C. Haase

Thin-film silicon solar cells with efficient periodic light trapping texture

For solar cells based on thin-film microcrystalline (μc-Si:H) or amorphous silicon (a-Si:H) with absorber layers in the micrometer range, highly effective light trapping and an optimal incoupling of the entire sun spectrum are essential. To investigate and optimize both effects the wave propagation in thin-film silicon solar cells is modeled in three dimensions (3D) solving the Maxwell equations rigorously. A periodic nanostructured texture is investigated as an alternative to the common randomly rough texture. Inverted 3D pyramids with a periodicity of 850nm and structure height of 400nm show promising high quantum efficiencies close to the Tiedje limit.

Advanced light trapping management by diffractive interlayer for thin-film silicon solar cells

Thin-film silicon solar cells made of amorphous and microcrystalline silicon in tandem cell configuration enable high efficiency and low-cost production. Precise control of the absorption in each diode by a wavelength-selective and diffractive interlayer provides optimized current matching. For this purpose, intermediate reflectors with periodically textured interfaces are investigated. The propagation of electromagnetic waves is simulated using a three dimensional Maxwell solver which considers both near field and far field optics. Design rules for intermediate reflectors and textured interfaces are presented.

Light trapping in thin-film silicon solar cells by nano-textured interfaces

In superstrate thin-film solar cells light scattering is introduced by the surface texture of the transparent conductive front contact. However, a prerequisite to discuss light trapping in thin-film solar cells is a deeper understanding of the scattering behavior of such randomly textured substrates. The haze, which is widely used to characterize the scattering properties of randomly textured substrates, is an inadequate criterion to correlate the optical quality of the substrate and the measured short circuit current of solar cells. It will be shown that the wavelength dependence of the haze can be used to classify different kind of substrates. Furthermore, the angular resolved scattering properties are analyzed by means of ray tracing based simulation approach. The gained results reveal new aspects for the assessment of light trapping in thin-film silicon solar cells.

Simple and Fast Method to Optimize Nanotextured Interfaces of Thin-Film Silicon Solar Cells

A simple and fast method was developed to determine the optimal surface texture of thin-film silicon solar cells. The optical wave propagation was studied for microcrystalline thin-film silicon solar cells with integrated line and triangular gratings. The developed method based on rigorous coupled wave analysis provides a good agreement with experimental data. The short circuit current is enhanced by 60% up to 20–21 mA/cm2 for grating periods of 500–700 nm and grating heights of 300–500 nm. The method facilitates an analysis of nanotextured solar cells which is 20 times faster than conventional approaches like finite difference and finite integral simulations.

Efficient light trapping scheme by periodic and quasi-random light trapping structures

Solar cells based on thin-film microcrystalline (μc-Si:H) or amorphous silicon (a-Si:H) with absorber layers in the micrometer range require highly efficient light trapping and an optimal incoupling of the entire sun spectrum. To investigate and optimize their optical properties the wave propagation in thin-film silicon solar cells is modelled in 3D solving the Maxwell equations rigorously. A periodic nanostructured texture with square based inverted pyramids is investigated as an alternative to the commonly used randomly rough texture. Different back contact designs were tested and their influence on the long wavelength light trapping was studied. A quasi-random light trapping structure that is composed of different pyramid period sizes was modelled and compared to a periodic single pyramid light trapping structure.

Optics of thin-film silicon solar cells with efficient periodic light trapping textures

The principle of interaction of light waves incident on a surface with a subwavelength nanostructure is a key question in the development of solar cells. Efficient thin-film solar cells based on microcrystalline silicon (μc-Si:H) or amorphous silicon (a-Si:H) with an absorber layer in the micrometer range require effective light trapping and an optimal incoupling of the entire sun spectrum. The established approach to achieve this is the application of randomly textured transparent conductive oxides (TCOs). Previous investigations of light trapping in thin-film devices have been conducted with often misleading far field measurements. Optical simulations based on the Finite Integration Technique (CST Microwave Studios) are a valuable approach to analyze the light propagation in thin-film devices and enable the study the subwavelength optics of nano-textured interfaces by solving the Maxwell equations rigorously in 3D. However, the question regarding the optimized lateral feature size, vertical height, resulting interface angle and shape of the texture is essential to reach high energy conversion efficiencies. Various texture designs are studied by numerical modeling. We present a 3D simulation analysis of thin-film silicon solar cell nano-optics that gives clear design criteria to reach high efficiencies.

Light Trapping in Thin-Film Silicon Solar Cells with Periodic Structures

The fundamental optics of microcrystalline thin-film silicon solar cells with integrated 1D line grating couplers, 2D and 3D pyramidal interfaces were investigated by means of numerical modelling. The simulated QE for a 1D grating is in good agreement with experimental data. The discussion of the blue response based on the measured and calculated reflection shows that a binary shaped grating does not lead to a significantly improved light in-coupling whereas the shape of the grating has a significant influence on the blue light incoupling properties of the solar cell. Only in the case of an insufficient diffraction at the textured TCO/p-interface an additional scattering at the back contact can improve the light trapping properties of the cell. As an alternative to randomly textured layers periodically textured structures were integrated in thin-film solar cells and their optical properties were investigated. The simulations predict that the highest current densities can be achieved for a solar cell with a 3D pyramidal interface