Browsing by Author "Liao, Yu-Kuang"

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A look into the origin of shunt leakage current of Cu(In,Ga)Se-2 solar cells via experimental and simulation methods
(2013-10-01)
Liao, Yu-Kuang
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Kuo, Shou-Yi
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Hsieh, Ming-Yang
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Lai, Fang-I
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Kao, Ming-Hsuan
This study investigates how to apply space-charge-limited (SCL) current to describe shunt leakage current in a CIGS solar cell. Possible factors inducing SCL current have been observed through conductive atomic force microscopy (C-AFM), which supports the SCL current theory, describing the shunt current of a CIGS solar cell. In simulations derived from experimental data, deviation of dark IV curves is due to flaws in the real device. These flaws are absent in simulation, but investigation verifies the characteristics of SCL current component's experimental IV curves within shunt leakage current. A device with a metal/CIGS/metal structure could simulate SCL current and confirm its characteristics. Such a simulated structure, representing flaws inserted into a CIGS solar cell, generates the same dark-current behavior revealed in experimental dark IV curves. This study investigates the response of dark current to varying sizes of the flaw within the CIGS solar cell. (C) 2013 Elsevier B.V. All rights reserved.
An Investigation of Carbon-Doping-Induced Current Collapse in GaN-on-Si High Electron Mobility Transistors
(2016-06-01)
Tzou, An-Jye
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Hsieh, Dan-Hua
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Chen, Szu-Hung
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Liao, Yu-Kuang
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Li, Zhen-Yu
This paper reports the successful fabrication of a GaN-on-Si high electron mobility transistor (HEMT) with a 1702 V breakdown voltage (BV) and low current collapse. The strain and threading dislocation density were well-controlled by 100 pairs of AlN/GaN superlattice buffer layers. Relative to the carbon-doped GaN spacer layer, we grew the AlGaN back barrier layer at a high temperature, resulting in a low carbon-doping concentration. The high-bandgap AlGaN provided an effective barrier for blocking leakage from the channel to substrate, leading to a BV comparable to the ordinary carbon-doped GaN HEMTs. In addition, the AlGaN back barrier showed a low dispersion of transiently pulsed I-D under substrate bias, implying that the buffer traps were effectively suppressed. Therefore, we obtained a low-dynamic on-resistance with this AlGaN back barrier. These two approaches of high BV with low current collapse improved the device performance, yielding a device that is reliable in power device applications.
Breakthrough to Non-Vacuum Deposition of Single-Crystal, Ultra-Thin, Homogeneous Nanoparticle Layers: A Better Alternative to Chemical Bath Deposition and Atomic Layer Deposition
(2017-04-01)
Liao, Yu-Kuang
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Liu, Yung-Tsung
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Hsieh, Dan-Hua
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Shen, Tien-Lin
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Hsieh, Ming-Yang
Most thin-film techniques require a multiple vacuum process, and cannot produce high-coverage continuous thin filmswith the thickness of a fewnanometers on rough surfaces. We present a new "paradigm shift" non-vacuum process to deposit high-quality, ultra-thin, single-crystal layers of coalesced sulfide nanoparticles (NPs) with controllable thickness down to a few nanometers, based on thermal decomposition. This provides high-coverage, homogeneous thickness, and large-area deposition over a rough surface, with little material loss or liquid chemical waste, and deposition rates of 10 nm/min. This technique can potentially replace conventional thin-film deposition methods, such as atomic layer deposition (ALD) and chemical bath deposition (CBD) as used by the Cu(In, Ga) Se2 (CIGS) thin-film solar cell industry for decades. We demonstrate 32% improvement of CIGS thin-film solar cell efficiency in comparison to reference devices prepared by conventional CBD deposition method by depositing the ZnS NPs buffer layer using the new process. The new ZnS NPs layer allows reduction of an intrinsic ZnO layer, which can lead to severe shunt leakage in case of a CBD buffer layer. This leads to a 65% relative efficiency increase.
