The effect on CdS/CdTe solar cell conversion efficiency of the presence of fluorine in the usual CdCl2 treatment of CdTe

The addition of CdF2 to the CdCl2 solution used in the well-known CdCl2 treatment of CdS/CdTe solar cells has been observed to drastically improve the conversion efficiency of fully fabricated CdS/CdTe solar cells. The observed improvement is as a result of further enhancement of structural and optoelectronic properties of the CdCl2+CdF2-treated CdTe layers compared to the CdCl2-treated CdTe layers. A set of CdS/CdTe samples were grown by electrochemical deposition under different conditions and each sample was divided into two. One set of these parts was treated with only CdCl2 solution while the other set was treated with the CdCl2+CdF2 mixture. Both sets were annealed at 450°C for 15min and the solar cells completed by evaporating Au back contact on the CdTe layers after chemical etching. The results of the device measurements show drastic improvements in all the solar cell parameters (open-circuit voltage, short-circuit current density, fill factor and conversion efficiency) for the devices treated with CdCl2+CdF2 mixture compared to those treated with only CdCl2. The conversion efficiencies increased from (1.0-3.0)% with CdCl2 treatment to about (5.0-7.0)% with CdCl2+CdF2 treatment. The observed improvements have been attributed to the unique properties of fluorine.