Dong Seop Kim

Factors Limiting the Formation of Uniform and Thick Aluminum–Back-Surface Field and Its Potential

Theoretical calculations reveal that the quality of an aluminum-back-surface field (BSF) in a silicon solar cell can be improved by either increasing the thickness of the deposited aluminum (Al), peak alloying temperature, or both. However, this study shows that there is a critical temperature for a given screen-printed Al thickness, above which the BSF quality begins to degrade because of nonuniformity triggered by the agglomeration of Al-Si melt in combination with the bandgap narrowing resulting from the high doping effect in the agglomerated regions. It is found that this critical temperature decreases with the increase in the thickness of the deposited Al layer and, therefore, limits the quality and thickness of the Al-BSF that can be achieved before degradation sets in. This nonuniformity of Al-BSF is observed in the form of scattered Al bumps with thick and thin BSF regions. A combination of experimental results and model calculations is used to provide improved understanding and guidelines for choosing the optimal combination of Al thickness and alloying temperature.

Ni/Cu metallization for low-cost high-efficiency PERC cells

Abstract Low-cost metallization process with excellent performance for high-efficiency passivated emitter and rear cell (PERC) is described. The fabrication processes of the high-efficiency silicon solar cell are too expensive and complicated to be commercialized widely. It is necessary to develop inexpensive metallization technique without degradation of the cell performance. In this paper, Ni/Cu contact system was used to fabricate low-cost high-efficiency solar cells instead of traditional solution that are based on evaporated Ti/Pd/Ag. The electroless plated Ni is utilized as the contact to silicon and the electroplated Cu served as the primary conductor layer. This metallization scheme has proven to be successful.

Hydrogenation of Si from SiNx(H) films: Characterization of H introduced into the Si

A promising method to introduce H into multicrystalline Si solar cells in order to passivate bulk defects is by the postdeposition annealing of a H-rich, SiNx surface layer. It has previously been difficult to characterize the small concentration of H that is introduced by this method. Infrared spectroscopy has been used together with marker impurities in the Si to determine the concentration and depth of H introduced into Si from an annealed SiNx film.

High-density inductively coupled plasma chemical vapor deposition of silicon nitride for solar cell application

Abstract The silicon nitride films were deposited by means of high-density inductively coupled plasma chemical vapor deposition in a planar coil reactor. The process gases used were pure nitrogen and a mixture of silane and helium. Passivated by silicon nitride, solar cells show efficiency above 13%. Strong H-atom release from the growing SiN film and Si–N bond healing are responsible for the improved electrical and passivation properties of SiN film. This paper presents the optimal refractive index of SiN for single layer antireflection coating as well as double layer antireflection coating in solar cell applications.

The Study of Silane-Free SiC x N y Film for Crystalline Silicon Solar Cells

We deposited plasma-enhanced chemical vapor deposition silicon carbon nitride (SiC x N y ) antireflection coating and passivation layers using a silane-free process. We used a solid polymer source developed at SiXtron Advanced Materials to eliminate the storage and handling of dangerous pyrophoric silane gas. We used ammonia flow rate as a control for the chemical and optical properties in the silane-free process. As NH 3 flow rate increases, the carbon content, refractive index, extinction coefficient, and surface charge density of the film decrease. At an ammonia flow rate of 3000 sccm, which is similar to the conventional SiN x , the extinction coefficients for the two films were similar. This led to an emitter dark saturation current density (J oe ) of 404 fA/cm 2 for the two films on 45 Ω/□ emitters. However, a stack passivation of SiO 2 /SiC x N y on an 80 Ω/□ emitter resulted in an emitter dark saturation current density of 95 fA/cm 2 , which is enough to provide a good surface passivation for high efficiency solar cells. An energy conversion efficiency of 17.4% was obtained for a 149 cm 2 textured Czochralski screen-printed solar cell with this stack passivation. For a 156 cm 2 nontextured multicrystalline silicon, with only SiC x N y and a 45 Ω/□ emitter, we obtained 14.9% efficiency.

