| In recent years, nano-silicon quantum dot (n-Si QDs) materials have attracted much attentionbecause of their potential applications in the fields of monolithic optoelectronic integration, the next generation solar cells, thin film transistors and other novel optoelectronic devices. So far, both the controllable preparation and optoelectronic properties of the n-Si QDs materials have been widely studied.One of the interesting topics currently is how to develop the novel optoelectronic devices and improve the device performance. In this thesis, we used the amorphous silicon carbide (a-SiC) instead of the conventional silicon dioxide to form the silicon quantum dot/silicon carbide (Si QDs/SiC) multi-layer structure by laser induced crystallization technique. The p-i-n structure devices with the multi-layer embedded between p-type and n-type silicon were fabricated and both optoelectronic and photovoltaic properties of this structure were studied.The main results of this thesis are as follows:1. The periodic a-Si/SiC multilayers were deposited in conventional plasma enhanced chemical vapor deposition (PECVD) system, the thickness of a-Si layer was controlled at 2-4 nm while amorphous SiC layer with thickness of 2nm. KrF pulsed excimer laser was used to crystallize a-Si layer to form Si QDs. It is found that the samples crystallizes when the laser energy density is controlled in a proper range (150mJ/cm2-183mJ/cm2) and the crystallization ratio can get up to 68% when the laser energy density is controlled at about 180mJ/cm2. The micro structures were investigated by cross-section transmission electron microscopy (TEM) and Raman spectroscopy, the periodic structure was still kept even after laser irradiation and the Si QDs with an average size of 3nm appeared in the intrinsic a-Si layer. Furthermore, we investigated the photoluminescence (PL) and the electroluminescence (EL) of the thin film at room temperature, the position of the luminescence band was located in the range of 850-950nm. We attributed the light emission to the recombination of electron-hole pairs in Si QDs and the recombination via interface states in the nc-Si/SiC interfacial region or the band tail states of a-SiC.2. Based on the performance of well-defined Si QDs/SiC multilayers utilizing laser induced crystallization technique, we fabricated the p-i-n structure on p-type silicon by using Si QDs/SiC multilayers as intrinsic lsyer. It was found that the turn-on voltage can be reduced to 5V and the EL intensity increased obviously under the same driven current compared with the reference samples without p-i-n structure. The Z-parameter model was used to analyse the recombination process of injected carriers, the Z-parameter was 1.3 for reference sample and it increased to 1.8 for p-i-n EL devices indicating that the radiative recombination dominates the recombination process of injected carriers in the p-i-n device.3. We prepared the p-i-n structure based on Si QDs/SiC multilayers on the Indium Tin Oxide (ITO) transparent electrode coated glass substrates utilizing the laser induced crystallization technique to get all-silicon quantum dots solar cells. The photovoltaic properties of the samples with different thicknesses and different QD sizes for the intrinsic absorption layer were studied. It was found from the external quantum efficiency (EQE) spectra of the cells that a red shift appeared while the size became larger, which was in a good agreement with the quantum size effect.We achieved a 456mV open-circuit voltage (Voc) which was very close to the highest Voc (492mV) reported by now and the short-circuit current density in our case was 0.078mA/cm2. These results laid a foundation for further research based on silicon quantum dot thin films. |
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