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Cu2O is a promising p-type semiconductor for low-cost photovoltaics and transparent optoelectronics. However, low-cost and low-temperature fabrication of Cu2O films with good transport properties remains challenging, thus limiting their widespread adoption in devices. Here, we report Cu2O thin films of 20–80 nm thickness with hole mobility up to 92 cm2V−1s−1 using atmospheric-pressure spatial atomic layer deposition at temperatures below 260 °C, from a copper (I) hexafluoro-2,4-pentanedionate cyclooctadiene precursor. Raman spectroscopy indicates the presence of copper split vacancies and shows that the high hole mobility can be correlated to a low concentration of shallow acceptor defects. The optical bandgap of deposited films can be tuned between 2.08 eV and 2.5 eV, depending on the deposition temperature. All-oxide semitransparent Cu2O/ZnO solar harvesters are fabricated, showing efficiency values comparable to devices that incorporate much thicker Cu2O layers. Our work provides a promising approach towards cost-efficient, all-oxide solar harvesters, and for other (opto)electronic devices. Semiconducting Cu2O is attractive for photovoltaic and optoelectronic devices, though balancing high hole mobility with low-cost fabrication is challenging. Here, Cu2O thin films with high hole mobility of 92 cm²V−1s−1 are deposited in air, and applied in a semi-transparent solar harvester.

Recent research focuses on finding alternative materials and fabrication techniques to replace traditional (p) and (n) doped hydrogenated amorphous silicon (a-Si:H) to reduce cost and boost the efficiency of silicon heterojunction (SHJ) solar cells. In this work, low-cost p-type Cu2O thin films have been investigated and integrated as a hole-transporting layer (HTL) in SHJ solar cells, using atmospheric-pressure spatial atomic layer deposition (AP-SALD), an open-air, scalable ALD approach. Phase pure Cu2O thin films have been deposited at temperatures below the degradation limit of the SHJ, thus maintaining the passivation effect of the a-Si:H layer. The effect of deposition temperatures and HTL thicknesses on the performance of the devices has been evaluated. The fabricated Cu2O HTL-based SHJ cells, having an area of 9 cm2, reach a power conversion efficiency (PCE) of 13.7%, which is the highest reported efficiency for silicon-based solar cells incorporating a Cu2O HTL.

Monolithic two‐terminal III‐V on Si dual‐junction solar cells, designed for low concentration applications, were fabricated by means of surface‐activated direct wafer bonding. The III‐V top cell is a heterojunction formed by an n‐Ga₀.₅In₀.₅P emitter and a p‐Al₀.₂Ga₀.₈As base. An efficiency of 21.1 ± 1.5% at one sun and 23.7 ± 1.7% at 10 suns is demonstrated, which to our knowledge is the best dual‐junction two‐terminal III‐V on Si tandem cell efficiency reported to date under verified reference conditions. The I‐V characterization of these 1‐cm² tandem cells under concentration required the development of a new method using a single‐source multiflash solar simulator and not perfectly matched component cells, also known as pseudo‐isotypes, formed by Si single‐junction cells and optical filters. In addition, the spectrum of the pulsed solar simulator was measured using a high‐speed CMOS spectrometer, allowing the calculation of the spectral mismatch correction factor. Merging these two techniques results in the hybrid corrected pseudo‐isotype (HCPI) characterization method, which shows a fast and accurate performance with a simplified procedure based on a single‐source solar simulator. Pseudo‐isotypes are easily adaptable to new cell designs by simply using a different filter, hence allowing the characterization of new multijunction solar cell architectures.

We report on the relevance of using Plasma Immersion Ion Implantation (PIII) technology for a simple and controlled Phosphorus doping process of polysilicon-based passivated contacts in high efficiency silicon solar cells. Three different ion doses were tested on polysilicon (poly-Si) layers (40 nm thick) / tunnel oxide stacks to study the influence of this parameter on the electron concentration and surface passivation properties. Excellent i-Voc of 732 mV and J0 = 4.1 fA.cm-2 were obtained for hydrogenated poly-Si (n+)/SiO2 structures on polished c-Si substrates after a firing step. For thinner poly-Si films (from 7 nm to 26 nm), only one ion dose was tested resulting in a slight decrease of the passivation levels with the layer thickness. Encouraging values of i-Voc = 700 mV and J0 ∼ 22 fA.cm−2 were also obtained for∼16 nm of poly-Si (n+) on textured c-Si substrates.

International audience; The optimization of solar cells properties with thermal criteria gives the possibility to achieve higher conversion efficiency in outdoor conditions. An innovative setup that allows the control of the surroundings of a solar cell is described. Under such specific conditions, the cell temperature can be stabilized and measured. The variation of the cell temperature with the applied bias is experimentally observed and quantified. Between short circuit current density (Jsc) and open circuit voltage (Voc), a slight difference of temperature is observed, revealing a variation of the thermal equilibrium between these two points. The resistivity of the absorber and the input power density are found to influence this temperature shift. From the experimental results, it appears that the emissivity of the solar cell increases with the applied voltage due to an increase in the excess carrier concentration. Consequently, the operating temperature at open-circuit is lower than at short-circuit.

International audience; Increasing modules voltage allows to reduce resistive losses and metal consumption. Reaching voltages of several hundreds of volts may also enable inverter simplification leading to more efficient and lower cost inverter systems. This study proposes to evaluate the feasibility of such modules using reduced area solar cells fabricated by laser scribing and subsequent mechanical cleavage of 6 inches Al-BSF silicon cells. First, we investigated the influence of the cell dimension on the efficiency. Due to a high recombination activity on the cell edges, a drastic loss in efficiency can be observed for samples area below 16 cm2. For a given area, the comparison of two cell formats (45×45 mm2 and 78×26 mm2) confirms the interest of reducing the peripheral length over the cell area (square shape). The optimization of the front metallization grid allows the fabrication of small area cells with similar efficiency as 6 inches cells. Finally, a high voltage module was fabricated with 45×45 mm2 cells allowing to reach an open-circuit voltage of 369 V, a high FF value of 78.7% and a gain of 1.3% absolute efficiency compared to the reference module.

International audience; Nowadays, compensated silicon (Si) is used in photovoltaic (PV) processes, whether it is through intentional co-doping of resistivity-adjusted Czochralski ingots for high efficiency n-type Si solar cells, as a result of alternative Si purification processes for the production of low-cost Si feedstock, or as a result of recycling end-of-life materials. Whatever the origin of the compensated Si, the doping concentrations need to be accurately and quickly characterized in order to control such processes. In this work, a rapid and highly sensitive characterization technique based on low temperature Hall Effect measurements is described in scientific details and compared to three well-established chemical methods: Glow Discharge Mass Spectrometry (GDMS), Inductively-Coupled Plasma Mass Spectrometry (ICP-MS), and Secondary Ion Mass Spectrometry (SIMS). The characterized samples were extracted from the n-type top part of a casted solar grade Si ingot. A very good agreement is observed between the dopants densities extracted from the electrical method and from the standard methods. With the advantage of a very low detection limit combined with a short measurement time, the advanced Hall Effect technique is promising for the rapid and accurate characterization of dopant concentrations in compensated Si.

International audience; This work explores the possibility to use the mechanism of hopping conduction – and particularly the transition temperature between band and hopping conduction – on low temperature resistivity measurements, for the control of dopants densities in p‐type compensated silicon. This work first establishes a parametric study of the hopping conductivity: the impact of the majority dopant density and of the compensation ratio is investigated. In the range of majority dopant concentration studied (5×1016 cm–3–5×1017 cm–3), a linear relation seems to appear between the majority dopant concentration and the transition temperature, and this, apparently whatever the compensation impurity type or the crystalline structure. It was then shown that both minority and majority dopant densities can be estimated from a single resistivity versus temperature curve. To our knowledge, this work presents the first experimental study of the feasibility of using such mechanisms to collect relevant information on the compensated Si composition.

