Fabrication And Photovoltaic Properties Of Si Quantum Dots/SiC Multilayers-based Heterojunction Solar Cells

Si quantum dots embedded in amorphous dielectric materials have attracted much attention since they can be potentially applied in many kinds of devices, such as Si-based light source and next generation solar cells due to their novel physical properties. Especially the silicon quantum dots based solar cells have a research hotspot in the worldwide range. As a dielectric material, amorphous silicon carbide has a low band gap compared with SiOx and SiNx, which is helpful for increasing carrier tunneling probability through the barrier layers to improve the cell efficiency consequently, In this thesis, hydrogenated amorphous silicon carbon thin films (a-SiC:H) were prepared in plasma enhanced chemical vapor deposition (PECVD) system and their microstructures, optical and carrier transport properties were investigated. Furthermore, Si quantum dots (Si QDs)/SiC multilayers were fabricated by annealing a-Si/SiC stacked structures prepared in PECVD system and their microstructures were examined. Finally, p-i-n devices containing Si QDs/SiC multilayers were fabricated, and their photovoltaic property was investigated.The main content and the results of this thesis are listed as follows: 1. Hydrogenated amorphous silicon carbide (a-SiC:H) thin film was deposited in plasma enhanced chemical vapor deposition system with the gas ratio R=[CH4]/[SiH4]=50:5. The microstructure and composition were characterized by Raman、FTIR and XPS spectra. It is demonstrated that the a-SiC:H thin film was Si-rich SiC with the element composition C/Si of0.36and the optical band gap of2.35eV. Room-temperature dark and photo conductivities is4×10-8S/cm and6×10-10S/cm, respectively. The photo-sensitivity is about102. It suggests that the prepared a-SiC:H thin film had high quality and could be used in Silicon-based nano-devices.2. The a-Si:H/SiC multilayers (MLs) with6periods were fabricated in PECVD system. The a-Si sublayer was4nm while the SiC sublayer was2nm and4nm, respectively. The post-treatment annealing was performed at900℃or1000℃for1h in order to get Si quantum dots. The structural change of the Si/SiC MLs before and after annealing was evaluated by Raman spectroscopy and the formation of Si QDs was revealed by transmission electron microscopy (TEM). It is found that crystallinity (Xc) for1000℃annealed Si/SiC MLs is higher than that for900℃annealed ones in both Si/SiC multilayered structures, which indicates that high annealing temperature promotes the crystallization process and the Si QDs with high density could be obtained. For1000℃annealed Si(4nm)/SiC(4nm) MLs, the Xc is69%and the size of Si QDs was around5.7nm.3. Solar cells containing Si QDs/SiC multilayers were fabricated, the optical absorption of Si QDs/SiC multilayers with various SiC thickness annealed at different temperature were measured. The sample annealed at1000℃exhibits the higher optical absorption than900℃annealed ones indicating that the absorption can be enhanced by forming dense Si QDs. The dark current-voltage (I-V) relationship of the samples were measured and it is revealed that FN (Fowler-Nordheim) tunneling process dominates the transport of samples when V is above0.6V. The illuminated Ⅰ-Ⅴ characteristics were measured under an AMI.5(100mW/cm2) illumination while the external quantum efficiency (EQE) spectra were collected by the spectral response measurement system. Both the external quantum efficiency and power conversion efficiency was obviously enhanced by reducing the SiC barrier thickness. It suggested that the reduction of the SiC barrier thickness indeed improved the carrier tunneling efficiency through the stacked structures and the cell device with3.73%conversion efficiency was achieved preliminarily.