Passivation of trap states in polycrystalline Si by cyanide treatments

The cyanide treatment in which polycrystalline Si is immersed in a KCN solution followed by rinsing in boiling water increases the energy conversion efficiency of 〈ITO/silicon oxide/polycrystalline Si〉 junction solar cells to 12.5%. The XPS measurements under bias show that the trap density in polycrystalline Si is markedly decreased by the cyanide treatment, especially the decrease near the Fermi level being remarkable. The dark current density for the cells without the cyanide treatment depends only weakly on the temperature, indicating that tunneling is a dominant mechanism for the current flow through the Si depletion layer. The cyanide treatment increases the temperature-dependence markedly, showing that thermal excitation of majority carriers becomes necessary for the current flow due to the elimination of the trap states in the Si depletion layer.