Tuning iron pyrite thin film microstructure by sulfurization of columnar iron precursors

Iron pyrite is a promising material for photovoltaic power production due to low material extraction and processing costs and high optical absorption. Reliable production of photovoltaic grade iron pyrite thin films has, however, been challenging. One potential fabrication route is the direct conversion of iron-to-iron pyrite by sulfur annealing (sulfurization). Bulk iron thin films are used typically but they can suffer from cracking or delamination. Herein we report the sulfurization of porous, columnar Fe films deposited with Glancing Angle Deposition (GLAD), which allows us to control the inter-column spacing (void-fraction) of the precursor film. We show that the morphology and microstructure of the iron pyrite films are strongly affected by the void-fraction. By precisely tuning the void-fraction of the precursor film at 82° oblique angle incidence deposition we can produce iron pyrite films with increased crystallite sizes >100 nm with a uniform, crack-free, facetted granular microstructure. Large crystallites may reduce carrier recombination at grain boundaries, which is attractive for photovoltaic devices. Further increasing the void-fraction produces a columnar iron pyrite structure. We also report composition, electrical and optical characterization including a 27 ps lifetime of photocarriers measured with ultrafast optical-pump/THz-probe. Structured, porous precursors offer an alternate route to control microstructure and film stress during fabrication of iron pyrite thin films.