Coating and Texturing
Fraunhofer Center for Silicon Photovoltaics CSP
The quality and characteristics of silicon wafer for photovoltaic applications can be changed significantly during solar cell production. In focus are the optical and electrical attributes of the wafers influencing the final efficiency of the solar cells strongly. The team »Coating and Texturing« is dealing for this with two essential approaches:
- Development of a plasma texturized surface having low surface recombination and high optical absorption
- Development of a light-induced hydrogenation process for passivating efficiency limiting recombination active defects in multi- and mono Si material
In order to achieve a fundamental scientific understanding of these approaches, the team »Coating and Texturing« has following scientific focuses:
- Plasma texturing of diamond sawn wafers and wafers produced by kerf-less technologies
- Influence of the chemical and physical ablation processes at the wafer surface on the optical and electrical characteristics
- Physical understanding (up to atomic scale) of the micro masking process taking place during mask-less plasma texturing
- Physical description and modelling of the light-induced hydrogen passivation
- Diffusion properties of hydrogen in silicon, silicon nitride and their interfaces
- Optimization of the plasma-induced chemical vapour deposited (PE-CVD) SiN layers
For this following methods are applied:
- Determination of the optical reflexion, absorption and transmission characteristics
- Sun simulator measurements
- Determination of lifetime by means of light beam induced current measurements (LBIC), microwave photo-conductance decay (µ-PCD) and quasi static state photo-conductance decay (QSSPC)
- Layer thickness measurements (ellipsometry, …)
- Surface morphology analyses (REM, laser microscopy, …)
- Chemical trace analyses (ICP-MS, ToF-SIMS, XPS)
- Nanoscopic analyses (REM, EDX, TEM…)
The work in this team is carried out in close collaboration with the group »Diagnostics of Solar Cells«.
Based on these scientific works both the efficiency and the costs of the solar cells can be optimized. Plasma texturing is an decisive step towards a dry and cost-effective solar cell process with low chemical consumption. High absorption values result in high solar cell efficiencies. The trend towards high efficiency solar cell technologies and thin wafers promote the using of the plasma texture additionally. Noteworthy plasma texturing is to a great extend independent on surface morphology or damage and crystal orientations which makes plasma texturing an ideal solution for diamond sawn wafers or kerf-less technologies.
A successful and stable passivation of B-O pairs as dominating efficiency limiting recombination centres in mono-Si material would be a breakthrough for p-type mono-Si materials in PV application. Further recombination active defects in mc-Si silicon material can be passivated as well.
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Moth-eye Effect for Crystalline Solar Cells
A nanostructure for crystalline silicon is imitating the surface morphology of moth eyes and is decreasing optical losses in solar cells.
Scientists at Fraunhofer Center for Silicon Photovoltaic CSP and the University of Applied Science Anhalt have developed a nanostructuring for silicon solar cells which is imitating the optical characteristics of moth eyes. In nature, this structure protects the moth from predators. Goal of the technological development is an implementation in current industrial processes. Due to this technology low reflectivity values can be achieved leading to an improved efficiency of the solar cells.
The physical fundamental of moth-eye structures is a surface morphology with widths of 400 nm and smaller. The width of the texture is in the order of the visible light, making the texture works as a so-called effective medium. This effective medium causes a gradual transition of the refractive index between air and silicon. The incident light is almost entirely absorbed.
Since the moth-eye structures are formed by an isotropic plasma etching by SF6 and oxygen, this technology is also suitable for both mono-crystalline and multi-crystalline silicon. Moreover, the plasma etching process is independent of the surface characteristics of the wafer. As a result, the technology is particularly suitable for texturing of diamond wire sawn wafers and wafers from kerf-less technologies.
The next step is to investigate the influence of the moth-eye structures on subsequent process steps in the industrial manufacturing of solar cells. As an example, surface passivation and metallization must be adapted to the moth-eye structure. Therefore, the SiN surface passivation and the cleaning of the nanostructures are being optimized. The goal of this optimization is the complete abandonment of wet chemical process steps, as these represent a cost factor and safety issue in industrial production.
This development is the result of the cooperation of the University for Applied Science Anhalt and the Fraunhofer Center for Silicon Photovoltaics CSP within the framework of the research project StrukturSolar (BMBF 03SF0417A).