Yi-Jiun Chen

Enhancing Optical Out-Coupling of Organic Light-Emitting Devices with Nanostructured Composite Electrodes Consisting of Indium Tin Oxide Nanomesh and Conducting Polymer.

A nanostructured composite electrode consisting of a high-index indium-tin-oxide nanomesh and low-index high-conductivity conducting polymer effectively enhances coupling of internal radiation of organic light-emitting devices into their substrates. When combining this internal extraction structure and the external extraction scheme, a very high external quantum efficiency of nearly 62% is achieved with a green phosphorescent device.

CHARACTERIZATION OF PIEZOELECTRIC (111)B INGAAS/GAAS P-I-N QUANTUM WELL STRUCTURES USING PHOTOREFLECTANCE SPECTROSCOPY

Strained-layer (111)B In0.2Ga0.8As/GaAs p-i-n quantum well structures grown with exciton transitions well resolved at room temperature have been studied by photoreflectance spectroscopy. Using the reduced mass deduced from experiments, the built-in electric field in the barrier region is obtained from the above band-gap Franz–Keldysh oscillations. The strain-induced piezoelectric field is then determined directly from a comparison of the periods of Franz–Keldysh oscillations in different samples. Numerical solutions for the exciton transitions from the derived potential profiles are in good agreement with the experimental results. The piezoelectric constant is also determined using the piezoelectric field.

Photoreflectance characterization of an InAlAs/InGaAs heterostructure bipolar transistor

We have measured the photoreflectance spectrum at 300 K from a lattice‐matched InAlAs/InGaAs heterostructure bipolar transistor grown by molecular beam epitaxy. The energy features of photoreflectance spectra have been identified and the built‐in dc electric fields and associated doping profiles have been evaluated in the n‐InAlAs emitter from the observed Franz–Keldysh oscillations. The undoped InGaAs spacer between emitter and base was added on to change the built‐in electric field. The results showed that the energy features above the InGaAs band gap are the transitions from the valence band to the quantized state of the conduction band. The quantum well of the conduction band is in the interface of the InAlAs and InGaAs heterojunction. The interface charge densities in the spacer channel are determined to be 3.54×1011 cm−2 and 4.22×1011 cm−2, corresponding to the samples with spacer thicknesses of 300 and 500 A, respectively. A triangular potential profile model was used to calculate the microstructur...

White Organic Light-Emitting Diode With Linearly Polarized Emission

An organic light emitting diode (OLED) with a linearly polarized white light emission is demonstrated using a nanograting structure. The aluminum based grating structure is fabricated by laser interference lithography and formed on the back side of the glass substrate of the OLED. The nanograting structure functions as a polarizer to select the transverse magnetic wave in the wavelength range of 400-700 nm. The polarization characteristics are studied experimentally and theoretically in detail. The experimental results agree well with the simulation by a rigorous coupled wave analysis. The polarization ratio of the polarizer can reach as high as 93.4%.

Plasmonic ITO-free polymer solar cell

The aluminum and sliver multilayered nano-grating structure is fabricated by laser interference lithography and the intervals between nanoslits is filled with modified PEDOT:PSS. The grating structured transparent electrode functions as the anti-reflection layer which not only decreases the reflected light but also increases the absorption of the active layer. The performances of P3HT:PC₆₁BM solar cells are studied experimentally and theoretically in detail. The field intensities of the transverse magnetic (TM) and transverse electrical (TE) waves distributed in the active layer are simulated by rigorous coupled wave analysis (RCWA). The power conversion efficiency of the plasmonic ITO-free polymer solar cell can reach 3.64% which is higher than ITO based polymer solar cell with efficiency of 3.45%.

Photoreflectance spectroscopy of strained-layer (111)B InGaAs/GaAs quantum well diodes

InGaAs/GaAs (111)B quantum well p-i-n structures grown by gas source molecular beam epitaxy have been investigated with a photoreflectance technique. Using the reduced mass deduced from experiments, the built-in electric field is obtained from the above band-gap Franz–Keldysh oscillations (FKOs). The strain-induced piezoelectric field is then determined directly from the comparison of the periods of FKOs in different samples. Numerical solutions for exciton transition energies with the experimentally derived potentials are in good agreement with experimental results. Hence, the piezoelectric constant can be determined using the piezoelectric field. The temperature dependences of the quantized energy levels indicate that the influence of temperature on exciton transitions is essentially the same as that of the gaps of the relevant bulk constituent materials.

