The Properties Of SiN_x Films With Silicon Quantum Dots And Their Applications In Solar Cells

Based on the quantum confinement effect, luminescence efficiency of silicon-based materials and photoelectric conversion efficiency of silicon-based solar cells can be enhanced greatly by using silicon quantum dots. Therefore, silicon quantum dot possess promising applications in silicon-based optoelectronic devices, and is becoming an immediate areas of research focus in semiconductor industry. However, the correlation between the photoluminescence features of silicon quantum dots and the structural traits of the films with silicon quantum dots is not well clear. Therefore, the reliability of a photoelectric device with silicon quantum dots cannot be guaranteed. In addition, the application of silicon quantum dots in solar cells is only at the initial stage of theoretical and experimental research. In view of those, silicon-rich silicon nitride films were prepared by plasma enhanced chemical vapor deposition (PECVD) at low temperature, and then silicon quantum dots can be precipitated from the silicon-rich silicon nitride films under high temperature annealing treatments. The main research work of this dissertation is about the effect of high temperature annealing treatments on photoluminescence of silicon quantum dot and the variation characteristics in chemical component and bonding configuration of silicon nitride thin films (SiNx films). Finally, according to the deposition process of silicon quantum dots we designed and prepared a new structure silicon heterojunction solar cell with silicon quantum dot. The dissertation's main reseults as follow:(1) The SiNx films embedded with silicon quantum dots were prepared with a gas mixture of SiH4 and NH3 by plasma enhanced chemical vapor deposition (PECVD) method at 200℃. The formed silicon quantum dos embedded in the SiNx films can be simply and quickly characterized by Raman spectroscopy, photoluminescence (PL) spectroscopy and Fourier transform infrared spectroscopy (FTIR). Two kinds of luminescent mechanisms of quantum confinement effect and defect states can be found for the SiNx films by analyzing these spectral datas. Moreover, the defect types in the SiNx films increase with increasing the flow rate of NH3 under the same substrate temperature, radio frequency power, and chamber pressure and silane flow rate. For the SiNx films prepared by the lower flow rate of NH3, the sizes of silicon quantum dots embedded in the films are larger and these silicon quantum dots lost their characteristics of quantum confinement effect due to growth under the higher temperature annealing treatments.(2) The influence of different temperature annealing treatments on chemical component and bond configuration of SiNx film was investigated by FTIR and XPS spectra at room temperature. On the basis of the arguments about FTIR and XPS spectroscopy we conclude that:as the annealing temperature lower than 800℃, the breaking of Si-H and N-H bonds in the SiNx films mainly form Si-N bonds; as the annealing temperature higher than 800℃, the breaking of Si-H and N-H bonds are helpful to effusion of N atoms and the formation of silicon nanoparticles; as the annealing temperature equaling to 1100℃, the N2 react on the SiNx films to cause the increase in the ratio R of nitrogen to silicon atoms. The relationship between the evolutions of FTIR, XPS spectroscopy of the samples with different annealing temperatures and the variations of bonding configurations of Si, N and H atoms was discussed in detail, and the formation mechanisms of silicon quantum dots embedded in SiNx films were investigated.(3) Silicon heterojunction solar cell with silicon quantum dots has been made by PECVD method and vacuum evaporation technology. The solar cell's structure is Al/SiNx antireflection film/N-a-Si:H/SiNx film with silicon quantum dots/P-c-Si/Al. Under the standard AM1.5G spectrum,Ⅰ-Ⅴcharacterization of the heteroj unction solar cell was measured at room temperature. The short circuit Isc, open circuit voltage Voc, the maximum power point current Imp, the maximum power point voltage Voc, fill factor FF and conversion efficiency is 1.9mA/cm2,457.5mV,1.49 mA/cm2,288.6 mV,0.495 and 0.43%, respectively. We think that the higher series resistance and interface barrier are two main factors which lead to the poor performance of the heterojunction solar cell. Meanwhile, the methods for improving the solar cell's performance also are given.