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조은환,Ajeet Rohatgi,옥영우 한국물리학회 2018 Current Applied Physics Vol.18 No.12
This paper reports on a systematic and quantitative assessment of light induced degradation (LID) and regeneration in full Al-BSF and passivated emitter rear contact cells (PERC) along with the fundamental understanding of the difference between the two. After LID, PERC cells showed a much greater loss in cell efficiency than full Al-BSF cells (∼0.9% vs ∼0.6%) because the degradation in bulk lifetime also erodes the benefit of superior BSRV in PERC cells. Three main regeneration conditions involving the combination of heat and light (75 °C/1 Sun/48 h, 130 °C/2 Suns/1.5 h and 200 °C/3 Suns/30 s) were implemented to eliminate LID loss due to BO defects. Low temperature/long time (75 °C/48 h) and high temperature/short time (200 °C/30s) regeneration process was unable to reach 100% stabilization. The intermediate temperature/time (130 °C/1.5 h) generation achieved nearly full recovery and stabilization (over 99%) for both full Al-BSF and PERC cells. We discussed the effect of temperature, time and suns in regeneration mechanism for two cells.
A Prospective Study of Anterior Segment Ocular Parameters in Anisometropia
Neha Singh,Jolly Rohatgi,Vinod Kumar 대한안과학회 2017 Korean Journal of Ophthalmology Vol.31 No.2
Purpose: The aim of this study was to investigate the differences in anterior segment ocular parameters inanisometropia >1 D. Methods: This study included 202 eyes of 101 subjects ranging from 10 to 40 years of age with anisometropiaof 1 D or more. The subjects were divided into groups according to anisomyopia, anisoastigmatism, andanisohypermetropia. After providing informed consent, each patient underwent a detailed ophthalmologicalexamination including cycloplegic refraction, best-corrected visual acuity, cover test, axial length (AL) measurementusing A-scan ultrasound biometer, keratometry, anterior chamber depth, and central corneal thicknessmeasurement. For each participant, the eye with greater refractive error was compared to the fellow eyevia paired t-tests. Correlations between parameters were studied using the Pearson correlation coefficient. Results: The average age of subjects was 21.7 ± 9.3 years. Of 101 subjects, 31 had anisomyopia; 42 had anisohypermetropia;and 28 had anisoastigmatism. A predisposition toward greater myopia in right eyes was noted inanisomyopia (24 of 31 subjects, 77%). The inter-ocular acuity difference was significant in all three groups (p < 0.01). As the degree of anisometropia increased, there was significant positive correlation in the difference in AL inmyopes (r = 0.863, p < 0.01) and hypermetropes (r = 0.669, p < 0.01) and the difference in corneal curvaturein anisoastigmatism (r = 0.564, p = 0.002) and hypermetropes (r = 0.376, p = 0.014). A significant positive correlationwas also present between the anterior chamber depth difference and refractive difference in hypermetropes(r = 0.359, p = 0.020). Conclusions: This study showed that anisomyopia is correlated only with anterior chamber differences. Anisohypermetropiais correlated with AL differences as well as corneal curvature difference and anterior chamberdepth difference. The amount of anisoastigmatism correlates only with corneal curvature difference.
Young-Woo Ok,Rohatgi, A,Yeon-Ho Kil,Sung-Eun Park,Dong-Hwan Kim,Joon-Sung Lee,Chel-Jong Choi IEEE 2011 IEEE electron device letters Vol.32 No.3
<P>We investigated 2-D dopant distribution in a -diffused emitter formed on textured Si solar cells using transmission electron microscopy (TEM) combined with selective chemical etching. TEM and simulation results demonstrate that the convex and concave regions of a pyramid in the textured Si surface show deeper and shallower junctions, respectively. By considering a strong dependence of phosphorus (P) diffusion on the Si interstitials, the abnormal profile of emitter in the textured Si surface could be attributed to the inhomogeneous distribution of Si interstitials caused by the geometry of the pyramid texture.</P>
String Ribbon Silicon Solar Cells with 17.8% Efficiency
Dong Seop Kim,Ajeet Rohatgi,Andrew Gabor 한국물리학회 2005 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.47 No.1
We have fabricated 4-cm2 cells on String Ribbon Si wafers with eciencies of 17.8 % by using a combination of laboratory and industrial processes. These are the most ecient String Ribbon devices made to date, demonstrating the high quality of the processed silicon and the future potential for industrial String Ribbon cells. A rapid thermal process was used to co-fire silicon nitride (SiNx), which was deposited using plasma-enhanced chemical vapor deposition, and Al to boost the minority carrier lifetime of bulk Si. Front contacts formed by using photolithography were used to achieve low shading losses and low contact resistance with a good blue response. The firing temperature and time were studied with respect to the trade-off between hydrogen retention and aluminum back-surface field (Al-BSF) formation. Bulk defect hydrogenation and deep Al-BSF formation took place in a very short time ( 1 sec) at temperatures higher than 740C.