Session 6: Process Integration and Low-cost Manufacturing; International audience; A key step to achieve high conversion efficiencies for silicon solar cells is the junction formation. Plasma Immersion Ion Implantation (PIII) enables fine variations of the surface dopant concentration and depth of the doping profiles. Since no “dead-layer” with electrically inactive dopant species remains after an activation annealing step, high electrical quality of doped regions can easily be performed. We studied whether advanced Phosphorus emitters obtained via PIII could be optimized to increase p-type crystalline silicon (c-Si) solar cell performances. The doping profiles were tuned by changing ion dose and post implantation activation annealing temperature (Tanneal). A detailed analysis of carrier recombination in the different part of Al-BSF solar cells has been performed. Thus, saturation current densities (J0) were extracted (passivated emitter J0e,passivation, metallized emitter J0e,metallization, c-Si bulk and Back Surface Field J0,bulk+BSF) by coupling IC-QssPC and Suns-Voc measurements on specific test structures. An increase in Voc and Jsc was obtained by lowering the ion dose for a given Tanneal, whereas Voc, Jsc and the Fill Factor (FF) were improved by lowering Tanneal for a given dose. In the first case, photovoltaic (PV) conversion efficiencies did not change because of an increase in series resistance. However in the second case, they were increased by 0.2% absolute by reducing Tanneal from 850°C to 800°C, with a high FF value of 79.3% despite a relatively high emitter sheet resistance (115 Ω/square).

International audience; Due to their nanostructured surface, nanowire-based solar cells are good candidates to increase light absorption in thin film solar cells. Among these structures, III-V nanowires grown on silicon substrates, in the form of a tandem solar cell, are particularly interesting to reach high efficiencies. The aim of this work is to perform optical and electrical simulations of tandem solar cells based on III-V nanowire arrays grown on silicon and to compare two different semiconductor compounds (GaAs0.8P0.2 and Ga0.8Al0.2As) with a band gap of 1.7 eV (optimal on Si) for the nanowire array.The simulated structure is composed of a periodic core-shell GaAs0.8P0.2 or Ga0.8Al0.2As nanowire (NW) array on a silicon substrate. In our simulations we are also taking into account a thin (5nm) passivating layer, the oxide used for the encapsulation of the nanowires and the top transparent conducting oxide. The height H of the nanowires is equal to 1.5 µm which is a realistic value from a technological point of view.Optical simulations are performed with an in-house Rigorous Coupled Wave Analysis (RCWA) software. To optimize the absorption of light in the structure, we are taking into account the current matching between the two solar cells in order to find the best geometry of the nanowire array. From the optical simulation, the generation rate is calculated and used as an input for the electrical simulation performed with the TCAD software Sentaurus. From the electrical simulation the power conversion efficiency is extracted for various doping profiles allowing its optimization. The influence of recombination in the multijunction structure are also analysed. Opto-electrical simulations demonstrate that optimal geometries and efficiencies are very similar for the two semiconductors used for the nanowires.

International audience; Time-resolved photoluminescence (TRPL) is used to evaluate the injection-dependent effective minority carrier lifetime of high resistivity epitaxial silicon layers grown on highly doped CZ-Si substrates. Effective lifetimes ranging from 10 μs to 200 μs are estimated for excess carrier densities between 1x1017 cm-3 and 2x1016 cm-3. Standard models are used to separate the contribution from the different recombination mechanisms. The influence of the epitaxial layer and substrate parameters on the minority carrier effective lifetime measurement is discussed.

session poster (I43); International audience; Monolithic two-terminal III-V on Si dual-junction (2J) solar cells were fabricated by means of Surface-Activated direct wafer Bonding (SAB). Al 0.2 Ga 0.8 As single-junction cells are grown on GaAs substrate by Metal-Organic Vapor Phase Epitaxy (MOVPE) and bonded at room temperature to independently fabricated Si solar cells. The n+-GaAs//n+-Si bonding interface is characterized by Transmission Electron Microscopy (TEM) revealing a 2-3 nm thick amorphous interlayer. The performance of the 1 cm 2 tandem cells, designed for low concentration applications, was studied by External Quantum Efficiency (EQE) and J-V measurements showing a power conversion efficiency of 17% under one-sun AM1.5G spectrum. To our knowledge, this is the highest efficiency ever reported for a wafer-bonded 2J III-V on Si solar cell. Limitations to performance have been identified and therefore higher efficiencies are expected.

International audience; Mastering the properties of ZnO nanowires grown by the low temperature chemical bath deposition (CBD) is of crucial importance but is still challenging. We show that the shape, dimensions, and doping of ZnO nanowires can simultaneously be tuned by the addition of aluminum nitrate in the standard chemical system using zinc nitrate, hexamethylenetetramine, and ammonia in aqueous solution. The formation and doping mechanisms of ZnO nanowires are thoroughly investigated by combining chemical, structural, and optical analyses with in situ pH measurements correlated with thermodynamic simulations. We reveal that the electrostatic interactions of Al(OH)4– complexes with the positive m-plane sidewalls of ZnO nanowires at a given pH favor their adsorption as capping agents, reducing the radial growth and promoting the elongation, while favoring the aluminum uniform incorporation. Importantly, the aluminum doping is found to be thermally activated above the low temperature of 200 °C under oxygen atmosphere, as indicated by the occurrence of six related additional modes in the range of 200–900 cm–1 in temperature-dependent Raman spectroscopy. These findings show that CBD using aluminum nitrate is of high potential for tuning both the morphology of ZnO nanowires and their physical properties via the aluminum doping, which paves the way for their more efficient use into sensing, electronic, and optoelectronic devices on both flexible and rigid substrates.

Specific photovoltaic (PV) applications can require the use of limited-area solar cells. In that case, the edge of the device can have a considerable influence on the conversion efficiency. In this study, limited-area solar cells were fabricated by laser scribing and subsequent mechanical cleavage of large aluminum back surface field solar cells. First, laser parameters were optimized in order to limit the loss in conversion efficiency. It was shown that laser scribing must be performed on the rear side of the device in order to avoid the formation of shunts. The variation in the laser conditions could not improve the cell efficiency, as the discontinuity of the crystal lattice has a prominent impact compared with defects induced by laser scribing and cleaving. The fabrication of various cell geometries confirmed that the reduction of edge recombination was possible by simply limiting the cell periphery over its area. Specific characterization of the fabricated devices was carried out in order to understand the influence of the cell area on the performance. Using the two-diode model, two methods were used to extract the saturation current densities induced by metallization-related recombination ( $J_{{\rm{0m}}}$ ) and edge recombination ( $J_{{\rm{0e-dr}}}$ ). From this model, it is thus possible to predict the open-circuit voltage of any cell format. This study is particularly relevant for specific PV applications requiring the use of small-area solar cells and can be applied to other silicon cell technologies.

Effect of geometrical and structural parameters on the efficiency of the tandem solar cell based on the III-V nanowire array on silicon is studied by the means of coupled opto-electrical simulations. A close to realistic structure, consisting of AlGaAs core-shell nanowire array, connected through a tunnel diode to a Si subcell is modelled, revealing the impact of top contact layer, growth mask and tunnel junction. Optical simulation of the tandem structure under current matching condition determine optimal geometrical parameters of the nanowire array. They are then used in the extensive electrical optimization of the radial junction in the nanowire subcell. Device simulations show the necessity of high doping of the junction in order to avoid full shell depletion. The influence of bulk and surface recombination on the performance of the top subcell is studied, exposing the importance of the good surface passivation near the depleted region of the radial p - n junction. Finally, simulations of the fully optimized tandem structure show that a promising efficiency of 27.6% with the short-circuit current density of 17.1 mA/cm^2 can be achieved with reasonable bulk and surface carrier lifetime., Comment: 18 pages, accepted version

International audience; The elucidation of the fundamental processes in aqueous solution during the chemical bath deposition of ZnO nanowires (NWs) using zinc nitrate and hexamethylenetetramine is of great significance: however, their extrinsic doping by foreign elements for monitoring their optical and electrical properties is still challenging. By combining thermodynamic simulations yielding theoretical solubility plots and speciation diagrams with in situ pH measurements and structural, chemical, and optical analyses, we report an in-depth understanding of the pH effects on the formation and aluminum doping mechanisms of ZnO NWs. By the addition of aluminum nitrate with a given relative concentration for the doping and of ammonia over a broad range of concentrations, the pH is shown to strongly influence the shape, diameter, length, and doping magnitude of ZnO NWs. Tuning the dimensions of ZnO NWs by inhibition of their radial growth only proceeds over a specific pH range, where negatively charged Al(OH)4– complexes are predominantly formed and act as capping agents by electrostatically interacting with the positively charged m-plane sidewalls. These complexes further favor the aluminum incorporation and doping of ZnO NWs, which only operate over the same pH range following thermal annealing above 200 °C. These findings reporting a full chemical synthesis diagram reveal the significance of carefully selecting and following the pH to control the morphology of ZnO NWs as well as to achieve their thermally activated extrinsic doping, as required for many nanoscale engineering devices.

