Solution based synthesis of Si quantum dots and down-shifting layers for solar cell applications
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Silicon is the main component of electronics and photovoltaics, but since bulk Si is a poor light emitter it cannot be adapted into opto-electronic devices and applications. Si nano-crystals on the other hand have exceptional optical and electronic properties which can introduce silicon into new opto-electronic applications. Due to their size and surface dependent tunable light emission, Si nano-crystals could revolutionize the electronic and photovoltaic industry. One of the main challenges is to produce stable Si nano-crystals with good size and surface control by a simple method feasible for scale up. A wide range of synthesis routes of Si nano-crystals have been established but a proper bottom-up solution based method has not been developed in terms of controlling photoluminescence in the whole visible spectrum. The origin of photoluminescence in Si nano-crystals is still debated. Since size and surface properties both affect the opto-electronic properties, intrinsic structural properties are also playing a major role which has to be carefully investigated. Development of methods for the preparation of stable and monodisperse Si nano-crystals as well as hybrid polymer Si nano-crystal films was the main goal of this thesis. Furthermore, the optical properties of these nanostructures, of importance for applications dependent on light shifting, were explored. In Paper-I, a synthesis route based on homogeneous reduction of SiCl4 by Knaphthalide to monodisperse diamond structured Si nano-crystals with controlled size and surface properties was developed. STEM and HRTEM confirmed a high monodispersity with size control from 3 to 27 nm of the diamond structured Si nanocrystals. The reducing agent and monomer concentration were optimized for the size control and with increased concentration of reducing agent; K-naphthalide, it was found that the size and dispersity of the Si nano-crystals decreased. Octanoxy and methoxy surface groups of Si nano-crystals were determined by FTIR spectroscopy. Steady state and time decay photoluminescence measurements indicated blue (350- 450 nm) and very fast emission in the nano-second (1-7 ns) range regardless of size and surface functionalization groups. Very low photoluminescence quantum efficiencies (1.63 ± 0.16)x10−3 %) were detected. Also a simple solution based reduction route to simple fcc Si nano-crystals based on homogeneous reduction of SiCl4 by Na-cyclopentadienide in tetrahydrofuran (THF) was explored in Paper-II. Not only size control but also structure control was shown for the first time for Si nano-crystals using a solution based method at ambient conditions (diameter of 2-7 nm). Energy filtered imaging (EFTEM) confirmed that the fcc nano-crystals consisted of Si. FTIR spectra proved the octanoxy capping on the surface of the Si nano-crystals. Steady-state photoluminescence response of the fcc structured Si nano-crystals showed a wide-range emission in the 400-700 nm range with half width maximum of 550 nm, consistent with the wide size distribution obtained. Time-resolved emission spectroscopy revealed very fast decays (0.2-6 ns). Photoluminescence quantum efficiency was determined to be ∼ 1 %. In Paper-III, diamond and fcc Si nano-structures were synthesized by homogeneous reduction of SiCl4 by alkali (Li, Na, K)-cyclopentadienides using THF or 1-2 dimethoxyethane (glyme) as solvents. The effect of the type of solvent, concentration of reactants and termination agent was investigated. Structure and size distribution of uncapped fcc structured Si nano-crystals were found to be between 5 and 25 nm by transmission electron microscopy (TEM). Using the Na-cyclopentadienide route both fcc and diamond structured Si nano-crystals with a wide size distribution (2-7 nm) was obtained after octanol surface functionalization. Stick like nano-structures were produced by increasing the concentration of the reactants. Increasing the temperature to 80 oC, polymerization and embedment of the Si nano-crystals into a polymer network was observed by STEM. The Li-cyclopentadienide route gave most probably also simple fcc structured Si nano-crystals, but it was challenging to image the nano-crystals due to polymerization and LiCl contamination. No fcc structured Si nano-crystals were observed by using K-cyclopentadienide as the reducing agent in THF. It was hard to study the products formed using K-cyclopentadienide in this solvent due to rapid oxidation of Si. FTIR spectra of uncapped fcc Si nano-crystals revealed the Si-C stretching band, but this band disappears/weakens with octanoxy capping. Initial cyclopentadienyl capping of the surface was found to play a major role on the fcc Si nano-crystal formation. All the synthesized Si nano-crystals showed a broad continuous PL between 350 and 750 nm. Photoluminescence contribution from the organic polymer was found to be negligible. The fcc structured Si nano-crystals could not be reproducibly prepared using a new batch of chemicals. A method for the production of stable Si nano-crystal hybrid films was further developed in Paper-IV. Characterization of these films with respect to their optical properties was performed. A fast and efficient synthesis procedure of the direct polymer Si nano-crystal hybrid film formation was established. Heterogeneous reduction of SiCl4 by K-cyclopentadienide in cyclopentadiene, gave white light emitting Si nano-crystal hybrid films upon excitation with UV. The hybrid films were produced from octanol, oleic acid, acrylic acid and ethylene glycol terminated Si nano-crystals. The size of the Si nano-crystals (∼2-10 nm) estimated by Raman spectroscopy was smallest for the ethylene glycol capped Si nano-crystal films. The dielectric functions were determined in the 0.73-5.9 eV range by ellipsometric studies. The calculated Tauc bandgaps of the Si nano-crystals in the hybrid films varied between 1.51 and 2.35 eV. Photoluminescence response of the hybrid films showed a broad emission in the 450-800 nm range consistent with the wide size distribution of Si nano-crystals. The photoluminescence response was tunable depending on the surface termination group and ethylene glycol termination of the surface resulted in a red shift in photoluminescence response at room temperature. The excited-state lifetimes consisted of very fast decay in the 2-9 nano-seconds range and a slow decay time in the range of 0.2-0.3 micro seconds at 10 K.
SerieDoctoral thesis at NTNU