Development of tools to improve the efficiency of hydrogenated amorphous silicon solar cells

Resumo

One of the alternative energies with more potential and less environmental impact is photovoltaic energy. In this sector, thin-film hydrogenated amorphous silicon (a-Si: H) solar cells are especially attractive due to their low cost. This project aims to improve the efficiency of such cells by two ways: simulation and characterization of these devices in real time. The studied cells are based on an a-Si: H p-i- n junction. Specifically they consist of a float glass substrate about 3.2mm thick, over which we find deposited: -A front contact transparent conductive oxide (TCO) of a thickness between 500-1000 nm, in particular made up of tin oxide doped with fluorine. -Three layers of a-Si: H: A p-type (10-20 nm) layer, an intrinsic (150-300 nm) and a n-type (10-20 nm) one. -The Back contact, formed by another TCO, in this case zinc oxide doped with aluminum, a reflective metal layer (aluminum) and a aluminum nickel vanadium layer which protects from oxidation. Simulations can save time and money of experimental tests. These were made with Sentaurus simulator (from Synopsys), which allows simulation 1D, 2D and 3D of different electronic devices. This software specializes in crystalline silicon devices, but has an advanced interface to introduce other materials. The solar cells which we study are made up of materials out of this database, so it was necessary to obtain the physical properties. Experimental data were introduced using data from atomic force microscopy (AFM) measurements and optical characterization of the layers. Theoretical models of the materials, composing the density of states and recombination and generation of carriers, were also used. In particular the simulations search for the negative effects of series and parallel resistances in the cell’s performance, and analyze the use of other materials in the p and n layers. The second path of the thesis was to design a device for real time characterization of the cells. In most solar panels cells are connected in series, so a very uniform current generation is required. The short circuit current density (Jsc) of a solar cell can be calculated from the measured spectral response (SR) of a small illuminated area. However, this kind of measurement is generally performed using a monochromator and lock-in amplifiers, a time-consuming method. Therefore, is not suitable for creating Jsc mappings ​​in large area modules. We develop a very fast spectral response system based on the electronic control of a group of LEDs that operate simultaneously and are focused on a small area of ​​the solar cell. The spectra of LEDs with narrow bandwidth cover the spectrum of silicon solar cells (from 300 nm to 1100 nm). Furthermore, each is diode is supplied with a single frequency. The time dependent current generated by the solar cell under study is sampled and subsequently a FFT analysis is performed to determine the current generated by each LED. In short, with this thesis we pursuit a better understanding of a-Si:H solar cells in order to obtain more efficient devices.