高活性金属源与低温快速生长CIGS结构缺陷的钝化及太阳电池研究

High efficiency thin film solar cells fabricated by lower energy consumption and higher yield lower are always expected. Now, the efficiency of the CuInGaSe2 (CIGS) prepared by low temperature process has been achieved 20.4%, but which is completed with very low deposition rate. If the CIGS absorber layer prepared by low-temperature and high-rate deposition processes, it will lead to poor device performance because of the metal elements cluster, poor crystallization, high defect density of interface and surface. In our project proposal, the high active metal evaporation source is designed to crack the metal cluster and enlarge the metal flue so that the CIGS thin films can be realized by low-temperature and high-rate deposition processes. Firstly, rapid reaction paths and polycrystalline structure will be investigated during this process. Secondly, the alkali elements are posted deposition treatment on absorber to passivate the defects in surface induced by this process. Thirdly, high work function and wide band gap metal oxide thin film is inserted between the back electrode and absorber to lower the interface barrier and decrease the back recombination, and then the carrier transport characterization can be improved. According to the results of device simulation, the mechanism of the high active metal source, different alkali elements incorporation into the absorber sequentially and the buffer layer of the back contact used in this process are investigated and the work principle are to be comprehended. This results contribute to controlling the crystallization of absorber, optical and electrical properties of pn Junction and back contact characterization, finally to improve the device efficiency. The achievements of the project proposal not only are helpful to rich the scientific connotation of the compound thin film solar cells but also can offer a new idea and scientific evidence for further decrease of the production cost ,energy consumption and improvement of the production yield.

低能耗、高产率的高效薄膜太阳电池是降低生产成本的大趋势。低温生长的铜铟镓硒电池效率已达20.4%，但沉积速率很慢。如在低温下高速生长铜铟镓硒会导致金属元素堆积、结晶质量低劣、结构缺陷剧增，电池难以高效。本项目设计新型高活性金属蒸发源，裂解金属原子蒸汽、提高活性原子束流，实现低温下高速生长铜铟镓硒。本项目研究低温高速生长铜铟镓硒的快速反应路径与多晶结构；碱金属对吸收层后处理，钝化低温高速沉积引起的表面结构缺陷；以高功涵、宽带隙金属氧化物为背接触缓冲层，降低接触势垒、抑制背界面复合，提高载流子输运特性。采用理论模拟仿真，深入揭示低温高速生长吸收层中高活性金属反应源、碱金属系列表面处理、背接触缓冲层等对器件的作用机理，实现吸收层结晶、异质结光电性能，及背界面接触特性的调控，有效提升器件效率。本项目不仅丰富化合物半导体薄膜太阳电池的科学内涵，还为降低生产能耗与成本、提高产率提供新思路和科学依据。

本项目针对低温快速生长高质量化合物半导体铜铟镓硒（CIGS）薄膜及其光伏器件进行了研究。通过对蒸发源的热学分析，设计出一种新型单加热努森源，有效降低了金属蒸发源口部的冷凝，改变了吸收层内缺陷类型，降低了缺陷密度。在低温450、230nm/min高速沉积条件下，以新源制备的CIGS薄膜太阳电池相对效率提升了29%；还通过在金属Mo和CIGS吸收层之间引入高功函材料（MoOx）的方法，结合器件仿真，分析了CIGS/Mo背界面能带排列及其功函数变化与厚度的关系，获得了最佳的背缓冲层厚度，有效降低了低温制备CIGS/Mo背界面的势垒，改善了载流子输运特性。而且还尝试了简单的化学处理，使Mo背电极处形成的Mo-O化合物形成类“水库”效应，可实现CIGS薄膜吸收层中碱金属Na与K掺杂浓度的调节；此外，在低温生长CIGS吸收层过程中，我们提出交叠工艺提升沉积速率，建立新工艺下CIGS薄膜的反应模型，分析了低温过程中CIGS薄膜的生长机理，实现了Ga梯度的调控，有效提升了器件的Voc和Jsc；为了进一步提高吸收层与异质结质量，我们研究了CsF后掺杂工艺（Cs-PDT）中重碱金属Cs对于CIGS薄膜特性及其表面电子结构的影响。发现：适量Cs进入到薄膜会钝化吸收层内的施主型缺陷如InCu，增加了吸收层内空穴载流子的浓度，从而提升器件性能，尤其对CIGS器件的VOC提升更明显，在CsF-PDT后，器件的VOC增加了70 mV。但大量的Cs掺杂会导致薄膜结构发生改变，形成新的深能级缺陷，恶化器件的性能。同时发现CsF-PDT更适合提吸收层表面高Ga含量的CIGS电池器件性能。其原因为降低了表面电子势垒，因此抑制了高Ga样品的高界面复合；为提高低温制备的CIGS)薄膜的结晶度，同时改善Ga梯度，我们通过Ag掺杂降低CIGS薄膜熔点温度，拓宽其带隙，改善薄膜的低温沉积质量。并借助碱金属Na改善了Ag掺杂CIGS薄膜载的流子浓度，提升了器件的开路电压。最终我们在蒸发速率约110nm/min、衬底温度为450 °C下，蒸制备出了效率为17.5%（无防反层）的太阳电池。