Characterization of large area Cu(In,Ga)Se-2 nanotip arrays via photoluminescence
(2012)
Liao, Yu-Kuang
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Lin, Woei-Tyng
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Hsieh, Dan-Hua
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Kuo, Shou-Yi
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Lai, Fang-I
In this study, photoluminescence (PL) measurements were used to characterized Cu(In,Ga)Se-2 nanotip arrays (CIGS NTRs). Large area CIGS NTRs were yielded by using one step Ar+ milling process without template. Formation mechanism of NTRs is due to a highly anisotropic milling effect of the quaternary components compound. In both CIGS thin films with and without NTRs, saturation effects of Donor-Acceptor Pair (DAP) had been observed while excitation power dependence PL measurements. Comparing with thin film CIGS without NTRs, it has revealed that the concentration DAP had been changed after formation of CIGS NTRs and had enhanced its carrier density of the p-type semiconductor.
Efficiency droop behavior improvement through barrier thickness modification for GaN-on-silicon light-emitting diodes
(2015-03-05)
Tzou, An-Jye
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Lin, Bing-Cheng
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Lee, Chia-Yu
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Lin, Da-Wei
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Liao, Yu-Kuang
Crack-free GaN-based light-emitting diodes (LEDs) were grown on 150-mm-diameter Si substrates by using low-pressure metal-organic chemical vapor deposition. The relationship between the LED devices and the thickness of quantum barriers (QBs) was investigated. The crystal quality and surface cracking of GaN-on-Si were greatly improved by an AlxGa1-xN buffer layer composed of graded Al. The threading dislocation density of the GaN-on-Si LEDs was reduced to < 7 x 10(8) cm(-2), yielding LEDs with high internal quantum efficiency. Simulation results indicated that reducing the QB thickness improved the carrier injection rate and distribution, thereby improving the droop behavior of the LEDs. LEDs featuring 6-nm-thick QBs exhibited the lowest droop behavior. However, the experimental results showed an unanticipated phenomenon, namely that the peak external quantum efficiency (EQE) and light output power (LOP) gradually decreased with a decreasing QB thickness. In other words, the GaN-on-Si LEDs with 8-nm-thick QBs exhibited low droop behavior and yielded a good peak EQE and LOP, achieving a 22.9% efficiency droop and 54.6% EQE. (C) 2015 Society of Photo-Optical Instrumentation Engineers (SPIE)
Enhanced broadband and omnidirectional performance of Cu(In,Ga)Se-2 solar cells with ZnO functional nanotree arrays
(2013)
Hsieh, Ming-Yang
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Kuo, Shou-Yi
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Han, Hau-Vei
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Yang, Jui-Fu
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Liao, Yu-Kuang
An effective approach is demonstrated for enhancing photoelectric conversion of Cu(In,Ga)Se-2 (CIGS) solar cells with three-dimensional ZnO nanotree arrays. Under a simulated one-sun condition, cells with ZnO nanotree arrays enhance the short-circuit current density by 10.62%. The omnidirectional anti-reflection of CIGS solar cells with various ZnO nanostructures is also investigated. The solar-spectrum weighted reflectance is approximately less than 5% for incident angles of up to 60 degrees and for the wavelengths primarily from 400 nm to 1000 nm. This enhancement in light harvesting is attributable to the gradual refractive index profile between the ZnO nanostructures and air.
Growth and characterization of Cu(In,Ga)Se-2 thin films by nanosecond and femtosecond pulsed laser deposition
(2014-06-02)
Chen, Shih-Chen
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Hsieh, Dan-Hua
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Jiang, Hsin
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Liao, Yu-Kuang
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Lai, Fang-I
In this work, CuIn1-xGaxSe2 (CIGS) thin films were prepared by nanosecond (ns)- and femtosecond (fs)-pulsed laser deposition (PLD) processes. Different film growth mechanisms were discussed in perspective of the laser-produced plasmas and crystal structures. The fs-PLD has successfully improved the inherent flaws, Cu-2 (-) Se-x, and air voids ubiquitously observed in ns-PLD-derived CIGS thin films. Moreover, the prominent antireflection and excellent crystalline structures were obtained in the fs-PLD-derived CIGS thin films. The absorption spectra suggest the divergence in energy levels of radiative defects brought by the inhomogeneous distribution of elements in the fs-PLD CIGS, which has also been supported by comparing photoluminescence (PL) spectra of ns- and fs-PLD CIGS thin films at 15 K. Finally, the superior carrier transport properties in fs-PLD CIGS were confirmed by fs pump-probe spectroscopy and four-probe measurements. The present results indicate a promising way for preparing high-quality CIGS thin films via fs-PLD.