International audience; Nanowire-based solar cells are interesting structures for photovoltaic applications as they enhance properties such as light absorption, trapping efficiency and carrier collection. Consequently, the potential to decrease the cost of photovoltaic energy thanks to these structures is not negligible. However, up to now, their efficiency has been limited mainly because of the recombination at the interfaces and in the volume. The effective minority carrier lifetime is a key parameter which is strongly connected to volume, interface and surface recombination properties. In this work, we have used a purely electrical approach called reverse recovery transient (RRT) to perform measurements of minority carrier lifetime in core-shell nanowire-based solar cells under dark conditions. The structures are based on crystalline silicon nanowires grown on silicon wafers and embedded in a radial amorphous silicon shell. The electrical contacts for this hetero-junction structure are transparent conductive oxide for the front surface and aluminum for the backside. A planar solar cell has also been fabricated to be used as a reference. By comparing RRT measurement on the nanowire-based solar cell and on the planar reference solar cell with simulations, we extract the lifetime of the nanowires.

In this chapter, we present the advantages of semiconducting nanostructures (nanowires) for energy harvesting applications. Three sources of energy are considered: mechanical inputs, light and thermal energy. Different simulation approaches are used to discuss the prospects of these energy transduction solutions at nanoscale. Some guidelines are brought out for the improvement of energy conversion efficiency by nanowires, when integrated into functional devices.

International audience; The absorption properties of ZnO nanowire arrays covered with a semiconducting absorbing shell for extremely thin absorber solar cells are theoretically investigated by optical computations of the ideal short-circuit current density with three-dimensional rigorous coupled wave analysis. The effects of nanowire geometrical dimensions on the light trapping and absorption properties are reported through a comprehensive optical mode analysis. It is shown that the high absorptance of these heterostructures is driven by two different regimes originating from the combination of individual nanowire effects and nanowire arrangement effects. In the short wavelength regime, the absorptance is likely dominated by optical modes efficiently coupled with the incident light and interacting with the nearby nanowires (i.e. diffraction), induced by the period of core shell ZnO nanowire arrays. In contrast, in the long wavelength regime, the absorptance is governed by key optically guided modes, related to the diameter of individual core shell ZnO nanowires.

session poster; International audience

Despite the emergence of quantum dot optoelectronics, so far very few attempts have been made to estimate theoretically the carrier mobility in nanocrystal solids. This paper presents a method to s...

The systematic computations of the short-circuit current density have been performed for Si and ZnO/CdTe core shell nanowire arrays of 1 μm height in order to optimize the structural morphology in terms of nanowire diameter and period. It is found that the best structural configuration for Si leading to the ideal short-circuit current density of 19.6 mA/cm 2 is achieved for a nanowire diameter and period of 315 nm and 350 nm, respectively. In case of ZnO/CdTe, the ideal short circuit current density is of 24.0 mA/cm 2 , the nanowire diameter and period is of 210 nm and 350 nm, respectively. It is shown that the optimal configuration is more compact in the case of Si nanowire arrays than in the case of ZnO/CdTe nanowire arrays. Since Si has a smaller absorption coefficient than CdTe, a larger amount of material is needed and thus more compact nanowire arrays are required. It is also revealed that core–shell nanowire arrays made of ZnO/CdTe more efficiently absorb light than that of Si, making this device a good candidate for the next generation of nanostructured solar cells.

Nanowires (NWs) solar cells are expected to outperform the thin-film counterparts in terms of optical absorptance. In this theoretical study we optimize the geometry of vertical crystalline-amorphous silicon core-shell NW arrays on doped ZnO:Al (AZO)-Glass substrate by means of 3-D optical simulations in order to maximize the photon absorption. The optimized geometry is investigated by means of 3-D TCAD numerical simulation in order to calculate the ultimate efficiency and the main figures of merit by taking into account recombination losses. We show that optimized 10 μm-long crystalline – amorphous silicon core-shell (c-Si/a-Si/AZO/Glass) NWs can reach photo-generated current up to 22.94 mA/cm 2 (above 45% larger than that of the planar counterpart with the same amount of absorbing material) and conversion efficiency of 13.95%.

session Th3: Th3-1; National audience

International audience; Time-resolved photoluminescence (TRPL) was investigated on passivated silicon wafers under modulated square-wave laser illumination. It is shown that time-correlated single-photon counting can be used to record the transient signals on silicon wafers with doping levels commonly used for photovoltaic applications. This article reports the self calibrated evaluation of the injection-dependent effective minority carrier lifetime from the TRPL measurements. The method only requires knowing the doping level, the incident laser power, the reflection coefficient and the sample thickness. TRPL results were found to be in good agreement with photoconductance lifetime measurements. The effect of the surface recombination velocity on the generation of the PL signal was shown experimentally and discussed with PC1D calculations.

Nanowire (NW) based solar cells are an attractive approach to fabricating solar cells with efficient light trapping scheme and high collection efficiency in case of radial junctions. This chapter focusses on the designing and characterization of a‐Si/c‐Si and ZnO/CdTe NW‐based solar cells. Silicon solar cells will be considered as a reference structure and ZnO/CdTe as a prospective structure based on a direct bandgap absorber. In order to demonstrate the potentiality of c‐Si/a‐Si and of ZnO/CdTe core–shell NW arrays and to improve light absorption for photovoltaic applications, an optimization of light absorptance is usually performed by numerical optical simulation. The theoretical ideal photo‐generated current improvement exhibited by NW arrays can be predicted by numerical simulation. There are two main approaches to elaborate the NW arrays: a bottom‐up approach based on the growth of the NWs usually by Chemical Vapor deposition (CVD) and a top‐down approach based on etching methods.

session poster (322-A44); International audience; In this contribution, the absorption and electrical transport mechanisms are investigated as key elements for predicting the photoconversion efficiency of core shell ZnO CdTe nanowire based solar cells. It is shown that the absorption of the optimized morphological dimensions originates from the combination of individual nanowire effects and arrangement effects. Individual nanowire effects, related to the nanowire diameter are revealed by the large absorption of an optical key mode in the long wavelength regime. The nanowire arrangement effects, related to the period of the array, occur at short wavelengths, through diffraction processes. The ZnO CdTe nanowire arrays were grown on top of FTO/glass substrate in order to study the electrical transport mechanisms. The current-voltage characteristics were measured and simulated for various temperatures. Both the measured and the simulated saturation current show similar variation with temperature, revealing that the transport mechanism in core shell ZnO CdTe nanowire arrays are dominated by trap-assisted tunneling. These findings will be used in optoelectronic simulations, in order to predict the potentialities of the core shell ZnO CdTe nanowire arrays for solar cells.

CdTe is an important compound semiconductor for solar cells, and its use in nanowire-based heterostructures may become a critical requirement, owing to the potential scarcity of tellurium. The effects of the CdCl2 heat treatment are investigated on the physical properties of vertically aligned ZnO/CdTe core-shell nanowire arrays grown by combining chemical bath deposition with close space sublimation. It is found that recrystallization phenomena are induced by the CdCl2 heat treatment in the CdTe shell composed of nanograins: its crystallinity is improved while grain growth and texture randomization occur. The presence of a tellurium crystalline phase that may decorate grain boundaries is also revealed. The CdCl2 heat treatment further favors the chlorine doping of the CdTe shell with the formation of chlorine A-centers and can result in the passivation of grain boundaries. The absorption properties of ZnO/CdTe core-shell nanowire arrays are highly efficient, and more than 80% of the incident light can be absorbed in the spectral range of the solar irradiance. The resulting photovoltaic properties of solar cells made from ZnO/CdTe core-shell nanowire arrays covered with CuSCN/Au back-side contact are also improved after the CdCl2 heat treatment. However, recombination and trap phenomena are expected to operate, and the collection of the holes that are mainly photo-generated in the CdTe shell from the CuSCN/Au back-side contact is presumably identified as the main critical point in these solar cells., This work has been supported by the Nanosciences Foundation of Grenoble through the project II-VI Photovoltaic and by Grenoble INP with a Bonus Qualité Recherche grant through the project CELESTE. This work has also been partially supported by the Spanish Ministry under contract MAT2010-16116.

