SPLTRAK Abstract Submission
Monte Carlo evaluation of multijunction solar systems in tandem and 4-terminal configurations
Roberto Corso, Marco Leonardi, Andrea Scuto, Salvatore A. Lombardo
Institute for Microelectronics and Microsystems, National Research Council, Catania, Italy

In this work, we perform a comparison between the tandem and the 4-terminal configuration of multijunction systems. Multijunction systems are made up by cells of different materials, each working on the portion of solar spectrum it is more suited to, and are a promising solution to overcome the efficiency limit of single-junction silicon photovoltaics. To this end, we developed an algorithm which performs Monte Carlo simulations of photons traveling through the layers of a multijunction system. The model is specifically designed for textured surfaces, for whom it implements a unit-cell approach and customizable pyramidal structures. The solar cell is made up by a certain number of layers, and for each layer its complex refractive index, thickness and texturing details are required. At each step, the intersection between the traveling direction and the surrounding surfaces is calculated; then, as the photon travels to the intersection point, a random absorption event is performed (where the absorption chance is based on the Lambert-Beer law) and, if the photon has not been absorbed in the current layer, its position is updated. Then, a reflection-transmission event is performed (with probabilities calculated from the Fresnel equations). When the photon reaches the boundaries of the unit-cell, periodic boundary conditions are applied.  This model has then been used to evaluate and optimize the performance of a multijunction solar system. We simulated the absorption spectra of the layers of the system while varying the thicknesses of the layers in the top cell, and from the simulated absorptions the current-voltage characteristics and power conversion efficiencies have been calculated. These steps have been performed for the tandem as well as for the 4-terminal configuration. Noticeable differences between the two cases are found and discussed in the paper.