Pierre Cowache

One step electrodeposition of CuInSe2: Improved structural, electronic, and photovoltaic properties by annealing under high selenium pressure

Films of Cu–In–Se alloys can be electrodeposited in a wide range of controlled composition. Annealing treatments under Se pressure transform these precursor films in large grain CuInSe2 films with improved electronic properties. These modifications are shown to depend on the Se pressure imposed during the treatment allowing a certain tailoring of the electronic properties of the films. The properties of electrodeposited/selenized films are presented as obtained from luminescence measurements, Hall effect, and photoelectrochemical characterization. An efficiency of 6.5% (total area, without antireflecting coating) is reported for the best CuInSe2/CdS/ZnO solar cell. An analysis of the device is also presented where limitations by interface recombination are shown to be the dominant loss mechanism.

Atomic layer deposition of zinc oxide and indium sulfide layers for Cu(In,Ga)Se2 thin-film solar cells

Atomic layer deposition (ALD) of ZnO and indium sulfide layers has been investigated. In situ monitoring at the monolayer level has been made by using quartz crystal microgravimetry (QCM), with a special focus on extrinsic doping of ZnO with Al. Cu(In,Ga)Se2/In2S3 (ALE)/ZnO (ALE) cells present efficiencies up to 13.5%. Indium sulfide layers used in these cells are characterized by a high band-gap value (up to 3.3 eV). They possess an amorphous like structure and a composition close to In2S3 as determined by Rutherford Back Scattering measurements. Lowering of the band-gap and crystallization take place under annealing, indicating that the high band-gap value of ALE indium sulfide layer is most likely related to structural effects.

Cadmium-free buffer layers deposited by atomic later epitaxy for copper indium diselenide solar cells

As a soft and highly controllable deposition technique, atomic layer epitaxy (ALE) is well suited to deposit buffer and window layers on CIS thin solar films with high interface quality. In this work we have investigated ALE buffer layers of zinc oxysulfide, indium sulfide and aluminum oxide deposited at low temperature (160°C). The most promising results have been obtained with using indium sulfide buffer layers, with a record efficiency of 13.5% (30.6 mA /cm2, 604 mV, FF=0.73 under 100 mW /cm2, without AR coating) achieved on a standard CIGS absorber. This opens a route for a dry cadmium-free buffer process fully compatible with the other vacuum deposition techniques (coevaporation, sputtering).

Investigation of the influence of the electrodeposition potential on the optical, photoelectrochemical and structural properties of as-deposited CdTe

Abstract The electrodeposition of cadmium telluride on cadmium sulphide deposited from a chemical bath is investigated with the aim of improving the structural and optical properties of the CdTe layers for solar cell applications. The CdTe layers are characterized by X-ray diffraction spectroscopy, optical transmission measurements and photoelectrochemical measurements. The influence of the electrodeposition potential on the conductivity type of the deposited CdTe is demonstrated. Spectral response measurements are used as an analytical method to study the optoelectronic properties of CdTe. Thickness-dependent spectral responses reflect a change of doping type for electrodeposition potentials more positive than +5 mV vs. the deposition potential of elemental cadmium. A front- and back-side illumination technique is used to determine the position of the active junction which varies between a n-CdS/p-CdTe and a n-CdTe|electrolyte junction. The width of the space charge layers and the doping concentrations are determined by using the Gartner equation for fitting the quantum efficiency vs. absorption coefficient plots.

Modelling of the structure of CdS thin films

Abstract The structure of CdS thin films produced by chemical bath deposition for window layers for photovoltaic devices has been investigated using the techniques of standard θ –2 θ X-ray diffraction (XRD) and glancing angle XRD (GAXRD). The relative peak intensities observed in the diffraction patterns were consistent neither with a cubic nor a hexagonal microstructure. By employing both diffraction geometries on the same sample it was established that the CdS layers were un-textured, thus a structure different from either the cubic or hexagonal modifications was inferred. In order to elucidate the crystalline structure of the material a Rietveld analysis program was employed, together with a special program written to generate appropriate input parameter files. It was found that a good match between experimental and simulated patterns could be obtained by postulating a polytype CdS structure, consisting of nearly random stacking sequences of the hexagonal planes that form the basis for both the cubic (zinc-blende) and hexagonal (wurzite) modifications. The GAXRD results indicate that little structural change occurs in the polytype CdS upon annealing in air, despite a pronounced colour change. The structural difference between films deposited under different conditions is much more pronounced than that between as-deposited and annealed material. The results are discussed with reference to investigations by other authors of CdS thin films and of CdS nanoparticles using the techniques of transmission electron microscopy and/or X-ray or electron diffraction.

