UC Irvine

Iron pyrite is an earth-abundant, non-toxic, and inexpensive semiconductor that has the potential to produce large photocurrents in photovoltaic devices. However, the efficiency of pyrite solar cell devices has been limited by its photovoltage (< 0.2 eV). The surface layer of pyrite has been identified as the reason behind the low efficiency of pyrite devices. Highly pure single crystals of pyrite were synthesized, characterized, and studied in this work to determine ways to boost the photovoltage of pyrite. Based on a numerical modeling of Hall coefficient and conductivity data, it was found that there is a surface inversion layer on the surface of the pyrite crystals. The presence of the inversion layer, plus the ionization of deep bulk donor states at the surface, has resulted in an extreme upward band bending and a sharp triangular potential, across which charge carriers can tunnel and thereby, degrade the VOC. A clear route involving the reduction of the density of the bulk donors by changing the condition of the crystal growth was demonstrated. Theoretical calculations showed that the reduction in the bulk donor density could indeed increase the photovoltage in pyrite photovoltaic devices. Lastly, pyrite single crystals were doped to understand the effect of several elemental impurities (Co, Cr, and Ni) on the optical and electronic properties of pyrite. A combination of experimental characterization techniques along theoretical calculations was necessary to understand the effect of each dopant fully. Overall, this work demonstrates a clear path forward to overcome the problems associated with pyrite and achieve high efficient pyrite solar devices.