Iron pyrite (FeS<sub>2</sub>) has been the subject of increased attention in the quest for efficient solar energy harvesting materials. A near-IR bandgap (~0.9 eV), large absorption coefficient at visible wavelengths, earth abundance and non-toxicity all serve to bolster its appeal. Harnessing pyrite's full potential will require a detailed understanding of its electronic band structure and the dynamics of photoexcited carriers which are still unclear. We seek to address this problem using ultrafast transient reflection experiments on natural single crystal pyrite with excitation energies above and below the accepted bandgap energy and a white light visible probe. Interpretation of the transient optical response is made simpler by first transforming transient reflection to transient absorption (Δα). This transformation is accomplished using the Kramers-Kronig constraint relating index of refraction to absorption coefficient and, for the first time, applies to materials with any ratio of n and κ. Our experiments show that the carriers generated by the 2.64, 1.55 and 0.89 eV excitation energies arrive at a common carrier distribution within the first 20 ps. Following this initial relaxation, all dynamics are spectrally uniform indicating that the carriers have fully relaxed either to the band extrema or to trap states. We also show that an excitation of 0.58 eV, well below the bandgap energy, gives rise to a long-lived signal suggesting either the possibility of direct excitation to midgap defect states or that the bandgap is well below the generally accepted one. Our results shed additional light on pyrite’s band structure and carrier dynamics, including the effects of defect and surface states, which should ultimately assist in creating efficient pyrite-based energy harvesting materials.

Broadband transient absorption of iron pyrite

251st National Meeting (2016)

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