Investigation of Modified Screen-Printing Al Pastes for Local Back Surface Field Formation

This paper reports on a low-cost screen-printing process to form a self-aligned local back surface field (LBSF) through dielectric rear surface passivation. The process involved formation of local openings through a dielectric (SiNx or stacked SiO2/SiNx) prior to full area Al screen-printing and a rapid firing. Conventional Al paste with glass frit degraded the SiNx surface passivation quality because of glass frit induced pinholes and etching of SiNx layer, and led to very thin LBSF regions. The same process with a fritless Al paste maintained the passivation quality of the SiNx, but did not provide an acceptably thick and uniform LBSF. Al pastes containing appropriate additives gave better LBSF because of the formation of a thicker and more uniform Al-BSF region. However, they exhibited somewhat lower internal back surface reflectance (<90%) compared to conventional Al paste on SiNx. More insight on these competing effects is provided by fabrication and analysis of complete solar cells

Structural, electrical and optical properties of In-doped CdS thin films prepared by vacuum coevaporation

Abstract In-doped CdS films of 1 gmm thickness for a window layer of solar cells have been prepared by vacuum coevaporation of CdS and In on glass substrates at 150 °C. The In concentration in CdS films was varied from 1018 to 1021 cm−3. Structural, electrical and optical properties of CdS films have been investigated by X-ray diffraction, scanning electron microscopy, electrical resistivity measurement, the Hall effect and optical transmittance spectra. As the In concentration increased the preferred orientation of the films change from the (002) plane to the (110) plane. Also, the grain size became smaller and the grain shape changed. The electrical conductivity, carrier concentration and Hall mobility increased with increasing In concentration and then decreased with further increase in In concentration. CdS films became degenerate semiconductors as electron concentration exceeded 2 × 1018cm−3 and the optical band gap increased with increasing electron concentration due to the increase of the Fermi level in the conduction band. The optimum In concentration turned out to be 3 × 1020cm−3, which showed the lowest resistivity of 5 × 10−3ωcm and the largest optical band gap of 2.6 eV.

Development of a Phosphorus Spray Diffusion System for Low-Cost Silicon Solar Cells

Phosphoric acid was used an n-type doping source to make an emitter for silicon solar cells. This paper reports on a cold spray method to coat phosphoric acid on the silicon wafer without any additional complicated and corrosive heating system. The key spray parameters such as belt speed, flow rate of the carrier gas, and concentration of phosphoric acid are optimized to get uniform and reproducible sheet resistance for the emitter. The diffusion process has been studied by firing silicon wafers that are spray-coated with phosphoric acid. screen printed solar cells have been fabricated using the emitter formed by the spray coating. The effects of the emitter on cell performance have been investigated and compared with those of the conventional POCl 3 -diffused emitter. screen printed/spray-diffused cells give impressive cell efficiencies of ∼ 16.6% on 1 Ω cm float zone wafers, which is nearly equal to those of POCl 3 emitter cells (∼16.9%). Compared with the POCl 3 emitter cell, the spray-diffused cells show slightly lower quantum efficiency at the short-wavelength response and slightly higher emitter saturation current density (∼3.59 X 10 -13 A/cm2 compared with 2.73 X 10 -13 A/cm2). Further optimization can eliminate this small difference in efficiency.

Hydrogen diffusion in silicon from plasma-enhanced chemical vapor deposited silicon nitride film at high temperature

The stable hydrogen isotope deuterium (D), which is released during the annealing of deuterated silicon nitride films, diffuses through the crystalline silicon and is captured by a thin, amorphous layer of silicon sputtered on the rear surface. We report on the measurement of the concentration of “penetrated” D by secondary ion mass spectrometry to monitor the flux of D diffusing through single-crystalline silicon wafers. The penetrated D content in the trapping layer increases with the annealing time. However, the flux of D injected into the silicon from the silicon nitride layer decreases as annealing time increases.

Electrical and optical properties of vacuum-evaporated CdS films

Abstract1 Μm CdS films for the window layer of CdS/CulnSe2 solar cells have been prepared by vacuum evaporation at various deposition conditions. Deposition rates were 0.73 and 3.3 nms−1, and substrate temperature ranged from 50 to 225 ‡C. The effect of the deposition conditions on the properties of CdS films was investigated by measuring electrical resistivity, optical transmittance and reflectance.The resistivity of the evaporated CdS films strongly decreased as substrate temperature decreased and the films with high deposition rate showed lower resistivity compared to the films with low deposition rate. Interestingly, the combination of high deposition rate and very low substrate temperature resulted in an increase of resistivity. The optical transmittance of CdS films increased as substrate temperature decreased and then decreased with further decrease in substrate temperature. The transmittance strongly depended on deposition rate at low substrate temperature (<100‡C), while it was independent of deposition rate at high substrate temperature (>100‡C). In particular, high transmittance can be extended to lower substrate temperature by reducing deposition rate. Low optical reflectance can be obtained by lowering substrate temperature. The results indicate that CdS films of low resistivity and high transmittance can be produced by vacuum evaporation at low substrate temperature and low deposition rate.