High-performance InGaN-based green light-emitting diodes with quaternary InAlGaN/GaN superlattice electron blocking layer
(2016-05-30)
Tzou, An-Jye
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Lin, Da-Wei
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Yu, Chien-Rong
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Li, Zhen-Yu
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Liao, Yu-Kuang
In this study, high-performance InGaN-based green light-emitting diodes (LEDs) with a quaternary InAlGaN/GaN superlattice electron blocking layer (QSL-EBL) have been demonstrated. The band structural simulation was employed to investigate the electrostatic field and carriers distribution, show that the efficiency and droop behavior can be intensively improved by using a QSL-EBL in LEDs. The QSL-EBL structure can reduce the polarization-related electrostatic fields in the multiple quantum wells (MQWs), leading to a smoother band diagram and a more uniform carriers distribution among the quantum wells under forward bias. In comparison with green LEDs with conventional bulk-EBL structure, the light output power of LEDs with QSL-EBL was greatly enhanced by 53%. The efficiency droop shows only 30% at 100 A/cm(2) comparing to its peak value, suggesting that the QSL-EBL LED is promising for future white lighting with high performance. (C) 2016 Optical Society of America
Highly Efficient Flexible Hybrid Nanocrystal-Cu(In,Ga)Se-2 (CIGS) Solar Cells
(2015-01-21)
Liao, Yu-Kuang
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Brossard, Mael
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Hsieh, Dan-Hua
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Lin, Tzu-Neng
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Charlton, Martin D. B.
A novel scheme for hybridizing inkjet-printed thin film Cu(In,Ga)Se-2 (CIGS) solar cells with self-assembled clusters of nanocrystal quantum dots (NQDs), which provides a 10.9% relative enhancement of the photon conversion efficiency (PCE), is demonstrated. A non-uniform layer of NQD aggregates is deposited between the transparent conductive oxide and a CdS/CIGS p-n junction using low cost pulsed-spray deposition. Hybridization significantly improves the external quantum efficiency of the hybrid devices in the absorption range of the NQDs and in the red to near-IR parts of the spectrum. The low wavelength response enhancement is found to be induced by luminescent down-shifting (LDS) from the NQD layer, while the increase at longer wavelengths is attributed to internal scattering from NQD aggregates. LDS is demonstrated using time-resolved spectroscopy, and the morphology of the NQD layer is investigated in fluorescence microscopy and cross-sectional transmission electron microscopy. The influence of the NQD dose on the PCE of the hybrid devices is investigated and an optimum value is obtained. The low costs and limited material consumptions associated with pulsed-spray deposition make these flexible hybrid devices promising candidates to help push thin-film photovoltaic technology towards grid parity.
Large Scale Single-Crystal Cu(In,Ga)Se(2) Nanotip Arrays For High Efficiency Solar Cell
(2011-10-01)
Liu, Chin-Hung
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Chen, Chia-Hsiang
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Chen, Szu-Ying
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Yen, Yu-Ting
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Kuo, Wei-Chen
In this paper, we demonstrated direct formation of large area Cu(In,Ga)Se(2), nanotip arrays (GIGS NTRs) by using one step Ar(+) milling process without template. By controlling milling time and incident angles, the length of CIGS NTRs with adjustable tilting orientations can be precisely controlled. Formation criteria of these CIGS NTRs have been discussed in terms of surface curvature, multiple components, and crystal quality, resulting in a highly anisotropic milling effect. The CIGS NTRs have very low reflectance <0.1% at incident wavelengths between 300 to 1200 nm. Open circuit voltage and short circuit current of CIGS NTRs solar cell were measured to be similar to 390 mV and similar to 22.56 mA/cm(2), yielding the filling factor and the efficiency of 59 and 5.2%, respectively. In contrast to CIGS thin film solar cell with efficiency of 3.2%, the nanostructured CIGS NTRs can have efficiency enhancement of similar to 160% due to the higher light absorption ability because of the nanostructure. The merits of current approach include the latest way via template-free direct creating process of nanostructured CIGS NTRs with controllable dimensionality and large scale production without postselenization process.
