Electrochemical synthesis of Cu-doped ZnTe films as back contacts to CdTe solar cells

Electrochemical deposition of Cu-doped ZnTe films was investigated as a back contact material for CdTe solar cells. The deposition system consisted of a potentiostat, a substrate (cathode), a Pt counter electrode, and an Ag/AgCl reference electrode. Experimental conditions were: pH=2.5∼4, deposition temperature=70 ∼80°C electrolyte concentration=0.02 M Zn²⁺, 10^-4 M TeO₂, and deposition potential (V_a) = -0.85 ∼-1.0 V vs. Ag/AgCl. A cyclic voltammogram obtained from an ITO substrate showed a ZnTe reduction peak at Va = -0.7 ∼ -0.9 V. A film obtained at pH=2.5, V_a=-0.975 V showed the strongest ZnTe X-ray diffraction peak, but its surface morphology was not as smooth as the films made under the condition of pH=3∼4, V_a=-0.950V. Triethanolamine did not form a complex with Cu²⁺ in acidic solution as needed for uniform Cu-doping, but oxalate anion and 1,10-phenanthroline did at pH=2∼3. The efficiency of CdTe solar cells increased from 3.4% to 7.1% when Cu-doped ZnTe film was used as the back contact.