Investigation of charge transport between nickel oxide nanoparticles and CdSe/ZnS alloyed nanocrystals
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Investigation of charge transport between nickel oxide nanoparticles and CdSe/ZnS alloyed nanocrystals R. Vasan*1, F. Gao2, M. O. Manasreh1, C. D. Heyes2 1 Department of Electrical Engineering, University of Arkansas, Fayetteville, AR, USA-72701 2 Department of Chemistry and BioChemistry, University of Arkansas, Fayetteville, AR, USA72701 ABSTRACT Charge transport between nickel oxide nanoparticles and CdSe/ZnS alloyed core/shell nanocrystals is investigated. The crystal structure and composition of the nickel oxide nanoparticles are evaluated using X-ray diffraction, Raman and X-ray photoelectron spectroscopies. The nanoparticles are near-stoichiometric with very low defect densities. The optical properties of the materials are studied by measuring the absorbance and time resolved photoluminescence spectra. The band gap of the nickel oxide nanoparticles is around 4.42 eV. The CdSe/ZnS nanocrystals exhibit shorter average lifetimes when mixed with nickel oxide nanoparticle powder. The lifetime quenching can be attributed to the efficient charge transport from the CdSe/ZnS nanocrystals to nickel oxide nanoparticles due to the relative valence band alignment. INTRODUCTION Recently inorganic colloidal quantum dots (QDs) were investigated as a replacement for existing organic active layers in photovoltaic devices and organic light emitting devices. One of the main reasons for this change is the environmental stability and high current operation of these devices with inorganic QDs [1]. The most commonly employed QDs are colloidally grown CdSe/ZnS and CdSe/CdS core-shell nanocrystals. Even though an alternative inorganic active material is employed, these devices still use organic charge transport layers [1]. The charge transport layers facilitate the extraction of carriers in photovoltaic devices and injection of carriers in light emitting devices. In the case of light emitting devices, the charge transport layers reduce the barrier to holes and electrons that are injected from the electrode into the QD emissive layer [2-4]. Suitable inorganic electron transport layers reported so far are zinc oxide, titanium dioxide, and tin oxide, which are intrinsically n-type materials [1-4]. The conduction band of the electron transport layers align with the conduction band of the QD and thereby reduce the barrier to electrons injected from the cathode, but there are not enough hole transport layers (HTLs) with suitable valence band alignment with the QDs. Even though some p-type materials are used as HTLs, the efficiency of the devices is much lower due to valence band mismatch [1, 2-4]. Non-stoichiometric nickel oxide (NiO) has been successfully employed as an HTL in quantum light emitting devices (QLEDs), but imbalance in the charge injection into the QDs created by the high barrier to holes resulted in low efficiency [2-4]. In this study, near-stoichiometric NiO nanoparticles are synthesized and characterized for application as a potential HTL for all-inorganic QLEDs. The structural and compositional studies of the synthesized n
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