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Atmospheric-Pressure Chemical Vapor Deposition of Iron Pyrite Thin Films Nicholas Berry, Ming Cheng, Craig L. Perkins, Moritz Limpinsel, John C. Hemminger, and Matt Law* multiterawatt PV deployment.[1–4] Unfortunately, pyrite devices suffer from a low Iron pyrite (cubic FeS2) is a promising candidate absorber material for open-circuit photovoltage (VOC): under earth-abundant thin-film solar cells. In this report, single-phase, large-grain, standard test conditions, the best pyrite and uniform polycrystalline pyrite thin films are fabricated on glass and photoelectrochemical and solid-state molybdenum-coated glass substrates by atmospheric-pressure chemical Schottky solar cells show large photocurvapor deposition (AP-CVD) using the reaction of iron(III) acetylacetonate rents (30–42 mA cm−2) but VOC < 200 mV and tert-butyl disulfide in argon at 300 °C, followed by sulfur annealing at (about 20% of the bandgap) and efficiencies of ≈3%.[5–10] Since 1984, a number 500–550 °C to convert marcasite impurities to pyrite. The pyrite-marcasite of studies have explored possible causes phase composition depends strongly on the concentration of sodium in the of the low VOC, including bulk or neargrowth substrate and the sulfur partial pressure during annealing. Phase surface nonstoichiometry (usually ascribed and elemental composition of the films are characterized by X-ray diffraction, to sulfur vacancies),[11–13] midgap defect Raman spectroscopy, Auger electron spectroscopy, secondary ion mass specstates that cause surface Fermi level pinning, thermionic-field emission, and large trometry, Rutherford backscattering spectrometry, and X-ray photoelectron dark currents,[11,14–16] metallic FeS-like surspectroscopy. The in-plane electrical properties are surprisingly insensitive face layers,[17,18] and domains of small-gap to phase and elemental impurities, with all films showing p-type, thermally phases in the pyrite bulk (including pyractivated transport with a small activation energy (≈30 meV), a roomrhotite, marcasite, and amorphous iron temperature resistivity of ≈1 Ω cm, and low mobility. These ubiquitous elecsulfide phases).[11,19] To date, there is no trical properties may result from robust surface effects. These CVD pyrite thin consensus as to the cause of the low VOC or the nature of the alleged gap states. films are well suited to fundamental electrical studies and the fabrication of Enhancing the photovoltage and efficiency pyrite photovoltaic device stacks. of pyrite solar cells requires basic research on the growth, structural and electronic characterization, and bulk and surface defect passivation of pyrite thin films. 1. Introduction Pyrite can be described as Fe 2+ S22−, in which the sulfur ions Iron pyrite (FeS2) is a promising photovoltaic (PV) material are paired into persulfide dimers. The crystal structure is simbecause of its suitable bandgap (Eg = 0.95 eV), strong light ilar to rock salt (space group Pa 3¯ ), with a face-centered cubic absorption (α > 105 cm−1 for hν > 1.4 eV), long minority car(FCC) sublattice of Fe2+ ions and sulfur dumbbells pointed rier diffusion lengths (100–1000 nm), and essentially infinite along the directions occupying the anion sites.[20] This elemental abundance, which makes it particularly exciting for arrangement results in a slightly distorted octahedral coordination for Fe2+, which exists in its diamagnetic d6 configuration, and tetrahedral coordination of each sulfur atom to three N. Berry iron ions and its dimer partner. The basic electronic structure Department of Physics University of California of pyrite has been the subject of extensive experimental[21–25] Irvine, CA 92697, USA and theoretical studies.[18,26–30] The top of the valence band is M. Cheng, M. Limpinsel, Prof. J. C. Hemminger, formed by the overlap of mostly nonbonding Fe 3d t2g states, Prof. M. Law while the bottom of the conduction band is composed primarily Department of Chemistry of Fe e ∗g and S ppσ∗ states. University of California Pyrite thin films have been prepared by many techniques, Irvine, CA 92697, USA including the sulfurization of iron thin films,[31–37] sputE-mail:
[email protected] tering,[38–41] flash evaporation,[42,43] electrodeposition,[44,45] spray Dr. C. L. Perkins pyrolysis,[46] molecular beam epitaxy,[47] and chemical vapor depNational Renewable Energy Laboratory osition (CVD). Pyrite growth by CVD has been studied mainly Golden, CO 8040, USA by three research groups using three different CVD chemistries DOI: 10.1002/aenm.201200043 (see Table 1). The Tributsch group at the Hahn-Meitner Institut
Adv. Energy Mater. 2012, DOI: 10.1002/aenm.201200043
© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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www.MaterialsViews.com Table 1. Pyrite thin films made by CVD. IPC, iron pentacarbonyl; TBDS, tert-butyl disulfide; TAA, thioacetamide. Chemistry
Substrate
T [°C]
P [Torr]
Composition and properties
Ref
IPC, H2S/S
Glass
130−140
760
FeS1.9,a)ρ-type,d)ρ = 2 Ω cm
[48]
IPC, TBDS
Steel, Al
480
5
–
[49]
IPC, TBDS
Glassy C
500–600
38
FeS2.0 – 2.04,b)ρ-typed)
[64]
IPC, S
Glassy C
300 + Af)
760
FeS2.0 – 2.04,b)ρ-typed)
[64]
IPC, TBDS
Various
525–630
38
IPC, TBDS
Si, GaP, ZnS
450–500
38
FeS1.99-2.01,b) no epitaxy
[53]
IPC, TBDS
Pyrite, ZnS
475
38
Epitaxy on pyrite, not ZnS; ρ = 2 Ω cm
[54]
IPC, TBDS
Glass, carbon
475
38
FeS1.98-2.02,b)ρ-type,d)ρ = 1 Ω cm
[55]
[52]
IPC, TBDS
Glass
425
38
Fe0.92Co0.078S2, η-typed),e)ρ = 0.005 Ω cm, μH = 2.4 cm2 V−1 s−1
[56]
FeCl3, TAA
Glass
450–550
760
FeS1.94,c)η-type,e)ρ = 10 Ω cm μH = 20 cm2 V−1 s−1
[57]
b)
−1 −1
epilayer, FeS1.98, η-type ρ = 10 Ω cm, μH = 280 cm V s c)
e)
2
FeCl3, TAA
Silicon
400
760
Fe(acac)3, TBDS, H2
Glass
300–340
8
FeS1.98 – 2.08,b)ρ-type,d)ρ = 1 Ω cm
[59]
Glass, Si, moly/glass
300 + Af)
760
FeS2.00 ± 0.06, ρ-type,d)ρ = 1 Ω cm
This work
Fe(acac)3, TBDS a)α
[58]
= by microprobe; b)by RBS; c)by energy dispersive sepctroscopy (EDS); d)by thermopower; e)by Hall effect; f)annealed in sulfur vapor.
in Berlin used iron pentacarbonyl (IPC) as an iron source and H2S, sulfur, tert-butyl sulfide (TBS), or tert-butyl disulfide (TBDS) as the sulfur source at temperatures of 130–600 °C at both atmospheric and low pressure.[48–55] Nominally undoped films made with IPC were reported to be p-type with low values of dark resistivity (0.1–1.0 Ω cm), thermopower (