Large Scale Single-Crystal Cu(In,Ga)Se-2 Nanotip Arrays For High Efficiency Solar Cell
(2011-10-01)
Liu, Chin-Hung
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Chen, Chia-Hsiang
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Chen, Szu-Ying
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Yen, Yu-Ting
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Kuo, Wei-Chen
In this paper, we demonstrated direct formation of large area Cu(In,Ga)Se-2, nanotip arrays (GIGS NTRs) by using one step Ar+ milling process without template. By controlling milling time and incident angles, the length of CIGS NTRs with adjustable tilting orientations can be precisely controlled. Formation criteria of these CIGS NTRs have been discussed in terms of surface curvature, multiple components, and crystal quality, resulting in a highly anisotropic milling effect. The CIGS NTRs have very low reflectance <0.1% at incident wavelengths between 300 to 1200 nm. Open circuit voltage and short circuit current of CIGS NTRs solar cell were measured to be similar to 390 mV and similar to 22.56 mA/cm(2), yielding the filling factor and the efficiency of 59 and 5.2%, respectively. In contrast to CIGS thin film solar cell with efficiency of 3.2%, the nanostructured CIGS NTRs can have efficiency enhancement of similar to 160% due to the higher light absorption ability because of the nanostructure. The merits of current approach include the latest way via template-free direct creating process of nanostructured CIGS NTRs with controllable dimensionality and large scale production without postselenization process.
Modeling and Optimization of Sub-Wavelength Grating Nanostructures on Cu(In,Ga)Se-2 Solar Cell
(2012-10-01)
Kuo, Shou-Yi
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Hsieh, Ming-Yang
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Lai, Fang-I
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Liao, Yu-Kuang
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Kao, Ming-Hsuan
In this study, an optical simulation of Cu(In,Ga)Se-2 (CIGS) solar cells by the rigorous coupled-wave analysis (RCWA) method is carried out to investigate the effects of surface morphology on the light absorption and power conversion efficiencies. Various sub-wavelength grating (SWG) nanostructures of periodic ZnO:Al (AZO) on CIGS solar cells were discussed in detail. SWG nanostructures were used as efficient antireflection layers. From the simulation results, AZO structures with nipple arrays effectively suppress the Fresnel reflection compared with nanorod- and cone-shaped AZO structures. The optimized reflectance decreased from 8.44 to 3.02% and the efficiency increased from 14.92 to 16.11% accordingly. The remarkable enhancement in light harvesting is attributed to the gradient refractive index profile between the AZO nanostructures and air. (C) 2012 The Japan Society of Applied Physics
Non-antireflective Scheme for Efficiency Enhancement of Cu(In,Ga)Se-2 Nanotip Array Solar Cells
(2013-08-01)
Liao, Yu-Kuang
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Wang, Yi-Chung
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Yen, Yu-Ting
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Chen, Chia-Hsiang
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Hsieh, Dan-Hua
We present systematic works in characterization of CIGS nanotip arrays (CIGS NTRs). CIGS NTRs are obtained by a one-step ion-milling process by a direct-sputtering process of CIGS thin films (CIGS TF) without a postselenization process. At the surface of CIGS NTRs, a region extending to 100 nm in depth with a lower copper concentration compared to that of CIGS TF has been discovered. After KCN washing, removal of secondary phases can be achieved and a layer with abundant copper vacancy (V-Cu) was left. Such compositional changes can be a benefit for a CIGS solar cell by promoting formation of Cd-occupied Cu sites (Cd-Cu) at the CdS/CIGS interface and creates a type-inversion layer to enhance interface passivation and carrier extraction. The raised V-Cu concentration and enhanced Cd diffusion in CIGS NTRs have been verified by energy dispersive spectrometry. Strengthened adhesion of Al:ZnO (AZO) thin film on CIGS NTRs capped with CdS has also been observed in SEM images and can explain the suppressed series resistance of the device with CIGS NTRs. Those improvements in electrical characteristics are the main factors for efficiency enhancement rather than antireflection.
