Hydrothermal Growth of ZnO Nanorod Arrays: Substrate, Nucleation Layer and Density Control Effects in Their Structural, Crystallographic, and Optical Characteristics

Abstract

ZnO nanorod arrays (NRAs) have attracted much attention due to their potential applications as building blocks for nanoscale electronic, optoelectronic, nanoscale sensors, solar cells and spintronics, among others. The precursor concentration, seed layer annealing, growth temperature and growth duration affects the morphology, distribution, density, shape and size of the final products. ZnO NRs grown on different ZnO-coated substrates (Glass, Si, FTO and ITO) show that the hydrothermal method is a suitable growth process for ZnO NR arrays on different substrates. The density of ZnO NRAs is successfully controlled in the hydrothermal growth process by depositing Ti (0.0, 0.3, 0.5, 1.0) nm and Au (4.0, 8.0, 12.0, 16.0) nm inhibitor buffer layers onto bare glass AZO, ZnO-coated AZO, FTO and ITO substrates, respectively. The areal density of NRs decreased with the increase in the thickness of Ti and Au buffer layers and density of NRAs grown on glass FTO and ITO substrates is approximately 10-times lower than the initial density and has a more uniform distribution. Importantly, no significant variation in NRAs quality is observed as a function of density, as determined by x-ray diffraction (XRD), Raman and photoluminescence (PL) measurements; only intensity variations are observed with the density change. However, maximum static water contact angle was found to be 135° for Ti buffer layers and 142° for Au buffer layers. Based on the aforementioned knowledge, alkali metal Li-doped ZnO NRAs were grown on p-Si substrate for 5h and their contribution studied in terms of Schottky photodiode performance. Li-doped photodiode shows low leakage current, a negative shift in turn-on voltage and large capacitance, which are a great benefit to minimize power loss and improve switching characteristics. Interestingly, the responsivity of Lidoped photodiode is very high and greater than the commercial GaN photodiode.

Doping with the transition metals (TM) Co and Ni affect the length of the final ZnO NRs product. No secondary Co- or Ni-related phase is observed in XRD patterns. EDX spectra showed a peak of Co (3.86 at %) and Ni (3.22 at %) in their respective spectrum. The doped ZnO NRs based Schottky diodes show lower rectifying properties than undoped diode, which may be due to an increase of barrier inhomogeneity, density of interface states, turn-on voltage, and series resistance. The obtained ideality factor is greater than unity and the barrier height obtained from current-voltage (I-V) measurements is lower than that obtained from capacitance-voltage (C-V) measurements.

A high-density of ZnO/ZnSe NWs is used for the fabrication of PbS quantum dots sensitized solar cells (PbS QDSSCs). The results demonstrate that the absorption increases for wavelength below 740 nm after loading PbS QDs. The photovoltaic parameters obtained are: Jsc= 20.60 mA/cm2, Voc= 155 mV, FF= 34.7% and the power conversion efficiency is η= 1.1%. The obtained results confirm that PbS QDs can be used as an effective inorganic sensitizer for future QDSSCs.

The most promising photoactive layer, Sb2S3, was deposited onto TiO2/ITO/glass via the thermal evaporation method for inorganic-organic hybrid Ag/P3HT/Sb2S3/TiO2/ITO/glass solar cell fabrication. To investigate the effect of air on the device performance, device processing was conducted in ambient conditions. Both P3HT and Sb2S3 are oxidized. The absorption coefficient of Sb2S3 is found to be > 105 cm-1 and the optical band gap was ~2.0 eV (from UV-vis), 2.05 eV (from spectroscopic ellipsometry) and 1.34 eV (from theoretical calculations). The experimental values of other optical constants, such as refractive index, extinction coefficient and dielectric constant were larger than the theoretical results. The values of solar cell parameters varied with the absorber thickness variation. The maximum power conversion efficiency was found to be 1.94% and the external quantum efficiency (EQE) was ~ 34%.

Moreover, the low band gap of Sb2S3, high absorption coefficient, large dielectric constant and refractive index values suggest that exciton binding energy in Sb2S3 should be small and an anti-reflective coating is recommended to enhance the light absorption for the very promising photoactive material for n-i-p hybrid solar cells. Furthermore, to improve the device efficiency, the process should be performed and optimized in an air protective environment.