M. Zanuccoli

Performance Analysis of Rear Point Contact Solar Cells by Three-Dimensional Numerical Simulation

The adoption of local point contacts at the back surface of high-efficiency monocrystalline silicon solar cells is strategic in order to reduce the recombination losses at the rear side of the device. However, the reduction of the rear-contact surface leads to an increase of series resistance losses. In this paper, we present an extensive analysis based on 3-D optoelectronic numerical device simulations in order to highlight the dependence of the conversion efficiency on the main geometrical and technological parameters of the cell, such as the pitch and the size of the rear point contacts and the substrate resistivity. A state-of-the-art device simulator has been successfully adopted in order to accurately solve the transport equations in the semiconductor by taking into account all the loss mechanisms that are crucial in order to address the design of the cell.

Open issues for the numerical simulation of silicon solar cells

The improvement of solar cell efficiency requires device optimization, including the careful design of contacts and doping profiles, and the implementation of light trapping strategies. In this context, electro-optical numerical simulation is essential to analyze the physical mechanisms that limit the cell efficiency and lead to design trade-offs. In this work we discuss the calibration of the relevant physical models for electrical simulation and we put in evidence important limitations of the most common adopted optical simulators.

Understanding the impact of double screen-printing on silicon solar cells by 2-D numerical simulations

In this work we investigate c-Si solar cells in which a two steps (double) screen-printing is implemented for the front contact in order to increase the aspect ratio of the fingers. To this purpose numerical simulations are performed on a solar cell and metallization properties are defined according to experimental data. The analysis of solar cells with conventional single screen-printing is also performed for comparison. In agreement with experiments we show the efficiency improvement of double printing over single printing. The analysis of electrical parameters (fill factor, short-circuit current and open-circuit voltage) allows to understand the source of the enhanced efficiency.

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.

2-D Numerical analysis of the impact of the highly-doped profile on selective emitter solar cell performance

Two-dimensional (2-D) numerical simulations have been performed to investigate the impact of the doping profile in the metal-contacted highly-doped regions for c-Si selective emitter (SE) solar cells. Numerical results show that the doping profile under the metallization significantly influences the recombination effects on the front-side of the SE cell and consequently its performance. A strong impact on the short-circuit current density of the solar cell has been seen. This is mainly due to the inclusion of large alignment tolerances used in the SE diffusion for the subsequent metallization process, leading to broad highly-doped areas. In this regard, a quantitative analysis of this effect has been carried out. The results reveal that an improved alignment, allowing a reduction of the alignment tolerances, leads to a wider process window of doping profiles and hence better SE cell performance.

Numerical simulation and modeling of rear point contact solar cells

High efficiency silicon monocrystalline solar cells commonly adopt point contacted rear surfaces to reduce the recombination losses in the rear side of the device. However, the reduction of the rear contact surface leads to an increase of series resistance losses. Modeling and analysis of rear point contact solar cells is strategic to optimize the device design by taking into account several competing physical mechanisms. Owing to their complicated geometries, the analysis of these devices requires three-dimensional (3-D) numerical simulation. In this work we analyze the influence of the most important geometrical and electrical parameters on the conversion efficiency of rear point contact solar cells.

Computationally efficient method for optical simulation of solar cells and their applications

This paper presents two novel implementations of the Differential method to solve the Maxwell equations in nanostructured optoelectronic solid state devices. The first proposed implementation is based on an improved and computationally efficient T-matrix formulation that adopts multiple-precision arithmetic to tackle the numerical instability problem which arises due to evanescent modes. The second implementation adopts the iterative approach that allows to achieve low computational complexity O(N logN) or better. The proposed algorithms may work with structures with arbitrary spatial variation of the permittivity. The developed two-dimensional numerical simulator is applied to analyze the dependence of the absorption characteristics of a thin silicon slab on the morphology of the front interface and on the angle of incidence of the radiation with respect to the device surface.

Fabrication, Simulation, and Experimental Characterization of EWT Solar Cells With Deep Grooved Base Contact

In this paper, we present a new solar cell design conceived for low-medium concentrator photovoltaic applications. The proposed cell is based on the emitter-wrap-through concept featuring grooved p-doped hole base contacts. The cell is fabricated by realizing an array composed of holes of two alternating doping types obtained by means of a deep reactive ion etching technique. Measurements under 1-sun illumination confirm the advantages of the considered architectures in terms of short-circuit current density and photon collection properties with respect to conventional frontand back-contact solar cells. Three-dimensional numerical simulations, calibrated starting from the measured dark J-V characteristics, are exploited to investigate the performance under concentrated light (maximum efficiency 21.4% at 44 suns) and to understand, especially in the case of highly resistive substrates, the impact of the p-doped holes depth in terms of resistive losses.

Efficient implementation of the Fourier modal method (RCWA) for the optical simulation of optoelectronics devices

This paper presents a new implementation of the Fourier modal method to solve the Maxwell equations in nano-structured solid state devices like solar cells and image sensors. The proposed method is based on an improved T-matrix approach in which the well-known stability problems are solved by adopting multi-precision arithmetic. Examples of application to the analysis of surface texturing in silicon solar cells are reported.

Simulation of micro-mirrors for optical MEMS

In this paper we present the application of a computationally efficient Rigorous Coupled-Wave Analysis method to solve the electromagnetic radiation scattering problem in devices featuring nano-structured interfaces and multilayer structures. The method, already successfully used to simulate photon propagation and absorption in thin-film solar cells, is used in this work to compute the optical reflectivity of micro- mirrors in Micro-Opto-Electro-Mechanical Systems (MOEMS).