• Korean Journal of Materials Research
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• v.31 no.1
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• pp.16-22
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• 2021

In this study, quantum dot-sensitized solar cells (QDSSC) using CdSe/ZnS quantum dots (QD) of various sizes with green, yellow, and red colors are developed. Quantum dots, depending their different sizes, have advantages of absorbing light of various wavelengths. This absorption of light of various wavelengths increases the photocurrent production of solar cells. The absorption and emission peaks and excellent photochemical properties of the synthesized quantum dots are confirmed through UV-visible and photoluminescence (PL) analysis. In TEM analysis, the average sizes of individual green, yellow, and red quantum dots are shown to be 5 nm, 6 nm, and 8 nm. The J-V curves of QDSSC for one type of QD show a current density of 1.7 mA/㎠ and an open-circuit voltage of 0.49 V, while QDSSC using three type of QDs shows improved electrical characteristics of 5.52 mA/㎠ and 0.52 V. As a result, the photoelectric conversion efficiency of QDSSC using one type of QD is as low as 0.53 %, but QDSSC using three type of QDs has a measured efficiency of 1.4 %.

• Journal of the Korean Institute of Electrical and Electronic Material Engineers
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• v.27 no.2
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• pp.110-114
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• 2014

In this study, we report the doping effect of graphene quantum dots (QDs) in nematic liquid crystal (NLC) system on rubbed polyimide (PI) surface. The good LC alignment and high thermal stability in QD-LC cell system on rubbed PI surfaces can be measured. Also, the low threshold voltage of QD-TN cell was observed about 2.77 V. The fast response time of 13.2 ms for QD-TN cell can be achieved. Finally, the good voltage holding ratio of QD-TN cell on rubbed PI surface was measured.

• Proceedings of the Korean Vacuum Society Conference
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• 2010.02a
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• pp.92-92
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• 2010

Replacing the existing illumination with solid-state lighting devices, such as light-emitting diodes (LEDs) are expected to reduce energy consumption and environmental pollution as they provide better efficiency and longer lifetimes. Currently, white light emitting diodes are composed of UV or blue LED with down-converting materials such as highly luminescent phosphors White light-emitting diodes (LED) were fabricated with multi-shell nanocrystal quantum dots for enhanced luminance and improved stability over time. Multi-shell quantum dots (QDs) were synthesized through one pot process by using the Successive Ionic Layer Adsorption and Reaction (SILAR) method. As prepared, the multi-shell QD has cubic lattice of zinc-blend structure with semi-spherical shape with quantum yield of higher than 60 % in solution. Further, highly fluorescent multi-shell QD was deposited on the blue LED, which resulted in QD-based white LED with high luminance with excellent color rendering properties.

• Journal of the Semiconductor & Display Technology
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• v.20 no.3
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• pp.11-17
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• 2021

I performed a semi-analytical numerical analysis of the effects of core size and electric field intensity on the light emission wavelength of CdSe/ZnS quantum dots (QDs). The analysis used a quantum mechanical approach; I solved the Schrödinger equation describing the electron-hole pairs of QDs. The numerical solutions are described using a basis set composed of the eigenstates of the Schrödinger equation; they are thus equivalent to analytical solutions. This semi-analytical numerical method made it simple and reliable to evaluate the dependency of QD characteristics on the QD core size and electric field intensity. As the QD core diameter changed from 9.9 to 2.5 nm, the light emission wavelength of CdSe core-only QDs varied from 262.9 to 643.8 nm, and that of CdSe/ZnS core/shell QDs from 279.9 to 697.2 nm. On application of an electric field of 8 × 10⁵ V/cm, the emission wavelengths of green-emitting CdSe and CdSe/ZnS QDs increased by 7.7 and 3.8 nm, respectively. This semi-analytical numerical analysis will aid the choice of QD size and material, and promote the development of improved QD light-emitting devices.