International audience; The absorption processes of extremely thin absorber solar cells based on ZnO/CdTe core–shell nanowire (NW) arrays with square, hexagonal or triangular arrangements are investigated through systematic computations of the ideal short-circuit current density using three-dimensional rigorous coupled wave analysis. The geometrical dimensions are optimized for optically designing these solar cells: the optimal NW diameter, height and array period are of 200 ± 10 nm, 1–3 μm and 350–400 nm for the square arrangement with CdTe shell thickness of 40–60 nm. The effects of the CdTe shell thickness on the absorption of ZnO/CdTe NW arrays are revealed through the study of two optical key modes: the first one is confining the light into individual NWs, the second one is strongly interacting with the NW arrangement. It is also shown that the reflectivity of the substrate can improve Fabry–Perot resonances within the NWs: the ideal short-circuit current density is increased by 10% for the ZnO/fluorine-doped tin oxide (FTO)/ideal reflector as compared to the ZnO/FTO/glass substrate. Furthermore, the optimized square arrangement absorbs light more efficiently than both optimized hexagonal and triangular arrangements. Eventually, the enhancement factor of the ideal short-circuit current density is calculated as high as 1.72 with respect to planar layers, showing the high optical potentiality of ZnO/CdTe core–shell NW arrays.

In this paper the optical optimization of vertical ZnO/CdTe and crystalline silicon/amorphous silicon (c-Si/a-Si) core-shell heterojunction nanowires array solar cells is presented. Optical simulations have been performed by means of a Rigorous Coupled-Wave Analysis (RCWA) numerical simulator which allows the modeling of nanostructured optoelectronic devices with a reasonable trade-off between accuracy and computational resources requirement. The optical optimization addresses the design of the array geometry in order to maximize the light absorption within the semiconductor. The paper describes the adopted simulation approach for the optical optimization of the investigated devices and analyzes the impact of the employed material and of the geometry on optical performance.

The optical absorption of an ultra-thin active layer solar cell can be significantly increased by engineering properly both the in plane patterning of the active layer and the vertical stack. This structuration both controls incident light coupling into slow light modes of the photonic crystals and photon lifetime in the absorbing material. Numerical results on optical and electrical properties will be presented, as well as preliminary optical and electrical characterizations of silicon based demonstrators.

Arrays of III-V direct-bandgap semiconductor nanowires are promising candidates for future photovoltaic devices due to their high optical absorption and their ability to be grown on low cost semiconductor substrates like silicon. The core-shell structure is particularly interesting as the electron-hole pair separation occurs in the radial direction and the photogenerated minority carriers have to travel short distances (the radius of the nanowires) thus improving the collection probability in case of well passivated nanowire surfaces. The aim of this study is to find the optimal geometry (length, height and diameter) of a GaAs nanowire array grown on a silicon substrate in order to have the best absorption of the incident photons. For this purpose, we have performed electromagnetic simulations with a homemade Rigorous Coupled Wave Analysis (RCWA) software. Our simulations take into account the core-shell structure, the passivation layer (GaAlAs) and the anti-reflection coating, but also the necessity to achieve current matching between the GaAs nanowire-based and the silicon substrate solar cells. This requirement is justified by the fact that the final goal is to process a tandem solar cell with junctions connected in series.

International audience; The low-cost fabrication of ZnO nanowire/CuSCN heterojunctions is demonstrated by combining chemical bath deposition with impregnation techniques. The ZnO nanowire arrays are completely filled by the CuSCN layer from their bottoms to their tops. The CuSCN layer is formed of columnar grains that are strongly oriented along the [003] direction owing to the polymeric form of the β-rhombohedral crystalline phase. Importantly, an annealing step is found essential in a fairly narrow range of low temperatures, not only for outgassing the solvent from the CuSCN layer, but also for reducing the density of interfacial defects. The resulting electrical properties of annealed ZnO nanowire/CuSCN heterojunctions are strongly improved: a maximum rectification ratio of 2644 at ±2 V is achieved following annealing at 150 °C under air atmosphere, which is related to a strong decrease in the reverse current density. Interestingly, the corresponding self-powered UV photodetectors exhibit a responsivity of 0.02 A/W at zero bias and at 370 nm with a UV-to-visible (370–500 nm) rejection ratio of 100 under an irradiance of 100 mW/cm2. The UV selectivity at 370 nm can also be readily modulated by tuning the length of ZnO nanowires. Eventually, a significant photovoltaic effect is revealed for this type of heterojunctions, leading to an open circuit voltage of 37 mV and a short circuit current density of 51 μA/cm2, which may be useful for the self-powering of the complete device. These findings show the underlying physical mechanisms at work in ZnO nanowire/CuSCN heterojunctions and reveal their high potential as self-powered UV photodetectors.

International audience

poster; International audience

The main objective of this thesis work is to study ways of improvement for the fabrication of n+ doping used as emitter zone in industrial PV cells made of crystalline silicon (c-Si). The plasma immersion ion implantation (PIII) technique allows precise control of the doping profiles of the implanted areas. The influence of the implantation dose and the activation annealing temperature of dopants on the doping profiles produced on p-type c-Si substrates were first studied. These dopings were integrated as emitters in Al-BSF (Aluminum Back Surface Field) and PERC (Passivated Emitter and Rear cells) cells. A detailed analysis of the losses by recombination of the charge carriers as well as the resistive losses was carried out. For an optimized doping profile, the best values ​​of emitter saturation current densities were 70 fA / cm². After cells optimization, record conversion efficiencies of 19.7% and 21% were obtained with Al-BSF and PERC cells, respectively. The PIII technique is particularly suitable for making ultra-thin junctions, compared to implantation by ion beams. Thus, different dopings were tested by variation of the PIII dose and of the annealing temperature on stacks consisting of layers of polysilicon (poly-Si) deposited by PECVD on p-type c-Si substrates, whith surfaces previously passivated by a tunnel oxide. Excellent state-of-the-art passivation properties (i-Voc~730mV and J0 ~ 5fA/cm²) were obtained after passivation of the surface of poly-Si layers by hydrogenated SiNx layers and firing annealing. With an optimized doping profile, the study of losses by recombination on Al-BSF cells integrating the polyslicon layer doped with PIII as an emitter revealed an improvement in the values ​​of saturation current densities of the emitter (54 fA/cm²)., L’objectif principal de ces travaux de thèse est d’étudier des voies d’améliorations pour la fabrication du dopage n+ utilisé comme zone d’émetteur dans les cellules PV industrielles en silicium cristallin (c-Si). La technique d’implantation ionique par immersion plasma (PIII) permet un contrôle précis des profils de dopage des zones implantées. L'influence de la dose d'implantation et de la température de recuit d'activation des dopants sur les profils de dopage fabriqués sur des substrat c-Si de type p ont d'abord été étudiées. Ces dopages ont été intégrés en tant qu'émetteur dans des cellules Al-BSF (Aluminium Back Surface Field) et PERC (Passivated Emitter and Rear cells). Une analyse détaillée des pertes par recombinaisons des porteurs de charges ainsi que des pertes résistives a été menée. Pour un profil de dopage optimisé, les meilleures valeurs de densités de courant de saturation de l'émetteur ont été de 70 fA/cm². Après optimisation des cellules, des rendements de conversion records de 19,7% et 21% ont été obtenus avec des cellules Al-BSF et PERC, respectivement. La technique PIII est particulièrement adaptée à la réalisation de jonctions ultra-minces, comparé à l'implantation par faisceaux d'ions. Ainsi, différents dopages ont été testés par variation de la dose PIII et de la température de recuit sur des empilements constitué de couches de polysilicium (poly-Si) déposée par PECVD sur des substrats c-Si de type p, dont la surface a été préalablement passivée par un oxyde tunnel. D'excellentes propriétés de passivation à l'état de l'art (i-Voc ~ 730mV et J0 ~ 5fA/cm²) ont été obtenues après passivation de la surface de la couche de poly-Si par des couches de SiNx hydrogénées et un recuit de firing. Avec un profil de dopage optimisé, l'étude des pertes par recombinaisons sur des cellules Al-BSF intégrant la couche de polyslicium dopée par PIII en tant qu'émetteur a révélé une amélioration des valeurs de densités de courant de saturation de l'émetteur (54 fA/cm²).

