Siao Li Liew

Photovoltaic characteristics of p-β-FeSi2(Al)/n-Si(100) heterojunction solar cells and the effects of interfacial engineering

Heterojunction solar cells with Al-alloyed polycrystalline p-type β-phase iron disilicide [p-β-FeSi2(Al)] on n-Si(100) were investigated. The p-β-FeSi2(Al) was grown by sputter deposition and rapid-thermal annealing. Photocurrent of ∼1.8 mA/cm2 and open-circuit voltage of ∼63 mV were obtained for p-β-FeSi2(Al)/n-Si(100)/Ti/Al control cells with indium-tin-oxide (ITO) top electrode. Open-circuit voltage increased considerably once thin Al layer was deposited before amorphous-FeSi2(Al) deposition. Furthermore, device performances were found to improve significantly (∼5.3 mA/cm2 and ∼450 mV) by introducing germanium-nitride electron-blocking layer between ITO and p-β-FeSi2(Al). The improvement is attributed to the formation of epitaxial Al-containing p+-Si at p-β-FeSi2(Al)/n-Si(100) interface and suppressed back-diffusion of photogenerated electrons into ITO.

Texture of NiGe on Ge(001) and its evolution with formation temperature

Texture of NiGe on Ge(001) and its evolution with formation temperature have been investigated. Pole figure investigation showed that NiGe formed by rapid thermal annealing of Ni(35nm)∕Ge(001) largely consists of epitaxial grains with orientation relationships: NiGe(111)[01¯1]∕∕Ge(001)[110],NiGe(020)[001]∕∕Ge(001)[100],NiGe(201)[102¯]∕∕Ge(001)[110],NiGe(211)[011¯]∕∕Ge(001)[110],NiGe(112)[201¯]∕∕Ge(001)[110], and NiGe(210)[001]∕∕Ge(001)[100]. For NiGe formed at 400 °C, NiGe(111)[01¯1]∕∕Ge(001)[110],NiGe(020)[001]∕∕Ge(001)[100],NiGe(201)[102¯]∕∕Ge(001)[110], and NiGe(211)[011¯]∕∕Ge(001)[110] were found to be the preferred orientations, while NiGe formed at 600 °C was dominated by NiGe grains with NiGe(111)[01¯1]∕∕Ge(001)[110] orientation. The increasing dominance of the grains with NiGe(111)[01¯1]∕∕Ge(001)[110] orientation is attributed to the minimum lattice mismatch with this orientation.

Impact of Al Passivation and Cosputter on the Structural Property of β-FeSi2 for Al-Doped β-FeSi2/n-Si(100) Based Solar Cells Application

The aluminum (Al) doped polycrystalline p-type β-phase iron disilicide (p-β-FeSi2) is grown by thermal diffusion of Al from Al-passivated n-type Si(100) surface into FeSi2 during crystallization of amorphous FeSi2 to form a p-type β-FeSi2/n-Si(100) heterostructure solar cell. The structural and photovoltaic properties of p-type β-FeSi2/n-type c-Si structures is then investigated in detail by using X-ray diffraction, Raman spectroscopy, transmission electron microscopy analysis, and electrical characterization. The results are compared with Al-doped p-β-FeSi2 prepared by using cosputtering of Al and FeSi2 layers on Al-passivated n-Si(100) substrates. A significant improvement in the maximum open-circuit voltage (Voc) from 120 to 320 mV is achieved upon the introduction of Al doping through cosputtering of Al and amorphous FeSi2 layer. The improvement in Voc is attributed to better structural quality of Al-doped FeSi2 film through Al doping and to the formation of high quality crystalline interface between Al-doped β-FeSi2 and n-type c-Si. The effects of Al-out diffusion on the performance of heterostructure solar cells have been investigated and discussed in detail.

Phase and Texture of Er–Germanide Formed on Ge ( 001 ) Through a Solid-State Reaction

The phase and texture of of Er-germanide formed on Ge(001) through a solid-state reaction between Er thin films and Ge(001) via rapid thermal annealing (300-600°C) were investigated. It was found that amorphous ErGe 1.2 forms at 300°C, followed by the formation of AlB 2 hexagonal ErGe 1.5 at 400-500°C and then orthorhombic ErGe 1.8 at 600°C. X-ray diffraction pole figure measurement revealed that the ErGe 1.5 5 films formed at 400-500°C consist predominantly of epitaxial grains with an orientation relationship of ErGe 1.5 (1100)[0001]//Ge(001)[011], although the presence of a considerable amount of epitaxial grains with an orientation relationship of ErGe 1.5 (1012)[0110]//Ge(001)[010] was also observed in the film formed at 400°C. The germanide film formed at 600°C was found to consist of randomly orientated orthorhombic ErGe 1.8 grains. Among the three different germanide phases, ErGe 1.5 showed minimum resistivity values as low as ∼ 19 μΩ cm.

Effect of La-Doping on optical bandgap and photoelectrochemical performance of hematite nanostructures

La-doped hematite nanotubes are fabricated by electrospinning of a sol–gel solution consisting of La(III) acetylacetonate hydrate/polyvinylpyrrolidone(PVP)/ferric acetylacetonate, and subsequent sintering at 500 °C for 5 h in air. Further grinding of these nanotubes affords La-doped hematite nanoparticles. FESEM EDX indicates that the La content is 3.66 mol% in La-doped hematite. HRTEM and XRD reveal that La3+ cations are doped into the hematite crystal lattice. UV-Vis diffuse reflectance shows increased light absorption for La-doped hematite, with the bandgap reduced from 2.58 eV to 2.46 eV. EIS and four-probe characterization demonstrate that La-doping reduces charge transfer resistance and increases the electrical conductivity, thus leading to improved charge transportation. Photoelectrochemical (PEC) water splitting studies show that under 100 mW cm−2 simulated solar irradiation, La-doped hematite nanoparticles demonstrate a net photocurrent density up to 0.112 and 0.270 mA cm−2 at 1.23 and 1.60 V vs. RHE, which are 187% and 63% higher than pristine hematite nanoparticles, respectively. The effect of La-doping on improving electrical conductivity, light absorption, and PEC performance is mainly attributed to the intensification of crystal orientation along the (110) plane and the lattice expansion caused by the La3+ cations, which have much larger radii and are more electron-rich than Fe3+.

Studies into the Yb-doping effects on photoelectrochemical properties of WO3 photocatalysts

Yb-doped WO3 photocatalysts were prepared by co-sputtering Yb and WO3 on FTO glass and then sintering at elevated temperatures in air. At sintering temperatures between 450 and 550 °C, the photocurrent densities of Yb-doped WO3 photocatalysts were higher and more stable than those of pristine WO3 photocatalysts. The Yb-doping effects on the photoelectrochemical properties were investigated using Raman, transient absorption and electrochemical impedance spectroscopies. Compared to pristine WO3 photocatalysts, Yb-doped photocatalysts sintered at up to 550 °C consist of sub-stoichiometric WO3−x due to the substitution of W6+ cations with Yb3+ dopants, displaying shorter electron–hole recombination times and higher levels of donor densities.