A. J. M. van Erven

Design of periodic nano- and macro-scale textures for high-performance thin-film multi-junction solar cells

Surface textures in thin-film silicon multi-junction solar cells play an important role in gaining the photocurrent of the devices. In this paper, a design of the textures is carried out for the case of amorphous silicon/micro-crystalline silicon (a-Si:H/mu c-Si:H) solar cells, employing advanced modelling to determine the textures for defect-free silicon layer growth and to increase the photocurrent. A model of non-conformal layer growth and a hybrid optical modelling approach are used to perform realistic 3D simulations of the structures. The hybrid optical modelling includes rigorous modelling based on the finite element method and geometrical optics models. This enables us to examine the surface texture scaling from nano- to macro-sized (several tens or hundreds of micrometers) texturisation features. First, selected random and periodic nanotextures are examined with respect to critical positions of defect-region formation in Si layers. We show that despite careful selection of a well-suited semi-ellipsoidal periodic texture for defect-free layer growth, defective regions in Si layers of a-Si: H/mu c-Si: H cell cannot be avoided if the lateral and vertical dimensions of the nano features are optimised only for high gain in photocurrent. Macro features are favourable for defect-free layer growth, but do not render the photocurrent gains as achieved with light-scattering properties of the optimised nanotextures. Simulation results show that from the optical point of view the semi-ellipsoidal periodic nanotextures with lateral features smaller than 0.4 mu m and vertical peak-to-peak heights around or above 0.3 mu m are required to achieve a gain in short-circuit current of the top cell with respect to the state-of-the-art random texture (>16% increase), whereas lateral dimensions around 0.8 mu m and heights around 0.6 mu m lead to a > 6% gain in short-circuit current of the bottom cell.

Gen5 production tool for light management textures

OM&T | Moser Baer Technologies is investigating the approach of texturing glass up to Gen5 size (1.1 × 1.3 m2) by nano-imprinting techniques. For the application of thin film silicon solar cells nano-imprinted 2D periodic gratings were successfully developed and tested as light trapping textures that can realize an increased solar cell efficiency and/or allow to reduce the thickness of the absorber layers. Reliable materials were found and compatibility of the nano-imprinted light management layers with solar module manufacturing processes was demonstrated. Scaling-up towards Gen5 size was realized by first moving from laboratory to Gen2 scale and to incorporate the findings into a processing line that can be used for the production of Gen5 size textured glass.

Controlling the silicon nitride film density for ultrahigh-rate deposition of top quality antireflection coatings

In this contribution we address the importance of a high mass density for silicon nitride films used as an antireflection coating on crystalline silicon solar cells. Two approaches for finding the optimized deposition conditions are presented. The outcome of these optimization studies clearly show that both the bulk and surface passivation benefit from a high mass density and that top quality antireflection coatings can be obtained at deposition rates up to 5 nm/s.

Nano-imprint lithography for advanced light management concepts in multi-junction solar cells

Nano-imprint lithography is a versatile tool for the realization of light management textures in photovoltaic devices. Various types of nano-structures of variable geometries and arrangements can be fabricated using this technology. In this contribution, we show how to apply nano-imprint lithography at different levels in the fabrication chain of thin-film silicon solar cells. For the realization of advanced light management concepts it is necessary to texture the front- and backside of a solar cell device or the intermediate reflector in a tandem solar cell structure. Two examples of advanced light management concepts for thin-film silicon tandem solar cells are shown in this study. In the first example periodic light trapping structures are applied at the front-side of the solar cell device which results in a higher conversion efficiency driven by an increased short-circuit current. In the second example the intermediate reflector of the solar cell is textured by a nano-imprint process, such that two different light-trapping textures are used for the top cell and the bottom cell of the tandem photovoltaic device. With this work we show how the fabrication of advanced light management concepts can benefit from an industrial relevant nano-imprint process.

Periodically textured glass for 20% bottom cell current increase in a-Si:H/μc-Si:H tandem solar cells

OM&T | Moser Baer Technologies is investigating the approach of using periodically textured glass in combination with an optimized TCO for an improved light trapping. Diffraction next to scattering can then be used to enhance the path-length of the incoming light and this approach enables the use of relatively thin front TCO layers. Optical absorption measurements by integrating sphere were used to evaluate different textures in combination with different TCO/silicon stacks. Solar cell processing was used to verify the results. It was found that an optimized 2D grating texture in combination with a thin front TCO layer can lead to 20% higher bottom cell currents in a-Si:H/μc-Si:H tandem solar cells compared to Asahi U-type FTO.