Method of Providing Dual Thickness Anti-Reflection (AR) Coating for Silicon Solar Cells
Publication Date: 2010-Oct-13
The IP.com Prior Art Database
A method of providing a dual thickness Anti-Reflection (AR) coating for silicon solar cells is disclosed. The dual thickness AR coating has a first thickness optimized for faceted surfaces and a second thickness optimized for horizontal surfaces.
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Method of Providing Dual Thickness Anti -Reflection (AR) Coating for Silicon Solar Cells
Disclosed is a method for providing a dual thickness Anti-Reflection (AR) coating for silicon solar cells.
Silicon solar cells typically utilize textured front surfaces and AR coatings to increase the efficiency of light absorption. Textured surfaces decrease reflection losses for the incident light and increase internal absorption, since angled surfaces cause the photons transmitted into the Si to propagate at an angle. The propagation of the photons at an angle increases the probability of photons reflected from the back surface of the Si to impinge on to the front surface at angles compatible with total internal reflection and light trapping.
The optimum thickness of an AR coating depends on the angle of incident light. Therefore, AR coatings optimized for one set of surfaces may not be optimized for another set of surfaces. This effect is non-negligible for a single layer quarter-wave dielectric coating on Si. In this case, the optimum coating thickness is as follows:
where "λ" is the wavelength of the light, "n" is the refractive index of the AR coating, and "ϕ" is the angle of incidence with respect to surface normal. Consider an example where λ = 633 nm and the AR coating is SiO2, with n=1.45. The quarter wave oxide thickness is 109 nm for light incident at ϕ=0° and 132 nm for light incident at ϕ=54.7° (a typical angle of incidence for faceted surfaces).
Disclosed is a method for providing a dual thickness AR coating for silicon solar cells. In an instance, the method provides a dual thickness SiO2 coating having a first thickness "dh" on horizontal surfaces and a second thickness "df" on faceted surfaces. The thicknesses "dh" and "df" may be individually optimized for their respective angular orientations.
The oxidation rate of Si depends on its crystallographic surface orientation. Under normal oxidation conditions providing oxide thicknesses in the range of 100 nm, the oxidation rate is slower on 100-oriented surfaces, such as horizontal surfaces of a 100-oriented silicon wafer. However, the oxidation rate is faster on 111-oriented surfaces, such as faceted surfaces oriented at an angle of 54.7° with respect to the horizontal. In an instance, thermal oxidation may be used with time and temperature conditions that simultaneously provide the desired thicknesses of "dh" and "df" on the horizontal surfaces and surfaces oriented at an angle. The method of providing the desired thicknesses of "dh" and "df" on the surfaces using thermal oxidation is illustrated in Fig. 1.
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Alternatively, a combination of thermal oxidation under conditions providing initial oxide thicknesses dh1 and df1 having the correct differential in thickness, followed by deposition of an approximately conformal oxide for a thickness "d" satisfying th...