AFM：Interface Energy-Level Management toward Efficient Tin Perovskite Solar Cells with Hole-Transport-Layer-Free Structure
Lead-free tin perovskite solar cells (PSCs) have emerged as a promising candidate toward high-performance and eco-friendly photovoltaic technology with a great potential for future application, and the highest efficiency of tin PSCs has approached 15%. However, the tin PSCs with over 10% efficiency usually used organic hole transport layer (HTL) at the illumination side that may induce device degradation during the heating or light soaking process. Therefore, development of the HTL-free device become an important choice to solve the potential stability issues.
Recently, a team led by Prof. Liyuan Han at Shanghai Jiao Tong University has developed an inverted HTL-free structure to realize highly stable PSCs with over 10% efficiency, this research work was published in 《Advanced Functional Materials》. Prof. Liyuan Han is the corresponding author, Dr. Tianhao Wu and Dr. Xiao Liu (The University of Tokyo, Japan) are the first authors. They used a heterogeneous ammonium doping strategy to produce a compact and dense formamidinium tin iodide (FASnI3) perovskite film on the bare ITO substrate, and then found an upward band-bending structure formed at the ITO/FASnI3 interface to selectively extract the photo-generated hole to the electrode without the use of HTL, resulting in suppression of the charge recombination in HTL-free PSCs. In contrast, they found that the formamidinium lead iodide (FAPbI3) perovskite with a deeper band position formed a downward band-bending on ITO surface, which may increase the energy barrier of hole extraction from FAPbI3 to ITO, and increase the series resistance at the contact area as well. As a result, the FASnI3-based HTL-free device achieved a PCE up to 10.11% with a much better stability against the continuous light soaking for 40 days and 80 ℃ annealing for 300 hours compared to that of the tin PSCs with an organic HTL.
Figure 1. a) Device configuration of the HTL-free tin PSCs with a structure of ITO/FASnI3-based perovskite/PCBM/BCP/Ag. b) SEM cross-section image of the HTL-free tin PSCs. c) XRD patterns (the diffraction peaks of ITO glass were also marked) of tin perovskite coated on ITO and ITO/PEDOT:PSS substrates. d) Energy-level alignment and current density-voltage curve of the HTL-free tin PSCs.
Figure 2. a) Stability test of the FASnI3-based PSCs with and without HTL under light soaking in N2 atmosphere. b) Stability test of the FASnI3-based PSCs with and without HTL at 80 oC in N2 atmosphere. Normalized PL intensity of the corresponding perovskite samples c) before and d) after thermal annealing for 300 hours.