THE CHALLENGE AND THE SOLUTION
Over the past decade, Halide perovskite photovoltaic materials have rapidly emerged as one of the leading contenders for next generation solar applications. Rapid increases in photovoltaic conversion efficiencies from 3.9% in 2009, to 25.7% in 2021, have been achieved.
Nonetheless, in the late 2010s, it was conjectured that nanoscale defect sites acted as sites of ultrafast photocarrier capture, impacting efficiency and long-term stability. Using our time-resolved multi-dimensional photoemission spectroscopy instrumentation, we sought to study the ultrafast photocarrier dynamics at the nanoscale defect sites in state-of-the-art perovskite photovoltaic films.
First, we used an ultrafast probe pulse in the UV (4.5-6.0eV) to photoemit electrons from the static, unphotoexcited perovskite sample. These photoemitted electrons were collected by an objective and imaged in real space. Prior to the imaging, the electrons were energy filtered using a analyzer, thus generating energy resolved images of the occupied electronic states in space in the disordered material. Next, for the time-resolved images, we photoexcited the sample with a femtosecond pulse in the visible. We then performed the above imaging technique with a time-delayed UV probe pulse, which gave us the distribution of carriers in real space as a function of time-delay after photoexcitation.