Optoelectronic Tunability of Hf-Doped ZnO for Photovoltaic Applications

Abstract

The need for a high optical spectrum throughput, high conductivity, and controlled energy levels of transparent conductive oxide used in solar cells stresses the development of novel materials that help reduce the existing dependency on indium-based oxides. ZnO is a promising material in this context, and in this work, we demonstrate how Hf doping of ZnO films allows engineering both electrical and optical properties to fit the requirements of different solar cell architectures and materials. We focus on the lightly doped domain where Hf substitution is believed to be the key for band gap tunability without negatively affecting the carrier transport behavior. We provide experimental analysis of controlled changes in the optical and electrical properties, including work function, and a detailed analysis of the structural behavior resulting from the deposition at elevated temperature. We finally present first-principles density functional theory simulations to elucidate the mechanisms responsible for the obtained electronic and electrical properties that predict a modification in the band structure of ZnO when Hf is substituted and/or embedded in the ZnO matrix as HfO2 phases.

Spectroscopic Ellipsometry Study on Tuning the Electrical and Optical Properties of Zr-Doped ZnO Thin Films Grown by Atomic Layer Deposition

“…)/( ) opt 2 0 e (10) where ε ∞ and ε 0 are the dielectric constants of the medium and free space, respectively, and m e * is the effective mass of the charge carriers. Furthermore, N opt , ρ, and μ, τ exhibit the following relationships: 1/τ = −e/(m e *μ)] and ρ = 1/ (N opt e μ).…”

“…In addition, ZnO exhibits high chemical stability, good surface uniformity, and a large exciton binding energy of 60 meV. ZnO also has excellent electrical conductivity generated by its high concentration of donor native point defects, such as oxygen vacancies (V O ) and zinc interstitials (Zn i ), enhanced by doping with group IV elements including titanium, 7 zirconium, 8 hafnium, 9,10 and ruthenium. 11 Furthermore, Zr incorporation in ZnO lattice is chemically favorable due to the comparable ionic radii of Zn 2+ and Zr 4+ ions (74 and 73 pm, respectively) for the four-coordinate tetrahedral substitution, which generates two extra electrons by each ionic substitution.…”

Solution Combustion Synthesis of Hafnium-Doped Indium Oxide Thin Films for Transparent Conductors

“…To study the stability of the thin films, the measurements were repeated after 3 weeks and, as can be seen in Figure 6, all electrical properties were improved. This slight enhancement can be attributed to fewer weakly bound oxygen species after the annealing treatments and more deep-level defects passivated with strong bonds [57][58][59][60][61][62].…”

“…Electrical properties of Hf-doped transparent conductive thin films prepared by various methods. References [32,34,46,[58][59][60][61][62] have been cited in the Supplementary Materials.…”

Role of Hafnium Doping on Wetting Transition Tuning the Wettability Properties of ZnO and Doped Thin Films: Self-Cleaning Coating for Solar Application

“…Hafnium, being a transition metal that exists in a + 4 oxidation state (charge mismatch: Hf 4+ and Zn 2+ ), a similar ionic radius (0.78 Å) to Zn (0.74 Å), low electronegativity (1.3), and high basicity can be a promising doping candidate to ZnO for self-cleaning applications. Several studies showed that doping with hafnium at a lower concentration can aid in increasing defects (oxygen vacancies) due to charge mismatch and, thus, will aid in producing hydrophilic surfaces. − Further, doping with high hafnium concentration will lead to the formation of HfO 2 , which is a strong Lewis base and an efficient suppressor of oxygen vacancy will lead to the development of hydrophobic surfaces. − Additionally, strong affinity of hafnium (−1.70 eV) toward oxygen in comparison to that of zinc (−0.76 eV), results in strong bonding between Hf and oxygen ions, thereby leading to a decrease in the concentration of oxygen vacancies.…”