Analyses of optical out-coupling of organic light-emitting devices having micromesh indium tin oxide and conducting polymer as composite transparent electrode.

UNLABELLED We report the characterization and analyses of organic light-emitting devices (OLEDs) using microstructured composite transparent electrodes consisting of the high-index ITO (indium tin oxide) micromesh and the low-index conducting polymer PEDOT PSS [poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)], that are fabricated by the facile and convenient microsphere lithography and are useful for enhancing light extraction. The rigorous electromagnetic simulation based on the three-dimensional finite-difference time-domain (FDTD) method was conducted to study optical properties and mechanisms in such devices. It provides a different but consistent viewpoint/insight of how this microstructured electrode enhances optical out-coupling of OLEDs, compared to that provided by ray optics simulation in previous works. Both experimental and simulation studies indicate such a microstructured electrode effectively enhances coupling of internal radiation into the substrate, compared to devices with the typical planar ITO electrode. By combining this internal extraction structure and the external extraction scheme (e.g. by attaching extraction lens) to further extract radiation into the substrate, a rather high external quantum efficiency of 46.8% was achieved with green phosphorescent OLEDs, clearly manifesting its high potential.

Design and fabrication of birefringent nano-grating structure for circularly polarized light emission

Three different nano-grating structures are designed as phase retarders that can transform linearly polarized light to circularly polarized emission for the wavelengths of 488 nm, 532 nm and 632.8 nm, respectively. Gold based nano-grating structures with various periods are fabricated by utilizing laser interference lithography. The ellipticity of all circularly polarized emission can reach around 90% such that the structure has great potential in the applications of three-dimensional (3D) display. The effects of the slit width and metal thickness modulations are simulated by rigorous coupled wave analysis (RCWA) method. Besides, the field intensity and phase of the transmitted TM and TE waves are also simulated to understand their polarization characteristics.

Improvement of power efficiency and reduction of blur effect in OLED with micro-lens array films by reducing substrate thickness

This study demonstrates that attaching micro-lens array films (MAFs) on the substrate and reducing the substrate thickness of OLED can significantly increase the power efficiency, while simultaneously reduce image blurring. Using a point source model, based on Monte-Carlo ray-tracing method, the power efficiency enhancement and reduction of blur effect are respectively discussed in three different regions of the MAFs attached substrate: partially reflecting region, transmitting region, and light guiding region of micro-lenses. According to the equations, derived with regard to the substrate thickness and the displacement from the point source and based on geometric relations corresponding to different regions, reducing the substrate thickness will result in different levels of enhancement for power efficiency in different regions. By comparing OLED with MAFs and bare OLED, the overall enhancement ratio of power efficiency is 1.46, which can be further improved to 1.78 by reducing the substrate thickness from 700 μm to 50 μm, and the blurlength is reduced from 942 μm to 255 μm. The simulation results demonstrate the possibility of applying MAFs to OLED for higher power efficiency without image degradation in display and lighting applications.

A Vision toward Ultimate Optical Out‐Coupling for Organic Light‐Emitting Diode Displays: 3D Pixel Configuration

Abstract Despite stringent power consumption requirements in many applications, over years organic light‐emitting diode (OLED) displays still suffer unsatisfactory energy efficiency due to poor light extraction. Approaches have been reported for OLED light out‐coupling, but they in general are not applicable for OLED displays due to difficulties in display image quality and fabrication complexity and compatibility. Thus to date, an effective and feasible light extraction technique that can boost efficiencies and yet keep image quality is still lacking and remains a great challenge. Here, a highly effective and scalable extraction‐enhancing OLED display pixel structure is proposed based on embedding the OLED inside a three‐dimensional reflective concave structure covered with a patterned high‐index filler. It can couple as much internal emission as possible into the filler region and then redirect otherwise confined light for out‐coupling. Comprehensive multi‐scale optical simulation validates that ultimately high light extraction efficiency approaching ≈80% and excellent viewing characteristics are simultaneously achievable with optimized structures using highly transparent top electrodes. This scheme is scalable and wavelength insensitive, and generally applicable to all red, green, and blue pixels in high‐resolution full‐color displays. Results of this work are believed to shed light on the development of future generations of advanced OLED displays.