Development of Ion Beam Sputtered Indium-Tin Solar Cell Technology.

Indium Tin Oxide (ITO) has been successfully used in place of a metal for solar cells of the conductor-insulator -semiconductor (CIS) type. Since ITO is a degenerate, but wide band gap semiconductor, it combines metal-like conductivity properties with the transparence of a glass. It also acts as an anti-reflection coating on silicon and a stable encapsulating layer. The goal of this study was to evaluate whether efficient and stable ITO based solar cells could be fabricated reproducibly with large area on polycrystalline silicon substrates before the technological viability of these devices could be established. This dissertation decribes the development of fabrication techniques and supporting analytical studies which were used to attain the goals of this study. Ion beam sputtering technology was used to fabricate the cells. This technology offered important degrees of flexibility over conventional sputtering technology. To obtain efficient solar cells, it was necessary to develop sputter deposition fixturing, optimize temperature, pressure and operation time parameters, and incorporate hydrogen in the sputtering gas. The more efficient solar cells apparently consist of an induced n-on-p junction in the silicon. Poorer cells with smaller barriers likely resemble Schottky barriers in that the metalurgical and electronic junctions coincide. Various electrical and optical characterizations of initially inefficient devices were used to optimize cell fabrication procedures. ITO/polycrystalline silicon solar cells were fabricated on a wide variety of substrates. On "Wacker Silso" substrates, cells exhibiting 14% (active area) and 12.5% (total area) efficiency were fabricated with a high degree of reproducibility. Static solar cell parameters at room temperature were: V(,oc) of 0.53 V, J(,sc) of 33 mA.cm('2) and fill factor of 0.72 for 4 cm('2) devices. Nearly identical results were obtained for cells ranging in area from 0.08 cm('2) to 20 cm('2). Cells remained stable over extended periods both in the dark under bias and under illumination.