Browsing by Subject "single crystals"

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    Understanding electronic transport and controlling doping in iron pyrite single crystals for ultra-low-cost photovoltaics
    (2020-11) Voigt, Bryan
    Iron pyrite is a potentially ideal absorber material for large-scale deployment of photovoltaics (PV) because it is composed of earth-abundant, non-toxic, ultra-cheap constituents, has a suitable band gap (1 eV), and absorbs sunlight strongly. Despite 30 years of research, however, doping control in pyrite is nearly non-existent, precluding pyrite p-n homojunction PV. This forced researchers towards heterojunction devices, which have failed to achieve PV efficiencies greater than 3 % (theoretically, ~30 % is possible) due to low open-circuit voltages. Recent progress, however, has perhaps finally identified the reason for low open-circuit voltages: a p-n junction internal to pyrite that is weakened by a high density of n-type defects. Electronic transport measurements have not yet measured this internal junction and confirmed it as the underlying issue, however, and the identity of the deleterious n-dopant remains outstanding. In this thesis, we identify S vacancies as n-dopants by growing high quality, phase-pure pyrite single crystals in variable S vapor pressures. Decreasing S pressure produces a strong increase in electron densities, and total impurity concentrations are too low to contribute measured donor densities, implicating S vacancies as the deleterious n-dopant in pyrite. We then present a systematic density functional theory study that pinpoints sulfur vacancy clusters, not simple point defects, as capable of producing experimentally-observed transport properties. Next, we use deliberate n-doping, via sulfur vacancies and cobalt, to reveal the internal junction as an exponential rise in sheet resistance below 200 K. In characterizing its properties, we implicate the junction as directly responsible for low open-circuit voltages. In the next chapter, we demonstrate this junction can be eliminated by near-surface Co doping, affording access to rich electronic transport phenomena at low temperatures. Lastly, crystals are controllably doped with P from <5 ppm to >100 ppm, and p-type behavior is exclusively observed at concentrations >60 ppm in both Hall effect and thermopower measurements, identifying P as a suitable p-dopant. This thesis thus directly implicates the internal junction as producing low voltages, informs strategies to improve or eliminate it, and demonstrates comprehensive n- and p-doping control, the latter finally making possible pyrite p-n homojunction PV.