Gae Hwang Lee

Organic-on-silicon complementary metal-oxide-semiconductor colour image sensors

Complementary metal–oxide–semiconductor (CMOS) colour image sensors are representative examples of light-detection devices. To achieve extremely high resolutions, the pixel sizes of the CMOS image sensors must be reduced to less than a micron, which in turn significantly limits the number of photons that can be captured by each pixel using silicon (Si)-based technology (i.e., this reduction in pixel size results in a loss of sensitivity). Here, we demonstrate a novel and efficient method of increasing the sensitivity and resolution of the CMOS image sensors by superposing an organic photodiode (OPD) onto a CMOS circuit with Si photodiodes, which consequently doubles the light-input surface area of each pixel. To realise this concept, we developed organic semiconductor materials with absorption properties selective to green light and successfully fabricated highly efficient green-light-sensitive OPDs without colour filters. We found that such a top light-receiving OPD, which is selective to specific green wavelengths, demonstrates great potential when combined with a newly designed Si-based CMOS circuit containing only blue and red colour filters. To demonstrate the effectiveness of this state-of-the-art hybrid colour image sensor, we acquired a real full-colour image using a camera that contained the organic-on-Si hybrid CMOS colour image sensor.

Dipolar donor–acceptor molecules in the cyanine limit for high efficiency green-light-selective organic photodiodes

We report on two novel p-type small molecules with a donor–acceptor molecular structure for application to green-light-selective organic photodiodes (OPDs). To achieve the requirement of high light selectivity and sensitivity, an electron-donating aryl amino moiety is combined with two respective electron-accepting heterocycles so that the molecules approach cyanine-like character, characterized by intense and sharp absorption. Molecular stacking is controlled by the addition of bulky aryl functional groups to the main backbone to further control the electrical charge transport properties. With this molecular design, a maximum external quantum efficiency close to 61% (λmax = 550 nm) and a dark-current density below 1.6 nA cm−2 (or specific detectivity D* = 1.19 × 1013 cm Hz1/2 W−1) at an applied reverse bias of 3 V are obtained when mixed with fullerene (C60) in an inverted-structure bulk heterojunction OPD composed of two transparent electrodes. The potential construction of a full-color photodetector or an image sensor is demonstrated by combining the green-light-selective OPD with a silicon photodiode containing solely blue and red color filters in a stacked architecture.

Applications of multidirectional reflective light-control films on reflective polymer-dispersed liquid crystal displays for enhancement in image quality at lower viewing angles

In this paper, multidirectional light-control reflective (LCR) films are developed in order to create an active reflective structure that will enhance the image brightness and contrast ratio of reflective dye-doped polymer-dispersed liquid crystal (D-PDLC) displays at lower viewing angles. Advantages of LCR films are that their production is low cost and they require a simple photolithographic fabrication method. The optimum design prism-type light-control reflective film succeeded in minimising the surface scattering effect; thus, the contrast ratio is much enhanced. The symmetric and asymmetric LCR films produced multidirectional scattering that enhances the reflectance at lower viewing angles, which has importance in future display applications. In particular, the prism LCR film has been found to be more influential on the reflectance of D-PDLC films due to multidirectional scattering of light by non-symmetric arrays. The improvement in contrast ratio has been confirmed by the enhancement of optical properties for reflective D-PDLC displays at lower viewing angles below 30°.

40.4: The Research of Various Mode Color PDLC Structures for Flexible Reflective Display

Various type color PDLC structures have been characterized on a glass and a plastic substrate for reflective mode color display. The standard PDLC and the black diachronic dye mixed PDLC structure with color filter (RGB) show a good black state with high color purity and 256 gray levels, even though both structures have a dark color images. Color dichroic dye (RGB/CMY) mixed PDLC cells promise a good white reflectance. The problem of using color dye system is the enhancement of a poor black state. Using new CMY color based filter with black dye PDLC shutter structure can improve a white state with a proper color vision.

Bright color optical switching device by polymer network liquid crystal with a specular reflector.

The color optical switching device by polymer network liquid crystal (PNLC) with color filter on a specular reflector shows excellent performance; white reflectance of 22%, color gamut of 32%, and contrast ratio up to 50:1 in reflective mode measurement. The view-angle dependence of the reflectance can be adjusted by changing the PNLC thickness. The color chromaticity shown by the device is close to the limit value of color filters, and its value nearly remains with respect to the operating voltage. These optical properties of the device can be explained from the prediction based on multiple interactions between the light and the droplets of liquid crystal. The high reflectance, vivid color image, and moderate responds time allow the PNLC device to drive good color moving image. It can widely extend the applications of the reflective device.

Enhancement of diffuse reflectance using air tunnel structure.

Submicrometer air gap structure has formed on diffuse reflection structure to improve light reflectance. Covering polymer or liquid on a diffuse reflector to make optical components induces the severe decrease of the total reflectance, since the diffuse reflected angle of some light rays is larger than the critical angle and the rays travel to the medium until meeting a proper small incident angle. The reflectance drops to 68% of the original value with just a polymer coating on the diffuse reflector. The formation of an air tunnel structure between the polymer layer and the diffuse reflector makes a symmetrical reflective index matching state and recovers 95% of the original reflectance. Due to the simple fabrication process and the chemical stability, the structure can be applied to various optical components and reflective display devices.

Recent developments in green light sensitive organic photodetectors for hybrid CMOS image sensor applications(Conference Presentation)

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.

Color Polymer-Stabilized Liquid Crystal for the Optical Shutter with Fast Response Time

A color-reflective optical shutter structure has been studied using polymer-stabilized liquid crystal (PSLC) devices with a specular reflector. The combined effect of an LC, a polymer, a color dye, and a specular reflector results in a high performance of a reflective-mode optical shutter as well as of a simple structure with a fast response time capable of smooth moving images. The reflectance is up to 40%, one of highest for reflective devices, whose response times are fast enough to provide video speed response (<16.6 ms). This effective and simple shutter concept can be applied in reflective-mode flexible displays and optical equipment with high performance.