铜锌锡硫在晶界面的能带结构研究

Thin film solar cells made of polycrystalline materials are studied because of the perceived advantages of cheap and easy preparation and ease of scale-up over single crystal-based cells. One of most promising material for thin film solar cells, CuIn1-xGaxSe2 (CIGS) is the absorber of choice for high efficiency thin film polycrystalline solar cells and high cell efficiencies have been achieved with CIGS cells by several groups (~20%). But the elements, In and Ga in CIGS are rare and costly, furthermore, they face earth abundant problem for large scale production in the future. Recently, quaternary compound Cu2ZnSnS4(CZTS), with In and Ga substituted by cheap and abundant Zn, Sn has achieved great interest and been studied as an alternative photovoltaic material. CZTS has similar electrical properties as CIGS as an excellent cheap substitute for CIGS material. But unfortunately, the convert efficiency of CZTS solar cell still remains a low level compared with CIGS solar cells at present. Grain boundaries (GB) with presence a lot of defects were responsible for the low solar cell efficiency. While for CIGS solar cells, high efficiency has been achieved. The physical reason lying behind is a special energy band alignment at GB which was introduced by the local charge distribution and composition non-uniformity at GB. How is the energy band aligned at GB in CZTS thin film and how does it affect the CZTS photovoltaic performance is the key question for understanding the low efficiency of the CZTS solar cell at present and direction for optimizing our fabrication method to further improve the solar cell efficiency. This requires a comprehensive nano-scale electrical characterization of the CZTS at GB. In this project, the scanning probe microscopy (SPM) techniques, which have been extensively used and appoved in the CIGS research, will be applied to study the microscopic electrical characteristic a the CZTS solar cells at grain and GB. With different application of the SPM techniques, such as STS, SKPM and CP-AFM, different electrical properties of the thin film at GBs will be characterized for the analyzing the alignment of the conducting band, valance band and band gap of CZTS at GBs. Furthermore, the band alignment and local and overall photovoltaic performance will be correlated. Through our experiments, we expect to bring new understanding for the low effeciency of the CZTS solar cell and further more to provide guidance for the direction of the optimization in the future.

在高效率低成本的铜铟镓硒(CIGS)薄膜太阳能电池基础上，由廉价锌，锡替代地球稀有元素铟，镓演化而来的铜锌锡硫(CZTS)薄膜太阳能电池不仅进一步降低了成本，同时也解决了CIGS薄膜太阳能电池生产原料铟镓资源丰度问题。然而目前其转化效率仍然远远低于CIGS太阳能电池。过去的实验研究表明CIGS太阳能电池高效率的关键在于对晶界面光生电子与空穴复合的抑制，而这种机制是通过CIGS晶界面特殊的能带结构得以实现。本项目将使用在CIGS研究中证实可行的扫描探针方法，利用对CZTS薄膜微观晶粒，晶界面的各种电学性质的测量，系统的研究CZTS薄膜在晶界面的能带结构，比较其与CIGS能带结构的异同；设计实验直接测量太阳能电池在晶界面的局域光电性能，并分析其与能带结构之间的关系，揭示CZTS薄膜太阳能电池低效率的微观物理机理；并以此机理为指导，提出CZTS薄膜生长工艺优化的方向。

在高效率低成本的铜铟镓硒(CIGS)薄膜太阳能电池基础上，由廉价锌，锡替代地球稀有元素铟，镓演化而来的铜锌锡硫(CZTS)薄膜太阳能电池不仅进一步降低了成本，同时解决了CIGS薄膜太阳能电池生产原料铟镓资源丰度问题。然而其转化效率仍然远远低于CIGS太阳能电池。过去的实验研究表明CIGS太阳能电池高效率的关键在于对晶界面光生电子与空穴复合的抑制。.本项目开发并应用多种扫描探针的测量方法，通过对CIGS,CZTS多晶薄膜微纳尺度的晶界面进行电学性质测量，实现了对晶界面能带结构的定量测量。我们发现在CIGS晶界面，导带向下偏置200meV, 价带向下偏置340meV， 在晶界面形成II类能带结构。这种能带结构在晶界面形成空穴势垒，降低晶界面处的空穴浓度，抑制电子空穴复合，是CIGS太阳能电池高效率的微观物理机理。然而在CZTS晶界面，导带向下偏置，与CIGS相同，价带向上偏置，与CIGS完全相反，形成I类能带结构。这种能带结构在晶界面形成空穴陷阱，反而增强了再晶界面的电子空穴复合，这是目前CZTS太阳能电池效率较低的主要原因之一。.近一步我们提出利用调控CZTS晶界面能带结构的方法优化太阳能电池的光伏性能。我们利用空气退火的方法，将CZTS薄膜晶界面的价带向上偏置调控为向下偏置，并利用扫描探针方法对晶界面的光伏性能进行测量。我们发现，随着晶界面能带结构的调控，晶界面处的漏电流大大降低，同时器件的总体效率也存在显著的提升。这不仅解释了业界长期利用空气退火提高太阳能电池效率的微观物理机理，同时也验证了利用能带结构调控优化太阳能电池性能的可行性。同时我们利用硫化处理方法，完成了晶界面富硫的CZTSS太阳能电池的制备。利用富硫材料能带较宽的原理，实现了晶界面价带的向下偏置。该方法制备的富硫晶面CZTSSe太阳能电池效率达到了8.5%，在目前真空法制备的CZTSSe太阳能电池中效率最高。