Structural, optical, electrical properties, and strain/stress of electrochemically deposited highly doped ZnO layers and nanostructured ZnO antireflective coatings for cost-effective photovoltaic device technology

High quality polycrystalline and nanostructured ZnO thin films were electrochemically deposited from aqueous solution of zinc nitrate (Zn(NO3)(2)) at negative electrochemical potential (E-C = -0.6 to -1.4 V) and moderate temperature (75-80 degrees C) on various substrates (Cu, Si, Mo/glass, ZnSe/Mo/glass, F: SnO2/glass). Undoped (i-ZnO) films were grown free of strain on Cu and Mo/glass using intermediate lattice matched buffer (ZnSe). The i-ZnO films on Si exhibited high tensile in-plane stress of sigma= 4 GPa. Growth rates depended on substrate orientation and electrochemical potential varying from E-C = -1.1 V for deposition on Si(100) to E-C= -1.3 V for optimum deposition on Si(111). Growth rates of undoped and doped (n-ZnO) films with Al and In dopant (Al: ZnO, In: ZnO) on Mo/glass depended on the solute dopant (AlCl3, InCl3) concentration (0.2 nm/s for i-ZnO, 0.7 nm/s for n-ZnO with n = 5 mM, 1.2 nm/s for n-ZnO with n= 9 mM). Hydrostatic compressive strains by incorporation of 0.4-12.0 at.% Al were in the range of epsilon(h) = -0.070 to -2.000. The resistivity of metal-pin contacted n-ZnO/i-ZnO films (Me/Al: ZnO/i-ZnO/Mo/glass, Me/In: ZnO/i-ZnO/ZnSe/Mo/glass, Me: Au/Ni) was in the order 10(5) Omega.cm. In the UV-VIS-NIR region (1.5-5.0 eV), the reflectivity of the films did not exceed 20% and the (optical) band gap of E-g = 3.5 eV indicated their high optical quality and material purity. The optical properties of electrochemically grown ZnO nanorod arrays (transparency: 70-80% at 1.5-3.0 eV and band-gap: 3.3-3.7 eV) were additionally analyzed by photoreflectance spectroscopy. By application of optical modulation techniques, both spectrally (4meV at 300 K) and spatially (7 meV/cm) resolved gap energies were quantified and the role of nitric (HNO3) and nitrate (NH4NO3) additives was extensively reinforced. Antireflective coatings (ARCs) of ZnO nanorods assembled in Cu(In, Ga)Se-2 thin film solar cells were found to quench the surface reflectivity by 5% at least. Integration of electrochemically processed transparent conductive ZnO films and ZnO ARCs in solar cell device technology is anticipated to provide key-solutions for cost effective energy harvesting. (C) 2015 Elsevier B.V. All rights reserved.