Observation of unusual optical transitions in thin-film Cu(In,Ga)Se-2 solar cells
(2012-11-05)
Liao, Yu-Kuang
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Kuo, Shou-Yi
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Lin, Woei-Tyng
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Lai, Fang-I
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Hsieh, Dan-Hua
In this paper, we examine photoluminescence spectra of Cu(In,Ga)Se-2 (CIGS) via temperature-dependent and power-dependent photoluminescence (PL). Donor-acceptor pair (DAP) transition, near-bandedge transition were identified by their activation energies. S-shaped displacement of peak position was observed and was attributed to carrier confinement caused by potential fluctuation. This coincides well with the obtained activation energy at low temperature. We also present a model for transition from V-Se to V-In and to V-Cu which illustrates competing mechanisms between DAPs recombinations. (C) 2012 Optical Society of America
Photon management of GaN-based optoelectronic devices via nanoscaled phenomena
(2016-09)
Tsai, Yu-Lin
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Lai, Kun-Yu
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Lee, Ming-Jui
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Liao, Yu-Kuang
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Ooi, Boon S.
Photon management is essential in improving the performances of optoelectronic devices including light emitting diodes, solar cells and photo detectors. Beyond the advances in material growth and device structure design, photon management via nanoscaled phenomena have also been demonstrated as a promising way for further modifying/improving the device performance. The accomplishments achieved by photon management via nanoscaled phenomena include strain-induced polarization field management, crystal quality improvement, light extraction/harvesting enhancement, radiation pattern control, and spectrum management. In this review, we summarize recent development, challenges and underlying physics of photon management in GaN-based light emitting diodes and solar cells. (C) 2016 Elsevier Ltd. All rights reserved.
Seeing pressure in color based on integration o highly sensitive pressure sensor and emission tunable light emitting diode
(2019-11-25)
Shen, Tien-Lin
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Chen, Chien-Tung
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Liao, Yu-Kuang
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Su, Teng-Yu
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Liu, Che-Yu
We demonstrate a highly sensitive, low-cost, environmental-friendly pressure sensor derived from a wool-based pressure sensor with wide pressure sensing range using wool bricks embedded with a Ag nano-wires. The easy fabrication and light weight allow portable and wearable device applications. Wth the integration of a light-emitting diode possessing multi-wavelength emission, we illustrate a hybrid multi-functional LED-integrated pressure sensor that is able to convert different applied pressures to light emission with different wavelengths. Due to the high sensitivity of the pressure sensor, the demonstration of acoustic signal detection has also been presented using sound of a metronome and a speaker playing a song. This multi-functional pressure sensor can be implemented to technologies such as smart lighting, health care, visible light communication (VLC), and other internet of things (IoT) applications. (C) 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Self-Powered, Self-Healed, and Shape-Adaptive Ultraviolet Photodetectors
(2020-02-26)
Tsai, Meng-Shian
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Shen, Tien-Lin
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Wu, Hsing-Mei
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Liao, Yu-Ming
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Liao, Yu-Kuang
The emergence of self-healing devices in recent years has drawn a great amount of attention in both academics and industry. Self-healed devices can autonomically restore a rupture as unexpected destruction occurs, which can efficiently prolong the life span of the devices; hence, they have an enhanced durability and decreased replacement cost. As a result, integration of wearable devices with self-healed electronics has become an indispensable issue in smart wearable devices. In this study, we present the first self-powered, self-healed, and wearable ultraviolet (UV) photodetector based on the integration of agarose/poly(vinyl alcohol) (PVA) double network (DN) hydrogels, which have the advantages of good mechanical strength, self-healing ability, and tolerability of multiple types of damage. With the integration of a DN hydrogel substrate, the photodetector enables 90% of the initial efficiency to be restored after five healing cycles, and each rapid healing time is suppressed to only 10 s. The proposed device has several merits, including having an all spray coating, self-sustainability, biocompatibility, good sensitivity, mechanical flexibility, and an outstanding healing ability, which are all essential to build smart electronic systems. The unprecedented self-healed photodetector expands the future scope of electronic skin design, and it also offers a new platform for the development of next-generation wearable electronics.
