Interconnecting layer design and low-bandgap perovskite solar cells optimization for 2-terminal perovskite-perovskite tandem devices

Abstract

Energy resources are of critical importance to the development of human civilization and the improvement of human well-being. However, the excessive use of fossil fuels has led to severe problems, such as energy crisis and environmental pollutions. It is highly desirable to develop sustainable energy resources as alternatives to the fossil fuels. Solar energy as a clean and renewable resource, is a promising candidate for the future energy consumption. To directly convert solar energy into electricity, various solar cell technologies have been developed. The commercialization of solar cells is dependent on the performance, cost and long-term stability. Although conventional solar cell technologies, such as crystalline silicon, copper indium gallium diselenide and cadmium telluride, have been intensively investigated, it is essential to develop new solar cell technology with low-cost and high performance to boost the practical applications of solar cells. Organic-inorganic hybrid halide perovskite solar cells (PSCs), as an emerging solar photovoltaic technology, have achieved remarkable performances with soaring efficiencies. With the advantages of versatile and low-cost fabrication process, superior optoelectronic properties and convincing performances comparable to conventional solar cells, PSCs have the potential to revolute a new generation of photovoltaics. Especially, the 2-terminal perovskite-perovskite tandem devices offer a proven strategy to further improve the efficiency and reduce the cost. However, the challenge of efficient 2-terminal perovskite-perovskite tandem devices lies on the a) efficient interconnection of two subcells and b) highly efficient and stable mixed Pb-Sn low-bandgap PSCs. To promote the development and application of 2-terminal perovskite-perovskite tandem devices, we have conducted the following research work: 1. Demonstrate a concept of thermionic emission for efficient and low-cost interconnecting layer for promoting tandem solar cells A novel fluoride silane incorporated polyethylenimine ethoxylated hybrid system is introduced into the interconnecting layer (ICL) to a) improve solvent resistance to protect perovskite films and b) passivate the defects at the perovskite surface to enhance the electrical property. The fundamental operation mechanism of thermionic emission assisted electron transport is demonstrated theoretically and experimentally. This ICL fulfills the complex requirements of high electrical property, optical transmission and solvent resistance for efficiently interconnecting two subcells. 2. Develop new in-situ tin(II) complex antisolvent process for highly performed and stable low-bandgap PSCs The performance of low-bandgap PSCs is severely restricted by the Sn2+ oxidation and trap states at grain boundaries. The proposed in-situ tin(II) process produces a) quasi core-shell structure at perovskite grain boundaries to passivate the trap states and protect perovskite films from oxidation, and b) a heterojunction at the perovskite top region to facilitate carrier extraction. The performance and stability of low-bandgap PSCs are highly improved. 3. Investigate the synergistic effect of bulk and surface additive engineering to improve the stability and performances of low-bandgap PSCs The bulk additive engineering introduces ferrocene as the reducing agent to improve the perovskite stability against oxygen. While, the surface additive engineering incorporates (2,5-difluorophenyl)thiourea to passivate the perovskite surface defects and improve the perovskite film stability against water. Consequently, the low-bandgap PSCs demonstrate high efficiency and good stability.