Detecting and gettering chromium impurities in photovoltaic crystalline silicon

Abstract

Photovoltaic (PV) modules provide a source of renewable electricity by harnessing solar energy. Currently, crystalline silicon dominates the PV market with an approximate market share of 90% and record solar cell efficiencies greater than 20%. However, the PV market must decrease the cost to the consumer to maintain growth and meet global electricity demands. Increasing the solar-to-electricity conversion efficiency is one of the most significant cost levers. Transition metal impurities can degrade silicon wafer-based solar cell efficiencies at concentrations as low as 1010 cm3 . By removing interstitial metals from the bulk and/or collecting interstitial metals at heterogeneous nucleation sites, phosphorous diffusion gettering renders them less detrimental in the final solar cell. While they exist for iron, kinetics process simulation tools do not yet exist for chromium, which has higher capture cross-sections for minority carriers and is therefore more detrimental in both p- and n-type materials. In this thesis, I employ synchrotron-based X-ray fluorescence microscopy to study chromium (Cr) distributions in multicrystalline silicon in as-grown material and after two phosphorous diffusion profiles. I complement quantified precipitate size and spatial distribution with interstitial Cr concentration and minority carrier lifetime measurements to provide insight into chromium gettering kinetics and offer suggestions for minimizing the device impacts of chromium. The data presented in this thesis can be used in development of kinetics process simulation tools for chromium gettering. Finally, I describe a new technique for detecting low concentrations of impurities in n- and p-type silicon. The development of high-performance silicon materials, including n-type, necessitates more sensitive impurity detection techniques, capable of measuring interstitial contaminations below 1010 cm-3. I propose the development of a free-carrier absorption-based technique that incorporates a temperature stage. By measuring injection-dependent lifetimes at a wide range of sample temperatures, the identifying parameters of lifetime-limiting defects can be deduced.