Mauro Zanuccoli

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.

Understanding the Impact of the Doping Profiles on Selective Emitter Solar Cell by Two-Dimensional Numerical Simulation

The selective emitter (SE) design, featuring lower doped areas between the front contact fingers and higher doped areas underneath the front metallization, is crucial to improve the performance at the front side of a monocrystalline (c-Si) silicon solar cell. One of the most interesting and promising low-cost SE process consists of the screen printing of a phosphorus-doped paste, allowing a separate optimization of the doping profiles in the metallized and nonmetallized front-side areas. By referring to this kind of process, this paper presents a simulation study with a decoupled analysis on the effect of the lowly doped and highly doped profiles on the performance of an SE solar cell, by means of 2-D electro-optical numerical device simulations. Moreover, by exploiting the 2-D modeling, the effect of the alignment tolerance used in the SE diffusion process for the subsequent metallization process has been also investigated. Numerical results show that the adoption of an optimized design for the SE cell can lead to an efficiency improvement above 0.4%abs compared with the 75 Ω/sq homogeneous emitter reference cell.

2-D numerical simulation and modeling of monocrystalline selective emitter solar cells

This paper presents a detailed analysis of the dependence of the performance of crystalline silicon (c-Si) selective emitter solar cells on geometrical parameters and doping profiles. Based on two dimensional drift-diffusion TCAD simulations, we report the effects of the front contact pitch and doping profiles on the most important output parameters of solar cells. Simulations show that a significant gain in terms of output power of the cell may arise compared to the homogeneous emitter solar cell. We also present an analysis of the main loss mechanisms and trade-offs for this solar cell.

Numerical Simulation on the Influence of Via and Rear Emitters in MWT Solar Cells

In this paper, we analyze, by means of numerical simulations, metal wrap through (MWT) silicon solar cells without a rear emitter and/or via an emitter that feature a Schottky contact between the Ag metal and the p-base. We show how the effective Schottky barrier height affects both dark and illuminated properties of the cell. An equivalent electrical model for the dark analysis is proposed, which accounts for the shunting effects and the thermionic-emission current at Ag/p-base contact. We investigate the figures of merit of MWT solar cells for different via configurations, highlighting the influence of the Ag/p-base barrier height. Moreover, the influence of the rear busbar width, as well as of the operating temperature, is analyzed.

Loss analysis of silicon solar cells by means of numerical device simulation

Numerical modeling represent a powerful approach to investigate the physical loss mechanisms that limit the conversion efficiency of solar cells. In this paper, a comprehensive analysis of the different loss mechanisms affecting the performance of a conventional c-Si solar cell, such as optical losses, recombination losses and parasitic resistive losses, has been performed by means of numerical device simulations. Moreover, a detailed quantification of the impact of the different recombination mechanisms within the cell has been carried out through dark current-voltage simulations, both for a conventional c-Si solar cell and a high-efficiency selective emitter c-Si solar cell.

Computationally efficient finite-difference modal method for the solution of Maxwell’s equations

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.

Iterative approach as alternative to S-matrix in modal methods

The continuously increasing complexity of opto-electronic devices and the rising demands of simulation accuracy lead to the need of solving very large systems of linear equations making iterative methods promising and attractive from the computational point of view with respect to direct methods. In particular, iterative approach potentially enables the reduction of required computational time to solve Maxwell’s equations by Eigenmode Expansion algorithms. Regardless of the particular eigenmodes finding method used, the expansion coefficients are computed as a rule by scattering matrix (S-matrix) approach or similar techniques requiring order of M3 operations. In this work we consider alternatives to the S-matrix technique which are based on pure iterative or mixed direct–iterative approaches. The possibility to diminish the impact of M3 -order calculations to overall time and in some cases even to reduce the number of arithmetic operations to M2 by applying iterative techniques are discussed. Numerical results are illustrated to discuss validity and potentiality of the proposed approaches.

Optical simulation of ZnO/CdTe and c-Si/a-Si vertical nanowires solar cells

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.