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ECN publication
Comparison of alkaline etches on multi-crystalline wafers
Published by: Publication date:
ECN 1995
ECN report number: Document type:
ECN-RX--95-061 Other
Number of pages: Full text:
6 Download PDF  

It is widely recognized that during further development of crystallinesilicon solar cell technology, reflection reduction and light trapping are important issues. ECN and R and S Renewable Energy Systems developed low cost technology for 16% encapsulated multi-crystalline Si (mc-Si) cells. Normally, the surface morphology for mc-Si wafers is determined by a concentrated NaOH or KOH saw damage etch. Within this project we have looked into reflection reduction by texture etching with diluted NaOH and KOH solutions and found a 0.2% increase in efficiency for AR-coated un-encapsulated cells. The purpose of the research described in this paper is to explore in detail how the difference in macroscopic properties of the cell such as reflectance can be related to the microscopic structure produced by saw damage and texture etches. Cheap texturing methods based on alkaline etches produce faceted surfaces. The texture varies from crystallite to crystallite depending on the local crystal orientation. The composition and temperature of the specific etchant used have a large influence on the shape of the texture. It has been demonstrated elsewhere in ray-tracing studies that rather small tilt angles of the facets (about 20 degrees) can give rise to very significant encapsulated reflection reduction and light-trapping. This suggests that even on multi-crystalline wafers with non-ideal crystallite orientations significant reflection reduction can be achieved. To compare various textures and explain their influence on cell properties it is important to characterise the texture in detail. Multicrystalline neighbouring wafers manufactured by Bayer were textured with different alkaline solutions. Crystal orientations of individual crystallites were obtained by means of Laue photography. Reflection measurements were made, both with the surface bare and encapsulated. Both total and angularly resolved reflection measurements were made. Optical, scanning electron and AFM microscopy were used to study the surface morphology. We will first describe the experimental techniques used. Next, results of the measurements will be presented and discussed. 12 figs., 7 refs.

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