Joon Sung Lee

Transmission Electron Microscope Study of Screen-Printed Ag Contacts on Crystalline Si Solar Cells

Microstructural and chemical properties of screen-printed Ag contacts on an n + emitter surface in crystalline Si solar cells are investigated using a transmission electron microscope. The Pb-based glass layer, where many Ag precipitates are randomly distributed, is formed between a Ag thick film and textured Si. For both textured and nontextured Si surfaces, the Ag crystallites are epitaxially grown on Si with an abrupt interface along the {111} atomic plane. Based on high resolution electron microscopy images combined with fast Fourier transform patterns, the registry of Ag on Si driven by a geometrical matching condition leads to minimization of the effective lattice mismatch between Ag and Si, resulting in the formation of a Ag/Si epitaxial superlattice near the interface region.

Abnormal Dopant Distribution in

We investigated 2-D dopant distribution in a -diffused emitter formed on textured Si solar cells using transmission electron microscopy (TEM) combined with selective chemical etching. TEM and simulation results demonstrate that the convex and concave regions of a pyramid in the textured Si surface show deeper and shallower junctions, respectively. By considering a strong dependence of phosphorus (P) diffusion on the Si interstitials, the abnormal profile of emitter in the textured Si surface could be attributed to the inhomogeneous distribution of Si interstitials caused by the geometry of the pyramid texture.

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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.

-Diffused

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.

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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.