SURFACE PASSIVATION OF CRYSTALLINE SILICON BY AMORPHOUS SILICON CARBIDE FILMS FOR PHOTOVOLTAIC APPLICATIONS

During the last two decades the photovoltaic market has rapidly increased, partly because of a new vision of solar energy as a useful and competitive source of electricity production and partly because of the reduction of manufacture costs while keeping efficiencies high. Crystalline silicon solar cells are the most widely studied cells and up to date they offer one of the best performances in terrestrial applications. The key issue of cost reduction in these cells is the reduction of silicon costs, by using thinner substrates and/or by working with solar-grade low-cost silicon material. In both cases reduction of recombination of charge carriers at the surface, what is called surface passivation, is compulsory to achieve the high efficencies (> 20%) expected. The state of the art in surface passivation is done by thin films of amorphous silicon nitride grow by Plasma Enhanced Chemical Vapour Deposition (PECVD). This material offers excellent surface passivation in most of the solar cell schemes and antireflective properties at the same time. Despite silicon nitride is very well established in photovoltaic field, in this thesis we offer an alternative that is based on amorphous silicon carbide (aSiC), also grown by PECVD. The passivation properties of silicon carbide have been already studied in our group finding that excellent results can be obtained when the films are rich in silicon, especially for those doped with phosphorus to form a n-type material. Because this feature leads to undesirable absorption of solar light within the films that does not contribute to the photocurrent, silicon carbide would then be relegated to passivate only the rear side of the solar cell. The aim of this work is to improve surface passivation properties developed previously at UPC, and add compulsory requisites for the application of crystalline solar cells. These requisites are: uniformity, transparency and antireflective properties, stability under long term operation and stability under high temperature steps (allowing screen printing processes). Also it is the willing to provide a better understanding of the fundamental properties of these films. The main results achieved are enumerated hereafter: Surface passivation improves with the film thickness and then saturates for films thicker than 50 nm. The mechanism responsible for this improvement is not an increase of the electric charge in the film, as in principle could be thought, but a better saturation of defects by the presence of hydrogen. The amount of charge density seems to be independent of the film. Experiments of corona charge reveal some treats about the nature of the charge density to provide the field effect passivation. The origin of the charge seems to be a continuous density of states at the interface, rather a fixed charge allocated in the film. None of the attempts using carbon rich films, which are transparent and with antireflective properties, resulted in excellent surface passivation. Such attempts included variation of the deposition parameters, dilution of plasma with hydrogen, and introduction of nitrogen of in the phosphorus doped a-SiC films. Therefore, up to now it becomes apparent that it is a fundamental property of silicon carbide films the necessity to be rich in silicon to perform surface passivation. The way to combine surface passivation and antireflective properties was applying stacks of different a-SiC layers: one silicon rich and one carbon rich. The thickness of the silicon rich layer was optimized to reach a trade-off between passivation and lost of photocurrent due to the absorption in the film. The stacks were used to passivate p-type bases, with reasonably good results, and n-type emitters, with very good results. A new material was tested: a ternary alloy of silicon, carbon and nitrogen doped with phosphorus. This material was applied to nand p-type bases and n-type emitters, presenting the best results in surface passivation achieved by our group, and comparable to surface passivation record achieved by amorphous silicon carbide. Best composition was rich in silicon, and again the use stacks of silicon rich and carbon rich films was combined successfully. Stability against thermal processes was tested on different passivation schemes. After the treatment, the passivation is strongly reduced for single silicon rich films, which were offering good initial results. On the other hand, the stacks with a second carbon rich film maintain reasonably well the surface passivation properties. The good dielectric properties of the carbon rich films, not provided by the silicon rich ones, resulted crucial to perform the first silicon solar cell with a-SiC rear side passivation with efficiencies above 20%.