Fluorine-doped tin oxide—selenium heterojunction for photovoltaic conversion

Abstract

Selenium, being a low-cost material and readily available in high purity form, is attracting interest for solar cell applications. It has been calculated theoretically [1] that selenium-based heterojunction solar cells can have an efficiency as high as 20%. Shaw and Ghosh [2] have reported that cells with efficiency better than 10% can be fabricated with selenium in the form of heterojunctions. The results on CdO-CdSe-Se and SnO2-CdSe-Se heterojunctions [2, 3] indicate a conversion efficiency of the order of 3%. Nakada and Kunioka [4] have developed an indium tin oxide (ITO)-selenium heterojunction solar cell having an efficiency of 3.3%. In this letter we report the results of fluorine-doped tin oxide (FTO)-selenium heterojunctions. Transparent FTO films were prepared on chemically and ultrasonically cleaned glass substrates using the spray pyrolysis technique described earlier [5]. The FTO films so obtained show optical transmittance of 84% for a wavelength of 540 nm with a sheet resistance of 10 f~/D at room temperature. No antireflection coating was done prior to deposition of FTO. High-purity selenium (99.999%) obtained from Nuclear Fuel Complex (India) was thermally deposited from a quartz crucible onto the FTO-coated glass slides kept at room temperature. On some of the samples, an ultrathin tellurium layer (1 to 5 nm) was deposited prior to selenium coating. The selenium film was intentionally grown as amorphous to have better control over thickness and surface morphology. A silver overlayer was grown on the selenium to give electrical contact and back-field reflectance. A conducting silver paste was used to take electrical contacts from the silver film. All the vacuum deposition was done in a Hind high-vacuum (India) coating unit 12A4 at a pressure better than 10 -5 torr using a liquid nitrogen trap. The thickness of the film was measured using an FECO interferometer. The selenium sandwich was subjected to thermal shocks at 200 ° C to convert high resistivity amorphous selenium to low resistivity polycrystalline selenium.