• Proceedings of the Korean Vacuum Society Conference
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• 2014.02a
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• pp.467.1-467.1
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• 2014

Quantum dots (QD) solar cells has received considerable attention due to their potential of improving the overall conversion efficiency by harvesting excess energy via multiple excitons generation (MEG). Although there have been many reports which show MEG phenomena by using optical measurement of quantum dots themselves, carrier multiplication in real QD photovoltaic devices has been sparsely reported due to difficulty in dissociation of excitons and charge collection. In this reports, heterojunction QD solar cells composed of PbS QD monolayer on highly crystalline $TiO_2$ thin films were fabricated by using Langmuir-Blodgett deposition technique to significantly reduce charge recombination at the interfaces between each QD. The PbS CQDs monolayer was characterized by using UV-vis, transmission electron microscopy (TEM) and atomic force microscopy (AFM). The internal quantum efficiency (IQE) for the monolayer QD solar cells was obtained by measurement of external quantum efficiency and determining light absorption efficiency of active layer. Carrier multiplication was observed by measuring IQE greater than 100% over threshold photon energy. Our findings demonstrate that monolayer QD solar cell structure is potentially capable of realizing highly efficient solar cells based on carrier multiplication.

• Proceedings of the Korean Vacuum Society Conference
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• 2011.08a
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• pp.265-265
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• 2011

자발형성법으로 InP (001) 기판에 성장한 InAs/InAlGaAs 양자점(QDs: quantum dots)의 광학적 특성을 PL (photoluminescence)와 TRPL (time-resolved PL)을 이용하여 분석하였다. InAs QDs 시료는 single layer InAs QDs (QD1)과 7-stacked InAs QDs (QD2)를 사용하였다. 두 시료 모두 저온 (10 K)에서 1,320 nm에서 PL 피크가 나타나고, 온도가 증가함에 따라 PL 피크는 적색편이 (red-shift)를 보였다. 양자점의 온도를 10 K에서 300 K까지 증가하였을 때 QD1은 178 nm 적색편이 하였으며, PL 스펙트럼 폭은 온도가 증가함에 따라 증가하였다. 그러나 QD2는 264 nm 적색편이를 보였으며 PL 스펙트럼의 폭은 QD1 시료와 반대로 온도가 증가함에 따라 감소하였다. QD2의 아주 넓은 PL 스펙트럼 폭과 매우 큰 적색편이는 InAs 양자점 크기의 변화가 QD1에 비해 훨씬 크기 때문이다. QD2의 경우 InAs 층수(layer number)가 증가함에 따라 InAs QD의 크기가 점차 증가하므로 QD 크기의 변화가 single layer인 QD1 시료보다 훨씬 크다. QD1의 PL 소멸은 파장이 증가함에 따라 점차 느려지다가 PL 피크 근처에서 가장 느린 소멸 곡선을 보이고, 파장이 더 증가하였을 때 PL 소멸은 점차 빠르게 소멸하였다. 그러나 QD2의 PL 소멸곡선은 파장이 증가함에 따라 점차 빠르게 소멸하였다. 이것은 QD2는 양자점 크기의 변화가 매우 크기 때문에 (lateral size=18~29 nm, height=2.8~5.9 nm) 방출파장이 증가함에 따라 양자점 사이의 파동함수의 겹침이 증가하여 캐리어의 이완이 증가하기 때문으로 설명된다. 온도에 따른 TRPL 결과는 두 시료 모두 10 K에서 150 K 까지는 소멸시간이 증가하였고, 150 K 이후부터는 소멸시간이 감소하였다. 온도가 증가함에 따라 소멸시간이 증가하는 것은 양자점에서 장벽과 WL (wetting layer)로 운반자(carrier)의 이동, 양자점들 사이에 열에 의해 유도된 운반자의 재분배 등으로 인한 발광 재결합으로 설명할 수 있다. 150 K 이상에서 소멸시간이 감소하는 것은 열적효과에 의한 비발광 재결합 과정에 의한 운반자의 소멸이 증가하기 때문이다. 온도에 따른 TRPL 결과는 두 시료 모두 150 K까지는 발광재결합이 우세하고, 150 K 이상에서 비발광재겹합이 우세하게 나타났다.