Si solar cells with record efficiencies over 26% have been recently demonstrated, approaching the Si single-junction limit of 30%. Multi-junction solar cells (MJSC) based on III-V materials can overcome this limit: efficiencies over 45% have been reported for a 5-junction under 1 sun and for a 4-junction under a concentrated illumination of 300 suns. Due to their elevated cost, these cells could be used in terrestrial applications only if operated under very high sunlight concentration for commercial terrestrial applications, which in turn increases the module and system complexity.An intermediate solution consists in fabricating high efficiency III-V solar cells on Si substrates, which are less expensive than the III-V or Ge substrates used in conventional MJSC. Mechanical-stacked and wafer-bonded solar cells, which avoid the unresolved issues of III-V on Si epitaxy, have already demonstrated efficiencies over 33%. This, combined with the recent advancements in the field of substrate reuse, predict a promising future for III-V on Si tandem solar cells, which could lead the next generation of high-efficiency and low-cost photovoltaics.In this PhD work, 2-junction (2J) AlGaAs//Si and 3-junction (3J) GaInP/AlGaAs//Si tandem solar cells were fabricated. The Si bottom subcell and the III-V top subcell(s) were joined together by wafer bonding, resulting in a 2-terminal (2T) III-V//Si solar cell configuration.Different wafer bonding techniques were studied, including an innovative bonding approach showing promising industrialization potential and thus, opening a new path for III-V on Si processing. The GaAs//Si bonding interface electrical properties were analyzed using dedicated test devices originally conceived at CEA, allowing to evaluate the interface resistance and the conduction mechanism.Experimental characterizations and simulations were performed in order to optimize the design and fabrication process, leading to record efficiencies. For the AlGaAs top subcell of the 2J, this includes the use of an AlInP window together with a GaInP emitter, forming an n-GaInP/p-AlGaAs heterojunction, which improved the short wavelength performance. In addition, the reduction of the GaAs bonding layer thickness and the use of a higher bandgap AlGaAs tunnel junction resulted in a higher transparency and a bottom subcell photocurrent improvement.For the Si bottom subcell, simulations allowed to identify the key factors that limit the performance, being the bulk lifetime the most critical characteristic in the thick Si cells used. In the case of III-V//Si interfaces, a highly doped emitter is crucial to minimize the surface recombination and maximize the open-circuit voltage, outweighing the drop in short-circuit current due to lifetime degradation. Back surface passivation is also important, specially to increase the infrared response. Different diffusion and implantation processes for the emitter formation were studied. Implantation processes showed less bulk lifetime degradation and smoother surfaces, thereby allowing bonding without chemical-mechanical planarization and thus higher doping levels at the surface.Finally, in order to correctly assess the efficiency of these III-V on Si tandem cells, a fast and low-cost current-voltage characterization method adapted for MJSC under low concentration was developed. This method does not require perfectly matched component cells and instead, Si single-junction cells with optical filters are used as pseudo-isotypes. An efficiency of 23.7% under 10 suns was demonstrated this way for the AlGaAs//Si cell, which is the highest efficiency reported to date for a 2J 2T Si-based tandem cell., Des rendements record à plus de 26% ont récemment été démontrés avec des cellules solaires en Si, approchant la limite théorique de 30% pour une seule jonction. Les cellules solaires à multi-jonctions (MJSC) fabriquées à base de matériaux III-V peuvent dépasser cette limite: des rendements supérieurs à 45% ont été reportés pour une cellule à 5 jonctions sous un soleil et pour une cellule à 4 jonctions sous lumière concentrée. Cependant, pour des applications terrestres, le coût élevé de ces technologies impose l’utilisation d’une haute concentration, ce qui augmente la complexité du système.Une solution intermédiaire consiste à fabriquer des cellules solaires III-V à haut rendement sur des substrats Si, moins coûteux que les substrats III-V ou Ge utilisés dans les MJSC classiques. Des rendements supérieurs à 33% ont déjà été démontrés pour des MJSC fabriquées par collage direct. Ceci, combiné aux progrès récents dans la réutilisation des substrats III-V, présage un avenir prometteur pour les cellules solaires tandem III-V sur Si, ce qui pourrait mener à la prochaine génération de systèmes photovoltaïques à haut rendement et faible coût.Dans ce travail de thèse, des cellules solaires tandem AlGaAs//Si à 2 jonctions (2J) et GaInP/AlGaAs//Si à 3 jonctions (3J) ont été fabriquées par collage direct, ce qui a donné lieu à une configuration à 2 terminaux (2T).Différentes techniques de collage ont été étudiées, notamment une approche innovante présentant un potentiel d'industrialisation prometteur pour l’intégration des matériaux III-V sur Si. Les propriétés électriques de l'interface de collage GaAs//Si ont été analysées à l'aide de dispositifs de test dédiés conçus au CEA, permettant d'évaluer la résistance d'interface et le mécanisme de conduction.Des caractérisations et simulations expérimentales ont été effectuées afin d'optimiser le design et le processus de fabrication, conduisant à des rendements record. Pour la sous-cellule supérieure en AlGaAs de la 2J, cela comprend l'utilisation d'une fenêtre en AlInP avec un émetteur en GaInP, formant une hétérojonction n-GaInP/p-AlGaAs, qui améliore les performances pour les faibles longueurs d'onde. De plus, la réduction de l'épaisseur de la couche de collage en GaAs et l'utilisation d'une jonction tunnel en AlGaAs, avec bande interdite plus large, augmentent la transparence et donc le photocourant de la sous-cellule inférieure.Pour la sous-cellule inférieure en Si, les simulations ont permis d'identifier les facteurs clés qui limitent les performances, la durée de vie étant la caractéristique la plus critique dans les cellules Si épaisses utilisées. Dans le cas des interfaces III-V//Si, un émetteur fortement dopé est essentiel pour minimiser la recombinaison de surface et donc augmenter la tension en circuit ouvert. La passivation de la surface arrière est également importante, notamment pour augmenter la réponse dans l’infrarouge. Différents processus de diffusion et d'implantation ont été étudiés pour former l'émetteur. Les processus d'implantation ont montré moins de dégradation de la durée de vie et des surfaces moins rugueux, permettant ainsi le collage sans planarisation chimico-mécanique et donc des niveaux de dopage plus élevés en surface.Finalement, afin d’évaluer correctement le rendement de conversion de ces cellules tandem III-V sur Si, une méthode de caractérisation courant-tension rapide et peu coûteuse, adaptée aux MJSC sous faible concentration a été développée. Cette méthode ne nécessite pas de cellules isotypes parfaitement identiques, à la place, des cellules Si à simple jonction avec filtres optiques sont utilisées. Une efficacité de 23,7% sous 10 soleils a été démontrée de cette manière pour la cellule AlGaAs//Si, qui est le rendement le plus élevé signalé à ce jour pour une cellule tandem à base de Si avec 2J et 2T.

