Preparation Of Chalcogenide Semiconductor Nanocrystals And Their Photovoltaic Application

In recent years, chalcogenide semiconductor nanocrystals have become the most active field among nanomaterials due to their unique photoelectric properties and potential applications in optoelectronic devices and biological imaging. It is important to develop simple methods of synthesizing chalcogenide semiconductor nanocrystals and know more about their physical and chemical properties for their practical applications. In this work, we researched the preparation ofâ…¡-â…¥andâ… -â…¡-â…¥chalcogenide semiconductor nanocrystals in high-boiling-point solvent systematically and introduced them into photovoltaic devices based on TiO2 nanorod arrays. The primary significant results are summarized as follows:(1) CdSxSe1-x nanorods consisting of CdSe-rich cores and CdS-rich shells were prepared in trioctylphosphine oxide and decylphosphonic acid with injection. The aspect ratio of the CdSxSe1-x nanorods increased with the increase of S/Se molar ratio which was attributed to the lower reactivity of S precursors compared to that of Se precursors. Moreover. it was confirmed that the formation of core-shell structure enhanced the fluorescence efficiency of the nanorods.(2) CdSxSe1-x nanocrystals were prepared in octadecene and oleic acid without injection. The morphology of CdS and CdSe nanocrystals changed from spherical to multi-armed and the phase changed from zinc-blende to wurtzite with the addition of halide ions, because the halide ions suppressed the reactivity of Cd precursors. The CdSxSe1-x nanocrystals were alloyed whose optical properties varied linearly with the change of S/Se molar ratio while their sizes were unchanged.(3) CuIn(SxSe1-x)2 nanocrystals were prepared in oleylamine. Taking CuInSSe for example, we investigated the growth mechanism of CuIn(SxSe1-x)2 nanocrystals and found the formation CuIn(SxSe1-x)2 nanocrystals involved the nucleation of CuSxSe1. and the subsequent reaction between CuSxSe1-x nuclei and the remaining In precursors. The variation of reaction temperature and addition of ligands such as oleic acid could influence the reactivity of In precursors.(4) CuInS2 nanocrystals showing quantum confinement effect were prepared in dodecanethiol. The strong complexing between dodecanethiol and Cu ions balanced the reactivity of Cu precursors and In precursors and limited the growth of CuInS2 nanocrystals. resulting in the small sizes of the nanocrystals. The CuInS2 nanocrystals emitted fluorescence at room temperature due to the effective passivation of the surface of the nanocrystals by dodecanethiol. The fluorescence was attributed to donor-acceptor pair recombination. The wavelength of PL peaks could be adjusted by the change of reaction temperature and the addition of ligands.(5) The chalcogenide semiconductor nanocrystals were introduced into photovoltaic devices based on TiO2 nanorod arrays. The organics on the surface of the nanocrystals were removed through thermal process and the nanocrystals were sintered to form films as absorbers. For n-type CdSe nanocrystals, p-type semiconductor CuSCN was used as hole transfer layer to form TiO2/CdSe/CuSCN trilayer structure and the conversion efficiency of this device is 0.21%. For p-type CuGaSe2 and CdTe nanocrystals, TiO2/nanocrystal bilayer structure was adopted. Because CuGaSe2 nanocrystals can not be sintered to form continuous films, the TiO2/CuGaSe2 bilayer structure did not display photovoltaic effect due to the serious carrier recombination. On the contrary, CdTe nanocrystals can be sintered to form continuous films and the conversion efficiency of TiO2/CdTe bilayer structure was 0.07%. The results are a step towards development of high efficiency all-inorganic TiO2 nanorod array-based solar cells.