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

• 10 mM AlCl3, InCl3 threshold in wet-processing of highly doped Al:ZnO, In:ZnO layers
• 9 mM AlCl3 doping rate 1.2 nm/s and Au/Ni/Al:ZnO/ZnO/Mo resistivity 0.9 × 105 Ω·cm
• Al:ZnO, In:ZnO microstructure dependent on Al, In at.% content and layer thickness
• 4 GPa biaxial stress of ZnO/Si and hydrostatic strain εh = − 2.000 of 12 at.% Al:ZnO/ZnO
• 7 meV/cm spatial resolution of ZnO nanorod energies by Optical Modulation techniques

High quality polycrystalline and nanostructured ZnO thin films were electrochemically deposited from aqueous solution of zinc nitrate (Zn(NO3)2) at negative electrochemical potential (EC = − 0.6 to ‑−1.4 V) and moderate temperature (75–80 °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 σ = 4 GPa. Growth rates depended on substrate orientation and electrochemical potential varying from EC = − 1.1 V for deposition on Si(100) to EC = ‑−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 εh = ‑−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 105 Ω·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 Eg = 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 (4 meV 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)Se2 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.