Toward Omnidirectional Light Absorption by Plasmonic Effect for High-Efficiency Flexible Nonvacuum Cu(In,Ga)Se-2 Thin Film Solar Cells
(2014-09-01)
Chen, Shih-Chen
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Chen, Yi-Ju
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Chen, Wei Ting
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Yen, Yu-Ting
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Kao, Tsung Sheng
We have successfully demonstrated a great advantage of plasmonic Au nanoparticles for efficient enhancement of Cu(In,Ga)Se-2(CIGS) flexible photovoltaic devices. The incorporation of Au NPs can eliminate obstacles in the way of developing ink-printing CIGS flexible thin film photovoltaics (TFPV), such as poor absorption at wavelengths in the high intensity region of solar spectrum, and that occurs significantly at large incident angle of solar irradiation. The enhancement of external quantum efficiency and photocurrent have been systematically analyzed via the calculated electromagnetic field distribution. Finally, the major benefits of the localized surface plasmon resonances (LSPR) in visible wavelength have been investigated by ultrabroadband pump-probe spectroscopy, providing a solid evidence on the strong absorption and reduction of surface recombination that increases electron hole generation and improves the carrier transportation in the vicinity of pn-juction.
Ultrafast carrier dynamics in Cu(In,Ga)Se-2 thin films probed by femtosecond pump-probe spectroscopy
(2012-06-04)
Chen, Shih-Chen
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Liao, Yu-Kuang
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Chen, Hsueh-Ju
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Chen, Chia-Hsiang
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Lai, Chih-Huang
Ultrafast carrier dynamics in Cu(In,Ga)Se-2 films are investigated using femtosecond pump-probe spectroscopy. Samples prepared by direct sputtering and co-evaporation processes, which exhibited remarkably different crystalline structures and free carrier densities, were found to result in substantially different carrier relaxation and recombination mechanisms. For the sputtered CIGS films, electron-electron scattering and Auger recombination was observed, whereas for the co-evaporated CIGS films, bandgap renormalization accompanied by band filling effect and hot phonon relaxation was observed. The lifetime of defect-related recombination in the co-evaporated CIGS films is much longer than that in the direct-sputtered CIGS films, reflecting a better quality with higher energy conversion efficiency of the former. (c) 2012 Optical Society of America
Ultrahigh-Performance Self-Powered Flexible Photodetector Driven from Photogating, Piezo-Phototronic, and Ferroelectric Effects
(1970-01-01)
Shen, Tien-Lin
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Chu, Yu-Wei
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Liao, Yu-Kuang
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Lee, Wen-Ya
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Kuo, Hao-Chung
Owing to the need for dynamic, real-time, and on-site data collection in internet of things applications, the realization of ultra-sensitive sensing networks with self-powered, flexible, and lightweight devices has become an important issue for the development of sensor systems. In this work, a novel, high-performance, self-powered photodetector is achieved through the combination of photogating, piezo-phototronic, and ferroelectric effect by incorporating a ferroelectric thin film of poly(vinylidene fluoride-co-trifluoroethylene) in a rationally designed device structure with suitable band alignment, which can modulate carrier transport behavior at the interface due to the internal electric field produced by light illumination, external strain, or voltage-poled dipole. This enables photocurrent and overall device performance to improve significantly. The unprecedented photodetector presented in this study has several merits, including mechanical flexibility and light weight that allow it to adapt to arbitrary surface topology; additionally, its self-powering capability and high reliability are urgently needed for the demanding functionality of devices for the development of next-generation optoelectronic devices, spanning from wearable communication to unattended harsh environments with a human-friendly interface.