Cu 2 (Sn,Si)S 3 is a potential material for obtaining cost effective and non-toxic absorber in th... more Cu 2 (Sn,Si)S 3 is a potential material for obtaining cost effective and non-toxic absorber in thin films solar cell devices. Furthermore tunable bandgap can be achieved in the whole optimal range 1.1 eV-1.5 eV depending on the Si/(Sn+Si) ratio. We report on the synthesis of thin Cu 2 (Sn,Si)S 3 films by a cosputtering-annealing route starting from Cu-Sn-Si precursors. The films consisted of densely packed grains. Combination of X-ray diffraction and Raman spectroscopy confirmed the formation of a Cu 2 Sn 0.7 Si 0.3 S 3 solid solution with a monoclinic structure. The presence of Cu 2 SnS 3 was also evidenced near the substrate and could be due to the formation kinetic of Cu 2 SnS 3 and Cu 2 (Sn,Si)S 3. Optical and electrical measurements showed that the substitution of 30 % Sn by Si leads to a band gap increasing from 0.9 eV to 1.2 eV while maintaining the p-type conductivity. Based on these results, It can be concluded that the Cu 2 (Sn,Si)S 3 compound is a promising absorber to be used in thin film solar cell applications. Keywords: Copper silicide; Cu 2 ZnSnS 4 ; Cu 2 ZnSiS 4 ; Cu 2 Zn(Sn,Si)S 4 ; Wide band gap; Solar energy materials Highlights Cu-Sn-Si-S films are synthetized by a co-sputtering and annealed route// Cu 2 Sn 0,7 S i0,3 solid solution crystallizes in the monoclinic structure// The films show both band gap of 1,2 eV and p-type conductivity// Cu 2 (Sn,Si)S 3 is a promising absorber for using thin film solar cells applications //
Macroporous layers are grown onto n-type silicon by successive photoelectrochemical etching in HF... more Macroporous layers are grown onto n-type silicon by successive photoelectrochemical etching in HF containing solution and chemical etching in KOH. This specific latter treatment gives highly antireflective properties of the Si surface. The duration of the chemical etching is optimized to render the surface as absorbent as possible and the morphology of the as-grown layer is characterized by scanning electron microscopy. Further functionalization of such structured Si surface is carried out by atomic layer deposition of a thin conformal and homogenous TiO 2 layer that is crystallized by an annealing at 450°C. This process allows using such surfaces as photoanodes for water oxidation. The 40 nm-thick TiO 2 film acts 2 indeed as an efficient protective layer against the photocorrosion of the porous Si in KOH, enhances its wettability and enlarge the light absorption of the photoelectrode. The macroporous Si has a beneficial effect on water oxidation in 1 M KOH and leads to a considerable negative shift of onset potential of ~400 mV as well as a 50 % increase in photocurrent at 1 V vs SCE.
Atomic Layer Deposition (ALD) of TiO 2 thin films on a Si substrate has been investigated using t... more Atomic Layer Deposition (ALD) of TiO 2 thin films on a Si substrate has been investigated using titanium isopropoxide (TTIP) and tetrakis(dimethylamino)titanium (TDMAT) in combination with water. The deposition rate and the chemical stability of the films are significantly different depending on the Ti precursor and process temperature (T ALD). A significant thickness shrinkage when the films are annealed is 1 reported for the first time on TiO 2. Comprehensive analysis of the films with XPS, FTIR, ellipsometry and porosimetry demonstrates that some precursor ligands are incorporated (most likely as isopropanol) when ALD is performed at low temperature (i. e. T ALD < 200 • C) using TTIP. The trapped ligand molecules can be removed by annealing, but make the film porous and thus have a detrimental effect on the dielectric properties. Higher quality non-porous films are grown by using TTIP at T ALD ≥ 200 • C or by using TDMAT. It is shown that measuring the refractive index is a simple, nondestructive and reliable way to determine film quality. Numerical simulations of ligand coverage show that the measured growth rates are consistent with a self-limiting ALD mechanism, albeit with partial incorporation of ligands from TTIP at low temperature (T ALD < 200 • C), which renders part of the surface inactive towards growth. Aside from this, the higher growth rate of TDMAT is due to more desorption of ligands during the Ti precursor pulse. The overall decrease in growth rate with temperature is related quantitatively to decreasing coverage of hydroxyl groups on TiO 2. Comparing the TTIP and TDMAT processes in this way reveals new aspects of the gas-surface chemistry during self-limiting ALD and how this affects film morphology and electrical properties.
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Papers by Maïmouna Diouf