THz laser reveals ultrafast lattice motion in lead halide perovskites

Researchers worldwide are investigating new materials to address global energy challenges and combat the impending environmental crisis. Among the most promising materials for efficient and cost-effective solar cell applications are lead halide perovskite (LHP) semiconductors.

Despite achieving record-breaking solar cell prototypes, the underlying reasons for the exceptional optoelectronic performance of LHPs remain partially unknown. However, an international team of physicists and chemists from institutions such as the Fritz Haber Institute of the Max Planck Society, École Polytechnique in Paris, Columbia University in New York, and the Free University in Berlin has made progress in understanding this by demonstrating laser-driven control over the atomic lattice motions of LHPs.

Using a rapid electric field spike that lasts less than a trillionth of a second (picosecond), delivered in the form of a single light cycle of far-infrared Terahertz radiation, the researchers observed the ultrafast response of the lattice. This discovery suggests the existence of a dynamic protection mechanism for electric charges. The ability to precisely control the atomic motions opens up possibilities for creating new non-equilibrium material properties, potentially leading to the development of advanced solar cell materials in the future.

LHP solar cell materials consist of a hybrid structure, combining an inorganic crystal lattice that acts as a periodic cage for hosting organic molecules. The interaction between free electronic charges and this hybrid lattice, along with its impurities, determines the amount of electricity that can be extracted from sunlight.

Understanding the complex interplay between these components is crucial for comprehending the remarkable optoelectronic performance of LHPs. The researchers from the Fritz Haber Institute and their international collaborators managed to isolate the lattice response to an electric field on timescales faster than 100 femtoseconds (one-tenth of a trillionth of a second).

The electric field was generated using an intense laser pulse that contained only a single cycle of far-infrared Terahertz light. This Terahertz field was so intense and rapid that it simulated the local electric field experienced by an excited charge carrier immediately after absorbing sunlight. This experimental approach revealed a coordinated motion of the crystal lattice, characterized by back-and-forth tilting of the octahedral building blocks of the inorganic cage. These nonlinear vibrations, previously overlooked, may result in higher order screening effects that contribute to a charge carrier protection mechanism, which has been widely discussed in the scientific community.

Furthermore, the tilting angle of the lattice plays a significant role in determining fundamental material properties, such as the crystallographic phase or electronic bandgap. Dr. Sebastian Maehrlein, the leader of the international research project, explains that instead of statically tuning material properties, this ultrafast dynamic material design approach enables the modulation of twist angles within a single Terahertz light cycle. As a result, it may be possible to control material properties as needed or discover new exotic states within this emerging class of materials.

By investigating these dynamic states of matter, the researchers hope to provide insights that will aid in the design of future energy materials. The findings have been published in the journal Science Advances.

Source: Max Planck Society

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