Characterization of cast polycrystalline silicon

Inexpensive solar cell wafers are required for large scale photovoltaic power. Cast poly-Si solar cells are made from such wafers. Inexpensive wafers are made from low quality starting material and the silicon growth method is simple and cheap compared with single silicon. Therefore, the wafers have many defects like grain boundaries and dislocations. There are also many precipitates which originate during the silicon growth from the impurities of the starting material and from the crucible.; These defects are examined here by several characterization methods. Diffusion length measurements show that high-efficiency cells have higher diffusion lengths than low-efficiency cells and cells have higher diffusion lengths than wafers. The surface photovoltage (SPV) method show a higher stress in low-efficiency wafers than in high-efficiency wafers. Deep level transient spectroscopy (DLTS) gives the defect energy levels and the impurity concentrations. Current-voltage (I-V) measurements show non-ideal I-V behavior of cast poly-Si solar cells. Activation energy measurements show a low activation energy and this implies current mechanisms in the diode other than space-charge recombination and quasi-neutral region recombination. Defect etch shows grain boundaries, dislocations and precipitates as expected and electron beam induced current (EBIC) shows these defects to be strong recombination centers.; Solar cell efficiency modeling including grain boundary recombination shows interesting results. The optimum base doping concentration for maximum cell efficiency decreases as the impurity concentration increases. This explains the reason why poly-Si solar cells have lower base doping concentrations than single crystal silicon solar cells.