Ultrafast hole transfer in CdSe/ZnTe type II core−shell nanostructure

We have synthesized thiol-capped CdSe/ZnTe quantum dot core−shell nanostructures by colloidal methods, have characterized them by steady-state absorption and photoluminescence (PL) spectroscopy and further confirmed by high resolution transmission electron microscopy and X-ray diffraction measurements. Clear red shift on shell formation was observed in optical absorption and photoluminescence studies. Time-resolved emission studies indicate longer emission lifetime of CdSe/ZnTe core−shell as compared to CdSe QD material where in both cases only CdSe gets excited, which indicates spatial charge separation in type-II core−shell. Ultrafast photoinduced charge transfer dynamics in type-II CdSe/ZnTe donor−acceptor core−shell were studied in real-time using femtosecond broadband pump−probe spectroscopy. Our transient absorption studies suggests that on photoexcitation core−shell hole transfer from CdSe core to ZnTe shell takes place in pulse-width limited time scale as evidenced by an increase in cooling dynamics of the charge carriers from 150 fs for CdSe to 300 fs for thickest CdSe/ZnTe core−shell. Increase in cooling dynamics in core−shell has been explained due to decoupling of electron and hole in photoexcited core−shell. Trapping dynamics play a major role in the excited dynamics of the photoexcited charge carriers of quantum dot materials. Bleach recovery kinetics of the photoexcited QD materials fitted multi-exponentially where 2.5 ps (first component) has been attributed to the electron trapping dynamics and the longer components (30−50 ps and >400 ps) attributed to the charge recombination dynamics

Investigation of detailed physical properties and solar cell performances of various type rare earth elements doped ZnO thin films

In this study the structural, optical, electrical properties and solar cell performance of undoped ZnO and rare earth (RE) doped ZnO (Zn0.95Yb0.05O, Zn0.95Eu0.05O and Zn-0.90 Eu0.05Yb0.05O) thin films prepared by sol-gel spin coating method were investigated. The structural characterizations of the obtained samples were examined by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD) analyses. The optical properties of these thin films were carried out with UV-vis transmittance spectroscopy technique in 350-800 nm range. The electrical properties of films were examined by resistance measurements at room temperature. XRD results show that all samples have single phase wurtzite (hexagonal) structure with (002) c-plane orientation. Detailed structural characterizations were examined from XRD data. SEM analysis represents that nanoparticles are formed on the thin films and the type of dopant affected the morphologies, thickness and sizes of ZnO nanostructures. Our optical results indicate the average optical transmittances of RE doped ZnO samples are 98% at different regions in the visible region. Also the band gap energy of all of thin films was calculated from optical transmittance spectroscopy data using Tauc equation. The band gap energies of undoped, Yb, Eu, and Eu/Yb co-doped ZnO were found as 3.307, 3.295, 3.29 and 3.28 eV, respectively. Urbach energy was calculated from spectral absorption coefficient and this value shows an increase with doping Eu and Yb elements. It was observed that the electrical resistivity of doped samples is low compared to ZnO thin film. Also, in this study the rare earth elements effects on the solar cell performance of ZnO nanostructures were investigated and it was seen that Yb and Eu elements improve the cell performance