The study of perovskite thin films growth techniques and its impacts on solar cell efficiency and stability

Abstract

Organometallic perovskite materials have been widely used as light absorbers for high-efficiency photovoltaic cells. To grow high-quality perovskite films, different growth techniques, such as the solution method (both two-step and one-step), thermal evaporation and hybrid chemical vapor deposition (HCVD), have been applied. Among those, HCVD is a novel growth technique for perovskite films with an average grain size as large as 1~2 µm, as clearly seen in secondary electron microscopy (SEM) images. Oxygen is employed to improve the crystallinity and passivate defect states in the grown films. X-ray diffraction (XRD) is used to analyze the crystallinity, while low-frequency noise (LFN), time-resolved photoluminescence (TRPL) and photothermal deflection spectroscopy (PDS) are used to characterize the trap density and carrier lifetime. We studied both planar and mesoporous devices, which are the two most common configurations of perovskite photovoltaic cells. The planar structure typically has the configuration FTO/c-TiO₂/perovskite/Spiro/Au, and the highest power conversion efficiency (PCE) is 15.4% for solution-based and 17.2% for HCVD-based CH₃NH₃PbI₃ (MAPI) photovoltaic cells. The mesoporous structure is achieved by inserting a mesoporous layer, such as TiO₂, as an electron transport layer (ETL) between the compact TiO₂ layer (c-TiO₂) and perovskite layer. The highest PCE of 17.6% was achieved by using TiO₂ as the mesoporous layer. Stability studies of perovskite photovoltaic cells are necessary due to the fast decomposition of the perovskite materials particularly when exposed to light and moisture. It has been found that annealing of perovskite films in dry oxygen gas increases the PCE as well as the stability of the resulting perovskite photovoltaic cells. Encapsulated photovoltaic cells treated with oxygen and stored in an N₂-filled glovebox can maintain 60% of their initial efficiency for as long as 7000 hours, while the efficiencies of the cells without oxygen treatment dropped to 10% of the initial values in approximately 1000 hours. Our work shows that that oxygen passivates traps that may accelerate the decomposition of the perovskite films in the photovoltaic cells. In addition, a porous TiO₂ layer can further enhance the stability of the photovoltaic cells since this layer enhances the collection of photo-carriers thereby significantly reducing the impact of the traps on their effectiveness in capturing the carriers. When the other conditions are unchanged, the PCE of devices with a porous TiO₂ layer can maintain 70% of the initial value for 7000 hours, which is a substantial enhancement over the planar perovskite photovoltaic cells.