Local investigation of monocrystalline silicon solar cells defects

We present results of microscale localization and characterization of defects in monocrystalline silicon solar cells using LBIC and Scanning near-field optical microscopy (SNOM). Although etched silicon is still most effective material for solar cells, some problems with their use in solar plant installation persist due to the defects. The basic characterization methods of silicon solar cells are electrical measurements. They represent integral measurements on the whole cell and only inform us about the presence of defects in the structure. Local defects in the p-n junction may be associated with structural imperfections (such as grain boundaries, dislocations, and scratches), chemical impurities, higher concentrations of donors and acceptors. Therefore, to develop investigation methods, a number of existing apparent and hidden imperfections in the bulk, surface or edges of the cell have to be examined, detected, localized with higher spatial resolution, characterized and classified. Moreover, it is important not only to find most harmful defects, but also to understand their nature and identify the factors which adversely affect recombination properties and consequently an overall efficiency of the cells. The suggested method combines an electric noise measurement with local topography and near-field optical beam induced current. To prove the feasibility of this method, we have chosen one bulk and one edge defect within the sample, which emitted light under low reverse-biased voltage. Light emission from edge spot exhibits strong positive I-U relation with temperature, while for a bulk spot this dependence is slightly negative. Moreover, it was found that one emitting macroscopic spot can consist of several small light emission spots which diameters are bellow 3 μm.