Recently, Professor Wu's research group published a new research of “Controllable Formation of Ordered Vacancy Compound for High Efficiency Solution Processed Cu(In,Ga)Se2 Solar Cells” (Adv. Funct. Mater. 2020, 2007928). IF=16.836, JCR District 1.

Article link：https://onlinelibrary.wiley.com/doi/10.1002/adfm.202007928。

High efficiencies over 20% CIGS solar cells are fabricated by vacuum based technologies (co-evaporation and sputtering), which are of high production cost. To make CIGS solar cells more competitive in the photovoltaic market, it is necessary to develop low cost CIGS synthesis technologies. Various solutions processed approaches have been developed and achieved a champion certified power conversion efficiency of 17.3%. Considering the development history of CIGS solar cells, it is found that there is a thin (2VCu+InCu) defect complex (ordered vacancy compound, OVC) layer existing on the surface of high efficiency vacuum prepared CIGS thin films. In solution process, controlling the distribution of elements along the thickness direction is difficult because of the intense atomic interdivision. Hence, accurate manipulation of OVC is a nontrivial challenge in solution process method.

In this work, an effective strategy is implemented to realize controllable OVC formation and achieve high efficiency solution processed CIGS thin film solar cells. We construct a Cu content gradient in the precursor film by introducing a distinct Cu-poor composition CIGS top layer on the normal Cu composition CIGS precursor film. The OVC phase could decrease the valence band maximum and repell the holes by creating a barrier in the interfaced. The formation of OVC facilitates the carrier collection in CIGS devices. Meanwhile, it significantly reduces the interface defects density. The highly improved interface quality was believed to promote the CIGS device performance.

The first author of this paper is Yunhai Zhao of Key Laboratory of Special Functional Materials, and the corresponding authors are Professor Sixin Wu and Doctor Shengjie Yuan. This work is supported by the National Natural Science Foundation of China, the Science and Technology Department of Henan Province, and the Education Department of Henan Province.

Figure 1. Schematic diagram for the OVC formation process.

Figure 2. The band diagrams of CIGS device without OVC (a) and with OVC (b).