This PhD work was developed in the context of research for novel characterization methods for ultra-thin dielectric films on semiconductors and their interfacial quality. Second harmonic generation (SHG) is a very promising non-invasive technique based on nonlinear optics. A laser emitting at the fundamental frequency is incident upon the sample which responds through its 2nd order polarization, generating a signal at twice the fundamental frequency. For centrosymmetric materials such as c-Si, amorphous SiO2 or Al2O3, the SHG signal is mainly due to the defects and to the static electric field Edc present at the interface (due to pre-existing charges Qox and/or photo-injected charge trapping/detrapping at interface traps Dit). Thus, SHG measurement gives access to the quality of dielectric/semiconductor interfaces. Nevertheless, the SHG signal is also dependent on multilayer optical propagation phenomena. For this reason, we have developed a simulation program which accounts for the optical phenomena and the static electric fields at the interfaces. We have used SHG to monitor the passivation quality of Al2O3/Si structures prepared with different processes and showed a correlation between SHG and minority carrier lifetime measurements. Qox and Dit were extracted from capacitance-voltage measurements and helped calculating the Edc values. The optical simulation, fed with known Edc values reproduced the experimental SHG data in these structures. The SHG was also used for Silicon-on-Insulator (SOI) substrates characterization. In thick SOI structures, both simulations and experimental results show that the SHG response is mainly given by optical interferences (Edc has no impact). In ultrathin SOI, the interfaces are electrically coupled and Edc is needed as input in the simulation in order to reproduce the experimental SHG data. This implies that in ultrathin SOI, SHG can access the interface electric fields in a non-destructive way.; Cette thèse s’intéresse à une technique de caractérisation particulièrement bien adaptée à l’étude de couches diélectriques ultra-minces sur semiconducteurs. La génération de seconde harmonique (SHG) est une méthode très prometteuse, basée sur l’optique non-linéaire. Un laser est focalisé sur l'échantillon à caractériser et le signal à deux fois la fréquence fondamentale est mesuré. Pour les matériaux centrosymétriques comme c-Si, SiO2 et Al2O3, le signal SHG est dû aux défauts et au champ électrique Edc d’interface (induit par les charges préexistantes Qox et/ou piégées au niveau des pièges d’interface Dit). La SHG donne ainsi accès à la qualité des interfaces entre diélectriques/semiconducteurs. Néanmoins, le signal SHG dépend aussi des phénomènes de propagation optique dans les structures multicouches. Pour cette raison, nous avons développé un programme de simulation qui prend en compte les phénomènes optiques et les champs électriques statiques aux interfaces. Nous avons utilisé la SHG pour analyser la qualité de passivation de structures Al2O3/Si préparées avec des procédés différents et nous avons montré une corrélation entre SHG et mesure de durée de vie des porteurs de charges. Les valeurs de Qox et Dit ont été extraites par des mesures de capacité-tension et elles ont permis de calculer le champ Edc. La simulation optique, avec les valeurs extraites de Edc a permis de reproduire les données expérimentales de SHG dans ces structures. La SHG a été utilisée également pour la caractérisation des substrats Silicium-sur-Isolant (SOI). Pour les structures SOI épaisses, la simulation et les résultats expérimentaux ont montré que la réponse SHG est dominée par les interférences optiques (faible impact de Edc). Pour les structures SOI ultraminces, les interfaces sont couplées électriquement et des valeurs de Edc sont nécessaires pour reproduire les données expérimentales par simulation. Cela implique que pour les SOI ultraminces, la SHG pourrait donner accès aux champs électriques au niveau des interfaces d’une manière non-destructive.

This PhD work was developed in the context of research for novel characterization methods for ultra-thin dielectric films on semiconductors and their interfacial quality. Second harmonic generation (SHG) is a very promising non-invasive technique based on nonlinear optics. A laser emitting at the fundamental frequency is incident upon the sample which responds through its 2nd order polarization, generating a signal at twice the fundamental frequency. For centrosymmetric materials such as c-Si, amorphous SiO2 or Al2O3, the SHG signal is mainly due to the defects and to the static electric field Edc present at the interface (due to pre-existing charges Qox and/or photo-injected charge trapping/detrapping at interface traps Dit). Thus, SHG measurement gives access to the quality of dielectric/semiconductor interfaces. Nevertheless, the SHG signal is also dependent on multilayer optical propagation phenomena. For this reason, we have developed a simulation program which accounts for the optical phenomena and the static electric fields at the interfaces. We have used SHG to monitor the passivation quality of Al2O3/Si structures prepared with different processes and showed a correlation between SHG and minority carrier lifetime measurements. Qox and Dit were extracted from capacitance-voltage measurements and helped calculating the Edc values. The optical simulation, fed with known Edc values reproduced the experimental SHG data in these structures. The SHG was also used for Silicon-on-Insulator (SOI) substrates characterization. In thick SOI structures, both simulations and experimental results show that the SHG response is mainly given by optical interferences (Edc has no impact). In ultrathin SOI, the interfaces are electrically coupled and Edc is needed as input in the simulation in order to reproduce the experimental SHG data. This implies that in ultrathin SOI, SHG can access the interface electric fields in a non-destructive way., Cette thèse s’intéresse à une technique de caractérisation particulièrement bien adaptée à l’étude de couches diélectriques ultra-minces sur semiconducteurs. La génération de seconde harmonique (SHG) est une méthode très prometteuse, basée sur l’optique non-linéaire. Un laser est focalisé sur l'échantillon à caractériser et le signal à deux fois la fréquence fondamentale est mesuré. Pour les matériaux centrosymétriques comme c-Si, SiO2 et Al2O3, le signal SHG est dû aux défauts et au champ électrique Edc d’interface (induit par les charges préexistantes Qox et/ou piégées au niveau des pièges d’interface Dit). La SHG donne ainsi accès à la qualité des interfaces entre diélectriques/semiconducteurs. Néanmoins, le signal SHG dépend aussi des phénomènes de propagation optique dans les structures multicouches. Pour cette raison, nous avons développé un programme de simulation qui prend en compte les phénomènes optiques et les champs électriques statiques aux interfaces. Nous avons utilisé la SHG pour analyser la qualité de passivation de structures Al2O3/Si préparées avec des procédés différents et nous avons montré une corrélation entre SHG et mesure de durée de vie des porteurs de charges. Les valeurs de Qox et Dit ont été extraites par des mesures de capacité-tension et elles ont permis de calculer le champ Edc. La simulation optique, avec les valeurs extraites de Edc a permis de reproduire les données expérimentales de SHG dans ces structures. La SHG a été utilisée également pour la caractérisation des substrats Silicium-sur-Isolant (SOI). Pour les structures SOI épaisses, la simulation et les résultats expérimentaux ont montré que la réponse SHG est dominée par les interférences optiques (faible impact de Edc). Pour les structures SOI ultraminces, les interfaces sont couplées électriquement et des valeurs de Edc sont nécessaires pour reproduire les données expérimentales par simulation. Cela implique que pour les SOI ultraminces, la SHG pourrait donner accès aux champs électriques au niveau des interfaces d’une manière non-destructive.

Zinc oxide is a semiconductor considered for a wide range of optoelectronic, biological, or gas sensor applications. Indeed, apart from being an abundant material, it has several remarkable properties as its electronic mobility (200 cm²/(V.s)) and his ability to grow under different nano-scale shapes thanks to low cost and easily scalable deposition techniques. ZnO nanowires grown by the chemical bath deposition technique are used in this thesis for their integration in 3rd generation solar cells. In these devices as in many other applications, the nanowire mophology and electrical property are essential to get interesting efficiency. This last aspect, in particular, has never been studied in details in the literature concerning chemical bath deposition. This work introduces a new way to control simultaneously the morphology and the doping level of ZnO nanowires by this deposition technique. A growth and doping mechanism has been identified thanks to thermodynamics simulations, in-situ pH measurements, and several characterization techniques like scanning electron microscope, X-ray diffraction, temperature dependant Raman spectroscopy, and atomic force microscopy. The ZnO nanowires were also embedded in dye sensitized solar cells to study the effect of morphology and doping level on the efficiency. ZnO nanowires combined with an Ag nanowire electrode can be deposited on a flexible substrate to make a flexible dye sensitized solar cell., L’oxyde de zinc est un semiconducteur pressenti pour une large variété d’applications optoélectroniques, biologiques ou encore détecteurs de gaz. En effet, en plus d’être un matériau abondant, il possède plusieurs propriétés remarquables comme sa large bande interdite de 3,33 eV, sa grande mobilité électronique de 200 cm²/(V.s) mais également sa capacité à croitre sous plusieurs formes nanométriques par des techniques de dépôt bas coût et facilement adaptables au milieu industriel. Les nanofils de ZnO élaborés par la technique de dépôt en bain chimique seront utilisés dans cette thèse pour leur intégration dans des cellules solaires de 3ème génération. Dans ces cellules, la morphologie des nanofils ainsi que leur dopage est primordial pour obtenir des rendements intéressants. Ce dernier aspect en particulier n’a cependant pas été étudié en détails dans la littérature concernant le dépôt en bain chimique. Ce travail présente donc une façon innovante de contrôler simultanément la morphologie et le dopage des nanofils par cette technique de dépôt. Un mécanisme de croissance et de dopage a été déterminé grâce à des simulations thermodynamiques, des mesures de pH in-situ et plusieurs méthodes de caractérisation telles que la microscopie électronique à balayage et à transmission, la diffraction des rayons X, la spectroscopie Raman en température et la microscopie à force atomique en mode électrique. Les nanofils de ZnO réalisés sont ensuite intégrés dans des cellules solaires à colorant pour étudier l’intérêt de l’optimisation des nanofils sur les performances des cellules solaires. Finalement, ces nanofils de ZnO combinés à une électrode en nanofils d’argent peuvent être intégrés sur substrat flexible pour réaliser une cellule à colorant plus légère et maniable et donc visant davantage d’applications.

