• Current Photovoltaic Research
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• v.2 no.2
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• pp.69-72
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• 2014

We investigated current-voltage (I-V) and capacitance (C)-V characteristics of solution-processed thin film solar cells, consisting of $Cu(In,Ga)S_2$ and $CuInS_2$ stacked absorber layers. The ideality factors, extracted from the temperature-dependent I-V curves, showed that the tunneling-mediated interface recombination was dominant in the cells. Rapid increase of both series- and shunt-resistance at low temperatures would limit the performance of the cells, requiring further optimization. The C-V data revealed that the carrier concentration of the $CuInS_2$ layer was about 10 times larger than that of the $Cu(In,Ga)S_2$ layer. All these results could help us to find strategies to improve the efficiency of the solution-processed thin film solar cells.

• Current Photovoltaic Research
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• v.6 no.1
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• pp.17-20
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• 2018

The dependency of the photovoltaic performance of p-/n-type silicon solar-cells on the metallic contaminant type (Fe, Cu, and Ni) and concentration was investigated. The minority-carrier recombination lifetime was degraded with increasing metallic contaminant concentration, however, the degradation sensitivity of recombination lifetime was lower at n-type than p-type silicon wafer, which means n-type silicon wafer have an immunity to the effect of metallic contamination. This is because heavy metal ions with positive charge have a much larger capture cross section of electron than hole, so that reaction with electrons occurs much more easily. The power conversion efficiency of n-type solar-cells was degraded by 9.73% when metallic impurities were introduced in the silicon bulk, which is lower degradation compared to p-type solar-cells (15.61% of efficiency degradation). Therefore, n-type silicon solar-cells have a potential to achieve high efficiency of the solar-cell in the future with a merit of immunity against metal contamination.

• Current Photovoltaic Research
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• v.8 no.1
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• pp.6-11
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• 2020

The power conversion efficiency (PCE) of a two-terminal tandem solar cell depends upon the tunnel-recombination junction (TRJ) between the top and bottom sub-cells. An optimized TRJ in a tandem cell helps improve its open-circuit voltage (V_oc), short-circuit current density (J_sc), fill factor (FF), and efficiency (PCE). One of the parameters that affect the TRJ is the buffer layer thickness. Therefore, we investigated various TRJs by varying the thickness of the buffer or intermediate layer (TRJ-buffer) in between the highly doped p-type and n-type layers of the TRJ. The TRJ-buffer layer was p-type nc-Si:H, with a doping of 0.06%, an activation energy (E_a) of 43 meV, an optical gap (E_g) of 2.04 eV, and its thickness was varied from 0 nm to 125 nm. The tandem solar cells we investigated were a combination of a heterojunction with intrinsic thin layer (HIT) bottom sub-cell and an a-Si:H (amorphous silicon) top sub-cell. The initial cell efficiency without the TRJ buffer was 7.65% while with an optimized buffer layer, its efficiency improved to 11.74%, i.e., an improvement in efficiency by a factor of 1.53.

• Korean Journal of Digital Imaging in Medicine
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• v.8 no.1
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• pp.51-55
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• 2006

The effects of Asaddition in amorphous selenium(a-Se) films for digital X-ray conversion material have been studied using the moving photocarrier grating(MPG) technique. This method utilizes the moving interference pattern generated by the superposition of the two frequency shifted laser beams for the illumination of the sample. This moving intensity grating induces a short circuit current, j$_{sc}$ in a-Se:As film. The transport parameters of the sample are extracted from the grating-velocity dependent short circuit current induced in the sample along the modulation direction. The electron and hole mobility, and recombination lifetime of a-Se films with arsenic(As) additions have been obtained. We have found an Increase in hole drift mobility and recombination lifetime, especially when 0.3% As is added into a-Se film, whereas electron mobility decreases with As addition due to the defect density. The transport properties for As doped a-Se films obtained by using MPG technique have been compared with X-ray sensitivity for a-Se:As device. The fabricated a-Se(0.3% As) device film exhibited the highest X-ray sensitivity out of 5 samples.

• Proceedings of the Korean Vacuum Society Conference
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• 2012.08a
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• pp.172-173
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• 2012

Currently GaN based LED is known to show high internal or external efficiency at low current range. However, this LED operation occurs at high current range and in this range, a significant performance degradation known as 'efficiency droop' occurs. Auger process, carrier leakage process, field effect due to lattice mismatch and thermal effects have been discussed as the causes of loss of efficiency, and these phenomena are major hindrance in LED performance. In order to investigate the main effects of efficiency loss and overcome such effects, it is essential to obtain relative proportion of measurements of internal quantum efficiency (IQE) and various radiative and nonradiative recombination processes. Also, it is very important to obtain radiative and non-radiative recombination times in LEDs. In this research, we measured the IQE of InGaN/GaN multiple quantum wells (MQWs) LEDs with PSS and Planar substrate using modified ABC equation, and investigated the physical mechanism behind by analyzing the emission energy, full-width half maximum (FWHM) of the emission spectra, and carrier recombination dynamic by time-resolved electroluminescence (TREL) measurement using pulse current generator. The LED layer structures were grown on a c-plane sapphire substrate and the active region consists of five 30 ${\AA}$ thick In0.15Ga0.85N QWs. The dimension of the fabricated LED chip was $800um{\times}300um$. Fig. 1. is shown external quantum efficiency (EQE) of both samples. Peak efficiency of LED with PSS is 92% and peak efficiency of LED with planar substrate is 82%. We also confirm that droop of PSS sample is slightly larger than planar substrate sample. Fig. 2 is shown that analysis of relation between IQE and decay time with increasing current using TREL method.

