(Invited) Charge Carrier Dynamics and Origin of Instability of Organo-Metal Perovskite Films and Quantum Dots

Abstract

Organo-metal perovskites are highly promising materials for photovoltaic applications, yet their degradation due to factors such as light, moisture, and oxygen remains a significant problem and the mechanism behind is not yet well understood. We have studied the degradation mechanism of methylammonium lead iodide (MAPbI3) perovskite films subjected to light in a dry environment from a structural perspective. The effect of mesoporous titanium dioxide (mp-TiO2) layer, commonly used in device applications, on the degradation is also investigated. The perovskite structural change was investigated using XRD, UV-Vis absorption spectroscopy, Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy, and Fourier Transform Infrared (FT-IR) spectroscopy. The mp-TiO2 plays a significant role in the degradation rate of the perovskite film. Our results indicate that the first step in the light-induced degradation process is the deprotonation of the methylammonium cations, which results in the evaporation of HI and CH3NH2 molecules leaving unreacted PbI2 behind. This is further supported by ultrafast dynamics studies of charge carriers using femtosecond transient absorption spectroscopy. In addition, have synthesized various organo-metal perovskite quantum dots (PQDs) and used them as a model system to understand the origin of the materials instability. We have found the photogenerated electrons and holes can cause material degradation through redox reactions that damage the ammonium ions acting as localized trap sites, which is likely the origin of instability from light. We also found that species that are likely to react with or extract the protons from the ammonium ions will cause degradation, such as water or alcoholic solvents. Therefore, a more acidic environment is desired for better stability. Based on this understanding, we have designed new strategy to passivate the surface of PQDs using branched ligands to improve stability. The new insight gained from our studies is also important for developing new perovskite materials for solar cell and other applications such as LEDs and sensors.