Structural, optical and transport properties of Ag2S films deposited chemically from aqueous solution

Abstract Ag 2 S films were deposited chemically from silver nitrate and thiourea aqueous solutions at low temperature (40–80°C). X ray diffraction shows that the films are well crystallized with the acanthite structure. They display good optical properties with a direct band gap in the range 0.9-1.05 eV. These properties are improved by a thermal annealing treatment in nitrogen at 250°C for 1 h, with an increase in the band gap up to 1.07 eV. The effective mobility of carriers was measured by time-resolved microwave photoconductivity (TRMC). Values around 5–10 cm 2 V −1 s −1 were estimated. A dependence on the deposition temperature is shown, with an optimal value around 60°C.

Cathodic Codeposition of Cadmium Telluride on Conducting Glass

Films of cadmium telluride up to 5 μm thick were electrodeposited on glass plates covered with a conductive tin oxide layer sensitized by forming a thin layer of tin through cathodic prereduction. Castaings probe analysis indicates that the stoichiometric ratio Cd/Te is close to unity in a narrow potential deposition range

Junction Formation Studies of One‐Step Electrodeposited CuInSe2 on CdS

Copper indium diselenide (CIS) is electrodeposited in a single step on chemical bath deposited cadmium sulfide layers in order to form a superstrate cell structure. The influence of the electrodeposition potential on junction formation is studied by solid-state photocurrent voltage and photocurrent spectral responses. Secondary ion mass spectroscopy depth profiling shows that cadmium and sulfur diffusion from the CdS layer into the CIS layer is small up to annealing temperatures of 320 C. The diffusion process is found to be related to the composition of the CIS film, which in turn is determined by the electrodeposition potential. Interdiffusion processes are hindered in indium-rich CIS films. A preliminary CIS superstrate cell characteristic is presented yielding an efficiency of 1.5%.

Wet treatment based interface engineering for high efficiency Cu(In,Ga)Se2 solar cells

Using atomic layer epitaxy (ALE) which is a soft deposition technique for ZnO it is possible to avoid the deposition of thick buffer layers by chemical bath deposition (CBD) and wet treatments can be almost reduced to surface treatments. In this work new electrochemical and chemical treatments have been designed with the objective of surface passivation and surface doping by using solutions with different reactivities (via pH, complexing agents, metallic cations). ZnO layers are then deposited by ALE to complete the junctions. The results show relations between the interface treatment and the cell characteristics. Efficiencies comparable and in some cases higher than those of the reference cells made with CBD CdS and sputtered ZnO have been obtained (up to 12.7% with indium treatments).

Indium-Based Interface Chemical Engineering by Electrochemistry and Atomic Layer Deposition for Copper Indium Diselenide Solar Cells

The key to achieve better Cu(In, Ga)Se2 (CIGS) cells is through the improvement of the CIGS/ZnO interface. In this work, we illustrate various approaches, wet and dry, to engineer that interface with processes that avoid the use of Cd containing compounds. Wet chemical treatments have been performed so as to test the possibility to improve that interface by surface doping of CIGS. X-ray photoelectron spectroscopy (XPS) and Kelvin probe studies show that such doping is not achieved in the conditions leading to best devices. Rather, the most desirable feature of the surface treatments appears to be surface passivation. We show that this can be achieved via CIGS surface reaction with In(III) ions, leading to 12.5% efficient devices. A well passivated interface can also be achieved directly, using an all dry process, by Atomic Layer Deposition (ALD) of In2S3 buffer layer, yielding to 13.5% efficient devices. The ALD growth of the buffer layers have been studied in situ with the help of a quartz crystal microgravimetry.