• Journal of the Korean Chemical Society
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• v.41 no.7
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• pp.329-336
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• 1997

Photoacoustic spectroscopy was utilized to investigate the carrier transport and the thermal diffusivity in GaAs and Si. From the frequency dependence of the photoacoustic signal, it is found that heat source was originated from the instantaneous thermalization process in low frequency region. In high frequency region, however, the heat was generated by the nonradiative bulk recombination and the nonradiative surface recombination processes. It was also shown that the photoacoustic effects in GaAs of a direct band gap were governed by all three processes and those in Si of an indirect band gap were produced by the instantaneous thermalization and the nonradiative bulk recombination only. The phase of the photoacoustic signal showed a minimum value in GaAs. In Si, the phase of the photoacoustic signal was monotonically decreased as the modulation frequency was increased, demonstrating the above-mentioned mechanisms of the generation of heat. By measuring the photoacoustic signal, thermal diffusivities of semiconductors were determined to be ∼0.35 ㎠/s for GaAs and ∼1.24 ㎠/s for Si. In addition, the similar values of thermal diffusivities were obtained from the curve fitting of photoacoustic phase spectra.

• Electrical & Electronic Materials
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• v.9 no.9
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• pp.885-890
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• 1996

Undoped p-type and Te doped n-type GaSb crystals were grown by the vertical Bridgman method. The lattice constant of the GaSb crystals was 6.096.+-.000373.angs.. The carrier concentration, the resistivity, and the carrier mobility measured by the van der Pauw method were p.iden.8*10$^{16}$ c $m^{-3}$ , .rho..iden.0.20 .ohm.-cm, .mu.$_{p}$ .iden.400c $m^{2}$ $V^{-1}$ se $c^{-1}$ for p-type, n.iden.1*10$^{17}$ c $m^{-3}$ , .rho..iden.0.15 .ohm.-cm, .mu.$_{n}$ .iden.500c $m^{2}$ $V^{-1}$ se $c^{-1}$ for n-type at 300K. In case of treatment with metal ion of R $u^{+3}$, P $t^{+4}$, the carrier concentration, resistivity and carrier mobility of the GaSb crystals were p.iden.2*10$^{17}$ c $m^{-3}$ , .rho..iden.0.08.ohm.-cm, .mu.$_{p}$ .iden.420c $m^{2}$ $V^{-1}$ se $c^{-1}$ for p-type, n.iden.2.5*10$^{17}$ c $m^{-3}$ , .rho..iden.0.07.ohm.-cm, .mu.$_{n}$ .iden.520c $m^{2}$ $V^{-1}$ se $c^{-1}$ for n-type respectively. GaSb crystals had a tendency to lower resistivity and higher mobility, for surface treatment with metal ion effectively diminished surface recombination centers.s.

• Proceedings of the Korean Vacuum Society Conference
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• 2016.02a
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• pp.404-404
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• 2016

