Minkyu Ju

Surface-concentrated light and efficient carrier collection in microhole-patterned Si solar cells

We investigate photovoltaic characteristics of crystalline Si solar cells with microhole-patterned surface. We compare patterned samples with different hole-widths and periods with a planar counterpart. From the finite-difference time-domain simulation, the patterned and planar samples are expected to have similar short circuit current density, J(sc) (difference: 1.2%). In contrast, the difference in the measured J(sc) is as large as 12.6%. The simulated optical field patterns reveal that the sample with more significantly concentrated light near the surface has higher quantum efficiency due to more efficient carrier collection. We report the highest efficiency of 15.6% among the hole-patterned solar cells.

The effect of small pyramid texturing on the enhanced passivation and efficiency of single c-Si solar cells

In this work, a simple method to form small random pyramid texturing (0.5–2 μm size) is proposed to enhance the surface passivation of commercial p-type Cz-Si wafers. Small pyramid texturing was generated with chemical nano-masking for anisotropic etching. The surface recombination velocity obtained after the passivation of the thermal oxide layer reduced from 65 and 10 cm s−1 for the large pyramids (10–15 μm size) and small pyramid (0.5–2 μm) texturing respectively. The solar cell fabricated with large pyramid texturing resulted in an efficiency of 17.82% with a current density (JSC) of 36.91 mA cm−2, an open circuit voltage (VOC) of 620 mV whereas small pyramid texturing resulted in an efficiency of 18.5% with JSC of 37.6 mA cm−2 and VOC of 628 mV. The low surface recombination velocity increases the VOC by 8 mV. The small pyramid textured wafers are found to enhance the quantum efficiency performance in both short and long wavelength regions.

Selective emitter using a screen printed etch barrier in crystalline silicon solar cell

The low level doping of a selective emitter by etch back is an easy and low cost process to obtain a better blue response from a solar cell. This work suggests that the contact resistance of the selective emitter can be controlled by wet etching with the commercial acid barrier paste that is commonly applied in screen printing. Wet etching conditions such as acid barrier curing time, etchant concentration, and etching time have been optimized for the process, which is controllable as well as fast. The acid barrier formed by screen printing was etched with HF and HNO3 (1:200) solution for 15 s, resulting in high sheet contact resistance of 90 Ω/sq. Doping concentrations of the electrode contact portion were 2 × 1021 cm−3 in the low sheet resistance (Rs) region and 7 × 1019 cm−3 in the high Rs region. Solar cells of 12.5 × 12.5 cm2 in dimensions with a wet etch back selective emitter Jsc of 37 mAcm−2, open circuit voltage (Voc) of 638.3 mV and efficiency of 18.13% were fabricated. The result showed an improvement of about 13 mV on Voc compared to those of the reference solar cell fabricated with the reactive-ion etching back selective emitter and with Jsc of 36.90 mAcm−2, Voc of 625.7 mV, and efficiency of 17.60%.

Fabrication of Crystalline Silicon Solar Cell with Emitter Diffusion, SiNx Surface Passivation and Screen Printing of Electrode

The amount of solar energy incident on the earth surface every second (1650 TW) is high‐ er than the combined power consumption by using oil, fossil fuel, and other sources of en‐ ergy by the entire world community (< 20 TW) in 2005. The solar photovoltaic power generation are ever increasing in capacity, yet at a lower scale. Thus there is a scope of fur‐ ther use of solar energy to produce more electricity. For this purpose a demand for a large scale commercial production of solar cells have emerged. There is a large variety of solar cell structures proposed with various types of materials, of which p-type c-Si solar cell has been one of the most popular and widely used in commercial production with screen print‐ ing technique.

SiNx Double Layer Antireflection Coating by Plasma-Enhanced Chemical Vapor Deposition for Single Crystalline Silicon Solar Cells

Double layer antireflection (DLAR) coatings have significant advantages over single-layer antireflection (SLAR) coatings owing to their coverage of a broad solar spectral range. In this study, the properties of SiNx are examined. DLAR coatings using two thin films of SiNx with different refractive indices are presented. Using the same materials is more cost-effective than using different materials. The total thickness of SiNx/SiNx films is kept at 80 nm. The top SiNx film has a refractive index of 1.9, while the bottom layer has a refractive index of 2.3. Single crystalline solar cells are fabricated with different thicknesses for the top and bottom layers. The solar cell with 60 nm/20 nm SiNx DLAR coatings has 18.3% efficiency, while that with 80 nm SiNx SLAR coating has 17.6% efficiency. The improvement of efficiency is due to the effect of better passivation and better antireflection of DLAR.

