Iinvestigation of ZnO, and AZnO and rare earth doped ZnO thin films for spectral conversion and application to solar cells

Abstract

Recently Zinc oxide has drawn a resurgent attention in semiconductor industry due to its interesting properties with diverse application potential. These properties include high exciton binding energy, high resistance against radiation, high breakdown voltage, insensitivity to visible light, and easy wet chemical etching. The high quantum efficiency for emission by ZnO has seen it being considered a strong candidate for solid-state white lighting applications as well as transparent conductor electrode in solar cells. In order to realize efficient utilization of the multi-functional properties of ZnO for electronic and opto-electric applications, ZnO is usually doped with different elements. Such doping is aimed at enhancing and controlling its electrical, optical and multi-functional properties. Typical dopants widely used are trivalent atoms categorized as group III in the periodic table (Al, In, Ga) through substitution of cations. The as-grown ZnO thin film is usually n-type semiconductor with structural, electrical and optical properties that can be varied depending on the growth conditions as well as post deposition treatment such as thermal annealing. The use of RF sputtering for ZnO deposition has been explored in this work through varying deposition time, RF power and the partial pressure of oxygen. The films were then subjected to ex-situ thermal annealing in Argon filled furnace leading to a significant increase in grain size. Rare earth (RE) doping of materials has been widely investigated owing to the prominent and desirable optical and magnetic properties. Typically trivalent rare earths elements such as Sm+, Tb3+ and Eu3+ are investigated in this research project. ZnO doped with RE has exhibited electroluminescence, thus highlighting its potential for photovoltaic applications as a bi-functional layer. A doped ZnO layer is thus simultaneously utilized as transparent conducting electrode and as a spectral conversion layer. The RE doped luminescent materials provide an opportunity to effectively use the high energy and sub-band gap energy photons from the solar spectrum that would have otherwise been lost in direct band gap absorbers. In solar cells, they have been applied with an intention to reduce the fundamental thermalization losses arising as a result of the intrinsic properties of the semiconductor material namely: (a) sub-bandgap photon loss (b) thermalization of charge carriers resulting from absorption of high energy photons. From the X-ray diffraction (XRD) patterns both pristine and doped ZnO thin films showed growth along the c-axis of the wurtzite structure. The peaks were found to match the reflection planes of (100), (002) and (102) with all the diffraction peaks being well indexed to the wurtzite structure of ZnO of the space group P63mc, which is consistent with the standard values reported in JCPDS, card no. 03-0888. The structural properties of the material were investigated using a -scanning electron microscope (SEM) and Atomic force microscopy (AFM) where the particle size, roughness, skewness and kurtosis were found to change with growth condition and annealing temperature. Most importantly, the results indicated that the photoluminescence (PL) properties reflect the quality of the pristine and doped ZnO. The films were then used in the fabrication of the solar cells as a bi-functional layer and thus as a proof of concept of good transparent conducting oxides (TCOs) and for spectral conversion. RBS measurements indicated the depth profile distribution of Zn, O and various rare earths which showed homogeneity in depth distribution without any external impurity.