ISBN 978-3-8439-4523-3

978-3-8439-4523-3, Reihe Ingenieurwissenschaften

Catherin Gemmel
Charge carrier lifetime in epitaxially grown silicon for photovoltaic solar cells fabricated by the porous silicon process

173 Seiten, Dissertation Universität Hannover (2020), Softcover, A5

Zusammenfassung / Abstract

In this thesis, we investigate the porous silicon (PSI) process as an alternative route to fabricate high-quality monocrystalline Si wafers. This process is a wafering method to fabricate kerfless wafers by epitaxial deposition on porous Si and subsequent detachment of the PSI wafer from a reusable substrate wafer. Due to reduced material losses and lower energy consumption compared to standard wire-sawn wafers from pulled crystals, solar cell manufacturing costs might be significantly reduced when using the PSI process.

However, the concept is economically viable only, if the quality of the PSI wafers is comparable to high-quality monocrystalline wafers, if the fabrication process is robust, and if the wafer price is competitive. Here, we measure carrier lifetimes of up to 8 ms on 150 µm-thick n-type wafers with a resistivity of 3 Ohm*cm. For p-type material we find lifetimes of up to 830 µs on 150 µm-thick wafers with a resistivity of 1.5 Ohm*cm. The carrier lifetime of wafers fabricated with the PSI process and parallel processed reference wafers without porous layer is at the same order of magnitude after external gettering. We find the same lifetime increase independent of the usage of phosphorus diffusion gettering or gettering with n-type polysilicon on oxide (POLO) junctions. We describe the bulk lifetime of n-type and p-type wafers from two suppliers with SRH defects limiting the extrinsic carrier lifetime. We observe that the contaminants in the wafers from the two suppliers differ and that they change over time. We determine for our PSI process at ISFH a detachment yield of >88% with an error probability of <5 %. Finally, we simulate solar cells using bulk material that is limited by the found defect parameters. We calculate efficiency potentials of 23.0% for a PERC+ solar cell, 24.2% for a p-type POLO2-IBC solar cell, and 24.7% for an n-type POLO2-IBC structure.