A novel method for crystalline silicon solar cells with low contact resistance and antireflection coating by an oxidized Mg layer

One of the key issues in the solar industry is lowering dopant concentration of emitter for high-efficiency crystalline solar cells. However, it is well known that a low surface concentration of dopants results in poor contact formation between the front Ag electrode and the n-layer of Si. In this paper, an evaporated Mg layer is used to reduce series resistance of c-Si solar cells. A layer of Mg metal is deposited on a lightly doped n-type Si emitter by evaporation. Ag electrode is screen printed to collect the generated electrons. Small work function difference between Mg and n-type silicon reduces the contact resistance. During a co-firing process, Mg is oxidized, and the oxidized layer serves as an antireflection layer. The measurement of an Ag/Mg/n-Si solar cell shows that Voc, Jsc, FF, and efficiency are 602 mV, 36.9 mA/cm2, 80.1%, and 17.75%, respectively. It can be applied to the manufacturing of low-cost, simple, and high-efficiency solar cells.

Effectiveness of Iodine Termination for Ultrahigh Efficiency Solar Cells as a Means of Chemical Surface Passivation

The use of iodine as a passivating agent for chemical modification of silicon surface is demonstrated. The measurement of carrier lifetime using microwave photoconductivity decay method shows an effective passivation with iodine treatment which is 5 times greater than hydrogen passivation. Unlike hydrogen termination, the negative charge created by the iodine termination enhances the solar cell performance. For n-type silicon, the charge effect results in electric passivation. For p-type silicon, the charge effect forms a barrier which acts as back surface field. For cells with the same area, open circuit voltage (VOC), short circuit current density (JSC), fill factor (FF), and efficiency (η) of iodine terminated one were 610 mV, 39.5 mA/cm2, 76.1%, and 18.3% while those of hydrogen passivated one were 600 mV, 33.4 mA/cm2, 73.1%, and 14.7%, respectively.

Analysis of Laser Injection Condition and Electrical Properties in Local BSF for Laser Fired Contact c-Si Solar Cell Applications

A crystalline silicon (c-Si) local-back-contact (LBC) solar cell for which a laser-condition-optimized surface-recombination velocity (SRV), a contact resistance (Rc), and local back surface fields (LBSFs) were utilized is reported. The effect of the laser condition on the rear-side electrical properties of the laser-fired LBC solar cell was studied. The Nd:YAG-laser (1064-nm wavelength) power and frequency were varied to obtain LBSF values with a lower contact resistance. A 10-kHz laser power of 44 mW resulted in an Rc of 0.125 ohms with an LBSF thickness of 2.09 μm and a higher open-circuit voltage (VOC) of 642 mV.

Enhanced Haze Ratio on Glass by Novel Vapor Texturing Method.

State-of-the-art optical trapping designs are required to enhance the light trapping capabilities of tandem thin film silicon solar cells. The wet etch process is used to texture the glass surface by dipping in diluted acidic solutions such as HNO3 (nitric acid) and HF (hydrofluoric acid). For vapor texturing, the vapor was generated by adding silicon to HF:HNO3 acidic solution. The anisotropic etching of vapor textured wafers resulted in an etching depth of about 2.78 μm with reduced reflectance of 5%. We achieved a high haze value of 74.6% at a 540 nm wavelength by increasing the etching time and HF concentration.

Analysis of Resistance and Surface Recombination Velocities by Contact Coverage for Optimizing Electrical Loss in c-Si Local Back Contact.

Recently, the importance of solar cell research has emerged due to emerging social issues such as environmental pollution problems and rising oil prices. Accordingly, each company is studying to make solar cell of high efficiency. In order to fabricate high-efficiency solar cells, the two major techniques have to be applied on the rear. One is complete passivation of the surface using a thermal oxide and the other one is the part that comes in contact with the electrode doped partially LBSF (Local BSF) formation. In this paper, LBC technology which is usually applied for high efficiency crystalline silicon solar cell, applied to mass productive solar cell to achieve high open circuit voltage and short circuit current with low surface recombination from rear side. Thermal SiO2/SiN(x) double layer which has superior thermal stability is formed on rear surface as passivation layer, then 1% of the whole rear surface area is locally contacted with aluminum. Finally, the cell has been fired at high temperature and the cell process has complete. The fabricated LBC cells conversion efficiency was 18.0% with 625 mV of open-circuit voltage (V(oc)), 37.58 mA/cm2 of current density (J(sc)), 76.3% of fillfactor (FF) at 5% contact coverage, respectively.