Formation and characterization of novel nanostructured un-doped and Ga-doped ZnO transparent conducting thin films for photoelectrode
In the present work, Ga-doped ZnO (GZO) thin films and ZnO nanorods (ZNRs) were fabricated for possible use as a transparent conducting oxide (TCO) film and a semiconductor oxide (SC) material, respectively, for photoelectrode component of the dye-sensitized solar cells (DSSCs). The GZO nanoparticles (NPs) were synthesized using the reflux-precipitation method, while ZNRs were grown on glass substrates seeded with an optimized GZO layer by the chemical bath deposition (CBD) method. The material properties of the samples were studied using various techniques. The X-ray diffraction patterns for all the GZO NPs and ZNRs exhibited a highly crystalline and hexagonal wurtzite structure of ZnO with no impurity phases. The values of the crystallite sizes range from 11 - 28 and 21 - 31 nm for the NPs and ZNRs samples, respectively. Peak shifts to lower diffraction angles from 47.45 - 47.35° with the increase in Ga/Zn solution pH was observed. A similar trend was observed also for other growth and deposition parameters studied. The studies on effects of annealing temperature and Ga-doping levels showed that the average lattice constants ‘a′ and ‘c′ were calculated as a = 3.263 and c = 5.208 Å, showing an increase in trend with the increase in Ga doping levels and annealing temperature. These values are in close range to the bulk ZnO lattice constants shown in the JCPDS card, No. 79-0205 (a = 3.264 and c = 5.219 Å). The XRD analysis revealed that the as-grown ZNRs were preferentially oriented along the c-axis in most cases with relative texture co-efficient values for (002) plane ranging from 0.309 - 0.418 and 0.37 - 0.56 with the increase in CBD growth concentration and growth time, respectively. The scanning electron microscopy (SEM), images revealed that, at the lower growth pH, the micrographs showed agglomerated tiny GZO NPs that formed on big slabs of nanorods, but fine and enlarged particles on nano-spherical bases formed at the higher pH values. The tiny hexagonal particles for un-annealed samples turned into bigger rods with increasing annealing temperatures. Similarly, the SEM micrographs and atomic force microscopy images of the ZNRs showed an increase in lengths and particle sizes as the growth concentration and time increased. It is important to note that, more regular and orderly nanorods with enhanced morphology and uniformity could be observed with the increase of the growth concentration to 0.05 M and time to 90 min. The PL results of the samples showed sharp and dominant UV-emission peaks which varied in position between 380 and 390 nm; and relatively smaller and broader deep level peak intensities (centred around 550 nm). The highest intensities of the UV-emission peaks were observed for GZO NPs prepared at 2 mol% Ga doping, annealing temperature of 300 ºC and 5 pH of growth solution, as well as ZNRs prepared at 0.05 M concentration and 90 min growth time. Interestingly, minor peaks located around 440 nm could also be seen in few samples. It was observed that the emission peaks of ZNRs and GZO NPs generally shifted towards higher wavelengths with the increase in the growth and deposition parameters. The UV-Vis analysis also demonstrated that the optical properties of the GZO NPs and ZNRs improved with the increase in growth and deposition parameters, as shown by the blue shift of the absorption edge of the reflectance spectra. The band gap energies of the GZO NPs were tuned from around 3.15 to 3.31 eV with the increase in the pH to 5 and Ga doping levels to 2 mol%. However, the band gaps of ZNRs varied between 3.1 and 2.0 eV with the increase in growth concentration and deposition time. It was observed that the GZO NPs’ sample prepared at 2 mol% Ga doping, 5 pH growth medium and annealed at 300 ºC presented an optimum crystallinity, minimum lattice stress, enhanced luminescence intensity and good optical properties and as such, was deposited on glass substrates to form the seed layer to produce GZO TCO film upon which ZNRs were grown. On the other hand, the sample that was deposited at the CBD growth concentration of 0.05 M and 90 min time presented good crystallization, good optical and luminescence properties with homogenous and well-aligned ZNRs. This could be used as a possible photoanode component of the dye-sensitized solar cell.