태양전지 제작 시 표면에 피라미드 구조를 형성하면 입사되는 광의 흡수를 높여 광 생성 전류의 향상에 기여한다. 일반적인 KOH를 이용한 습식 표면조직화 공정은 평균 10%의 반사율을 보였으며, 유도 결합 플라즈마를 이용한 RIE 공정은 평균 5.4%의 더 낮은 반사율을 보였다. 그러나 RIE 공정을 이용한 표면조직화는 낮은 반사율과 서브 마이크론 크기의 표면 구조를 만들 수 있지만 플라즈마 조사에 의한 표면 손상이 많이 발생하게 된다. 이러한 표면 손상은 태양전지 제작 시 표면에서 높은 재결합 영역으로 작용하게 되어 포화 전류(saturation currents, $J_0$)를 증가시키고 캐리어 수명(carrier lifetime, ${\tau}$)을 낮추는 결함 요소로 작용한다. 이러한 플라즈마에 의한 표면 손상을 제거하기 위해 HF, HNO3, DI-water를 이용하여 DRE(Damage Remove Etching) 공정을 진행하였다. DRE 공정은 HF : DI-water 솔루션과 HNO3 : HF : DI-water 솔루션의 두 가지 공정을 이용하여 공정 시간을 가변하며 진행하였다. 포화전류($J_0$), 캐리어 수명(${\tau}$), 벌크 캐리어 수명(Bulk ${\tau}$)을 비교를 하기위해 KOH, RIE, RIE + DRE 공정을 진행한 세 가지 샘플로 실험을 진행하였다. DRE 공정을 적용할 경우 공정 시간이 지날수록 반사도가 높아지는 경향을 보였지만, 두 번째의 최적화된 솔루션 공정에서 $2.36E-13A/cm^2$, $42{\mu}s$의 $J_0$, Bulk ${\tau}$값과 가장 높은 $26.4{\mu}s$의 ${\tau}$를 얻을 수 있었다. 이러한 결과는 오제 재결합(auger recombination)이 가장 많이 발생하는 지역인 표면과 불균일한 도핑 영역에서 DRE 공정을 통해 나아진 표면 특성과 균일한 도핑 프로파일을 형성하게 되어 재결합 영역과 $J_0$가 감소 된 것으로 판단된다. 높아진 반사도의 경우 $SiN_x$를 이용한 반사방지막을 통해 표면 반사율을 1% 이내로 내릴 수 있어 보완이 가능하였다. 본 연구에서는 RIE 공정 중 플라즈마에 의해 발생하는 표면 손상 제거를 통하여 캐리어 라이프 타임의 향상된 조건을 찾기 위한 연구를 진행하였으며, 기존 RIE 공정에 비해 반사도의 상승은 있지만 플라즈마로 인한 표면 손상을 제거하여 오제 재결합에 의한 발생하는 $J_0$를 낮출 수 있었고 높은 ${\tau}$값인 $26.4{\mu}s$의 결과를 얻어 추후 태양전지 제작에 향상된 효율을 기대할 수 있을 것으로 기대된다.

• Proceedings of the Korean Vacuum Society Conference
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• 2011.02a
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• pp.407-408
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• 2011

Natural boron consists of two stable isotopes 10B and 11B with natural abundance of 18.8 atom percent of 10B and 81.2 atom percent of 11B. The thermal neutron absorption cross-section for 10B and 11B are 3837 barn and 0.005 barn respectively. 10B enriched specific compounds are used for control rods and as a reactor coolant additives. In this work 2 methods for boron enrichment were analysed: 1) Gas irradiation in static conditions. Dissociation occurs due to multiphoton absorption by specific isotopes in appropriately tuned laser field. IR shifted laser pulses are usually used in combination with increasing the laser intensity also improves selectivity up to some degree. In order to prevent recombination of dissociated molecules BCl3 is mixed with H2S 2) SILARC method. Advantages of this method: a) Gas cooling is helpful to split and shrink boron isotopes absorption bands. In order to achieve better selectivity BCl3 gas has to be substantially rarefied (~0.01%-5%) in mixture with carrier gas. b) Laser intensity is lower than in the first method. Some preliminary calculations of dissociation and recombination with carrier gas molecules energetics for both methods will be demonstrated Boron separation in SILARC method can be represented as multistage process: 1) Mixture of BCl3 with carrier gas is putted in reservoir 2) Gas overcooling due to expansion through Laval nozzle 3) IR multiphoton absorption by gas irradiated by specifically tuned laser field with subsequent gradual gas condensation in outlet chamber It is planned to develop software which includes these stages. This software will rely on the following available software based on quantum molecular dynamics in external quantized field: 1) WavePacket: Each particle is treated semiclassicaly based on Wigner transform method 2) Turbomole: It is based on local density methods like density of functional methods (DFT) and its improvement- coupled clusters approach (CC) to take into account quantum correlation. These models will be used to extract information concerning kinetic coefficients, and their dependence on applied external field. Information on radiative corrections to equation of state induced by laser field which take into account possible phase transition (or crossover?) can be also revealed. This mixed phase equation of state with quantum corrections will be further used in hydrodynamical simulations. Moreover results of these hydrodynamical simulations can be compared with results of CFD calculations. The first reasonable question to ask before starting the CFD simulations is whether turbulent effects are significant or not, and how to model turbulence? The questions of laser beam parameters and outlet chamber geometry which are most optimal to make all gas volume irradiated is also discussed. Relationship between enrichment factor and stagnation pressure and temperature based on experimental data is also reported.

• Journal of the Korean Crystal Growth and Crystal Technology
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• v.8 no.4
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• pp.581-586
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• 1998

We investigated the effect of mechanical back side damage in Czochralski grown silicon wafer. The intensity of mechanical damage was evaluated by minority carrier recombination lifetime by laser excitation/microwave reflection photoconductivity decay method, degree of X-ray diffuse scattering, X-ray section topography, and wet oxidation/preferential etching methods. The data indicate that the higher the mechanical damage intensity, the lower the minority carrier lifetime, and the magnitude of diffuse scattering and X-ray excess intensity increased proportionally, and it was at Grade 1:Grade 2:Grade 3=1:7:18.4 that the normalized relative quantization ratio of excess intensity in damaged wafer was calculated, which are normalized to the excess intensity from sample Grade 1.