Iron pyrite (cubic FeS2) is an earth-abundant, non-toxic semiconductor with great potential as an absorber material in future large-scale deployment of solar photovoltaic panels.
The surprisingly small photo-voltage generated by this material (< 0.2 V) has limited its
solar cell efficiency and prevented its commercial development to date. The origin of this
limitation has been discussed over the past 30 years, and is addressed here. Electrical measurements of high-purity single crystals are used to show that there is a thin, conductive inversion layer on the surface of n-type pyrite. Tunneling of charge carriers across this inversion layer can result in losses that account for the low voltage of pyrite solar cells. This finding is in line with experiments presented here that show a clear dependence of electrical transport properties on the surface-to-bulk ratio of a given pyrite material. The hole-rich surface can explain why the great majority of pyrite thin films are reported to be p-type regardless of synthesis technique or composition. A quantitative model for multilayer transport in pyrite is established that can be used to predict the effect of surface passivation, and methodically evaluate a variety of suggested passivation treatments. Chemical etching, as well as annealing in certain atmospheres, can substantially reduce the conductivity of the inversion layer, and further treatments are proposed to eliminate it. Lastly, novel all-solid-state pn-heterojunction solar cells with pyrite absorbers are presented, that overcome the 0.2 eV photo-voltage limitation. Overall, this work represents an important step towards fully understanding the short-comings of pyrite absorbers. A clear path forward, along with the necessary to ols and methods, is shown to enable pyrite to live up to its potential of becoming a low-cost, non-toxic, earth-abundant absorber material for deployment of solar photovoltaics on the terawatt scale.