【作者】 王利；

【导师】 赵颖； 张晓丹；

【作者基本信息】 南开大学 ， 电子科学与技术， 2013， 博士

【摘要】 pin型硅基薄膜叠层太阳电池因高效率、低成本、光致衰退小等优点具有较大的产业化潜力。为进一步提升这种电池的光电转换效率,通过优化前电极的陷光特性,以实现在宽光谱范围内对太阳光子的高效利用成为研究的热点。其中作为顶电池的非晶硅子电池直接沉积于前电极之上,并且该子电池厚度通常较薄,以实现各子电池之间的电流匹配。此时前电极的绒面形貌结构将对电池输出特性产生较大影响。另外,该超薄非晶硅太阳电池自身的pin结特性也会极大影响到电池性能。针对这些问题,本论文将采用金属有机化学气相沉积(MOCVD)工艺自行研制的高绒度掺硼氧化锌透明导电薄膜(ZnO:B,记为BZO),用作pin型超薄非晶硅太阳电池的前电极,进行了如下三个方面的研究：首先,详细研究了高绒度BZO前电极表面大而尖锐的类金字塔形貌对电池开路电压Voc和填充因子FF的影响。结果表明：该前电极表面V型形貌会在非晶硅太阳电池中引入漏电,导致电池Voc和FF的恶化。氩气等离子体刻蚀工艺可将该V型形貌修饰成U型弹坑状,进而避免电池Voc、FF的下降。但该刻蚀工艺会大大消弱前电极的陷光能力,造成电池短路电流的下降。基于此,提出了一种新的表面形貌刻蚀工艺：H2/CH4混合气等离子干法刻蚀方法。深入研究了该刻蚀方法的工艺参数变化(刻蚀时间、刻蚀功率、H2/CH4比等)对刻蚀效果的影响,并对刻蚀后的BZO薄膜的光学、电学特性做了较为全面的探讨。采用该方法刻蚀后的BZO薄膜表面会形成大、小特征尺寸类金字塔交错分布的新型绒面形貌结构,不仅同时兼顾了短波区和长波区的陷光特性,而且避免了因V型绒面形貌对电池Voc和FF特性的不利影响。其次,分别从模拟和实验角度研究了界面特性(TCO/p、p/i、n/Metal)对超薄非晶硅电池Voc和FF的影响。针对TCO/p界面接触势垒,设计了三种不同界面结构：①BZO/p+-μc-Si/p-a-SiC；②BZO/n+-μc-Si/p+-μc-Si/p-a-SiC；③BZO/p-a-Si/p-a-SiC。对比了这三种结构对TCO/p界面改善的效果。其中方案2中BZO/n+-μc-Si/p+-μc-Si/p-a-SiC的界面结构,因将BZO表面和隧穿界面分离开,对应电池的Voc.FF最好。同时,还关注了p/i界面导带底位置处势垒和价带顶位置处势垒分别对电子反扩散过程和空穴外迁过程的影响。通过模拟这两处势垒,结果表明：导带底位置处势垒对Voc、FF的影响大于价带顶位置处。为了改善p/i界面,将宽带隙本征非晶硅碳材料插入该界面充当缓冲层,并对该缓冲层进行氢等离子体后处理,相应的电池Voc和FF特性进一步改善。最后,将自行开发的微晶硅氧(μc-SiOx)掺杂材料用作超薄非晶硅太阳电池的p层和n层。结果表明：该掺杂层材料不仅有利于改善电池Voc,同时还可增强电池对BZO前电极尖锐、粗糙表面的容忍度。通过改变沉积参数(氢稀释、CO2流量),重点研究了氢、氧元素在微晶硅氧材料中的掺杂行为。另外从实验中还可发现,p型微晶硅氧材料纵向电导比横向电导高约4个数量级,n型微晶硅氧材料的纵向电导比横向电导则高达5个数量级。而常用的n型非晶硅材料,两者电导值则处在同一数量级内。微晶硅氧掺杂层材料独特的两相分离的结构特点,即富氧的非晶硅相和富硅的微晶硅相呈分离分布状态,是造成该材料呈现出电导各向异性的主要原因。而恰恰是该特性提高了超薄非晶硅太阳电池对高绒度衬底的容忍度。通过优化p-μc-SiOx和n-μc-SiOx掺杂层,在本征层厚度约为140nm,仅有铝背反射电极的情况下,非晶硅太阳电池效率为7.76%(Voc:911mv；Jsc:12mA/cm2；FF:71%)。更多还原

