Jean-François Guillemoles

Comparison of optical and electrochemical properties of anatase and brookite TiO2 synthesized by the sol–gel method

Three polymorphs of TiO2, rutile, anatase and brookite, are well known. Each variety has its own physical properties, bandgap, surface states, etc. For photovoltaic applications, anatase is the phase most used. Up to now, the third polymorph of TiO2, brookite, could not be obtained by soft chemistry methods. We have now synthesized nanometric particles of pure brookite by ‘chimie douce’ in aqueous solution. For the first time, this phase will be characterized in view of its possible application in dye-sensitized cells. We discuss the potential of brookite as a good candidate for photovoltaic devices in comparison to other phases, in terms of its morphology, bandgap and electrochemical properties in water and acetonitrile before sensitization.

Oxygenation and air-annealing effects on the electronic properties of Cu(In,Ga)Se2 films and devices

Post-deposition air-annealing effects of Cu(In,Ga)Se2 based thin films and heterojunction solar cell devices are studied by photoelectron spectroscopy and admittance spectroscopy. Ultraviolet photoelectron spectroscopy reveals type inversion at the surface of the as-prepared films, which is eliminated after exposure of several minutes to air due to the passivation of surface Se deficiencies. X-ray photoelectron spectroscopy demonstrates that air annealing at 200 °C leads to a decreased Cu concentration at the film surface. Admittance spectroscopy of complete ZnO/CdS/Cu(In,Ga)Se2 heterojunction solar cells shows that the Cu(In,Ga)Se2 surface type inversion is restored by the chemical bath used for CdS deposition. Air annealing of the finished devices at 200 °C reduces the type inversion again due to defect passivation. Our results also show that oxygenation leads to a charge redistribution and to a significant compensation of the effective acceptor density in the bulk of the absorber. This is consistent wi...

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.

Intermediate band solar cells: Recent progress and future directions

Extensive literature and publications on intermediate band solar cells (IBSCs) are reviewed. A detailed discussion is given on the thermodynamics of solar energy conversion in IBSCs, the device physics, and the carrier dynamics processes with a particular emphasis on the two-step inter-subband absorption/recombination processes that are of paramount importance in a successful implementation high-efficiency IBSC. The experimental solar cell performance is further discussed, which has been recently demonstrated by using highly mismatched alloys and high-density quantum dot arrays and superlattice. IBSCs having widely different structures, materials, and spectral responses are also covered, as is the optimization of device parameters to achieve maximum performance.

Comparative investigation of solar cell thin film processing using nanosecond and femtosecond lasers

The purpose of the present study was to examine the possibility of laser-machining of CuInSe2-based photovoltaic devices. Therefore, ablation thresholds and ablation rates of ZnO, CuInSe2 and Mo thin films have been measured for irradiation with nanosecond laser pulses of ultraviolet and visible light and subpicosecond laser pulses of a Ti : sapphire laser. The experimental results were compared with the theoretical evaluation of the samples heat regime obtained from numerical calculations. In addition, the photo-electrical properties of the solar cells were measured before and after laser-machining. Scanning electron microscopy and energy dispersive x-ray analyses were employed to characterize the laser-induced ablation channels. As a result, two phenomena were found to limit the laser-machining process: (i) residues of Mo that were projected onto the walls of the ablation channel and (ii) the metallization of the CuInSe2 semiconductor close to the channel. Both effects lead to a shunt in the device that decreases the photovoltaic efficiency. As a consequence of these limiting effects, micromachining of CuInSe2-based solar cells was not possible with nanosecond laser pulses. Only subpicosecond laser pulses provided selective or complete ablation of the thin layers without a relevant change in the photoelectrical properties.

Hot carrier solar cells: Achievable efficiency accounting for heat losses in the absorber and through contacts

The hot carrier solar cell enables the photovoltaic efficiency limit to be approached by tapping into what are normally heat losses. Previous models neglected thermalization in the absorber and assumed ideally energy selective contacts that allow minimum heat losses upon carrier extraction. The proposed improved model includes both realistic contacts and thermalization rates. The heat flux due to carrier extraction is computed. Results show that spectrally broad semiselective contacts are compatible with an efficiency exceeding the single junction limit, which would considerably facilitate the realization of the device.

High resolution XPS studies of Se chemistry of a Cu(In, Ga)Se2 surface

Chemistry of co-evaporated Cu(In, Ga)Se2 (CIGS) surfaces submitted or not to chemical treatments was investigated by high resolution XPS. The surface analysis allowed us to compare the surface composition with the bulk one as a function of the treatments. We also studied too several standard compounds as Cu2−xSe, In2Se3, ZnSe, Se0 and CdSe. A Se XPS signal specific of the CIGS surfaces was identified. In this paper, we present a detailed study of Se signal in CIGS and show of the different spectroscopic contributions can be separated using standards information. Then, we discuss the chemical origin of these signals and the implications for the device processing.

Defects in Cu(In, Ga) Se2 semiconductors and their role in the device performance of thin-film solar cells

This contribution is a summary of an international, interdisciplinary workshop dedicated to defects in chalcopyrite semiconductors and their relation to the device characteristics of thin-film solar cells, held on 3–5 June 1996 in Oberstdorf, Germany. Results of different characterization methods were brought together to identify common observations. The comparison of results from electrical defect spectroscopy and luminescence investigations confirmed the presence of energetic distributions of defects throughout the bandgap of chalcopyrite thin films. Electrical defect spectroscopy detects a defect about 280 meV above the valence band edge of Cu(In, Ga)Se2 regardless of the preparation conditions of the sample. In a solar cell the density of this defect depends on the operation conditions. This observation might be related to the migration of copper in an electric field, which occurs even at room temperature. Other defects appear to be related to processing or impurities. Photoluminescence decay measurements yield time constants of several nanoseconds under low injection conditions. Modelling of the current–voltage characteristics of Cu(In, Ga)Se2-based thin-film cells suggests that compensating acceptor states in the CdS or at the heterointerface are responsible for the frequently observed cross-overs between the dark and illuminated curves.© 1997 John Wiley & Sons, Ltd.

Contactless mapping of saturation currents of solar cells by photoluminescence

We report in this letter the contactless measurement of spatially resolved photocurrent–photovoltage relationship. The method is based on hyperspectral imaging, from which we record cartography of absolute photoluminescence spectra from solar cells. Using the generalized Planck’s law, it is therefore possible to derive the quantitative value of the quasi-Fermi levels splitting, related to the voltage over the junction. It allows us to directly extract optoelectronics properties of the device with a solely optical method. As a proof of concept, we derive saturation currents of a GaAs solar cell and find a good agreement with the standard electrical measurements.

Ab initio calculation of intrinsic point defects in CuInSe2

Abstract Preliminary ab initio calculation of different point defects energy and electronic density of states have been performed on the prototype chalcopyrite semiconductor CuInSe 2 . The simulation method used is based on the density functional theory within the framework of pseudo-potentials and plane waves basis. The isolated neutral defects considered are: V Cu , V In , V Se , Cu i , In i , In Cu , Cu In and the complex defects are 2Cu i +Cu In , In Cu +Cu In and 2V Cu +In Cu , some of which being computed for the first time by advanced ab initio techniques. In agreement with previous results, we show that some point defects (such as V Cu ) and pair defects (2V Cu +In Cu ) have very low formation energies. Some energies of formation were found significantly lower than previous estimations. The comparison of the formation energies with the exchange correlation (LDA or GGA) is discussed. The perturbation induced by the presence of some of these ideal defects on the density of states is also presented.