Green Chemical Approaches Toward High-Quality Semiconductor Nanocrystats With Photovoltaic Property

Exploring novel approaches to produce low-cost, high-efficiency photovoltaic cellshas attracted great interests because of the urgent need for clean and renewable energysources. Recently, the advances of colloidal synthesis of semiconductor nanocrystalshave opened the door to address this challenge. Various II-VI and I-III-VI2semiconductors, such as CdSe, CdTe, CuInS2, CuInSe2, and Cu(InGa)Se2have beenstudied intensively. Despite their appealing photovoltaic performance, the intrinsictoxicity of the reagents used sheds a doubt on the future applicability of thesenanocrystals. Since1993, Murray et al. gave the first report for synthesizinghigh-quality CdSe nanocrystals, which involved the pyrolysis of cadmiumorganometallic precursor Cd(CH3)2in tri-n-octylphosphine oxide and a hot injection ofSe source, this method has been used for more than twenty years, although Cd(CH3)2isextremely toxic, pyrophoric, expensive, unstable at room temperature, and explosive atelevated temperature with releasing large amount of toxic gas. Later, Peng et al.developed a relatively simple and convenient “green chemistry” route, namely, usinginexpensive and little toxic CdO rather than Cd(CH3)2and using octadecene as thenon-coordinating solvent, while the alkylphosphine, such as trioctylphosphine andtributylphosphine, have still to be used, which are hazardous and unstable. In this thesis,we develop some environment-friendly and phosphine-free routes for synthesizing highquality semiconductor nanocrystals with the photovoltaic property. Because these routes are simple and convenient, it is reasonable to expect that they will promote the practicalapplications of semiconductor nanocrystals in solar cells.In chapter2, we demonstrate a “one-pot” strategy for synthesizing ZnSenanocrystals in liquid paraffin firstly. The resultant ZnSe nanocrystals possess highphotoluminescence quantum yields and narrow size distribution. Moreover, byregulating the experimental variables, it is found that the precursor concentration, theZn:Se ratio, and the heating rate greatly influence the growth kinetics of nanocrystals. Ingeneral, high precursor concentration facilitate the formation of small nanocrystals,whereas high Zn:Se ratio improve the PLQYs of nanocrystals. However, as the Zn:Seratio higher than1:0.25, the morphology of nanocrystals transfer from dots to rods,simultaneously with a crystal phase transition from zinc blende to wurtzite. Moreover,the increase of heating rate also facilitates the formation of rod-like nanocrystals.Secondly, we indicate a simple and convenient method for synthesizing high-qualityCu2-xSe Nanocrystals through modified hot-injection strategy. Systematic studies revealthat the key in the current synthesis is the ability to tune the reactivity of Se byregulating the experimental variables, such as the reaction time, Se concentration,reaction temperature, and particularly the addition of noncoordinating solvent of Se. Ingeneral, the formation of Cu2-xSe is facilitated at high Se concentration, high reactiontemperature, and low coordinative ability of the solvent. At last, a photoresponse deviceis fabricated by sandwiching the Cu2-xSe nanocrystals between two blank ITO glasseswith a device configuration of ITO/Cu2-xSe/ITO, which possesses the obviousphotoresponsive behavior.In chapter3, we demonstrate the dissolution of elemental Se in oleylamine byalkylthiol reduction at room temperature, which generate soluble alkylammoniumselenide. By using this Se precursor, we have successfully synthesized manyhigh-quality selenide nanocrystals, such as Cu2ZnSnSe4, Cu(InGa)Se2, Co2+-andFe2+-doped Cu2SnSe3nanocrystals. As a result from the existence of dodecanethiol, which suppress the growth of bigger nanocrystals by forming strong metal-S bondingon nanocrystal surface, the nanocrystals all possess small size and size monodispersity.The proposed reduction reaction between Se powder and dodecanethiol is investigated,which shows that Se powder is reduced by dodecanethiol, whereas dodecanethiol isoxidized to disulfides. The formation of disulfides is verified both by nuclear magneticresonance and mass spectra. Since the copper-based multinary selenide nanocrystalshave been regarded as potential candidates because of the suitable band gap, highabsorption coefficient, good photostability, and low toxicity, high photovoltaicefficiency, a photoresponse device with metal/semiconductor/metal structure isfabricated, which shows a promising photoresponsive behavior.In chapter4, we demonstrate the effect of electrostatic repulsion on the growth rateand photoluminescence quantum yields of aqueous nanocrystals. In general, the growthof aqueous nanocrystals involves two stages; the initial nucleation stage and thefollowing growth stage. Electrostatic factors play different role in these two stages. Atthe former stage, the growth is mainly through the agglomeration and fusion of smallclusters and monomers. The growth rate is dependent on the equilibrium of variousinterparticle interactions. The experimental variables that lower electrostatic repulsionfacilitate the formation of bigger nucleus. At the following growth stage, nanocrystalgrowth is mainly through monomer diffusion. The experimental variables that benefitmonomer diffusion increase nanocrystal growth rate. It could be understood on twoaspects. Firstly, a high ionic strength (the concentration effect, the salt effect, and the pHeffect) will reduce the surface potentials and the thickness of diffuse layer, making iteasier for monomer diffusion. Secondly, the neutral monomer is easier to migratethrough diffuse layer than the charged ones, thus presenting a rapid growth ofnanocrystals. High growth rate of nanocrystals not always lead to highphotoluminescence quantum yields, since the photoluminescence quantum yields aregoverned by the nature of nanocrystal adsorbed layer and the array of Cd and ligands on nanocrystal surface. In general, it should be considered both with the physical diffusionand with the chemical adsorption of monomers. Based on these understanding, we cansynthesize aqueous nanocrystals with various sizes, photoluminescence quantum yields,and ligands. At last, a bilayer photovoltaic device composed ofpoly(p-phenylene-vinylene) and CdTe nanocrystals heterojunction is prepared. Theopen-circuit voltage, short-circuit current, fill factor, and power conversion efficiency ofthe device are0.62V,16.77mA/cm2,0.38%, and3.99%respectively.