Anne Kaminski-Cachopo

Physical Properties of Annealed ZnO Nanowire/CuSCN Heterojunctions for Self-Powered UV Photodetectors

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/cm(2). 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/cm(2), 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.

Light trapping in ZnO nanowire arrays covered with an absorbing shell for solar cells

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.

Light absorption processes and optimization of ZnO/CdTe core–shell nanowire arrays for nanostructured solar cells

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.

Time-resolved photoluminescence for self-calibrated injection-dependent minority carrier lifetime measurements in silicon

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.

Improvement of the physical properties of ZnO/CdTe core-shell nanowire arrays by CdCl2 heat treatment 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.

Modeling of Edge Losses in Al-BSF Silicon Solar Cells

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 (J0m) and edge recombination (J0e-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.

Opto-electrical simulation of III-V nanowire based tandem solar cells on Si

Due to their nanostructured surface, nanowire-based solar cells are interesting structures to increase light absorption in thin film solar cells. Among these structures, III-V nanowires grown on silicon substrates to obtain a tandem solar cell are particularly interesting to absorb selectively different part of the solar spectrum and to reduce thermalization effects. The aim of this work is to perform optical and electrical simulations of tandem solar cells based on III-V nanowires on silicon and to compare two different III-V semiconductor compounds (GaAs0.8P0.2 and Ga0.8Al0.2As) with a band gap of 1.7 eV (optimal on Si) for the nanowire array. Optical simulations are performed with an in-house Rigorous Coupled Wave Analysis (RCWA) software by taking into account the current matching between the two solar cells. Electrical simulations are performed with the TCAD software Sentaurus. Opto-electrical simulations demonstrate that optimal geometries and efficiencies are very similar for the two semiconductors used for the nanowires.

Plasma immersion ion implantation (PIII): New path for optimizing doping profiles of advanced phosphorus emitters

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 concentr...

Effects of the pH on the Formation and Doping Mechanisms of ZnO Nanowires Using Aluminum Nitrate and Ammonia

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.

Toward an efficient extremely thin absorber solar cell based on ZnO nanowire arrays

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.