Band Gap Engineering of Group II-VI Nanostructured Thin Films for Photovoltaic Applications

Abstract

The nonlinear optical phenomena and quantum confinement effects in the group II-VI semiconductors make them a potential candidate for use in solar cell and photovoltaic applications. The effects of composition and processing parameters on structure, morphology and optical properties of the group II-VI thin films and nanostructures have been studied. Nano-crystalline Cd1-xZnxS (0 ≤ x ≤ 1) thin films have been deposited through the closed spaced sublimation and Sol-gel techniques. The samples have been characterized through X-ray diffraction, Scanning Electron Microscopy, Transmission electron microscopy, Raman Spectroscopy, UV-Visible absorption and Photoluminescence measurements. The samples deposited through the sublimation process exhibited larger crystallite size for intermediate compositions. For the samples deposited through the Sol-gel process, crystallite size was independent of the composition. The band gap inversely followed the crystallite size of the thin films. The band gap varied between 2.36 eV to 3.22 eV and 2.75 eV to 3.89 eV for Cd1-xZnxS thin films fabricated through sublimation and Sol-gel process respectively. Cd1-xZnxS (0 ≤ x ≤ 1) thin films prepared through sublimation revealed strong excitonic and phononic behavior compared with deposited through the Sol-gel process. The effect of vacuum annealing has been studied on CdZnS thin films annealed at 300 oC and 400 oC for two hours. New diffraction peaks along (002) and (103) planes and a decrease in substitution disorder have been observed after vacuum annealing. The band gap and Stoke‟s shift energies increased while the Photoluminescence peaks blue shifted with vacuum annealing. Thin films of CdS1-xSex have been fabricated through Sol-gel process. The XRD peaks shifted towards lower angles and lattice strain decreased by an increase in selenium concentration. The band gap energy showed a strong dependency on the lattice strain. Band gap decrease from 2.82eV to 1.7eV following the reduction in lattice strain. Longitudinal optical plus acoustic phonon mode peak emerged and the intensity of 2LO peak decreased by the increase in selenium concentration. A multi-band photoluminescence emission has been observed owing to Se dependent deeper accepter levels in nano-crystalline CdS1-xSex thin films. One-dimensional Cd1-xZnxS (0 ≤ x ≤ 1) nanostructures have been synthesized through the sublimation technique using Au as a seed layer. Their morphology changed from nanowires to nano-ribbons and band gap increased from 2.38 eV to 3.54 eV with Zinc concentration. An increase in Zinc concentration has been observed at higher substrate temperatures. The Stoke‟s shift energy raised from 45 meV to 302 meV and a blue shift of 53 meV in emission energy has been observed as substrate temperature increased from 510 oC to 550 oC. The low-temperature and time-resolved photoluminescence response of CdZnS nanostructures has been examined. The single-crystalline CdZnS nanostructures exhibited bi-exponential excitonic decay with an average lifetime of 2.56 ns and 4.11 ns, respectively. For single isolated nanostructures, emission band systematically shifted towards the higher-energy and defect-related lower energy emission peak disappeared with a decrease in nanostructure width. The detrimental effect of low temperature Photoluminescence measurements have been observed in individual nanostructures. The results from our study showed that CdZnS nanostructures could be potential candidates for nanostructure based optoelectronics and photovoltaic devices.