Teng-Yu Wang

High Efficiency Silicon Solar Cells with Bilayer Passivation Structure

The effects of passivation on monocrystalline silicon solar cells were investigated. Al 2 O 3 and SiO 2 films, prepared by atomic layer deposition and thermal oxidation, respectively, were used as passivation layers. Passivation using a monolayer (SiO 2 ) yielded a cell efficiency of 17.5%. However, the Al 2 O 3 /SiO 2 bilayer structure drastically increased cell efficiency to 20.1%. Quantum efficiency results revealed a significant improvement in the IR range, suggesting that bilayer passivation was effective on the back of a solar cell in providing a high energy conversion efficiency.

Fabrication of an ultrathin silicon wafer with a honeycomb structure by the thermal-stress-induced pattern transfer (TIPT) method

A 3 µm thick silicon thin film with a textured surface was fabricated successfully by three major steps. First, the silicon thin film was deposited on a sapphire substrate by a plasma-enhanced chemical vapor deposition system. Second, metal paste was printed on the silicon thin film. Third, a thermal treatment was applied on the sapphire substrate. After cooling, the silicon layer, combined with the metal paste, was peeled from the sapphire substrate because of the large differences in the thermal expansion coefficient between the silicon-metal composite layer and the sapphire substrate. An ultrathin silicon wafer of 3 µm thickness was obtained in this study. Furthermore, a silicon layer with a micro-scale and a nano-scale honeycomb structures can be obtained easily by transferring patterns from the well-designed patterned sapphire substrate.

Excellent passivation structure of high efficiency multicrystalline silicon solar cells

The passivation effects for multicrystalline silicon solar cell with different configurations are investigated. Al2O3 and SiO2 films are used as the passivation layer in this work. They are prepared by atomic layer deposition and thermal oxidation, respectively. Using monolayer (Al2O3 or SiO2) as the passivation layer, the cell efficiency is 16.33% and 17.41%, respectively. The excellent passivation structure shows the drastic enhancement in cell efficiency of 19.09%. The quantum efficiency results show the improvement in the IR range, which explains the high energy conversion efficiency for the excellent structure.

Surface treatment for multi-crystalline silicon wafer sliced by diamond wire saw

Silicon wafers sliced with diamond wire saw and SiC slurry are compared in this study. Diamond wire saw have high slicing speed and the cost is lower than slurry cut process. However it is not suitable for multi-crystalline wafer slicing process. In this study, a surface treatment process was used on the diamond wire cut multi-crystalline silicon wafer. With the modification of wafer surface, the surface quality and solar cell characteristics could be improved. The energy conversion efficiency was increased from 17.7% to 18.4%, which is comparable to the solar cell from conventional slurry cut. Furthermore, there is a decline in wafering cost.

Fabrication of 40 µm thin silicon solar cells with different crystal orientation by SLiM-cut process

Thin silicon foils with two different crystal orientation were fabricated by SLiM-cut method. The thickness of the <;100> and <;111> foils are 45 μm and 33 μm, respectively. The surface quality was compared in this study. The <;111> foil have smoother surface than the <;100> foil. 4cm2 thin silicon solar cells with heterojunction structure were fabricated. The efficiency was found to be 10.74% and 12.81% for the <;100> and <;111> solar cell, respectively. The surface quality of the silicon foils was found to affect the solar cell character. However, the Voc of <;100> solar cell is higher than <;111> solar cell. It was believed that the local epitaxial occur when the amorphous silicon was deposited on the <;111> surface. The increasing of dangling bonds will become the recombination center and reduced the Voc of solar cell.

The enhancement of thin silicon solar cell by selective emitter structure

Making solar cell with thinner wafer could reduce the material cost. However, with the thinning of the wafer thickness, the energy conversion efficiency (Eff) is also reduced. In this study, single crystalline silicon wafer with different thickness (180 μm, 130 μm, 100 μm, and 70 μm) was used. The Eff was varied from 17.35% to 16.98% as the decreasing of wafer thickness. This was due to the short circuit current density losing. The thinner wafer had less light absorption and caused Jsc reduction from 37.39 mA/cm2 to 36.83 mA/cm2. For improving the efficiency, selective emitter structure was used. The Eff could be increased to 17.48%, which was comparable to convention solar cell, as the wafer thickness was only 70 μm.

Fabrication of an ultra-thin silicon wafer with honeycomb structure by Thermal-Stress Induced Pattern Transfer (TIPT) method

A novel process of making ultra-thin silicon wafer by Thermal-stress Induced Pattern Transfer (TIPT) method was reported. The method employs three steps, first a silicon thin film was deposited on a sapphire substrate by plasma enhanced chemical vapor deposition. Second, metal paste was printed on the silicon layer, and finally a thermal annealing process was carried out. After cooling down, the silicon layer together with the metal paste will peeled off from the sapphire substrate because of the differences of thermal expansion coefficient between sapphire substrate and silicon film. An ultra-thin silicon layer of 3 μm thickness was obtained in this study. Furthermore, the silicon layer with the micro-scale and nano-scale honeycomb structure could be obtained by the well-designed of sapphire patterns. An ultra-thin silicon layer with honeycomb structure with 780 nm in diameter and 337 nm in depth was obtained by the TIPT process.

Deactivation Treatments of Silicon Solar Cells for Efficiency Recovery after Illumination Degradation

We applied the deactivation treatments to p-type single crystalline silicon solar cells for deactivating the recombination-active boron-oxygen complex. The methods we used include thermal annealing treatment, capacitively couple plasma (CCP) treatment, and plasma immersion ion implantation (PIII) treatment. The results showed that all the deactivation treatments were working and the energy transfer efficiency (Eff) was thereby increased by more than 1% absolute compared to the degraded state base on the increasing of the open-circular voltage (Voc) and short-current density (Jsc). The CCP deactivated treatment got better efficiencies than PIII treatment because the PIII treatment damaged the surface of solar cells. After the forming gas treatment, the samples could be improved to close to the PIII samples due to the surface damage repairing. However, the increased efficiency could not be kept and would be degraded again after illumination.