GIPO newsletter

Photo-stability study of colloidal CdSe/ZnS quantum dots passivated in Al₂O₃ using atomic layer deposition

Professor Ming-Hua Mao’s laboratory

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

We report photo-stability enhancement of colloidal CdSe/ZnS quantum dots (QDs) passivated in Al₂O₃ thin film using the atomic layer deposition (ALD) technique. 62% of the original peak photoluminescence (PL) intensity remained after ALD. The photo-oxidation and photo-induced fluorescence enhancement (PFE) effects of both unpassivated and passivated QDs were studied under various conditions. Unpassivated QDs showed rapid PL degradation under high excitation due to strong photo-oxidation in air while the PL intensity of Al₂O₃ passivated QDs was found to remain stable. Our experimental results demonstrated that passivation of colloidal QDs by ALD is a promising method to well encapsulate QDs to prevent gas permeation and to enhance photo-stability. This is essential for the applications of colloidal QDs in light-emitting devices. This work has been published in J. Appl. Phys., vol. 120, pp. 083103, 2016.

Fig. 1. PL spectra of unpassivated QDs (dotted line) and Al₂O₃ passivated QDs (solid line).

Fig. 2. Temporal evolution of PL intensities in air and in vacuum under CW excitation for unpassivated and passivated QDs at different power densities.

Development of Extremely High External Quantum Efficiency Blue Organic Light emitting Devices

Professor Chung-Chih Wu

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

We have developed highly efficient blue organic light emitting diodes (OLEDs) based on highly efficient thermally activated delayed fluorescent (TADF) emitters. Photophysical studies indicate that TADF emitters can simultaneously possess nearly unitary (100%) photoluminescence quantum yields (PLQYs), and strongly anisotropically/horizontally oriented emitting dipoles (with horizontal dipole ratio of 83%) that can help boost optical out-coupling efficiencies of OLEDs. Baed on such advanced TADF emitters, extremely efficient blue OLEDs have been developed with ideal internal quantum efficiencies (IQE) of nealy 100% and very high external quantum efficiencies (EQE) of nearly 37% in conventional planar OLED structures (without using any internal/external optical out-coupling schemes), and even higher EQE of nearly 63% by adopting simple external optical out-coupling schemes. Such results are by far the highest EQE ever reported for blue TADF OLEDs and blue OLEDs of any kind, indicating their high potential for OLED applications.

Fig. 1. Achieving very high external quantum efficiencies of blue OLEDs.

High-Mobility Graphene Field Effect Transistors on Bare SiO₂ Substrates

Professor Chih-I Wu

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

Here we use the chemical vapor deposition (CVD)-grown graphene on Cu foil and then transfer it via the polymer-free method to the SiO₂ substrate like Fig.1.

The Dirac point of the large size GFET has a significant shift at about 82V (see figure 2a). Carrier mobility was extracted as shown in figure 2 and the electron and hole mobility are 1400 cm^-2V^-1s^-1 and 250 cm^-2V^-1s^-1, respectively. The hole mobility is close to the CVD graphene transferring on SiO₂ substrates at 1.6K. This result implies that the large size GFET has too many defect and carbon cluster (see figure 1) at the channel, which will affect the electronic properties significantly.

Dirac point is around 28V and the shape of this curve is more symmetry than that shown in figure 2a. The carrier mobility of electron and hole is 5578cm^-2V^-1s^-1 and 7734cm^-2V^-1s^-1, respectively, and the mobility is much higher than the large size GFET. Fig.2b shows us that the small size GFET exhibits the Dirac point closer to the 0 point, because the defects or domain boundary will adsorb some particle on it such as H₂O molecular to cause the p-type doping. The p-type doping will also affect the carrier mobility and affect the electron mobility especially.

In this study, we have demonstrated a new transfer technique for transferring graphene to prevent the PMMA residuals on the graphene surface. The Dirac point of small size GFET is at about 28V which shows the graphene intrinsic electronic properties. And the carrier mobility of small GFET are 5578cm^-2V^-1s^-1 and 7734cm^-2V^-1s^-1, respectively and this value is much higher than the data reported in the previous researches on bare SiO₂ Substrates.

FIGURE 1. The OM image of CVD graphene transferred on a SiO₂substrate by polymer-free method.

FIGURE 2a & 2b. The V_g versus σ of large and small size graphene FET, and the Dirac point is shift to 79V and 28V respectively.

Variable focus microlens array with curved electrodes

Professor Guo-Dung J. Su's laboratory

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

A liquid crystal microlens array with a curved electrode is proposed and fabricated. The fabrication process consists of two parts: microlens array fabrication and liquid crystal cell assembly. Utilizing the hydrophilic confinement effect, an inkjet printer, and the replication process, a microlens array on a glass substrate is fabricated. PEDOT:PSS is spin-coated on the microlens array to form a curved electrode, and the microlens array is flattened by SU-8 photoresist and assembled with ITO glass. From interference patterns, the focusing power range is from −47.28 to −331 diopters under 10 V and it agrees well with theoretical calculation. This technology could be useful for optical zoom systems or focus-tunable lens applications.

Fig. 1. The image performance of the liquid crystal microlens array at (a) V = 0 and (b) V = 10

Fig. 2. Optical power at different voltages

Blue organic light-emitting diode by exciplex sensitized triplet-triplet annihilation

Professor Jiun-Haw Lee

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

Energy up-conversion was achieved by exciplex sensitized triplet-triplet annihilation (ESTTA) process in an organic light-emitting diode (OLED) with blue emission. As shown in Fig. 1 (a), carrier recombines to form the exciplex, which serves as the low-bandgap sensitizer. Then, triplet exciton transfers the energy to the wide-bandgap emitter with triplet-triplet annihilation characteristic for blue-light generation. Fig. 1 (b) shows the emission spectra from this blue ESTTA-OLED under different voltages. Emission with peak at 430 nm (corresponding to 2.9 eV) was observed under 2.2 V driving. The driving voltage was lower than the photon energy which demonstrated the up-conversion process in this ESTTA-OLED.

Fig. 1. (a) Schematic diagram of ESTTA process. (b) Emission spectra from the OLED under different driving voltages.