Sung Ju Tark

Enhanced broadband and omni-directional performance of polycrystalline Si solar cells by using discrete multilayer antireflection coatings

The performance enhancement of polycrystalline Si solar cells by using an optimized discrete multilayer anti-reflection (AR) coating with broadband and omni-directional characteristics is presented. Discrete multilayer AR coatings are optimized by a genetic algorithm, and experimentally demonstrated by refractive-index tunable SiO₂ nano-helix arrays and co-sputtered (SiO₂)x(TiO₂)₁₋x thin film layers. The optimized multilayer AR coating shows a reduced total reflection, leading to the high incident-photon-to-electron conversion efficiency over a correspondingly wide range of wavelengths and incident angles, offering a very promising way to harvest more solar energy by virtually any type of solar cells for a longer time of a day.

Effect of high-temperature annealing on ion-implanted silicon solar cells

P-type and n-type wafers were implanted with phosphorus and boron, respectively, for emitter formation and were annealed subsequently at 950~1050 for 30~90 min for activation. Boron emitters were activated at or higher, while phosphorus emitters were activated at . QSSPC measurements show that the implied of boron emitters increases about 15 mV and the decreases by deep junction annealing even after the activation due to the reduced recombination in the emitter. However, for phosphorus emitters the implied decreases from 622 mV to 560 mV and the increases with deep junction annealing. This is due to the abrupt decrease in the bulk lifetime of the p-type wafer itself from 178 μs to 14 μs. PC1D simulation based on these results shows that, for p-type implanted solar cells, increasing the annealing temperature and time abruptly decreases the efficiency (%), while, for n-type implanted solar cells, deep junction annealing increases the efficiency and , especially (%) for backside emitter solar cells.

Improvement of electrical properties in screen-printed crystalline silicon solar cells by contact treatment of the grid edge

In this study, the influence of HF treatment of Ag pastes after a firing process was investigated. It was shown that the HF treatment can improve the fill factors and efficiencies of various cells including those with high initial specific contact resistances. SEM images showed that this improvement is due to the etching of the thin glass layer at the Ag-Si boundary, which exposes the Ag crystallites and colloids. These colloids electrically connect the bulk Ag to the Si through a direct contact, which reduces both the transfer length and the specific contact resistance. A model of the current path was proposed to explain the effect of HF treatment on the edge of the Ag grid.

Effects of rapid thermal process on the junction properties of aluminum rear emitter solar cells

N-type silicon with aluminum emitters for rear junctions was studied; aluminum back surface fields were replaced with n-type silicon wafers. Aluminum rear emitters for n-type silicon solar cells were studied with various rapid thermal processing conditions. With fast ramping-up and fast cooling, an aluminum rear junction was formed uniformly with low emitter recombination current. The effects of junction quality on solar cell efficiency were investigated.

Aluminum fire-through with different types of the rear passivation layers in crystalline silicon solar cells

Aluminum penetration during dielectric layer annealing on silicon was studied for solar cell application. The thickness and uniformity of the aluminum-doped region was examined in variously annealed dielectric layers. Three types of silicon wafers were used with (1) bare Si, (2) SiO2 layer (80 nm)/Si, and (3) SiNX layer (80 nm)/Si. Local metal contacts were made through laser-drilled holes, and annealing was tested at four different temperatures. Reactions between aluminum and silicon were observed by cross-sectional scanning electron microscopy. Reactions occurred at 660 °C on bare Si and at ca. 690 °C on the SiO2 layer. However, the SiO2 did not withstand annealing at higher temperatures. The SiNX layer showed no Al-BSF region in samples annealed at up to 760 °C, making it a suitable material for rear passivation layers in local contact Si solar cells. A Si solar cell fabricated by laser drilling and screen printing showed an efficiency of 12.41% without optimization.

Effect of the Low-Temperature Annealing on Zn-Doped Indium--Tin-Oxide Films for Silicon Heterojunction Solar Cells

The effects of the low-temperature annealing on Zn-doped indium–tin-oxide (ITO) films such as the electrical, optical and structural properties were investigated. Zn-doped ITO films were fabricated by rf magnetron sputtering of ITO and Al-doped ZnO (AZO) targets on corning glass at room temperature. The content of Zn increased with increasing the power of AZO target. The carrier concentration of films shows the decreasing behaviour with increasing the content of Zn, due to a carrier compensation originating from the substitution of a doped Zn for an In or interstitial site. After the low-temperature annealing at 180 °C in vacuum, all films were slightly decreased a carrier concentration and increased the hall mobility because of the absorption of oxygen on the surface films. In addition, the average transmittance did not show a considerable change and had a high values over 80%. Especially, the Zn-doped ITO with atomic ratio of Zn/(In+Zn) of 6.8 at. % had the resistivity of 4×10-4 Ω cm, the highest hall mobility of 41 cm2 V-1 s-1, and the average transmittance of 82%.

