Toward Highly Efficient Low-Carbon Footprint Solar Cells : Impact of High-Temperature Processing on Epitaxially Grown p-Type Silicon Wafers

Abstract

Conventional silicon (Si) wafers are produced by energy-intensive ingot crystallization which is responsible for a major share of a solar cell's carbon footprint. This work explores Si epitaxially grown silicon wafers (EpiWafers) that are produced by direct epitaxial deposition of trichlorosilane on a reusable substrate. This approach requires less energy and material and hence offers a potential for reduced cost and carbon footprint. Solar cells made from EpiWafers usually suffer from efficiency losses due to recombination at structural crystal defects associated with epitaxial growth. The nature of these defects is investigated and defects at the EpiWafer's back surface are critical. Most of these defects are highly recombination-active, pairwise-connected misfit dislocations in the <110> direction. They originate from a lattice mismatch between the highly doped substrate and the less-doped epitaxially grown layer. In this contribution, the detrimental impact of these defects can be mitigated using typical manufacturing processes of high-efficiency solar cells, such as KOH etching, gettering, and oxidation. Local minority charge carrier lifetimes as high as 2.2 ms after industrially feasible processes are reported. Simulations using efficiency-limiting bulk recombination analysis implies that the material would allow conversion efficiencies of up to 25.6% considering tunnel oxide-passivated contact acting as rear emitter solar cell design.