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.

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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.

This work presents a study based on electro-optical numerical simulations of the impact of geometrical and doping parameters on main figures of merit of crystalline silicon back-contact back-junction solar cells. State-of-the-art physical models in combination with two-dimensional simulations performed by a TCAD tool have been adopted to carry out an extensive and detailed analysis of the influence of many fabrication parameters on performance. The studied design parameters are the doping level in front surface field (FSF), back surface field (BSF) and emitter, and the main geometrical parameters. A doping level value that allows the maximization of the efficiency for the three regions can be clearly identified. In particular, for BSF and emitter, an efficiency degradation is observed for relatively lower doping values and is ascribed to the higher contact recombination while for higher doping values the Auger recombination plays a significant role in reducing the ultimate efficiency. In FSF region the recombination due to defects at the front interface is the main limiting mechanisms for efficiency. On the basis of our analysis, a marked sensitivity of the efficiency to the gap and pitch size is caused by the series resistance increase. The efficiency exhibits a maximum value for an emitter coverage fraction (R) of 85 %. However, in the case of lower emitter coverage, Auger, Shockley–Read–Hall (SRH): in bulk and at interfaces are detrimental for the cell conversion efficiency

In this work, a new implementation of the finite-difference (FD) modal method (FDMM) based on an iterative approach to calculate the eigenvalues and corresponding eigenfunctions of the Helmholtz equation is presented. Two relevant enhancements that significantly increase the speed and accuracy of the method are introduced. First of all, the solution of the complete eigenvalue problem is avoided in favor of finding only the meaningful part of eigenmodes by using iterative methods. Second, a multigrid algorithm and Richardson extrapolation are implemented. Simultaneous use of these techniques leads to an enhancement in terms of accuracy, which allows a simple method such as the FDMM with a typical three-point difference scheme to be significantly competitive with an analytical modal method.

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