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We analyze photoluminescence (PL) and electroluminescence (EL) using a hyperspectral imager that records spectrally resolved luminescence images of solar cell absorbers. The system is calibrated to yield the luminescence flux in absolute values. This system enables to quantitatively image physical parameters such as the photovoltage with an uncertainty of less than 30mV. The wide field illumination, low power excitation and fast acquisition brings new insights compare to classical setups such as confocal microscope. Several types of absorbers have been analyzed. For instance, we can investigate spatial fluctuations of the Quasi Fermi Levels splitting in CIGS polycristalline absorbers and link those fluctuations to transport properties. The method is general to the point that third generation PV cells absorbers can also be evaluated. We illustrate the great potential of our setup by imaging quasi Fermi levels splitting in Intermediate Band Solar cells. Such techniques, directly evaluating the performance of photovoltaic absorbers and devices are needed for fast, high throughput investigations of combinatorial experiments such as the projects carried out for the material genomics programme.
We analyze photoluminescence (PL) and electroluminescence (EL) using a hyperspectral imager that records spectrally resolved luminescence images of solar cell absorbers. The system is calibrated to yield the luminescence flux in absolute values. This system enables to quantitatively image physical parameters such as the photovoltage with an uncertainty of less than 30mV. The wide field illumination, low power excitation and fast acquisition brings new insights compared to classical setups such as confocal microscope. Several types of absorbers have been analyzed. For instance, we can investigate spatial fluctuations of the Quasi Fermi Levels splitting in CIGS polycristalline absorbers and link those fluctuations to transport properties. The method is general to the point that third generation PV cells absorbers can also be evaluated. We illustrate the great potential of our setup by imaging carrier temperature in Hot Carriers Solar cells absorbers and quasi Fermi levels splitting in Intermediate Band Solar cells.
The purpose of the present paper is to focus on the impact of oxygen gas partial pressure during the sputtering of i-ZnO and ZnMgO on the transient behavior of solar cells parameters when a CBD-ZnS buffer layer is used. Based on electrical characterization of cells, we have observed that the effect of light-soaking is different on J-V characteristics depending on the quantity of oxygen present during the first deposition time of the i-ZnO or ZnMgO layers. In fact, we have noticed that, when cells are prepared with standard i-ZnO, the efficiencies are very low and a pronounced transient behavior is observed. However, when the i-ZnO or ZnMgO is first formed by a few nanometers sputtered layer without any additive oxygen, depending on the thickness of this layer, the transient effects strongly decrease. It is then possible to reach efficiencies quite similar to the CdS reference cells, especially with ZnMgO, without any post-treatments.
This article discusses the state of the art of solution growth of ZnO films and nanostructures. Chemical bath deposition (CBD), hydrothermal deposition, and electrodeposition (ED) are presented, where the interplay between experimental parameters and film properties can be highlighted. All of the methods allow the growth of ZnO with high structural quality and morphologies ranging from nanorods to dense films to nanoporous structures. The growth appears to be controlled by heterogeneous nucleation in supersaturated solutions, in the bulk for CBD, and at the interface for ED. Various emerging applications are presented, from light-emitting devices to solar cells and piezoelectric microgenerators.
This paper presents the influence of the solution chemistry of chemical bath deposition (pH and complexing agents) on the performance of CuIn(S,Se)2 cells after an initial CN treatment. It is shown that it is possible to modify the deposition conditions of the CdS by increasing the pH of the solution and by replacing the complexing agent (ammonia) by citrate ions. Both NH3 based and citrate based process give very homogenous and covering thin films. However, in the case of the citrate based process a decrease of open circuit voltage (Voc) and fill factor (FF) and thus of the cell efficiencies is observed. This points out that the main role of the buffer layer is not only related to the specific properties of the CdS itself but also to the near surface modifications of the CuIn(S,Se)2 caused by the presence of the complexing agent in the bath.
This paper presents optimization studies on the formation of cadmium free buffer layers for high efficiency copper indium diselenide (CIGS) thin film solar cells using a vapor phase route. Indium sulfide layers have been deposited on CIGS substrates by Atomic Layer Deposition (ALD) at substrate temperatures between 140 and 260 °C using indium acetylacetonate and hydrogen sulfide precursors. The parametric study of the deposition temperature shows an optimal value at about 220°C, leading to an efficiency of 16.4 % which is a technological breakthrough. The analysis of the device shows that indium sulfide layers give an improvement of the blue response of the cells as compared a standard CdS processed cell, due to a high apparent band gap (2.7-2.8 eV), higher open circuit voltages (up to 665 mV) and fill factor (78 %). This denotes high interface quality of the system. Atomic diffusion processes of sodium and copper in the buffer layer are evidenced.