Zinc oxide is a semiconductor considered for a wide range of optoelectronic, biological, or gas sensor applications. Indeed, apart from being an abundant material, it has several remarkable properties as its electronic mobility (200 cm²/(V.s)) and his ability to grow under different nano-scale shapes thanks to low cost and easily scalable deposition techniques. ZnO nanowires grown by the chemical bath deposition technique are used in this thesis for their integration in 3rd generation solar cells. In these devices as in many other applications, the nanowire mophology and electrical property are essential to get interesting efficiency. This last aspect, in particular, has never been studied in details in the literature concerning chemical bath deposition. This work introduces a new way to control simultaneously the morphology and the doping level of ZnO nanowires by this deposition technique. A growth and doping mechanism has been identified thanks to thermodynamics simulations, in-situ pH measurements, and several characterization techniques like scanning electron microscope, X-ray diffraction, temperature dependant Raman spectroscopy, and atomic force microscopy. The ZnO nanowires were also embedded in dye sensitized solar cells to study the effect of morphology and doping level on the efficiency. ZnO nanowires combined with an Ag nanowire electrode can be deposited on a flexible substrate to make a flexible dye sensitized solar cell.; L’oxyde de zinc est un semiconducteur pressenti pour une large variété d’applications optoélectroniques, biologiques ou encore détecteurs de gaz. En effet, en plus d’être un matériau abondant, il possède plusieurs propriétés remarquables comme sa large bande interdite de 3,33 eV, sa grande mobilité électronique de 200 cm²/(V.s) mais également sa capacité à croitre sous plusieurs formes nanométriques par des techniques de dépôt bas coût et facilement adaptables au milieu industriel. Les nanofils de ZnO élaborés par la technique de dépôt en bain chimique seront utilisés dans cette thèse pour leur intégration dans des cellules solaires de 3ème génération. Dans ces cellules, la morphologie des nanofils ainsi que leur dopage est primordial pour obtenir des rendements intéressants. Ce dernier aspect en particulier n’a cependant pas été étudié en détails dans la littérature concernant le dépôt en bain chimique. Ce travail présente donc une façon innovante de contrôler simultanément la morphologie et le dopage des nanofils par cette technique de dépôt. Un mécanisme de croissance et de dopage a été déterminé grâce à des simulations thermodynamiques, des mesures de pH in-situ et plusieurs méthodes de caractérisation telles que la microscopie électronique à balayage et à transmission, la diffraction des rayons X, la spectroscopie Raman en température et la microscopie à force atomique en mode électrique. Les nanofils de ZnO réalisés sont ensuite intégrés dans des cellules solaires à colorant pour étudier l’intérêt de l’optimisation des nanofils sur les performances des cellules solaires. Finalement, ces nanofils de ZnO combinés à une électrode en nanofils d’argent peuvent être intégrés sur substrat flexible pour réaliser une cellule à colorant plus légère et maniable et donc visant davantage d’applications.

This study aims both at developing alternative characterization techniques for the determination of dopant concentrations in compensated silicon, and at improving the understanding of the effect of compensation on transport mechanisms at the nanometer scale. To do so, the different electrical conduction mechanisms occurring in compensated silicon, and more precisely the influence of dopant concentration on the latter, are studied in details in the temperature range [15K-350K] thanks to an Hall effect device. A first step consisted in enriching the theoretical models used to describe the variation of charge carrier density with temperature. It lead to an optimization of an existing characterization technique based on the fitting of those models to the experimental data measured by Hall Effect. A second step consisted in studying the possibility to use hopping conduction mechanisms to quantify dopant densities, via the preparation of model samples with a controlled compensation ratio. The results allowed to construct three innovating techniques based on resistivity measurements with temperature. The latter have then been used to characterize industrial materials (one ingot originating from recycled photovoltaic cells, and one ingot coming from the purification of metallurgical grade silicon). The results have then been compared to usual characterization techniques. Finally, Monte-Carlo type simulations of the spatial distribution of the electrical potential in silicon, allowed to clarify the influence of compensation on the electrostatic disorder in the material, and particularly on the charge carrier mobility., L’objectif de ces travaux de thèse est double : développer des méthodes de caractérisation alternatives des teneurs en dopants dans le silicium compensé, et améliorer la compréhension de l’influence à l’échelle nanométrique de la compensation du dopage sur les mécanismes de transport. Pour cela, les différents mécanismes de conduction électrique à l’œuvre dans le silicium compensé, et plus précisément l’influence des teneurs en dopants sur ceux-ci, ont été étudiés en détail dans la gamme de température [15K-350K] à partir d’un dispositif à effet Hall. Un premier travail a consisté à enrichir les modèles théoriques utilisés pour décrire la variation avec la température de la densité de porteurs libres, et a permis d’optimiser une méthode de caractérisation préexistante basée sur l’ajustement de ces modèles aux données expérimentales mesurées par effet Hall. Un second volet a consisté à étudier la possibilité d’utiliser le phénomène de conduction électrique par « hopping » pour la quantification des teneurs en dopants, via la préparation d’échantillons d’étude à degrés de compensation contrôlés. Fort des résultats obtenus, trois techniques inédites basées sur la mesure de résistivité en température ont ainsi été proposées. Celles-ci ont ensuite été utilisées pour la caractérisation de matériaux issus de procédés industriels (lingot issu du recyclage de cellules photovoltaïques d’une part, et lingot issu de la purification bas coût de Si métallurgique d’autre part). Les résultats ont ensuite été confrontés aux techniques de caractérisation usuelles. Enfin, des simulations (de type Monte-Carlo) de la répartition spatial du potentiel électrique dans le matériau ont permis de préciser l’influence de la compensation sur le désordre électrostatique dans le matériau, et notamment sur la mobilité des porteurs de charge.

Photovoltaic energy is a very attractive way to produce renewable energy. The current increase in the photovoltaic energy production mainly takes advantage of the continuous decrease in the solar cell cost as well as to incentive policy. However, installed photovoltaic panels only contribute to a very small part of the global electricity production. Therefore, important technological developments are dedicated to the second generation of solar cells (i.e. thin film solar cells) in order to reduce more their manufacturing cost despite the resulting lower conversion efficiency owing to a weaker structural and optical material quality. One alternative way to increase the solar cell efficiency is to fabricate nanowire-based solar cells since they may benefit from a higher light absorption and carrier collection efficiency. The light absorption is actually increased thanks to the high surface/volume ratio of nanowires but also to light trapping related to the nanowire length. Furthermore, the collection of minority charge carriers is more efficient in radial structures (i.e. core-shell structures) since the nanowire diameter is very small. This PhD thesis aims at investigating the optoelectronic properties of silicon and ZnO/CdTe nanowires (absorption, lifetime of minority charge carriers, bulk and surface recombination…) in order to design an optimised nanowire-based solar cell structure. Electromagnetic simulations will be first performed to define the best nanowire geometry for the absorbance, and then compared to experimental measurements of the absorption coefficient. Electrical characterisations (lifetime measurements, surface recombination…) will be also achieved to analyse the structural quality and to simulate the solar cell electrical properties. Some prototypes of optimised solar cells will eventually be fabricated., La conversion photovoltaïque est un procédé très attractif pour la fourniture d’énergie propre et renouvelable. Cette filière est en plein essor grâce à une réduction constante des coûts de revient et des politiques incitatives de nombreux pays. Pourtant, l’ensemble des panneaux photovoltaïques installés ne produit qu’une faible part de la consommation mondiale en électricité. Les récents développements technologiques dans l’industrie photovoltaïque se sont surtout concentrés sur les cellules dites de seconde génération, à savoir les couches minces à base de CIGS, CdTe, a-Si, a-SiGe. Cette filière permet la fourniture d’électricité à coût inférieur à la technologie standard silicium, mais les rendements de conversion demeurent encore faibles, ce qui nécessite de larges surfaces disponibles. Il est à noter notamment que les cellules couches minces à base de matériaux semiconducteurs à gap direct comme le CIGS et le CdTe sont en plein essor puisqu’ils profitent en particulier d’une absorption accrue par rapport au silicium ; toutefois, ces matériaux sont présents en quantité limitée à la surface de la planète (In, Te). Dans ce contexte, les cellules à base de nanofils constituent une solution intéressante aux problèmes de l’absorption de la lumière, du transport et de la séparation des porteurs de charge photo-générés mais aussi de la quantité de matière utilisée. En effet, en utilisant une jonction radiale (i.e. entourant le nanofil), il est possible de séparer l’absorption de la lumière ( liée notamment à la longueur du nanofil) de la collecte des porteurs de charge (qui dépend du diamètre des nanofils). L’intérêt de ces structures réside également dans les propriétés de base des nanofils : la relaxation élastique favorable sur leur surface latérale ouvre le champ au dépôt de nanofils par hétéro-épitaxie sur tout type de substrat alors que la faible densité de défauts étendus en leur sein est propice à un transport efficace des porteurs de charges. Ainsi, la possibilité de réaliser des nanofils sur substrat souple en réduisant de manière importante la quantité de matière utilisée par rapport à une cellule en silicium cristallin massif peut être envisagée. Plusieurs laboratoires grenoblois ont déjà une expertise dans le domaine de la croissance des nanofils. Cette thèse a pour but de réaliser une analyse expérimentale approfondie des propriétés optoélectroniques des nanofils (par des mesures de réflectivité, de durée de vie des porteurs minoritaires et de recombinaisons en surface et aux interfaces) combinée à des simulations optiques (de type RCWA ou FDTD) et électriques (TCAD). L’objectif ultime étant de concevoir et de développer des cellules à base de nanofils de silicium et de ZnO/CdTe. Des démonstrateurs seront réalisés sur la base des simulations électro-optiques. Pour cela, les moyens d’élaboration, de caractérisation et de technologie des différents laboratoires et entités, ainsi que les compétences associées, seront mis en commun pour accompagner les travaux du doctorant.

