Metal oxide sol annealing on perovskite MAPbI(3) film with thermal stability enhanced by caffeine additive and PMMA interlayer

• Main Authors: Soo, Yew Hang, Ng, Chai Yan, Jun, Hieng Kiat, Ng, Soo Ai, Wong, Yew Hoong
• Format: Article
• Published: Kluwer (now part of Springer) 2022
• Subjects: QD Chemistry
• Online Access: http://eprints.um.edu.my/42149/
• Institution: Universiti Malaya

Methylammonium lead iodide (MAPbI(3)) is a popular light-absorbing layer in the perovskite solar cells (PSCs). However, the poor thermal stability of MAPbI(3) has inhibited the direct annealing of low-temperature processed (<= 150 degrees C) metal oxide sols as a charge transport layer (CTL) on top of it. In this work, caffeine as a nonvolatile and low-cost additive was added into the poly(methyl methacrylate) (PMMA)-encapsulated MAPbI(3) film to enhance its thermal stability at 150 degrees C. The caffeine-added MAPbI(3) (C-MAPbI(3)) film was prepared using antisolvent-assisted one-step spin-coating. With caffeine additive, the thermal degradation of the C-MAPbI(3) film was significantly reduced, and the film still retained dominating MAPbI(3) phase after 24 h of annealing. In addition, the caffeine additive improved the film morphology by promoting seamless grain boundary formation. The thickness of the PMMA encapsulation layer deposited on top of the C-MAPbI(3) film could be reduced from similar to 1550 nm (charge insulating layer) to similar to 60 nm (ultrathin charge tunneling layer) without sacrificing the thermal stability of the C-MAPbI(3) film. The vanadium oxide (VOx) sol was spin-coated on the similar to 60 nm PMMA-encapsulated C-MAPbI(3) film and annealed at 150 degrees C for 20 min to act as a hole transport layer (HTL). The VOx sol could be annealed without causing thermal degradation to the C-MAPbI(3) film. With this discovery, the VOx may replace the thermally unstable organic HTLs in n-i-p PSCs. This low-cost and facile metal oxide sol route would lead to economic and scalable production of all-metal oxide CTL-based PSCs with significantly improved device operational stability.