This invention relates generally to photovoltaic cells, and more particularly the invention relates to an improved method of forming a light-trapping surface for a photovoltaic cell.
The photovoltaic cell relies on photons of an impinging radiation source such as the sun to generate electron-hole pairs in a semiconductor body. The electrons and holes migrate to p- and n-doped regions in the semiconductor body to establish a voltage differential and current flow.
The efficiency of the photovoltaic cell depends in part on preventing photons from escaping the semiconductor body by reflection or otherwise prior to forming an electron-hole pair. It is known that the texturing of a surface of a semiconductor body will reduce photon loss as the textured surface acts as a light trap by rereflecting escaping photons back into the semiconductor body.
Heretofore, attempts to trap the light have included roughening a surface mechanically by sand-blasting or molding and by selective dissolution by acids or bases depending on the material utilized in fabricating the photosensitive device.
Chemical etching has been employed in forming the textured surface in a silicon substrate. It is known that an anisotropic etchant can be used to etch a silicon surface having a (100) crystallographic orientation to form pyramidal shapes having sides with (111) crystallographic orientation. See for example Green and Campbell, "Light Trapping Properties of Pyramidally Textured and Grooves Surfaces," Conference Record, 19th IEEE Photovoltaic Specialists Conference, May 4-8, 1987. Green and Campbell discuss the effectiveness of various textured surfaces including regular pyramids, random pyramids, and Lambertian surfaces. However, Green and Campbell do not discuss the processing of silicon bodies to achieve the optimal surfaces.
Hall U.S. Pat. No. 4,427,839 discloses a process for forming inverted pyramids on the surface of a silicon body by photoresist masking and etching. Other techniques omit the passivation and photolithography such as Bailey et al., U.S. Pat. No. 4,137,123. Bailey et al. teach etching a (100) silicon surface by an anisotropic etch including silicon in the etchant. The preferred texturing is accomplished by mixing 0-75% by volume ethylene glycol, 0.05-10% by weight potassium hydroxide, and the balance being water plus silicon particles or silicon-containing compounds such as potassium silicate and sodium silicate. Temperatures range from 50.degree. C. to 100.degree. C. An alternate to the ethylene glycol is mentioned, namely isopropyl alcohol.