CuInSe2 and Cu(In, Ga)Se2 precursor layers have been prepared by electrodeposition, with morphologies suitable for device completion. These precursor films were transformed into photovoltaic quality films after thermal annealing without any post-additional vacuum deposition process. Depending on the preparation parameters annealed films with different band gaps between 1eV and 1.5 eV have been prepared. The dependence of resulting solar cell parameters has been investigated. The best efficiency achieved is about 10,2 % for a band gap of 1.45 eV. This device presents an open circuit voltage value of 740 mV, in agreement with the higher band gap value. Device characterisations (current-voltage, capacitance-voltage and spectral response analysis) have been performed. Admittance spectroscopy at room temperature indicates the presence of two acceptor traps at 0.3 and 0.43 eV from the valance band with density of the order of 2. 1017 cm-3 eV-1.
State of the art CIGS thin films have been studied by means of the semiconductorelectrolyte junction. They appear as chemically robust allowing to use aggressive electrolyte compositions as for instance more acidic pH, down to 0 in sulfuric acid environment. In these electrolytes, reliable capacitance and photo-current related characterization techniques have been used. It has been shown that a short treatment in a gold (III) solution can facilitate the characterizations, and stabilize the surface composition. These results tend towards settling a standardized electrochemical testing procedure for CIGS layers
Growth and properties of indium sulfide layers (< 300 nm) prepared by atomic layer deposition (ALD) have been studied. Growth rate of about 0.6 A per cycle has been measured for films deposited at 160°C from indium acetylacetonate and hydrogen sulfide precursors. The films are crystalline with the β modification. They possess high band gap values (2.7-2.8 eV) which are related to small grain sizes (3-4 nm) through quantum size effects. Electrical properties have been addressed using the semiconductor electrolyte junction. They are n type with a doping level around 1016 cm−3 and possess a good blocking behavior under reverse bias. The flat band potential is close to -1 V vs MSE. These figures are close to those measured under similar conditions with CdS CBD buffer layers and could explain the good cell performances obtained with ALD In2S3.
Chemistry of co-evaporated CIGS surfaces submitted to chemical treatments relevant to fabrication steps were investigated by XPS and admittance spectroscopy. A Se XPS signal specific of the CIGS surfaces was identified. Surface states seen by Admittance and surface chemistry are seen to change significantly during the elaboration steps. Consequences for device elaboration are briefly discussed.
The control of the barrier height at the Cu(In,Ga)Se2(CIGS)/CdS interface, via the chemical state of the CIGS surface, is a key to improve solar cell performances. In this paper chemical modifications have been achieved by the electrochemical method, by varying both the pH of the solution and the applied potential. It is shown that the potential for electrochemical oxidation depends on the pH, in good agreement with the thermodynamic predictions. The resulting surface composition of CIGS, analyzed by XPS, also depends on the pH. Contact Potential Differences (CPD) and Surface Photovoltage (SPV) measurements show that the electronic properties of the CIGS surface are modified. The important result is that the oxidation leads to the formation of permanent positive surface charges, which increase the band bending up to 0.20 V in a direction favorable for the solar cell performances.
Chalcogenide semiconductors have been deposited epitaxially from aqueous solutions either chemically or electrochemically at growth rates of up to 0.7 μmhr−1. After recalling the basic principles of these deposition processes, results are presented concerning chemically deposited CdS on InP, GaP and CuInSe2 substrates, electrodeposited CdTe on InP, and CdSAnP heterostructures. Characterisation of these structures by RHEED, TEM, HRTEM, and glazing angle X ray diffraction allows to analyse the effects of substrate orientation, polarity, lattice match plus the influence of temperature on epitaxial growth. These results are discussed in terms of self organisation and a site selective growth mechanisms due to the free enegy of formation of each compound.
The introduction of II-VI semiconductor compounds into porous silicon layers has been investigated in order to obtain transparent and conducting contacts with the inner surface of the material. CdTe and ZnSe have been electrodeposited cathodically on n type nanoporous electrodes from acidic solutions containing the metallic cations and dissolved oxides of selenium or tellurium. CdS incorporation into p-type porous silicon has been achieved by chemical bath deposition, from solutions containing cadmium complexes and thioacetamid as a sulfur donor. Characterization of the deposits has been performed by SEM observations, X-ray analysis and RBS. Results confirm the penetration of the compounds into the porous films, with small to strong concentration gradients in thickness depending on the deposition method. After deposition and sample drying, the luminescence of CdTe embedded layers has almost disappeared, whereas those containing ZnSe and CdS show a photoluminescence efficiency which is not severely degraded.
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