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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.
Kyungsun Ryu,Keeya Madani,Ajeet Rohatgi,Young-Woo Ok 한국물리학회 2018 Current Applied Physics Vol.18 No.2
We present the fabrication and analysis of Passivated Emitter and Rear Totally Diffused (PERT) solar cells on n-type silicon using a co-diffusion process. In a single high temperature step, a BSG/SiOx stack deposited by APCVD and a POCl3 back surface field diffuse into the wafer to form the boron doped emitter and phosphorus doped back surface field. The SiOx layer on top of BSG acts as a masking layer to prevent cross-doping of phosphorus as well as a blocking layer for boron out-diffusion. This resulted in an initial sheet resistance of 76 Ω/□ with good uniformity and a final p+ emitter sheet resistance of 97 Ω/□ after boron rich layer removal. Additionally, bulk lifetime was investigated before and after the high temperature step that resulted in an increase from 1.2 ms to 1.5 ms due to a POCl3 gettering effect. A peak cell efficiency of 20.3% was achieved and each recombination component in terms of saturation current density was calculated and analyzed to understand the cell for further efficiency enhancement.
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, 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.
Ok Young-Woo,Kim Jong-Hee,Upadhyaya Vijaykumar D.,Rohatgi Ajeet,Hong chang-Hee,Choi Chel-Jong 한국물리학회 2023 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.82 No.7
The contact resistance and sheet resistance of a screen-printed fre-through Ag/Al paste metallization to the implanted B emitters of n-type crystalline Si solar cells were investigated as a function of B doses in the range of 2× 1014 to 5× 1015 cm−2 at a fxed implantation energy of 10 keV. For B emitters implanted with a B dose of>2× 1015 cm−2, the Ag/Al paste contacts showed very low contact resistance (<4.3 mΩ⋅cm2 ) and sheet resistance (<90 Ω/□), which resulted in a negligible contribution to the series resistance (<0.1 Ω⋅cm2 ) in a fnished cell with contacts made by foating and non-foating screen-printing techniques. A comparison of the sheet resistance measured between the transfer length method and four-point probe analyses indicated an increase in carrier concentration of B emitter surface associated with additional doping through the Al difusion from the Ag/Al paste into B emitter. Scanning transmission electron microscopy combined with energy-dispersive X-ray spectroscopy analysis revealed that after the fre-through process, the Al-containing Pb-based glass layer and numerous small Ag/Al crystallites were formed along the emitter surface without the formation of an Al-Si alloy layer.