• Applied Science and Convergence Technology
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• v.25 no.1
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• pp.1-6
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• 2016

Quantum dots (QDs) are considered as excellent color conversion and self-emitting materials for display and lighting applications. In this article, various technologies which can be used to realize white light emission with QDs are discussed. QDs have good color purity with a narrow emission spectrum and tunable optical properties with size control capabilities. For white light emission with a color-conversion approach, QDs are combined with blue-emitting inorganic and organic light-emitting diodes (LED) to generate white emission with high energy conversion efficiency and a high color rendering index for various display and lighting applications. Various device structures for self-emitting white QD light-emitting diodes (QD-LED) are also reviewed. Various stacking and patterning technologies are discussed in relation to QD-LED devices.

• ETRI Journal
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• v.26 no.5
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• pp.475-480
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• 2004

Self-assembled InAs quantum dots (QDs) embedded in an InAlGaAs matrix were grown on an InP (001) using a solid-source molecular beam epitaxy and investigated using transmission electron microscopy (TEM) and photoluminescence (PL) spectroscopy. TEM images indicated that the QD formation was strongly dependent on the growth behaviors of group III elements during the deposition of InAlGaAs barriers. We achieved a lasing operation of around 1.5 ${\mu}m$ at room temperature from uncoated QD lasers based on the InAlGaAs-InAlAs material system on the InP (001). The lasing wavelengths of the ridge-waveguide QD lasers were also dependent upon the cavity lengths due mainly to the gain required for the lasing operation.

• Journal of the Korean Vacuum Society
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• v.21 no.6
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• pp.342-347
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• 2012

The optical properties of InAs quantum dots (QDs) grown on a GaAs substrates by migration enhanced molecular beam epitaxy method have been investigated by using photoluminescence (PL) and time-resolved PL measurements. The luminescence properties of InAs/GaAs QDs have been studied as functions of temperature, excitation laser power, and emission wavelength. The PL peak of InAs QDs capped with $In_{0.15}Ga_{0.85}As$ layer (QD2) measured at 10 K is redshifted about 80 nm compared with that of InAs QDs with no InGaAs layer (QD1). This redshift of QD2 is attributed to the increase in dot size due to the diffusion of In from the InGaAs capping layer. The PL decay times of QD1 and QD2 at 10 K are 1.12 and 1.00 ns taken at the PL peak of 1,117 and 1,197 nm, respectively. The reduced decay time of QD2 can be explained by the improved carrier confinement and enhanced wave function overlap due to increased QD size. The PL decay times for both QD1 and QD2 are independent on the emission wavelength, indicating the uniformity of dot size.

• Journal of the Korean Vacuum Society
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• v.20 no.6
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• pp.442-448
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• 2011

Self-assembled InAs/InAlGaAs quantum dots (QDs) grown on an InP (001) substrate have been investigated by using photoluminescence (PL) and time-resolved PL measurements. The single layer (QD1) and seven stacks (QD2) of InAs/InAlGaAs QDs grown by the conventional S-K growth mode were used. The PL peak at 10 K was 1,320 nm for both QD1 and QD2. As the temperature increases from 10 to 300 K, the PL peaks for QD1 and QD2 were red-shifted in the amount of 178 and 264 nm, respectively. For QD1, the PL decay increased with increasing emission wavelength from 1,216 to 1,320 nm, reaching a maximum decay time of 1.49 ns at 1,320 nm, and then decreased as the emission wavelength was increased further. However, the PL decay time for QD2 decreased continuously from 1.83 to 1.22 ns as the emission wavelength was increased from 1,130 to 1,600 nm, respectively. These PL and TRPL results for QD2 can be explained by the large variation in the QD size with stacking number caused by the phase separation of InAlGaAs.