基于有序缺陷结构的高效CZTSSe薄膜太阳能电池

Kesterite CZTSSe is an emerging alternative solar-cell absorber due to the earth-abundant elements. There exist ordered defect compounds (ODC) under Cu-poor condition at grain surfaces in CIGS devices, thus ODC decrease both the valence and conduction band edge energies relative to the CIGS bulk. This leads to the potential barrier that attract electrons toward GBs, and repel holes to the bulk. The constituent atoms at the grain boundaries in CZTSSe create the large number of localized defects and wrong bonds that promote the recombination of photon-excited electron and hole carriers. Moreover, the p-CZTSSe absorber layer cannot have an p-to-n type inversion because high concentration form at the surface of CZTSSe. To this end, this proposal describes a general and effective strategy to suppress the surface recombination by designing the ODC in CZTSe absorber surface. The bulk of CZTSSe absorber layer was obtained through a composition grading strategy to achieve a V-shaped Ag-graded structure with a higher Ag content on both the backand front surfaces of the (Cu,Ag)2ZnSn(S,Se)4 (CAZTSSe) layer. In order to form the ODC layer at the surface of CZTSSe absorber, the high-content Ag was used substituted Cu to form n type inversion, then expect to obtain ODC through adjusting Cu ratio at Cu-poor condition. So, the CIGS-like ODC at the surface of CZTSSe is predicted to lower the valence and conduction band edge energy in a manner that attract electrons and repel holes, the surfaces or GBs should not contribute significantly to the recombination center in the device. The relationship of material design, structure optimization and performance of the CZTSSe device will be thoroughly investigated and established in this project for obtain a series of novel high-performance kesterite solar cells.

高元素丰度的铜锌锡硫硒（CZTSSe）有望取代铜铟镓硒（CIGS），成为最具潜力的薄膜光伏材料之一。高效的CIGS器件吸收层表面在贫铜时形成的n型有序缺陷化合物（n-ODC）能有效的降低表面复合。但CZTSSe器件吸收层表面复合严重，且过高的缺陷浓度使表面无法实现由p型到n型转化。本项目拟通过在表面上高含量银取代铜实现CZTSSe器件表面上的n-ODC结构。高含量银取代铜可使其表面由p型转化为n型，同时势必引起CZTSSe表面贫铜。在表面上，铜空位（VCu）和本身大量存在的铜锌反位缺陷（CuZn）形成有序缺陷化合物（2VCu+CuZn），从而得到类似CIGS器件中的n-ODC结构。该结构能有效分离电子和空穴，减少表面复合，获得高效的CZTSSe光伏器件，为该领域的发展提供新的方法和思路。

铜锌锡硫硒（CZTSSe）薄膜太阳能电池器件是由铜铟镓硒（CIGS）器件演化而来，但CZTSSe器件的光电转换效率仅为13%，远低于CIGS的23.35%。虽然CZTSSe器件的电池结构、制备工艺都继续完全沿用了CIGS器件的，由于CZTSSe吸收层表面过高的CuZn反位缺陷浓度无法形成CIGS表面n型OVC结构，促进电荷在界面的有效分离，从而导致界面复合严重。本项目中，我们设计了两种方案改善CZTSSe的表面，一是直接在硒化后表面热蒸一薄层的In2S3，然后PDT热处理，形成一层n型的贫铜OVC结构，从而改善了界面性能，减少电荷的复合，使开路电压由489提升至528mV，效率达到13.59%；二是在硒化后的表面热蒸一层AgF，PDT处理后，表面形成一层高Ag的CAZTSSe，高Ag使得由p-to-n转换，形成类似CIGS的n型的OVC结构，使开压达到500mV，转换效率达到12.34%。另外，在项目执行过程中，随着对CZTSSe器件的认识的逐步深入，我们意识到器件效率低的原因应该是多中元素共同作用的结果，如果只关注某一方面，会限制器件效率的大幅提升，为此，我们也围绕吸收层的体相、缓冲层、以及背界面都开展了研究，为下一步综合多种因素，协同作用大幅提升器件效率做了有益的尝试。