Numerical Simulation of Vertical Silicon Nanowires based Heterojunction Solar Cells

Abstract 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%

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

Abstract 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.Peer Reviewe

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

International audienceDue 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

Technological guidelines for the design of tandem III-V nanowire on Si solar cells from opto-electrical simulations

International audienceEffect 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 junction. Finally, simulations of the fully optimized tandem structure show that a promising efficiency of with the short-circuit current of can be achieved with reasonable bulk and surface carrier lifetime

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

International audienceThe 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

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

International audienceThe 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

Coupling Optical and Electrical Modelling for the study of a-Si:H-based nanowire Array Solar Cells

International audienceCoupled optical/electrical simulations have been performed on solar cells consisting in arrays of p‐i‐n radial nanowires based on crystalline p‐type silicon (c‐Si) core/hydrogenated amorphous silicon (a‐Si:H) shell heterojunctions. Three‐dimensional (3D) optical calculations based on rigorous coupled wave analysis (RCWA) are firstly performed and then coupled to a semiconductor device simulator that exploits the radial symmetry of the nanowires. By varying either the doping concentration of the c‐Si core, or the work function of the Al‐doped ZnO (AZO) back contact we can separate and originally highlight the contribution to the cells performance of the nanowires themselves (the radial cell) from the planar part in between the nanowires (the planar cell). We show that the short‐circuit current density (Jsc) only depends on the doping of the c‐Si core indicating that it is mainly influenced by the radial cell. On the contrary the open‐circuit voltage (Voc) is strongly affected by the back contact conditions (AZO work function), revealing an important impact of the interspacing between the nanowires on the characteristics of the entire nanowire array. We explain this strong influence of the back contact conditions by the fact that it determines the band‐bending in the a‐Si:H absorber shell touching the AZO, i.e. in the planar part. Therefore, it directly impacts the potential drop (Vbi) in the same area. For low AZO work functions, the dark current density (Jdark) is increased in the planar region, where Vbi is lower, which degrades the Voc of the entire cell

Opto-electrical simulations of tandem solar cells based on III-V nanowires on silicon substrate

session poster (C.P2.15)International audienc