Iron Sulfide for Photovoltaics
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by sp-hybrid orbitals occupies the Cl position. FeS2 has a low-temperature metastable orthorhombic polymorph—marcasite — which is also a semiconductor according to band-structure calculations.4 There are also many other Fe^Sy compounds possible in the Fe-S system, making it rather difficult to synthesize the pyrite phase. Nevertheless, the possibility of preparing pyrite single crystals by flux,5 hydrothermal,6 and chemical vapor transport (CVT)7 methods has been demonstrated. In all cases, the synthesized pyrite did not have the ideal stoichiometry due to a relatively large range of homogeneity. Synthetic single crystals (as well as most of the natural crystals) usually have a sulfur deficiency which results in FeS2-jr, with x < 0.08, depending on the growth temperature.8 Electrical Properties Any device application requires, in principle, careful control of the electronic properties of the semiconductor involved. In natural pyrite crystals, both n- and p-type conduction have been observed,3 whereas
Fe ^"'^
105 a [cm"1]
CuInSe,
_. — —.--•
.--' a-Si
104 U
CdTe
/ ' ..--"c'-Si
3
10
102
in' 1.0
1.5
2.0 hv [eV]
Figure 1. Absorption coefficient of pyrite after A. Schlegel and P. Wachter,31 compared with that of other photovoltaic semiconductors?1
2.5
synthetic crystals usually are only n-type. In comparison, thin pyrite films are p-type in most cases (see Table I). At present, it is still somewhat uncertain whether the change in conduction type in natural crystals is due to an impurity type or a stoichiometry-related doping mechanism.9 In support of the latter effect, it has been suggested that mineral pyrite formed at lower temperatures tends to be p-type (iron deficient) whereas high-temperature material tends to be n-type (sulfur deficient). A similar view has been taken by Bittner,5 who showed that in synthetic pyrite crystals, a sulfur deficiency resulted in n-type, and a sulfur excess, in p-type, conduction. This correlates with the above observation that pyrite crystals synthesized at temperatures above 500cC are usually n-type and thin pyrite films deposited at temperatures below 350°C are usually p-type (the stoichiometry is often uncertain). Doping studies in synthetically prepared material have been rather inconclusive so far. From a comparison between predominant impurity concentration and conduction type in natural specimens, it is expected that Cu, Ni, and Co act as donors, whereas P, As, and Sb should act as acceptors. The growth of p-type single crystals by CVT using phosphorus as a dopant has recently been demonstrated in our laboratory.10 In Figure 3, the room-temperature conductivity has been plotted as a function of carrier concentration (as deduced from Hall effect studies) for a series of natural and synthetic pyrite crystals. The carrier concentration found in natural crystals varies between 1017 and 1019/cm3, whereas that of synthetic crystals appears to be restricted to the 1016-1017/cm3 range. It can be seen that the electron Hall mobility is typically a factor of 100 larger
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