An international team of scientists from Fritz Haber Institute of the Max Planck Society, École Polytechnique in Paris, Columbia University in New York, and the Free University in Berlin have demonstrated laser-driven control of fundamental motions of the lead halide perovskite (LHP) atomic lattice.
Sketch of the experimental pump-probe configuration. Image from Science Advances
By applying a sudden electric field spike faster than a trillionth of a second (picosecond) in the form of a single light cycle of far-infrared Terahertz radiation, the team unveiled the ultrafast lattice response, which might contribute to a dynamic protection mechanism for electric charges. This precise control over the atomic twist motions could allow to create novel non-equilibrium material properties, potentially providing hints for designing the solar cell material of the future.
The investigated hybrid LHP solar cell materials consist of an inorganic crystal lattice, which acts as periodic cages for hosting organic molecules. The interplay of free electronic charges with this hybrid lattice and its impurities determines how much electricity can be extracted from the sun light's energy.
Understanding this complicated interaction might be the key for a microscopic understanding of the outstanding optoelectronic performance of LHPs. The researchers have now been able to isolate the lattice response to an electric field on timescales faster than 100 femtoseconds, that is one tenth of a trillionth of a second.
The electric field has been applied by an intense laser pulse containing only a single cycle of far-infrared, so-called Terahertz (THz), light. "This THz field is so strong and so fast that it may mimic the local electric field of an excited charge carrier immediately after the absorption of a quantum of sunlight," explains Maximilian Frenzel, one of the main authors performing the experiments.
Based on this approach, the investigators observed a concerted motion of the crystal lattice, mainly consisting of back and forth tilting of the octahedral building blocks of the inorganic cage. These nonlinearly excited vibrations can lead to—so far neglected—higher order screening effects, contributing to an often discussed charge carrier protection mechanism.
"Moreover, the related tilting angle plays a dominating role in determining the fundamental material properties, such as the crystallographic phase or electronic bandgap," clarifies Dr. Sebastian Maehrlein, leader of the international research project. Thus, instead of static chemical tuning of material properties, ultrafast dynamic material design comes into reach: "As we can now modulate these twist angles by a single THz light cycle," summarizes Dr. Maehrlein, "in future we might be able to control material properties on demand or even discover novel exotic states of this emerging material class."