The physicochemical properties of a spherical semiconductor nanocrystal, intermediate between the molecule and the solid depend on its size. Stacked or dispersed, these nanocrystals are building blocks of new functional materials with tunable properties, particularly appealing for optoelectronics. This thesis takes part in the development of these new materials. It mainly presents a methodology for the simulation of electronic transport in nanocrystal solids within the weak electronic coupling regime. It is applied to a material made of silicon nanocrystals embedded in silicon oxide and considered for photovoltaïc applications. The displacement kinetics of charge carriers is related to the tunneling transfer rate (hopping) between nanocrystals. These rates are calculated within the framework of Marcus theory and take into account the electron-phonon interactions, the effect of the bias field and the electron-electron interactions at short and long range. The calculation of electronic states (electrons and holes) in k.p theory associated with the use of Bardeen's formula provides, compared to previous works, results (mobility or current) in absolute terms. The mobility thus computed is far lower than the results of the literature and encourage to consider other materials. Furthermore, the device simulations show the significant impact of the electrodes on the current-voltage characteristics. Also, a new accelerated kinetic Monte-Carlo algorithm has been adapted in order to reproduce the disorder inherent in the manufacturing process while maintaining a reasonable simulation time. Thus the impact of the size disorder is poor at room temperature while the percolation paths shunt the contribution of other conduction paths. Characterization results compared to simulations tend to show that these paths concentrate carriers and exhibit Coulomb blockade phenomenon. Finally, the absorption cross section is calculated theoretically to obtain the generation rate under illumination. It is similar to the bulk silicon one. And a method employing a Kelvin probe microscope is described to characterize the carrier lifetime, namely the recombination rate. The results thus obtained are consistent with other experimental technics., Les propriétés physico-chimiques d'un nanocristal semi-conducteur sphérique, intermédiaires entre la molécule et le solide, dépendent de sa taille. Empilés ou dispersés, ces nanocristaux sont les briques architecturales de nouveaux matériaux fonctionnels aux propriétés ajustables, en particulier pour l’optoélectronique. Cette thèse s'inscrit dans le développement de ces nouveaux matériaux et présente avant tout une méthodologie pour la simulation du transport électronique dans un solide à nanocristaux en régime de faible couplage électronique appliquée à des nanocristaux de silicium dans une matrice de SiO2 pour les applications photovoltaïques. La cinétique du déplacement des porteurs est liée au taux de transfert tunnel (hopping) entre nanocristaux. Ces taux sont calculés dans le cadre de la théorie de Marcus et prennent en compte l'interaction électron-phonon dont l'effet du champ de polarisation dans la matrice ainsi que les interactions électrostatiques à courte et longue portée. Le calcul des états électroniques (électrons et trous) en théorie k.p associé à l'utilisation de la formule de Bardeen donne au code la capacité, par rapport à la littérature, de fournir des résultats (mobilité ou courant) en valeur absolue. Les résultats de mobilité ainsi obtenus pour des empilements cubiques idéaux viennent contredire les résultats de la littérature et incitent à considérer d'autres matériaux notamment en ce qui concerne la matrice pour obtenir de meilleurs performances. En outre, les résultats de simulation de dispositifs montrent l'impact considérable des électrodes sur les caractéristiques courant-tension. Aussi, un nouvel algorithme Monte-Carlo Cinétique accéléré a été adapté afin de pouvoir reproduire le désordre inhérent à la méthode de fabrication tout en maintenant un temps de simulation raisonnable. Ainsi l'impact du désordre en taille se révèle faible à température ambiante tandis que les chemins de percolation occultent la contribution des autres chemins de conduction. Des résultats de caractérisation comparés aux simulations tendent par ailleurs à indiquer que ces chemins peuvent concentrer les porteurs et exhiber un phénomène de blocage de coulomb. Enfin, la section efficace d'absorption est calculée théoriquement et permet d'obtenir le taux de génération sous illumination qui se révèle proche du silicium massif. Et une méthode en microscopie à sonde de Kelvin est décrite pour caractériser la durée de vie des porteurs c'est-à-dire le taux de recombinaison, les résultats ainsi obtenus étant cohérents avec d'autres techniques expérimentales.

This thesis is focused on the development and the optimization of electrochemical metallization techniques allowing the deposition of conductive metals, silver and the copper, by electrolytic deposition or by lip (light-induced plating). Two approaches were studied to realize the front side contacts of silicon solar cells: the thickening of screen-printed contacts and the fabrication of contacts completely by electrochemical deposition without screen-printing. For this solution, the deposition of a seed layer before thickening is necessary to insure a low contact resistivity, a satisfying adhesion and selectivity through the anti-reflection coating. These objectives were reached thanks to the optimization of electroless nickel-phosphorous (nip) deposits, including on low doped emitter. The investigations also allowed a better understanding of the NiP/Si contact formation mechanisms. The feasibility of electrochemical deposition techniques was demonstrated for various applications : cells with electrochemical front side contacts NiP/Ag, type n cells, thickening of fine line screen-printed contacts… very promising results of fill factor ff and efficiency improvement were obtained and allow to realize new structures of high efficiency photovoltaic cells : cells with low doped emitter, cells with selective emitter and with laser ablated anti-reflection coating, rear contact cells…, Cette thèse s’est focalisée sur le développement et l’optimisation de techniques de métallisation électrochimique permettant le dépôt de métaux conducteurs, l’argent et le cuivre, par voie électrolytique ou par la technique dite LIP (Light-Induced Plating). Deux approches ont été abordées pour l’élaboration des contacts en face avant : l’épaississement de contacts sérigraphiés d’une part, et la réalisation de contacts entièrement par voie électrochimique sans recours à la sérigraphie. Pour cette dernière solution, le dépôt d’une couche d’accroche avant l’étape d’épaississement est nécessaire afin d’assurer une résistivité de contact faible, une bonne adhérence et une bonne sélectivité au travers de la couche anti-reflet. Ces objectifs ont été atteints grâce à la mise en œuvre et l’optimisation de dépôts electroless de nickel-phosphore (NiP), y compris sur émetteur peu dopé. Les investigations menées ont également permis une meilleure compréhension des mécanismes de formation du contact NiP/Si. La faisabilité des techniques de dépôt électrochimique a été démontrée pour diverses applications: cellules avec contacts électrochimiques NiP/Ag en face avant, cellules de type n, épaississement de contacts fins sérigraphiés… Des résultats très prometteurs d’amélioration de facteur de forme FF et de rendement η ont été obtenus et permettent d’envisager une ouverture potentielle vers de nouvelles structures de cellules photovoltaïques à haut rendement : cellules à émetteur peu dopé, cellules à émetteur sélectif avec ouverture laser de la couche anti-reflet, cellules à contacts arrières….