Kang, M.H.,Rohatgi, A. North-Holland ; Elsevier Science Ltd 2016 Solar energy materials and solar cells Vol.154 No.-
The levelized cost of electricity (LCOE) of a commercial scale photovoltaics (PV) system is quantitatively investigated. The impact of the system and financial parameters, the installed system cost, solar insolation, system lifetime, system derate losses, module cost, module efficiency, balance of system (BOS) cost, inflation, discount rate, and loan rate, are quantitatively calculated using the System Advisor Model (SAM) from the National Renewable Energy Laboratory (NREL). 3D contour plots are generated to assess the impact of the key system and financial parameters on the LCOE. Calculations show that an installed system cost of 2.8, 2.3, and 2.1$/W can provide an LCOE of ~10@?/kWh (average price of electricity in the US) in Phoenix, Atlanta, and Boston, respectively, for a 30 years system lifetime, 20% system derate losses without investment tax credit (ITC). In addition, contour plots are generated to show what happens to the LCOE if the above parameters change. The study uses reasonable inputs for the current (2015) commercial scale PV system in the US.
Kang, Moon Hee,Ok, Young-Woo,Rohatgi, Ajeet IEEE 2016 IEEE journal of photovoltaics Vol.6 No.4
<P>Thermal and plasma-assisted atomic layer deposition (ALD) Al2O3 films for surface passivation of Si solar cells are investigated. ALD Al2O3 films were deposited on both Czochralski (Cz) and float-zone wafers with textured and planar surface, and then, surface recombination velocity (SRV) was measured. An extremely low SRV of 3 cm/s after annealing and 13 cm/s after contact firing was achieved from plasma-assisted ALD Al2O3 on a 2-Omega.cm Cz wafer. In addition, screen-printed large-area (239-cm(2)) textured solar cells were fabricated using plasma-assisted ALD Al2O3 films and compared with SiO2-passivated solar cells. Superior conversion efficiency of 20.0% (n-type) and 18.6% (p-type) was achieved from ALD Al2O3 passivation, while SiO2-passivated solar cells provide 19.8% (n-type) and 17.7% (p-type).</P>
Study of degradation in bulk lifetime of n-type silicon wafer due to oxidation of boron-rich layer
Kyungsun Ryu,Chel-Jong Choi,Ajeet Rohatgi,Young-Woo Ok 한국물리학회 2016 Current Applied Physics Vol.16 No.5
Various boron (B) diffusion techniques are being investigated to fabricate n-type Si solar cells. Thermal oxidation is often used to remove boron-rich layer (BRL) formed as a byproduct of B diffusion because BRL interferes with surface passivation of boron emitter. However, oxidizing the BRL can cause significant degradation in bulk lifetime. In this paper, high resolution electron microscopy (HREM) was performed to detect the presence of BRL after B diffusion and its removal after subsequent oxidation. In addition, bulk lifetime of n-type Si with BRL was measured after various oxidation conditions to systematically investigate the mechanism of oxidation-induced lifetime degradation in n-type Si. Detailed analysis of the oxidized samples revealed that iron (Fe) is primary metal impurity responsible for the bulk lifetime degradation after oxidation. This happens because Fe is gettered in BRL after B diffusion and during the oxidation, when the BRL is consumed, Fe is released into the bulk to degrade lifetime.
Study of degradation in bulk lifetime of n-type silicon wafer due to oxidation of boron-rich layer
Ryu, K.,Choi, C.J.,Rohatgi, A.,Ok, Y.W. Elsevier 2016 Current Applied Physics Vol.16 No.5
<P>Various boron (B) diffusion techniques are being investigated to fabricate n-type Si solar cells. Thermal oxidation is often used to remove boron-rich layer (BRL) formed as a byproduct of B diffusion because BRL interferes with surface passivation of boron emitter. However, oxidizing the BRL can cause significant degradation in bulk lifetime. In this paper, high resolution electron microscopy (HREM) was performed to detect the presence of BRL after B diffusion and its removal after subsequent oxidation. In addition, bulk lifetime of n-type Si with BRL was measured after various oxidation conditions to systematically investigate the mechanism of oxidation-induced lifetime degradation in n-type Si. Detailed analysis of the oxidized samples revealed that iron (Fe) is primary metal impurity responsible for the bulk lifetime degradation after oxidation. This happens because Fe is gettered in BRL after B diffusion and during the oxidation, when the BRL is consumed, Fe is released into the bulk to degrade lifetime. (C) 2016 Elsevier B.V. All rights reserved.</P>