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Rapid electrochemical texturing of Si for solar cells applications

Novel method of silicon etching in alkaline electrolytes with the application of negative high potential, (termed Negative Potential Dissolution [NPD]) can be applicable not only for polishing but rather also to texturing of multi silicon in the solar cells industry.

Targets and Objectives:
- Perform and understand electrochemical texturing of silicon single crystals.
- Evaluate the influence of the following parameters on mono-silicon texturing:
-- Depth of negative potential
-- Alkaline concentration
-- Process time
- Introduce the concept of electrochemical texturing into multi silicon
- Perform both anisotropic and isotropic texturing of multi silicon and EFG silicon
- Evaluate and study the parameters controlling both isotropic and anisotropic texturing
- Ultimately, to develop and introduce new inexpensive, ultra-rapid texturing technology which do not use either HF or nitric acid.

Achievements and Milestones:
- Anisotropic silicon texturing was obtained with all three mono-crystalline silicon. Characteristic texturing for each single crystal was obtained. For example, in <100> pyramids with a base length of more than 20 ’Ým could be obtained and for <110> we observed in the formation of a prism.
- Negative potentials were scanned and it was established that surface texturing starts only at potentials below -20V. As the potential scanned down to negative values of -100 V new textured surfaces are formed.
- Alkaline concentrations of 8-50% were studies as a function of the applied potential. We found that the optimal alkaline concentration was in the range of 20-24%. In this potential the current drawn from the system was the highest and thus fast removal of silicon from the surface was feasible.
- Texturing of single crystal as-cut silicon could be obtained in time scales of less than 60 seconds.
- Anisotropic texturing of both multi silicon and EFG silicon was successfully obtained with this novel electrochemical method. The obtained textured surfaces have no steps in grain boundaries.
- During the passed 18 months of FANTASI project we successfully isolated the parameters controlling the texturing of multi-silicon. We found the optimal conditions in terms of value of negative potential, alkaline concentration and process time.
- We successfully obtained isotropic texturing of IMEC multi silicon. This process is achieved at totally different conditions than anisotropic texturing is obtained. Namely, alkaline concentration was increased dramatically to 50% and the process time was shortened to 20-30 seconds with a silicon removal rate of 15 ’Ým/mi. Although the process is not optimizes yet we can observe that no steps are formed in-between grain boundaries during the process.

Future Technology work:
- Optimize IMEC multi silicon isotexturing with the use of NPD process.
- Study and investigate RWE EFG silicon isotexturing by NPD technology.
- Increase working area from 1 cm2 to 4 cm2.

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Reported by

Department of Materials Engineering
Technion-Israel Institute of Technology
32000 Haifa
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