emitting displays with colloidal quantum-dot (QD) have recently received considerable attention due to advantages
of QD property such as high quantum yields, extremely narrow emission, spectral tunability, and a higher stability than
other existing luminophores. However, the difficulty of patterning red, green, and blue (RGB) pixels of three individual
QDs with controlled interfaces has prevented from developing a full-color QD display with acceptable quantum
efficiency. In this talk, the issues of QD EL and successful embodiment of full-color QD display by the solvent-free
transfer printing of QD pattern will be presented. Modulated QD assemblies exhibit the excellent morphology, wellordered
QD structure, and clearly defined interfaces, which result in significant enhancements in the charge
transport/balance in the QD layer. A large-area full-color QD displays on a glass substrate, and even on a flexible
substrate can be realized in this manner with the control of nano-interfaces and carrier behaviors.
We developed an active matrix organic light-emitting diodes (AMOLEDs) on a glass using two organic thin-film transistors (OTFTs) and a capacitor in a pixel. OTFTs switching-arrays with 64 scan lines and 64 (RGB) data lines were designed and fabricated to drive OLED arrays. In this study, OTFT devices have bottom contact structures with an ink-jet printed polymer semiconductor and an organic insulator as a gate dielectric. The width and length of the switching OTFT is 500μm and 10μm, respectively and the driving OTFT has 900μm channel width with the same channel length. The characteristics of the OTFTs were examined using test cells around display area. On/off ratio, mobility, on-current of switching OTFT and on-current of driving OTFT were 106, 0.1 cm2/V-sec, order of 8μA and over 70 μA respectively. These properties were enough to drive the AMOLEDs over 60 Hz frame rate. AMOLEDs composed of the OTFT switching arrays and OLEDs made by deposition of small molecule materials were fabricated and driven to make moving images, successfully.
In order to realize the high-performance solution-processed transistors (OTFTs) on plastic substrate, it is essential to have a solution-processible organic gate insulator which can give high field-effect mobility and on/off ratio in the devices and should endure sever photolithography process. Our crosslinked gate insulator film has a good chemical resistance to electrode etchants. However, the etchant exposure of the gate insulators resulted in an increase of the off-current while keeping the on-current the same. We also demonstrate solution processed n-type OTFTs with high mobility based on the soluble derivatives of fullerene (C60) as n-type channel materials. We obtained high electron mobilities of 0.02-0.1 cm2/V.s depending on the workfunction of the source and drain metals, demonstrating that the electron injection is contact-limited. Furthermore, we fabricated n-type OTFTs by all solution deposition process including source and drain metals as well as gate insulators and organic semiconductors. These types of OTFTs can be well suited for a wide range of existing and future flexible circuits and display applications which require a simplified process and low-weight and low-cost products.
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