Preparation, Surface Manipulation And Photovoltaic Application Of Semiconductor Nanocrystals

The tunable band gap due to quantum size effect enables a tunable opticalabsorption offset wavelength for semiconductor nanocrystals. In addition, localizedsurface plasmon resonance absorption and local field enhancement are produced whenthe frequency of incident light couples with that of collective oscillation of freecarriers in nanostructures. Semiconductor nanocrystals have a key advantage oftunable free carrier concentrations, which means flexible tunability of localizedsurface plasmon resonance. Exciton absorption and localized surface plasmonresonance produce more excellent absorption performance than bulk material, whichis beneficial to reduce the thickness of absorption layer for solar cell, shortening thetransport distance for photo-generated carriers collection and reducing the cellmanufacturing cost.In this thesis, in order to improve the photoelectric conversion efficiency throughbroadening the spectrum of sunlight converted to photocurrent, we study the size andmorphology controllable preparation of nanocrystals using chemical methods. CdSequantum dots with exciton absorption in the visible region, PbS quantum dots withexciton absorption in near infrared region, CuS nanocrystals with localized surfaceplasmon resonance in the near infrared region, and Bi2Se3nanocrystals are obtained.The optical properties and chemical stability of as prepared nanocrystals arecontrolled through nanostructure surface engineerings. The nanocrystalline solar cellsare prepared, and the I-V characteristics are tested to estimate the deviceperformance. The interface transport properties of carriers are studied through acimpedance technical. The main results obtained are as follows.1. Controllable preparation of semiconductor nanocrystals.CdSe, PbS, CuS and Bi2Se3semiconductor nanocrystals are prepared in inertatmosphere using chemical method with1-octadecene and oleylamine as nonpolarsolvents. By controlling the reaction temperature, time and concentration of precursors the sizes and morphologies of nanocrystals are controlled. In the visible tonear infrared range, the exciton absorption peak of CdSe quantum dots is tunable from520nm to610nm, and the exciton absorption peak of PbS quantum dots is tunablefrom920nm to1055nm. The preparation process of CuS nanocrystals in nonpolarsolvent is further simplified using ultrasonic chemical method. And the localizedsurface plasmon resonance effect of CuS nanocrystals is established by Mie-Gansscattering theory and media dielectric constant experiments. Bi2Se3nanocrystals aresuccessfully prepared in nonpolar solvent by adjusting the reactivity of seleniumprecursors.2. Surface treatments and the effects on optical properties of nanocrystals.In order to improve the inter-particle carriers transport capability, surface ligandexchanges are carried out to change the surface ligands of the CdSe, PbS quantumdots. The surface modifications as well as the environment conditions such asatmosphere and pH value significantly influence the chemical stability and opticalproperties of nanocrystals. The chemical and optical stability of nanocrystals areimproved by inorganic shells coating. The localized surface plasmon resonancewavelength of CuS nanocrystals is tuned through varying free carrier concentrationvia ligands surface charge transfer doping.3. Preparation and characterization of nanocrystals solar cells.Nanocrystals/electron transport layer heterojunction solar cells are constructedby combining PbS quantum dot film prepared by layer-by-layer spin casting andtransparent TiO2electron transport layer deposited by atomic layer deposition (ALD).The performance of nanocrystals solar cells are estimated using I-V characteristics.The interface transfer characteristics of carriers are studied by ac impedancespectroscopy testing and equivalent circuit model fitting.