Beyond Lambertian light trapping for large-area silicon solar cells: fabrication methods

Abstract

Light trapping photonic crystal (PhC) patterns on the surface of Si solar cells provides a novel opportunity to approach the theoretical efficiency limit of 32.3%, for light-to-electrical power conversion with a single junction cell. This is beyond the efficiency limit implied by the Lambertian limit of ray trapping ~ 29%. The interference and slow light effects are harnessed for collecting light even at the long wavelengths near the Si band-gap. We compare two different methods for surface patterning, that can be extended to large area surface patterning: 1) laser direct write and 2) step-&-repeat 5 × reduction projection lithography. Large area throughput limitations of these methods are compared with the established electron beam lithography (EBL) route, which is conventionally utilised but much slower than the presented methods. Spectral characterisation of the PhC light trapping is compared for samples fabricated by different methods. Reflectance of Si etched via laser patterned mask was ~ 7% at visible wavelengths and was comparable with Si patterned via EBL made mask. The later pattern showed a stronger absorbance than the Lambertian limit 6 .

Laser Patterned Alumina Mask and Mask-less Dry Etch of Si for Light Trapping

“…This is caused by reduced reflectivity of surface due to Al 2 O 3 coating and larger pulse energy required to open a hole in the mask. The holes in Al 2 O 3 mask have not shown melting signatures as compared with Cr-mask used earlier [8]. The minority carriers lifetime tests showed strong dependence of τ mc values on surface treatments which are required for PhC patterning of light trapping surface.…”

“…Similar plasma etching conditions as used in the case of 30 nm of Cr showed very different tee-pees pattern for the 50 nm alumina mask ablated by E p ≈ 4.2 nJ pulses (at focus). Etched by SF 6 :CHF 3 :O 2 at 5:1:1 flow rate ratio for 15 min at ICP 180 W and 0 W bias (Cr-mask) [8] and 30 min at ICP 140 W and 5 W bias (Al 2 O 3 -mask); He pressure 2.70 kPa at a process pressure of 2.5 Pa. The ablation threshold fluence F th [J/cm 2 ] of Si and Gaussian beam waist w 0 were experimentally established from the single pulse ablation dependence of the diameter of the ablated crater D a vs. pulse energy ln E p : D 2 a = 2w 2 0 ln E p .…”

“…Version December 29, 2022 submitted to Journal Not Specified 2 of 14 (PhC) patterns [8]. Dry or wet etching of inverse pyramidal (or tee-pee) patterns were carried out through sub-1 µm openings in the mask.…”

“…Figure 5. Color-photo shows Si samples with different Al 2 O 3 masks before and after dry plasma etch.Left-SEM image show Si PhC made by plasma etch developed for an under-etch zero-ridge tee-pees PhC pattern using 30-nm-thick Cr mask[8]. Similar plasma etching conditions as used in the case of 30 nm of Cr showed very different tee-pees pattern for the 50 nm alumina mask ablated by E p ≈ 4.2 nJ pulses (at focus).…”

Si-Cr Alloys at Nanoscale

“…Direct fs-laser writing (Fig. 1) of etch masks in Cr and Al 2 O 3 films of tens-of-nanometre thickness was recently demonstrated for photonic crystal (PhC) patterns on a large area (2 × 2 cm 2 ) for Si solar cell applications 27 . These patterns rely on precisely controlled etching conditions using well defined masks.…”