Interpretation of the effect of back surface etching on barrier height, carriers concentration and defects density of the CdTe solar cell devices

Thin film CdS/CdTe photovoltaic devices have shown a potential interest by the researchers due to the low fabrication cost and high device stability. Developing an appropriate back contact for CdTe solar cells is crucial to achieve good device performance. In this work, we explore the impacts of hydrogen iodide acid etching on the back surface of CdTe solar cell devices to remove the back oxides and chlorides formed during CdCl2 treatment and form a Te-rich back surface. The back barrier height, carrier density and defect states are measured by temperature dependence of current-voltage (J-V-T), capacitance-voltage (C-V-T), admittance spectroscopy (AS) and impedance spectroscopy (IS) measurements. In C-V measurements, an appropriate AC modulation voltage and frequency will be selected, and DC bias voltage will be scanned. In AS and IS measurements, carriers or defects emission induces by applying small AC modulation voltage to be trapped and detrapped at certain frequencies and temperatures. An obvious increase in carrier concentration, suggesting an increased copper doping in the device. Additionally, the HI-treatment eliminates another defect level at 0.41 eV with respect to the valance band. All these improvements lead to better device performance of CdTe solar cells.