Smooth anti-reflective three-dimensional textures for liquid phase crystallized silicon thin-film solar cells on glass

Abstract

Recently, liquid phase crystallization of thin silicon films has emerged as a candidate for thin-film photovoltaics. On 10 μm thin absorbers, wafer-equivalent morphologies and open-circuit voltages were reached, leading to 13.2% record efficiency. However, short-circuit current densities are still limited, mainly due to optical losses at the glass-silicon interface. While nano-structures at this interface have been shown to efficiently reduce reflection, up to now these textures caused a deterioration of electronic silicon material quality. Therefore, optical gains were mitigated due to recombination losses. Here, the SMooth Anti-Reflective Three-dimensional (SMART) texture is introduced to overcome this trade-off. By smoothing nanoimprinted SiOx nano-pillar arrays with spin-coated TiOx layers, light in-coupling into laser-crystallized silicon solar cells is significantly improved as successfully demonstrated in three-dimensional simulations and in experiment. At the same time, electronic silicon material quality is equivalent to that of planar references, allowing to reach V oc values above 630 mV. Furthermore, the short-circuit current density could be increased from 21.0 mA cm−2 for planar reference cells to 24.5 mA cm−2 on SMART textures, a relative increase of 18%. External quantum efficiency measurements yield an increase for wavelengths up to 700 nm compared to a state-of-the-art solar cell with 11.9% efficiency, corresponding to a jsc, EQE gain of 2.8 mA cm−2.

Imprinted Nanostructures for Light Management in Crystalline Silicon Thin-Film Solar Cells on Glass

“…Finally, a 10 nm thin silicon oxide layer is sputtered on all three nanostructure types (1)-(3), serving as a passivation layer at the interface to the silicon absorber. More details can be found in references [4,5]. Silicon layers of 8-10 µm thickness are deposited onto these nanostructured superstrates by electron-beam evaporation at a temperature of 600°C resulting in nano-crystalline material.…”

“…All three nanotextures are assessed with respect to light trapping properties as well as the material quality of the silicon absorber. The morphologically flat SiO x -TiO x -layer stack is found to overcome the trade-off between optical gain and nanotexture-caused electronic losses best: 8 µm thin silicon solar cell test devices were fabricated exhibiting an excellent material quality with open-circuit voltage values above 640 mV and a short-circuit current density of 23.3 mA cm -2 , corresponding to a relative increase of 11% compared to 21.0 mA cm -2 of simultaneously processed optimized planar reference devices [4].…”

Angle-Resolved Reflectivity Analysis of Textured Substrates for Liquid-Phase Crystallized Silicon Thin-Film Solar Cells

“…However, the electronic material quality can be reduced by introducing nano-structures at this interface [2]. Two approaches were recently introduced that combine high electronic material quality LPC silicon with nanostructures, namely the sinusoidal structure [3] and the smooth anti-reflective three-dimensional (SMART) texture [4]. In this contribution, we investigate the angular dependence of the anti-reflective properties of these two structured LPC silicon absorbers compared to an optimized planar reference sample.…”

Optical simulations of advanced light management for liquid-phase crystallized silicon thin-film solar cells

“…(6) 2 and (iii) an atomic force microscopy (AFM) measurement of a sinusoidally nanotextured sol-gel layer on glass with 750 nm pitch and about 150 nm texture height, hence an aspect ratio of about a = 20%. (b) (i) An illustration and (ii) a coloured SEM picture of smooth anti-reflective three-dimensional textures (SMART), 3 which consist of silicon oxide (SiOx) nanopillars in a hexagonal array covered by titanium oxide. SMARTs are characterized by the pitch P [as in (a,ii)], the height h and the filling fraction, given as the surface fraction covered by the SiOx oxide nanopillars.…”

“…6 Figure 1 shows two types of periodic nanostructures between glass and LPC-Si, which we recently demonstrated: sinusoidal nanotextures 1,7 and smooth anti-reflective three-dimensional textures (SMART). 3 Both types are fabricated with nanoimprint lithography 8, 9 using high-temperature stable sol-gels, 10 which are suited for the LPC process. LPC-Si solar cells on these textures have excellent material qualities and hence electric properties, such that open circuit voltages exceeding 615 mV or even 640 mV are measured for sinusoidal or SMART structures, respectively.…”