Ultrathin Epitaxial Silicon Solar Cells with Inverted Nanopyramid Arrays for Efficient Light Trapping

Abstract

Ultrathin c-Si solar cells have the potential to drastically reduce costs by saving raw material while maintaining good efficiencies thanks to the excellent quality of monocrystalline silicon. However, efficient light trapping strategies must be implemented to achieve high short-circuit currents. We report on the fabrication of both planar and patterned ultrathin c-Si solar cells on glass using low temperature (T < 275 °C), low-cost, and scalable techniques. Epitaxial c-Si layers are grown by PECVD at 160 °C and transferred on a glass substrate by anodic bonding and mechanical cleavage. A silver back mirror is combined with a front texturation based on an inverted nanopyramid array fabricated by nanoimprint lithography and wet etching. We demonstrate a short-circuit current density of 25.3 mA/cm(2) for an equivalent thickness of only 2.75 μm. External quantum efficiency (EQE) measurements are in very good agreement with FDTD simulations. We infer an optical path enhancement of 10 in the long wavelength range. A simple propagation model reveals that the low photon escape probability of 25% is the key factor in the light trapping mechanism. The main limitations of our current technology and the potential efficiencies achievable with contact optimization are discussed.

Sunlight-thin nanophotonic monocrystalline silicon solar cells

“…They noted that the LTE of experimental cells is significantly lower than that of theoretical structures, which can approach the Lambertian limit, LTE=1. The present nanophotonic cell, and that of [14], present a clear improvement compared to the reported results of [37], by advancing from 0.2-0.3 to 0.56 and 0.57 respectively. We further focus on the lowly-absorbed photons and evaluate the light-path enhancement factor of the nanophotonic cells following the same method as in [14].…”

“…The present nanophotonic cell, and that of [14], present a clear improvement compared to the reported results of [37], by advancing from 0.2-0.3 to 0.56 and 0.57 respectively. We further focus on the lowly-absorbed photons and evaluate the light-path enhancement factor of the nanophotonic cells following the same method as in [14]. The factor F estimates the light-path enhancement by using a simple model of propagation of diffracted waves [38,39].…”

“…They cover a large variety of architectures but mostly with high aspect-ratio nanostructures, such as nanowires or nanocolumns, that offer superior optical performance. The best energy-conversion efficiencies are however achieved with shallower tapered textures, shaped as periodic nanocones [9] or periodic inverted nanopyramids [13,14] that better preserve the electrical performances. Recent findings now indicate that disturbing the periodicity of these patterns could further improve their light trapping effect [16,17].…”

“…Recent findings now indicate that disturbing the periodicity of these patterns could further improve their light trapping effect [16,17]. Critically, the thin crystalline-silicon films were mostly produced by wafer thinning or from SOI wafers, thus with high silicon waste, except for plasmaenhanced chemical vapor deposition growth [14]. The potential development of thin-film monocrystalline-Si photovoltaics then not only relies on the electrical integration of nanophotonics, but also on the integration of kerf-less thin-film fabrication processes and of up-scalable nanostructuring techniques.…”

Geometry-driven carrier extraction enhancement in photovoltaic cells based on arrays of subwavelength light funnels

“…was investigated both experimentally and numerically and broadband absorption enhancement of solar radiation was demonstrated. 5,[25][26][27][28][29][30][31][32][33][34][35][36][37][38][39][40][41][42][43] Photovoltaic cells based on subwavelength features were experimentally realized with carrier extraction schemes such as all back-side contacts, radial congurations in which the p-n junction is aligned in a core-shell geometry, and axial congurations where the junction is positioned along the height of the subwavelength structure. 28,29,41,42,[44][45][46] Kayes et al presented a model for pillar arrays with radial junctions and demonstrated carrier extraction enhancement due to the short collection lengths.…”