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.
Thin-film silicon solar cells require an effective light trapping and a low reflectivity over the entire sun spectrum.
As the optics in thin-film devices is not understood in detail optical simulations can be a useful tool to investigate
the wave propagation in textured layer stacks. For microcrystalline (μc-Si:H) and amorphous (a-Si:H) silicon
solar cells transparent conductive oxides (ZnO) with randomly rough textured interfaces are commonly used to
achieve an improved light in-coupling into the cell and light scattering at the rough interfaces. Since periodically
textured substrates offer the possibility to design the solar cell in accordance to a waveguide, the solar cells with
integrated grating coupler and Bragg reflector gain more and more in importance. To get more insight into light
propagation a detailed computational study focusing on the relation of the incoming light wave and the structure
size and structure shape of the interface texture is extremely valuable.
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.