Electrical and Optical Properties of Polysilicon Sheets for Photovoltaic Devices Grown From Powder

Abstract

In a search for low-cost and high efficiency solar cell manufacturing, several techniques of growing thin silicon sheets from powder have been adopted, such as: plasma spraying on various substrates, zone-melting using incoherent focussed light as heat source, and low-angle horizontal pulling of thin ribbon over the melted tin-lead support. Undoped, and n- and p-type silicon powders of various grain sizes have been used as starting materials, with pure graphite, quartz, or low-cost ceramics serving as temporary or permanent substrates. N⁺/p ⁺/n junctions, and back surface fields were formed by implanting PF ₅ and BF ₃⁺ ions in a glow discharge ion implanter at ultralow energies (less than 1 keg) and high total ion doses. Ion induced damage was annealed by RTP in the incoherent light furnace at temperatures as low as 700°C.

Structural and optoelectronic properties of as-grown and processed thin polysilicon sheets were determined by Raman spectroscopy, spectral response, and electrical transport measurements. A sharp peak at 520 cm⁻¹ in the Raman spectra, associated with TO ((Г) phonons, indicated a good crystallinity of polysilicon sheets. High values of short-circuit currents, and an almost flat spectral response down to 400 nm, without intentionally applying a surface passivating oxide, illustrate the advantage of the applied novel technologies.