Optimization of Electrochemically Deposited Highly Doped ZnO Bilayers on Ga-Rich Chalcopyrite Selenide for Cost-Effective Photovoltaic Device Technology

High quality polycrystalline bilayers of aluminium doped ZnO (Al:ZnO) were successively electrodeposited in the form of columnar structures preferentially oriented along the (10 (1) over bar1) crystallographic direction from aqueous solution of zinc nitrate (Zn(NO3)(2)) at negative electrochemical potential of E-C = (-0.8)-(-1.2) V and moderate temperature of 80 degrees C on gallium rich (30% Ga) chalcopyrite selenide Cu(In,Ga)Se-2 (CIGS) with chemically deposited ZnSe buffer (ZnSe/Cu(In,Ga)Se-2/Mo/glass). The aluminium doped ZnO layer properties have initially been probed by deposition of Al:ZnO/i-ZnO bilayers directly on Mo/glass substrates. The band-gap energy of the Al:ZnO/i-ZnO reference layers was found to vary from 3.2 to 3.7 eV by varying the AlCl3 solute dopant concentration from 1 to 20 mM. The electrical resistivity of indium-pellet contacted highly doped Al:ZnO sheet of In/Al:ZnO/i-ZnO/Mo/glass reference samples was of the order rho similar to 10(-5) Omega.cm; the respective carrier concentration of the order 10(22) cm(-3) is commensurate with that of sputtered Al:ZnO layers. For crystal quality optimization of the bilayers by maintenance of the volatile selenium content of the chalcopyrite, they were subjected to 2-step annealing under successive temperature raise and N-2 flux regulation. The hydrostatic compressive strain due to Al3+ incorporation in the ZnO lattice of bilayers processed successively with 5 and 12 mM AlCl3 dopant was epsilon(h) = -0.046 and the respective stress sigma(h) = -20 GPa. The surface reflectivity of maximum 5% over the scanned region of 180-900 nm and the (optical) band gap of E-g = 3.67 eV were indicative of the high optical quality of the electrochemically deposited (ECD) Al:ZnO bilayers.