Fabrication of CH3NH3SnI3 perovskite solar cells using vacuum process

Abstract

Recently, hybrid organic-inorganic perovskite solar cells have attracted the great attention because of their remarkably high power conversion efficiency (PCE) combined with simple and low temperature process. But most of the devices reported up to now are Pb-based perovskite (e.g., CH3NH3PbI3) solar cells which have potential risk such as heavy metal pollution, lead poisoning and lead accumulation. Therefore, replacing Pb in the perovskite is one of the most important issues in the field of perovskite solar cells for their commercialization and practical applications in the future. Until now, Sn has been considered the most viable element for replacing Pb in perovskite since Sn is the group 14 elements in periodic table and it is also known that Sn can adopt perovskite structure with CH3NH3+ and I- ions. Based on this knowledge, some papers have reported lead-free Sn-based perovskite solar cells since 2014, however, all reported devices have been fabricated by spin-coating process. In the spin-coating process, electron transporting layer (ETL) under the perovskite is limited to mesoporous TiO2 (mp-TiO2) which causes poor surface morphology of CH3NH3SnI3 perovskite film. Low stability to oxidation and short carrier diffusion lengths have been also pointed out as the critical problems for solution-processed CH3NH3SnI3 perovskite. In this regard, vacuum process can be appropriate method for deposition of lead-free CH3NH3SnI3 perovskite film because vacuum process has several advantages such as clean environment, wide selection of charge transporting layer and high reproducibility. The clean deposition condition in vacuum process is very effective to easily oxidizable CH3NH3SnI3 film and various charge transporting layers can be chosen in consideration of better morphology of perovskite and well-matched energy level alignment. In this work, various hole transporting layers (HTLs) are introduced and the effect of HTLs on the growth of vacuum-processed CH3NH3SnI3 perovskite film is investigated. As a result, CH3NH3SnI3 film shows the most improved crystallinity on ITO/MoO3/NPB sublayer compared to on ITO and on ITO/MoO3 sublayer. The initial growth of 2 nm-thick SnI2 on each sublayer strongly affects total growth of CH3NH3SnI3, which is confirmed by X-ray diffraction (XRD) patterns. In addition, the effect of working pressure and thickness on the growth and film qualities of CH3NH3SnI3 perovskite is investigated. Controlling the increase in working pressure by evaporated CH3NH3I, stoichiometric and unifrom CH3NH3SnI3 perovskite film with nearly crystalline morphology is successfully formed at the apparent working pressure of 510-5 torr. The perovskite film fabricated at this pressure also shows the highest hole mobility due to the increased crystallinity and band gap energy of 1.33 eV. Lastly, optical simulation and fabrcating real CH3NH3SnI3 perovskite solar cells are conducted. Although real lead-free device exhibits poor performance due to the lack of optimization, optical simulation results indicate that short-circuit current (JSC) of the device can be increased up to ~23 mA/cm2 when thickness of perovskite reaches around 500 nm. If open-circuit voltage (VOC) reaches 0.88 V and fill factor (FF) attains ~0.7, the PCE of lead-free perovskite solar cells is expected to be ~14.2 % which is comparable to the Pb-based perovskite solar cells.