LOW-TEMPERATURE SURFACE PASSIVATION FOR SILICON SOLAR-CELLS

Surface passivation at low processing temperatures becomes an importan t topic for cheap solar cell processing. In this study, we first give a broad overview of the state of the art in this field. Subsequently, the results of a series of mutually related experiments are given abou t surface passivation with direct Plasma Enhanced Chemical Vapour Depo sition (PECVD) of silicon oxide (Si-oxide) and silicon nitride (Si-nit ride). Results of harmonically modulated microwave reflection experime nts are combined with Capacitance-Voltage measurements on Metal-Insula tor-Silicon structures (CV-MIS), accelerated degradation tests and wit h Secondary Ion Mass Spectrometry (SIMS) and Elastic Recoil Detection (ERD) measurements of hydrogen and deuterium concentrations in the pas sivating layers. A large positive fixed charge density at the interfac e is very important for the achieved low surface recombination velocit ies S. The density of interface states D-it is strongly reduced by pos t deposition anneals. The lowest values of S are obtained with PECVD o f Si-nitride. The surface passivation obtained with Si-nitride is stab le under typical operating conditions for solar cells. By using deuter ium as a tracer it is shown that hydrogen in the ambient of the post d eposition anneal does not play a role in the passivation by Si-nitride . Finally, the results of CV-MIS measurements (Capacitance-Voltage mea surements on Metal-Insulator-Silicon structures) on deposited Si-nitri de layers are used to calculate effective recombination velocities as a function of the injection level at the surface, using a model that i s able to predict the surface recombination velocity S at thermally ox idized silicon surfaces. These results are not in agreement with the m easured increase of S at low injection levels.