Improved stability of CdTe solar cells by absorber surface etching

For CdTe solar cells copper seems to be necessary to achieve best energy conversion efficiencies, whilst it is known to be the main reason of cell performance degradation due to its tendency to diffuse through the bulk. Some studies have shown a direct connection between defect concentration and copper, but little has been discussed about its relation to the CdTe etching. Within this study many samples with Cu/Au back-contact have been prepared with different etching times and tested applying thermal, luminous and electrical stresses. We have analyzed the aging effects on the cell performance and on the nature and concentration of the defects by means of a variety of characterization techniques, like atomic force microscopy, Raman, current-voltage, capacitance-voltage, drive level capacitance profiling and admittance spectroscopies. Results of the accelerated lifetime tests show that different performance degradation is observed for cells with differently etched absorber. Solar cells made with optimized etching have very low degradation while the strongest performance reduction is detected for unetched cells despite their initial efficiency is as high as for the case of etched absorbers.