铜铟镓硒（CIGSe）薄膜太阳电池具有稳定性好、能带结构可调、光电转换效率高、可制备柔性基底太阳电池等优势。异质结界面的能带排布和界面缺陷状态对CIGSe太阳电池的性能具有重要影响。本文针对CIGSe异质结界面问题开展研究，通过Zn(O,S)缓冲层、In2S3基缓冲层和CIGSe:Cs吸收层的优化，深入分析异质结界面能带排布和界面缺陷对CIGSe太阳电池的影响机制，揭示了两者的耦合作用机理。主要研究内容及成果如下：研究了化学水浴法和反应溅射法Zn(O,S)的制备工艺参数对薄膜形貌、光学性能、结晶性能的影响。发现化学水浴沉积法制备的Zn(O,S)薄膜为非晶薄膜，成分稳定；反应溅射法制备的Zn(O,S)薄膜中存在二次相ZnSO3和ZnSO?4，扩大了Zn(O,S)的带隙，获得表面平整均匀、结晶性良好的宽带隙Zn(O,S)薄膜。研究了化学水浴沉积法和反应溅射法制备的Zn(O,S)缓冲层与CIGSe吸收层异质结能带排布情况。反应溅射法Zn(O,S)薄膜与CIGSe吸收层能带匹配较为良好。采用热处理工艺改善Zn(O,S)/CIGSe界面缺陷情况。热处理增强Zn、S等元素的扩散，使CIGSe表面的导电类型由p型转变为n型，促进发生载流子分离的p-n结电子界面进入吸收层，远离异质材料物理界面的较多缺陷。电池效率达到14.6%。研究了溅射法In2S3基薄膜的制备工艺参数对薄膜形貌、光学性能、结晶性能的影响。提出了采用间接带隙半导体作为CIGSe太阳电池的缓冲层，提高缓冲层透光性能和电池效率的新设计思路。发现In2S3基薄膜的间接带隙特性有效提高了缓冲层的透光性能，非晶特性显著优化了In2S3/CIGSe异质结界面质量。获得电池效率达到14.7%的全干法无镉In2S3/CIGSe太阳电池，其开路电压达到599.4 mV。研究了原位Cs掺杂对CIGSe吸收层、异质结界面及CIGSe太阳电池的影响。提出靶材原位Cs掺杂工艺，揭示Cs掺杂的作用机理。发现Cs与Na存在协同作用。适量的Cs掺杂可以抑制InCu2+、GaCu2+缺陷，并通过CsInSe2改善晶界和异质结界面能带排布；过量的Cs掺杂则会引入新的缺陷劣化CIGSe质量。当Cs掺杂量为0.75 atm‰时，获得效率达到16.2%的CIGSe:Cs太阳电池。研究了CIGSe太阳电池异质结界面处能带排布的优化区间，即相对吸收层而言，缓冲层导带能级提高0-0.4 eV，价带能级降低。分析其与界面缺陷的耦合作用，两者通过调控异质结界面处载流子分离、输运与复合，影响CIGSe太阳电池性能。
Cu(In,Ga)Se2 (CIGSe) thin film solar cells have been considered one of the best candidates for photovoltaic modules owing to their outstanding advantages, such as stability, tunable band alignment, high conversion efficiency, and flexible applications. The heterojunction is the core of CIGSe solar cells, at which the band alignment and interface defects play a vital role in the device performance. In order to reveal how the band alignment and interface defects govern the device performances, we have focused on the heterojunction interfaces of CIGSe solar cells in this work. Zn(O,S) buffer, In2S3-based buffer, and CIGSe:Cs absorber have been systemically studied and applied to the CIGSe solar cells, thus clearly elucidating the mechanisms of band alignment and interface defects at the heterojunction on the devices. The main research works and results are as follows:The effects of various process parameters on the morphology, optical properties, and microstructure have been investigated for Zn(O,S) films deposited by the chemical bath deposition and reactive sputtering, respectively. It was found that the Zn(O,S) films deposited by the chemical bath deposition method were amorphous films with stable compositions. Zn(O,S) films deposited by the reactive sputtering method exist in a wurtzite structure and contain secondary phases ZnSO3 and ZnSO4. The secondary phases lead to a wide-bandgap Zn(O,S) film with a flat and uniform surface.The band alignment at Zn(O,S)/CIGSe heterojunctions has been analyzed. The band alignment of Zn(O,S) films deposited by the reactive sputtering method matched well with those of CIGSe absorbers. Further, a heat treatment was introduced to reduce defects at Zn(O,S)/CIGSe interfaces. Heat treatment enhanced the element diffusion in the devices while transforming the CIGSe absorber near heterojunctions from p-type to n-type. The electronic interfaces of the p-n junctions were promoted into the absorbers, where carrier separation occurred. As a result, the depletion regions were moved away from the physical interfaces between heterogeneous materials with numerous defects. The device efficiency was increased to 14.6%.The effects of various process parameters on the morphology, optical properties, and microstructure have been investigated for In2S3-based films deposited by the sputtering method. A novel idea was proposed to improve the optical transmission of the buffer and device efficiency, namely employing an indirect bandgap semiconductor as the buffer for CIGSe solar cells. The indirect bandgap of the In2S3-based films significantly improved their optical transmission. The amorphous In2S3-based films benefited the In2S3/CIGSe interfaces by passivating defects. An all-dry process In2S3/CIGSe solar cell with a device efficiency of 14.7% was achieved. The open circuit voltage of Cd-free CIGSe devices reached 599.4 mV.The effects of in-situ Cs doping on CIGSe absorbers, heterojunction interfaces, and CIGSe solar cells have been investigated respectively. We have developed in-situ Cs doped CIGSe films and the CIGSe solar cells fabricated with these absorbers. The CIGSe:Cs absorbers were deposited by sputtering with in-situ Cs doped CIGSe ceramic targets. The mechanism for the effects of in-situ Cs doping has been proposed. It was found that Cs synergized with Na. An appropriate Cs amount could inhibit InCu2+ and GaCu2+ while optimizing the band alignment at CIGSe grain boundaries and p-n junction interface. Excessive Cs doping would introduce new defects to degrade the quality of CIGSe absorbers. CIGSe:Cs devices with the best efficiency of 16.2% were achieved when 0.75 atm‰ Cs was doped.The optimized band alignment at the heterojunction interfaces of CIGSe solar cells has been investigated, i.e., 0-0.4 eV upward shift in the conduction band and downward shift in the valence band of the buffer with respect to the absorber. The interaction between band alignment and defects at the heterojunction interface has been analyzed. Both of them affected the device performance together by controlling the carrier separation, transportation, and recombination at the heterojunction interfaces of CIGSe solar cells.
