Comparison of nano-structured solar cell anti-reflection coating based on graded-index or mode coupling approach

• Comparison between anti-reflection coating (ARC) by graded-index and mode coupling.
• Graded index ARCs lead to degraded light trapping at long wavelengths.
• The observation confirmed by two-way reflectance and absorbance calculations.
• The appropriate use of ARCs for thin-film and wafer-based photovoltaics.

Solar cell nano-structured anti-reflection coatings based on graded index (GI) and mode coupling are all proposed to significantly reduce the reflection from semiconductor–air interface. In this work, it is shown that purely graded index approach can lead to degradation of long wavelength absorption by eliminating quasi-guided mode excitation. The reason is that the physically graded layer not only provides low reflectance path from air to semiconductor, but also from semiconductor to air, leading to photon escape. This results in out-coupling of photons from the semiconductor to air. On the other hand, anti-reflection coating based on mode coupling does not suffer from degraded long wavelength absorption and it is capable of acting as one-way photon pass coating. It is found that the sidewall thickness of mode coupling anti-reflection coating has significant impact on its effectiveness for anti-reflection, and therefore the selection of process methods is critical for its low reflectance. It is proposed that the purely graded index coating is more suitable for wafer-based photovoltaics where full absorption is possible by two photon traces. The mode coupling coating is suitable for both wafer-based photovoltaics and thin-film photovoltaics since it provides not only low reflectance but also long wavelength quasi-guided mode excitations. In the end, new types of anti-reflection coating and light trapping structure are proposed to further enhance the performance.