Hee Jin Lim

The flexural strengths of silicon substrates with various surface morphologies for silicon solar cells

The influence of various surface morphologies on the mechanical strength of silicon substrates was investigated in this study. The yield for the solar cell industry is mainly related to the fracturing of silicon wafers during the manufacturing process. The flexural strengths of silicon substrates were influenced by the density of the pyramids as well as by the size and the rounded surface of the pyramids. To characterize and optimize the relevant texturing process in terms of mechanical stability and the fabrication yield, the mechanical properties of textured silicon substrates were investigated to optimize the size and morphology of random pyramids. Several types of silicon substrates were studied, including the planar type, a textured surface with large and small pyramids, and a textured surface with rounded pyramids. The surface morphology and a cross-section of the as-textured and fractured silicon substrates were investigated by scanning electron microscopy.

The properties of surface textured ZnO:Al films grown in hydrogen atmosphere

ZnO is used as a transparent conducting oxide (TCO) for the front electrode in silicon thin film solar cells because of its low resistivity and high optical transmittance. Hydrogenated Al-doped zinc oxide (AZO:H) thin films were grown on glass by rf magnetron sputtering using a ceramic target (98 wt. % ZnO, 2 wt.% Al 2 O 3 ) in H 2 /Ar ambient. We changed the ratio of H 2 /(Ar+H 2 ) and the substrate temperature from RT to 250 °C. As results, a resistivity of 3.21 × 10−4 Ω·cm was obtained at the 2% H 2 with the substrate temperature of 150□. UV-measurement showed that the optical transmission of AZO:H films was above 85% in the visible range with a widened optical band gap. After deposition, AZO:H films were etched in diluted HCl (0.5%) to investigate the variation of optical properties and surface morphology. The etched films had excellent scattering properties (spectral haze: 52%) and low reflectance. We will present results that demonstrate an improved light trapping due to the textured surface by an etching technique.

Minority carrier lifetime of silicon wafer passivated by PECVD amorphous silicon layers for silicon heterojunction solar cells

Passivation is the one of important factors in the solar cells. The measurement of effectiveness of the passivation layer was based on quasi-steady-state photoconductance. In the case of the sample which was passivated using p-type amorphous silicon(a-Si:H), the minority carrier lifetime was 8.9μs. When the passivation layer was intrinsic a-Si:H, the minority carrier lifetime was 221.3μs. Considering the minority carrier lifetimes of both samples, silicon heterojunction solar cells were fabricated with and without intrinsic a-Si:H layer. the efficiency of the sample passivated with intrinsic amorphous silicon was 13.52%, whereas it was only 11.58% when this layer was not employed.

Effect of Growth Temperature on the Properties of Hydrogenation Al-doped ZnO Films

This study examined the effect of growth temperature on the electrical and optical properties of hydrogenated Al-doped zinc oxide (AZO:H) thin films deposited by rf magnetron sputtering using a ceramic target (98 wt.% ZnO, 2 wt.% Al2O3). Various AZO films on glass were prepared by changing the substrate temperature from room temperature to 200 o C. It was shown that intentionally incorporated hydrogen plays an important role on the electrical properties of AZO : H films by increasing free carrier concentration. As a result, in the 2% H2 addition at the growth temperature of 150 o C, resistivity of 3.21 10 4 Ω cm, mobility of 21.9 cm 2 / V−s, electric charge carrier concentration of 9.35 10 20 cm 3 was obtained. The AZO : H films show a hexagonal wurtzite structure preferentially oriented in the (002) crystallographic direction.