Transformation of amorphous iron oxide thin films, pre-deposited by spray pyrolysis, into FeS2 and FeSe2 layers for solar cells applications Beya Ouertani (1) , Moncef Saadoun (2), Brahim Bessais (2) and Hatem Ezzaouia (1) (1) Laboratory of Semiconductors, Nanostructures and Advanced Technology (LSNTA), , Borj-Cedria Science and Technology Park, BP 95, 2050 Hammam-Lif, Tunisia (2) Photovoltaic Laboratory, Research and Technology Centre of Energy, Borj-Cedria Science and Technology Park, BP 95, 2050 Hammam-Lif, Tunisia
Abstract: Iron oxide thin films were deposited by a simple, non-toxic and low cost method: spray pyrolysis of FeCl 3.6H2O (0.03 M)-based aqueous solution onto glass substrates heated at 350 °C. Rust red amorphous films were obtained. After annealing in vacuum-sealed tube (~10−4 Pa) for 5 h at different temperatures ranging from 250 °C to 400 °C, Well crystallized Fe2O3 films were obtained. In a first step, sulphuration at 450 °C for 6 h under vacuum (10-4 Pa) transforms the amorphous iron oxide films into FeS2-pyrite films having good crystallinity. SEM images show that the FeS2 films are granular. The homogeneity and both dimension and distribution of the FeS2 grain sizes were found to depend on the sulphuration temperature of the predeposited amorphous iron oxide films. First optical analyses enabled us to deduce a large absorption coefficient (~10000 cm-1) and a direct energy band gap of about 0.73 eV. All the prepared FeS2 films show p-type conduction. In a second step, the amorphous iron oxide films were heat treated under a selenium atmosphere (10-4 Pa) at different temperatures for 6 h. X-ray diffraction (XRD) was used to investigate the structure of the obtained films. Single FeSe2-phase films having good crystallinity were obtained at a selenisation temperature of 550 °C. Optical analyses of the FeSe2 films obtained at 550 °C enabled us to deduce a large absorption coefficient (~100000 cm-1). Surface scanning electron microscopy (SEM) observations show inhomogeneous films. Electrical conductivity of the as-prepared films was measured at high and low temperatures. In a third step, in the aim of increasing the band gap value of FeS2-pyrite thin films having good crystallinity, high absorption coefficient (∼100000 cm−1) and a band gap of about (0.73 eV), we drawed attention to the fabrication of these films after alloying with Ru. We followed two methods: the first one consists of spraying aqueous RuCl3·3H2O solution, during shorter time, on heated pre-deposited oxide layer at the same spray conditions with molar ratio as RuCl3·3H2O:FeCl3·6H2O=x:1−x (x =0.3966, 0.1586, 0.0396, 0.0317, 0.0156 and 0.00). The second consists of spraying on heated substrate, an aqueous solution prepared by dissolving ferric chloride (FeCl3·6H2O) and Ruthenium(III) chloride hydrate (RuCl3·3H2O) with molar ratio as RuCl3·3H2O:FeCl3·6H2O=x:1−x (x=0.3171, 0.1586, 0.234, 0.0119, 0.0051, 0.0025 and 0.00). Afterward, the as obtained films are sulphured at the optimum conditions (pressure ≅ 10−4 Pa, duration =6 h, temperature =450 °C). Dark layers having granular structure, were obtained. The effect of alloying on atomic structure, as well as optical properties of Ru-alloyed FeS2-pyrite films were examined by XRD, optical and MEB characterisations. Our results show that the band gap value of Fe1−xRuxS2 layers increases versus the alloy percentage. An optimum band gap value was obtained according to the first method of about 1.48 eV for x=0.0156; which is considered a very interesting result for the photovoltaic applications of our films. An increase of the band gap value versus the Ru concentration with the second method was observed, as well.
*Fabrication of iron oxide films
extractor hood
Solution Pump
flowmeter
Jet
Hot
Substrats
Heat treatment (~ 10-4 Pa)
plate
Agitator •Power supply •Temperature regulation
wustite (FeO), hematite (α-Fe2O3), maghemite (γ-Fe2O3), magnetite (Fe3O4))
N2
refractory brick
during 5 hours
carrier gas
Amorphous iron oxide layer
At 350°C
*Transformation of amorphous iron oxide films into FeS2 and FeSe2 films Sulphuration (~ 10-4 Pa)
5 4
FeS2 Selenization (~ 10-4 Pa)
3
0
0.65
0.70
0.75
8
0.80
h (eV)
0.85
0.90
0.65 0.70 0.75 0.80 0.85 0.90 0.95 1.00 h (eV)
0.95
(a)
FeSe2
(ah)2 (eV.cm-1)2
6 a (104.cm-1)
during 5 hours At 550°C
1 é é 1 (1 - R ) 2 é (1 - R) é2 a = Ln é +é +R 2 é é 2 é 2T d é 4T é é é é
a (104.Cm-1 )
during 5 hours At 450°C
Amorphous iron oxide layer
(ah)2(eV.cm-1)2
6
4 2 0
E ~ 1.03eV 0.6
0.8
h (eV)
1.0
1.2
0
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4
FeOOH (T<250°C) Fe2O3 (300°C<T<600°C) Fe3O4 (T ~600°C)