【Abstract】 pin-type silicon based multijunction thin film solar cells has already shown high productive potential because of its excellent device properties, such as high conversion efficiency, low production cost, high stability. Recently, the research topic has been focused on fully utilizing the wide solar-spectrum. Generally, pin-type amorphous silicon thin film solar cells directly deposited on front contact is used as top cell. Along the direction of incident light, this top cell is located at the front of optical path. Its output characteristic has a huge influence on the performance of multijunction solar cells. In this thesis, high-haze boron doped zinc oxide deposited by metal organic chemical vapor deposition was used as front contact in amorphous silicon thin film solar cell with an absorber layer thickness only around150nm. The corresponding study can be divided into three parts as follows:First of all, the relationship between the electrical properties of amorphous silicon solar cell(Open circuit voltage; Fill factor) and the as-grown sharp edges of the pyramids at the rough texture surface of ZnO:B was carefully investigated. The result demonstrates that, the V-type surface morphology can cause large defect regions in amorphous silicon material, increase the device current leakage. Argon plasma post-treatment method was introduced to tailor the surface morphology from V-type to U-type, improve the open circuit voltage and fill factor. Unfortunately, this etching process can reduce the light trapping capability. In this thesis, we developed a new dry etching method to modify the surface of ZnO:B thin film:hydrogen and methane mixed gas (H2/CH4) plasma post treatment method. With this etching process, The light trapping capability of ZnO:B thin film in wide solar-spectrum improved simultaneously. The ultrathin amorphous silicon thin film solar cells fabricated on this substrate demonstrated excellent electrical properties. In order to obtain more information about this etching method, the plasma parameters were adjusted to investigate the change of etching process.Secondly, the influence of device interface properties on the open circuit voltage and fill factor was also focused on. Different optimized methods has been introduced to overcome the TCO/p contact potential and the comparison of those methods was also listed in the fourth chapter. The result presented that both the tunneling current and the decrease of potential barrier can both improve the open circuit and fill factor. The other part of fourth chapter was to study the p/i contact properties. The potentail barriers at the bottom of conduction band and the top of valence band were compared both from simulation and experiment. It was found that the former p/i potential barrier showed larger influence on the solar cell electrical performance. With the purpose of improving the p/i contact properties, large bandgap intrinsic layer material a-SiC:H was inserted into p/i interface and the mechanism behind this interface layer has been deeply discussed.Finally, the doped microcrystalline silicon oxide material (p-layer; n-layer) was introduced in ultrathin amorphous silicon thin film solar cells. The distinctive properties of those doped materials can be summarized into two points:firstly, the in-plane conductivity was smaller by five orders of magnitude than the transverse conductivity. As comparison, the in-plane conductivity of doped amorphous silicon is almost same as the transverse conductivity. Secondly, the doped microcrystalline silicon oxide material consisted of two different crystalline phases:microcrystalline silicon and amorphous silicon oxide. Those two phases distributed separately in the whole material region. On the other hand, we also focused on the performance of solar cells when those doped materials were used. It was presented that, the electrical properties of solar cells deposited on very high-haze front contact successfully avoided the deterioration of open circuit voltage and fill factor. With the optimization of those doped materials, the conversion efficiency of single ultrathin amorphous silicon solar cells can reach up to7.76%for an absorber layer thickness only around140nm(Voc:911mV;Jsc:12mA/cm2; FF:71%).更多还原