基于Cu2ZnSnS4薄膜的V字型能带构造及其对光伏性能的影响

Cu2ZnSn(S,Se)4 thin films is expected to become the next generation of low cost, high efficiency compound thin film solar cells, due to its abundant raw materials, excellent photoelectric properties. To date low open circuit voltage (Voc) is a major factor limiting access to high efficiency of Cu2ZnSn(S,Se)4 solar cell. The challenge is how to increase Voc, while maintaining the large short-circuit current (Isc). In this project, the Cu2ZnSnS4 thin films with V-shaped double band-gap depth profiles are proposed by doping Se or Ge, which will be prepared by low cost oxides nanoparticles-based route. The band gap structure of Cu2ZnSnS4 is tunable by changing the Se/S or Ge/Sn ratio in the depth profiles. The resulting V-shaped double band-gap depth profiles have a wide band gap at front and back side, while narrow band gap in middle, which can simultaneously increase Isc and Voc. According to the theoretical results and performance of solar cells, the mechanism of carrier transport in such double band-gap grading’s films will be investigated. Engineering the band-gap, the performance of heterojunction interface will be improved. The fabricated solar cells is expected to have higher Isc, Voc and efficiency. All the research above could provide instructions for the development of low-cost thin film solar cells with high conversion efficiency, which has important scientific significance and application value.

Cu2ZnSn(S,Se)4薄膜因其原料丰富、光电性能优异，有望成为下一代低成本、高效率化合物薄膜太阳能电池的典范。目前限制Cu2ZnSn(S,Se)4薄膜太阳能电池效率的主要因素是低的开路电压，在保持大短路电流的同时提高开路电压仍面临许多问题。本项目拟采用低成本的氧化物纳米颗粒法制备Cu2ZnSnS4薄膜，通过对Cu2ZnSnS4薄膜进行Se或Ge掺杂，控制Se/S或Ge/Sn在薄膜不同深度的比例，调节其带隙，构建表背面带隙宽、中间窄的V字型双梯度能带结构，以期有效解决传统单梯度能带结构中因为表面带隙窄引起的电池开路电压的降低。结合能带理论计算和电池性能结果分析双梯度能带结构中载流子输运的微观机制，仔细调控薄膜带隙，改善异质结界面的性能。预计所研制的电池将同时具有大的短路电流、高的开路电压和光电转换效率，为实现低成本、高效率薄膜太阳能电池的应用发展提供指导，具有重要的科学意义和应用价值。

以低成本的氧化纳米颗粒法制备CZTSSe薄膜为基础，本项目首先开展了CZTSSe薄膜中S/Se的调控，采用三步退火法（硫化-硒化-表面硫化）构建具有V字型能带结构的CZTSSe薄膜。浅层的表面硫化未对薄膜形貌结构产生根本的变化，然而微量的S取代CZTSSe表面的Se后，其表面带隙被扩宽，形成V字型能带。这使得CZTSSe器件在保持短路电流的同时，能够获得高的开路电压，并取得了6.4%转换效率。其次，采用GeO2粉末或GeSe2薄膜等锗源对CZTSSe薄膜进行Ge元素掺杂，形成组分可控的CZTGSSe薄膜。（i）在氧化物前驱膜的合成中，引入GeO2源获得混合氧化物，并结合后续硫化和硒化过程，实现Ge/Sn和S/Se在CZTGSSe薄膜的均匀掺杂和薄膜带隙的宽范围调控，从而实现对薄膜器件Voc和Jsc的调节。（ii）通过热蒸发获得一定厚度的GeSe2薄膜，并将其与所制备的CZTS薄膜形成“面对面”接触，构造三明治结构退火，实现对CZTS同时进行Ge和Se的掺杂，获得V字型能带薄膜。该项目研究为CZTSSe薄膜太阳能电池的能带调控提供了思路，加深了人们对器件中载流子输运等问题的理解。