【作者】 赵之雯；

【导师】 刘玉岭； 闻立时；

【作者基本信息】 河北工业大学 ， 微电子学与固体电子学， 2011， 博士

【摘要】 降低微晶硅太阳能电池的成本最直接有效的方法是提高沉积速率,这使微晶硅薄膜的高速沉积问题成为太阳能电池产业化进程中一个必须攻克的难题。本文采用高压高功率结合VHF-(甚高频)PECVD的技术路线,实现了微晶硅薄膜材料的高速沉积,并对影响微晶硅薄膜材料沉积速率的因素和影响高速沉积微晶硅薄膜质量的原因进行分析研究。论文主要研究内容和创新工作如下:系统研究了气压、功率、硅烷浓度、衬底温度、气体流量以及电极间距等因素对微晶硅生长速率的影响,并结合OES谱分析了影响生长速率的原因。提出增加反应气压和电极间距能够提高微晶硅生长速率。增加气压使硅烷分解增加,反应活性基团扩散到生长表面的几率增加,使沉积速率随着气压增加迅速提高。增大电极间距使等离子体反应空间增加,反应活性基团到衬底的通量增加,令沉积速率随电极间距增大而提高。本文通过调整气压和电极间距的方法将沉积速率从传统低压技术下的1～2?/s提高到23?/s以上。研究了高速沉积下微晶硅薄膜质量的影响因素,发现气体总流量是影响高速沉积薄膜缺陷态密度的关键因素。想在高速沉积情况下得到高质量的微晶硅薄膜,必须通过调整流量保证反应气维持适宜的气体滞留时间。本文制备了生长速率在23?/s以上,晶化率在40%以上,μτ乘积在10-4cm2/V左右,具有<220>晶相择优的高速高质量微晶硅薄膜样品。研究了不同沉积速率下制备的微晶硅薄膜特性的差异。结果发现,与低速沉积的微晶硅薄膜相比,高速沉积的微晶硅薄膜具有非晶孵化层较厚、纵向结构均匀性较差、晶粒尺寸较大、薄膜致密性较差及薄膜表面粗糙度较大的特点。研究了微晶硅薄膜材料稳定性的问题。进行光衰退试验的样品包括了从低晶化率至高晶化率微晶硅的不同晶化率范围,发现材料晶化率大小决定着电池光衰退的多少。光照一段时间后微晶硅电池在光照100小时以前几乎不衰退,100小时后逐渐衰退,且2000小时不饱和。综上所述,本论文采用高压高功率与VHF-PECVD相结合的方法实现了器件质量级微晶硅的高速沉积,并且研究了影响微晶硅沉积速率及其质量的关键因素,为大幅度降低硅薄膜太阳能电池成本奠定了实验基础。研究了不同沉积速率下制备的微晶硅薄膜的异同,找到了导致高速沉积微晶硅薄膜材料性能降低的影响因素。对微晶硅薄膜材料的稳定性进行了初步研究,发现材料的结构特性尤其是晶化率大小决定着材料的光衰退,微晶硅材料中的非晶硅组分是导致光衰退的主要原因。更多还原

【Abstract】 High-rate deposition process plays an important role in reducing production cost of microcrystalline silicon (μc-Si) solar cells. It is also an access to industrialize the amorphous/microcrystalline silicon tandem solar cells. High-rate deposition of microcrystalline silicon films and solar cells were realized in this paper using very high frequency plasma enhanced chemical vapor deposition (VHF-PECVD) technique combined with high pressure. After a series of experiments and analyses, impact factors on deposition rate and material quality ofμc-Si films and solar cells were discussed in detail. Research contents and main innovations were described follow.The influences of factors including pressure, power, silane concentration, temperature, total flow rate and the distance space between the cathode and the substrate to the deposition rate ofμc-Si:H were systematically investigated. On-line optical emission spectra (OES) measuring was used to detect the growth rate changes. It can be concluded that high pressure and a large distance space between the cathode and the substrate contribute to increasing the deposition rate of the microcrystalline silicon films. Increasing pressure can not only accelerate the silane decomposition, but also be in favor of ions diffusion from bulk plasma to the growth surface and simultaneously decrease the bombardment of electrons with high energy on the silicon films. As a result, the deposition rate is increased. A large space between the cathode and the substrate will enlarge the reaction space, more reactive ions could reach the growth surface, and therefore the deposition rate is increased. By increasing pressure and raising the space distance, highest deposition rate of over 23?/s was achieved in our experiments.Then factors that affect the material quality were investigated and the following conclusions were obtained. Our experiments demonstrated for the first time that the long resident time has great effect on the defect density of the silicon films. When the pressure or the electrode space is increased without changing the total flow rate, the gas residence time will be prolonged, which will lead to poor film quality with high defect density. So the total gas flow rate should be increased as well as a high pressure and a large distance space between the cathode and the substrate were applied to prepare high quality materials with a high deposition rate. Accordingly, device-qualityμc-Si:H films with crystalline fractions over 40%,μτaround 10-4cm2/V, and the <220> preferential orientation have been obtained at high deposition rate of 23?/s by optimizing the deposition conditions.The influence of different deposition rates on the properties ofμc-Si:H thin films and the profermance of their solar cells was studied carefully. Compared with the low deposition rate materials, the high deposition rate ones have a thicker amorphous incubation layer, worse uniformity of vertical (growth direction) structure, larger grains, looser microstructure and a rougher surface on top.The magnitude of relative light induced degradation is closely related to material structure. Amorphous fraction is the key determining factor to light induced degradation. The results showed clearly that the magnitude of relative efficiency degradation is increase with amorphous fraction. The more amorphous fraction located in material, the more degradation was been found. With better structure and optical properties, Microcrystalline silicon with transition region is more suitable for the manufacturing of stable Microcrystalline silicon solar cells due to the structure and optical properties.In summary, device-quality high-rate depositionμc-Si:H films and solar cells have been realized in this dissertation. Various methods to improve the deposition rate and material quality were investigated in detail. All these results will be valuable for the reduction of production cost of theμc-Si:H solar cells in the future. Amorphous fraction is the key determining factorto light induced degradation.,the magnitude of relative efficiency degradation is increase with amorphous fraction.更多还原