Grit Köppel

Optical characterization of high mobility polycrystalline ZnO:Al films

Optical methods are powerful and non-destructive means to characterize highly doped transparent conducting oxide thin films. In order to describe the optical properties of high-mobility ZnO films we present a dielectric function composed of different analytic expressions to describe the different contributions to the dielectric function of the films. This allows for the correct description of measured optical spectra and reduces the complex functions to a set of fitting parameters. In a second step we compare the obtained parameters to theoretical models. The basic theories are nicely reproduced and the basic link between optical and electrical properties can be understood. The findings can help on the route to a complete presiction of optical properties from the basic material properties or vice versa.

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

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 Voc 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.

Combining tailor-made textures for light in-coupling and light trapping in liquid phase crystallized silicon thin-film solar cells

We present tailor-made imprinted nanostructures for light management in liquid phase crystallized silicon thin-film solar cells providing both, increased jsc by enhanced absorption and excellent electronic material-quality with Voc-values >640mV. All superstrate textures successfully enhance light in-coupling in 10-20µm thick liquid phase crystallized silicon thin-films. Moreover, the effect of combining imprinted textures at the front side with individually optimized light trapping schemes at the rear side of the absorber layers on the optical properties is analyzed. With a silicon absorber layer thickness of 17µm maximum achievable short-circuit current density of 37.0mA/cm2 is obtained, an increase by + 1.8mA/cm2 (or 5.1%) compared to the optimized planar reference.

Periodic and Random Substrate Textures for Liquid-Phase Crystallized Silicon Thin-Film Solar Cells

A major limitation in current liquid-phase crystallized (LPC) silicon thin-film record solar cells is optical losses caused by their planar glass-silicon interface. In this study, silicon is grown on nanoimprinted periodically, as well as randomly textured glass substrates, and successfully implemented into state-of-the-art LPC silicon thin-film solar cells. Compared with an optimized planar reference device, both textures enhance absorption of light. Interlayer and process optimization allowed achieving a material quality comparable with the planar reference device. On the random texture, an open-circuit voltage above 630 mV was obtained, as well as an external quantum efficiency exceeding the planar reference device by +3 mA/cm2.

On accurate simulations of thin-film solar cells with a thick glass superstrate

The optical response of periodically nanotextured layer stacks with dimensions comparable to the wavelength of the incident light can be computed with rigorous Maxwell solvers, such as the finite element method (FEM). Experimentally, such layer stacks are often prepared on glass superstrates with a thickness, which is orders of magnitude larger than the wavelength. For many applications, light in these thick superstrates can be treated incoherently. The front side of thick superstrate is located far away from the computational domain of the Maxwell solvers. Nonetheless, it has to be considered in order to achieve accurate results. In this contribution, we discuss how solutions of rigorous Maxwell solvers can be corrected for flat front sides of the superstrates with an incoherent a posteriori approach. We test these corrections for hexagonal sinusoidal nanotextured silica-silicon interfaces, which are applied in certain silicon thin-film solar cells. These corrections are determined via a scattering matrix, which contains the full scattering information of the periodically nanotextured structure. A comparison with experimental data reveals that higher-order corrections can predict the measured reflectivity of the samples much better than an often-applied zeroth-order correction.

Nanophotonic light management for perovskite–silicon tandem solar cells

Abstract. Currently, perovskite–silicon tandem solar cells are one of the most investigated concepts for overcoming the theoretical limit for the power conversion efficiency of silicon solar cells. For monolithic tandem solar cells, the available light must be distributed equally between the two subcells, which is known as current matching. For a planar device design, a global optimization of the layer thicknesses in the perovskite top cell allows current matching to be reached and reflective losses of the solar cell to be minimized at the same time. However, even after this optimization, the reflection and parasitic absorption losses add up to 7  mA  /  cm2. In this contribution, we use numerical simulations to study how well hexagonal sinusoidal nanotextures in the perovskite top-cell can reduce the reflective losses of the combined tandem device. We investigate three configurations. The current density utilization can be increased from 91% for the optimized planar reference to 98% for the best nanotextured device (period 500 nm and peak-to-valley height 500 nm), where 100% refers to the Tiedje–Yablonovitch limit. In a first attempt to experimentally realize such nanophotonically structured perovskite solar cells for monolithic tandems, we investigate the morphology of perovskite layers deposited onto sinusoidally structured substrates.

Facing the challenge of liquid phase crystallizing silicon on textured glass substrates

A major limitation in current liquid phase crystallized (LPC) silicon thin-film record solar cells are optical losses caused by their planar glass-silicon interface. In this study, silicon is grown on nanoimprinted periodically as well as on randomly textured glass substrates and successfully implemented into state-of-the-art LPC silicon thin-film solar cell stacks. By systematically varying every layer the whole sample stack is optimized regarding its anti-reflection ability. Compared to an optimized planar reference device, a reduction of reflection losses by -3.5% (absolute) on the random and by -9.4% (absolute) on the periodic texture has been achieved in the wavelength range of interest.

On Accurate Simulations of Thin-Film Solar Cells With a Thick Glass Superstrate

The air-glass interface significantly affects the reflectivity of nanotextured layer stacks on thick glass superstrates. We estimate this effect with an a posteriori approach applied to results obtained with FEM. Further, we give experimental proofs.

Antireflective nanotextures for monolithic perovskite-silicon tandem solar cells

Recently, we studied the effect of hexagonal sinusoidal textures on the reflective properties of perovskite-silicon tandem solar cells using the finite element method (FEM). We saw that such nanotextures, applied to the perovskite top cell, can strongly increase the current density utilization from 91% for the optimized planar reference to 98% for the best nanotextured device (period 500 nm and peak-to-valley height 500 nm), where 100% refers to the Tiedje-Yablonovitch limit.* In this manuscript we elaborate on some numerical details of that work: we validate an assumption based on the Tiedje-Yablonovitch limit, we present a convergence study for simulations with the finite-element method, and we compare different configurations for sinusoidal nanotextures.

Improved Light Management in Crystalline Silicon Thin-Film Solar Cells by Advanced Nano-Texture Fabrication

We present a texturing method for liquid phase crystallized silicon thin-film solar cells enabling a maximum achievable short-circuit current density of 36.5mA cm−2 due to optimized light management compared to current textured devices.