Effect of the Phosphorus Gettering on Si Heterojunction Solar Cells

To improve the efficiency of crystalline silicon solar cells, should be collected the excess carrier as much as possible. Therefore, minimizing the recombination both at the bulk and surface regions is important. Impurities make recombination sites and they are the major reason for recombination. Phosphorus (P) gettering was introduced to reduce metal impurities in the bulk region of Si wafers and then to improve the efficiency of Si heterojunction solar cells fabricated on the wafers. Resistivity of wafers was measured by a four-point probe method. Fill factor of solar cells was measured by a solar simulator. Saturation current and ideality factor were calculated from a dark current density-voltage graph. External quantum efficiency was analyzed to assess the effect of P gettering on the performance of solar cells. Minority bulk lifetime measured by microwave photoconductance decay increases from 368.3 to 660.8 μs. Open-circuit voltage and short-circuit current density increase from 577 to 598 mV and 27.8 to 29.8 mA/cm2, respectively. The efficiency of solar cells increases from 11.9 to 13.4%. P gettering will be feasible to improve the efficiency of Si heterojunction solar cells fabricated on P-doped Si wafers.

Effect of Interface Reaction between ZnO:Al and Amorphous Silicon on Silicon Heterojunction Solar Cells

Silicon heterojunction solar cells have been studied by many research groups. In this work, silicon heterojunction solar cells having a simple structure of Ag/ZnO:Al/n type a-Si:H/p type c-Si/Al were fabricated. Samples were fabricated to investigate the effect of transparent conductive oxide growth conditions on the interface between ZnO:Al layer and a-Si:H layer. One sample was deposited by ZnO:Al at low working pressure. The other sample was deposited by ZnO:Al at alternating high working pressure and low working pressure. Electrical properties and chemical properties were investigated by light I-V characteristics and AES method, respectively. The light I-V characteristics showed better efficiency on sample deposited by ZnO:Al by alternating high working pressure and low working pressure. Atomic concentrations and relative oxidation states of Si, O, and Zn were analyzed by AES method. For poor efficiency samples, Si was diffused into ZnO:Al layer and O was diffused at the interface of ZnO:Al and Si. Differentiated O KLL spectra, Zn LMM spectra, and Si KLL spectra were used for interface reaction and oxidation state. According to AES spectra, sample deposited by high working pressure was effective at reducing the interface reaction and the Si diffusion. Consequently, the efficiency was improved by suppressing the SiOx formation at the interface.

Advanced Yield Strength of Interconnector Ribbon for Photovoltaic Module Using Crystallographic Texture Control

This paper reports a study on reducing the yield strength of Cu ribbon wire used for Si solar cell interconnections in solar panels. Low yield strength Cu core should be used as the interconnector ribbon to minimize the fracture of Si solar cells during the tabbing process. We lowered the yield strength of Cu ribbon by controlling the crystallographic texture without increasing the annealing time and temperature. The crystallographic texture was controlled by lubrication in a cold rolling process. The crystallographic texture was observed by scanning electron microscopy with electron back scattered diffraction. A tensile test was performed for the comparison of the mechanical properties of Cu with and without lubrication. The average yield strength was 91.2 MPa with lubrication whereas the yield strength was 99.6 MPa without lubrication. The lower value of the lubricated samples seemed to be caused by the higher cube texture intensity than that of the samples without lubrication.

Effects of in‐situ NH3 post plasma treatment on the surface passivation layer

Hydrogenated silicon nitride (SiNx:H) using plasma enhanced chemical vapor deposition is widely used in photovoltaic industry as an antireflection coating and passivation layer. The samples with or without in‐situ NH3 post‐plasma treatment had the following structures: SiNx/N‐type Si/SiNx versus in‐situ NH3 post‐plasma treated SiNx/N‐type Si/SiNx. The wafer was dipped in saw‐damage‐etching solution and wet cleaning process was treated. After the dry process with N2 atmosphere, SiNx thin film was deposited on back surface. Then SiNx thin film was deposited on the front surface with or without in‐situ NH3 post‐plasma treatment process. In order to minimize the plasma induced surface damage, we used lower power than the process power during the NH3 post‐plasma treatment. After the in‐situ NH3 post‐plasma‐treatment, we analyzed the effect of this in‐situ NH3 post‐plasma‐treatment for passivation. The minority carrier lifetime was observed by means of quasi‐steady‐state photoconductance and microwave photocond...