Structural inhomogeneities in polycrystalline silicon on glass solar cells and their effects on device characteristics

The effects of electrostatic fluctuations due to charged extended defects and strain-induced bandgap fluctuations are examined in polycrystalline silicon on glass solar cells. The analysis is based on models previously applied to Cu(In,Ga)Se2 solar cells, but with a new interpretation of the local ideality factor associated with electrostatic fluctuations. It is shown that electrostatic fluctuations become influential to the cell voltage properties as the absorber dopant concentration falls below a certain threshold (a few 10 15 cm � 3 ), and the degradations to the open circuit voltage and fill factor are expected to increase with further lowering of dopant density. It is equally plausible that the electrostatic fluctuations originate from charged dislocations or grain boundaries. Bandgap fluctuations on the other hand can be detrimental to the open circuit voltage of cells of any absorber dopant density. However, this voltage degrading effect is seen only in the cells deposited by electronbeam evaporation, and not amongst those made by plasma enhanced chemical vapour deposition. Copyright # 2011 John Wiley & Sons, Ltd.