Isabelle Gerard

Pore Formation on n-InP(100) in Acidic Liquid Ammonia at 223 K

For the first time, pore formation on n-InP(100) has been carried out by galvanostatic treatments in acidic liquid ammonia at 223 K. Voltage oscillations correlated to a specific current line oriented pore morphology have been evidenced by scanning electron microscopy. Whatever the anodic charge, a constant pore depth was formed (2-3 μm). Porous layers have been characterized by ex situ photoluminescence measurements that have revealed a dead layer behavior. This work demonstrates the crucial role of interfacial phenomena illustrated by the use of this uncommon nonaqueous electrolyte.

Thin anodic oxides on n-InP studied by photocurrent transients and surface analysis

Investigation of semiconductor/electrolyte interface modified by superficial oxides has been performed with photocurrent transients and surface analysis. Studies of the photocurrent transients before and after the modification of the semiconductor surface exhibit the electrical properties of the interface, whereas XPS analysis gives access to the chemical aspect. This work evidenced a correlation between the level of photocurrent, the surface oxidation and the texture (porosity, thickness) of the oxide layer.

Insights on the influence of surface roughness on photovoltaic properties of state of the art copper indium gallium diselenide thin films solar cells

The influence of Cu(In,Ga)Se2 (CIGSe) surface roughness on the photovoltaic parameters of state of the art devices is reported, highlighting the importance of the roughness of the as-grown CIGSe absorbers on solar cell efficiencies. As-grown CIGSe surface is progressively smoothed using a chemical etch, and characterized by SEM, AFM, XPS, μ-Raman spectroscopy, x-ray diffraction (XRD), and reflectivity. The decrease of roughness has no marked influence on crystal structure and surface composition of the absorber. The main effect is that the total reflectivity of the CIGSe surface increases with decreasing roughness. The samples are processed into solar cells and characterized by current-voltage measurements. While the open circuit voltage (Voc) and fill factor remain constant, the short circuit current (Jsc) decreases markedly with decreasing roughness, resulting in a reduction of the solar cell efficiency from 14% down to 11%, which exceeds the expected decrease from increased reflectivity. Quantum effici...

Investigation of oxide growth and stability on n-GaAs and n-InP by coupling transient photocurrent and surface analysis

Abstract Investigation of oxide chemistry and reactivity of III–V semiconductors has been undergone with an electrochemical approach coupled with surface analysis. Growth and stability of anodic oxides obtained on n-GaAs and n-InP was investigated with potensiostatic mode under illumination at pH 9. For the two compounds, the transient photocurrent, the recovery of the photocurrent and XPS analysis were performed in the same conditions. InP and GaAs present very different behaviours, which are linked to the stability of the oxidised interface. InP evidences a thin, stable and electrically blocking oxide film, while GaAs exhibits a thicker, unstable and porous oxide layer. This work point out that XPS analysis has to be coupled with in situ techniques as transient photocurrent, to understand accurately the oxide growth mechanism.

Study of a thin anodic oxide on n-InP by photocurrent transient, capacitance measurements and surface analysis

Abstract Investigation of the anodic oxide on n-InP has been made with an approach coupling photocurrent transient and capacitance measurements in the dark, associated to chemical surface analysis by photoelectron spectroscopy (XPS). Growth, formation and stability of anodic oxides obtained on n-InP were investigated under illumination at pH 9. A potentiostatic mode has been preferred to explore the chemical evolution of the interface. This study shows that the decay of the photocurrent transients, the modification of the capacitances and the chemical evolution of the surface composition are totally related. The growth of a thin and stable oxide provides a blocking behaviour of the oxidised n-InP toward the photo-transfer. Moreover, our study evidenced a formation and growing of a mixed In-P oxide layer, in three stages.

Photodissolution of n-GaAs electrodes under laser illumination: control of the etching profile

The etching profiles of n-GaAs electrodes are controlled under laser illumination and anodic polarization. Depending on the incident light power and pH of the solution two etching profiles are obtained: at strong alkali or acid pH the photon flux limits the photodissolution of GaAs. The etching profile is Gaussian, reproducing the photon distribution of the laser. This etching profile is associated with an immediate and constant photocurrent. At intermediate pH, the growth of an oxide layer limits the photodissolution of n-GaAs. Therefore we obtained a flat bottom etching profile related to a decrease of the photocurrent to a steady state.

Modelling of n-type CdTe photoluminescence variation with polarization: a probe of the shift of semiconductor band edges

Abstract Photoluminescence techniques are used as a probe of energetic surface band edge configuration for n-type CdTe in the presence of Ce 4+ ions. The band shift observed for a part of the potential range is perfectly detected by photoluminescence. The results agree with those obtained from capacitance measurements. The photoluminescence response has been modelled according to the dead layer model, using as fitting parameters the donor density N D and the flat band potential V FB . The best agreement between our experimental results and those predicted by the model is obtained with the donor density derived from the capacitance measurements.

Broadband light-trapping in ultra-thin nano-structured solar cells

Conventional light trapping techniques are inefficient at the sub-wavelength scale. This is the main limitation for the thickness reduction of thin-film solar cells below 500nm. We propose a novel architecture for broadband light absorption in ultra-thin active layers based on plasmonic nano-cavities and multi-resonant mechanism. Strong light enhancement will be shown numerically for photovoltaic materials such as CIGSe and GaAs. First experiments on ultrathin nano-patterned CIGSe solar cells will be presented.

Pyrolysis of coal tar pitch with graphite-MoCl5 intercalation compound

Coal tar pitch (CTP) was pyrolysed in the presence of graphite-MoCl5 intercalation compound (MoCl5-GIC). Only a third-stage GIC could be obtained using the classical two-temperature method. GIC concentrations in the starting mixtures were 2, 5 and 10 wt%. The samples were characterized by elemental analysis, X-ray diffraction and scanning electron microscopy. The distribution of GIC particles in the CTP matrix is homogeneous and their presence leads to a significant increase in green coke yield on pyrolysis, in agreement with many published results. The influence of heating rate on the green coke yield was also studied: the lower the heating rate, the higher the yield. After thermal treatment at 750°C, molybdenum is always present in the green coke at a content close to the initial one, whereas most of the chlorine has been outgassed. It appears that CTP-MoCl5-GIC mixtures have a behaviour similar to that of CTP-FeCl3-GIC mixtures on pyrolysis. The final aim of this work is to synthesize catalysts supported on carbonaceous materials.

Enrichment in tellurium during photodissolution on n-CdTe in sulfuric acid solution

Dissolution of n-type CdTe samples has been investigated under illumination in 2 M H2SO4 solution. A Te0 layer is formed which leads to a decay of the photocurrent. This enrichment has been investigated by cyclic voltammetry and by X-ray photoelectron spectroscopy (XPS). Photons are absorbed within the Te0 layer leading to a decrease of the photocurrent and to a characteristic modification of the spectral response that can be used as an in situ probe of the presence of such a layer.