Space-time visualization of photo-excited carrier dynamics in ferroelectric solar-energy converters by ultrafast electron microscopy

Project Information

SpaceTimeFerro

Grant agreement ID: 101064961

DOI

10.3030/101064961

Project closed

EC signature date 26 July 2022

Start date 1 January 2023

End date 31 December 2024

Funded under

• Marie Skłodowska-Curie Actions (MSCA)

Total cost

No data

EU contribution

€ 189 687,36

Coordinated by

UNIVERSITAT KONSTANZ
Germany

Periodic Reporting for period 1 - SpaceTimeFerro (Space-time visualization of photo-excited carrier dynamics in ferroelectric solar-energy converters by ultrafast electron microscopy)

Reporting period: 2023-01-01 to 2024-12-31

Summary of the context and overall objectives of the project

The global demand for sustainable and efficient energy solutions has intensified due to the limitations of fossil fuels and the growing need for renewable alternatives. Current solar energy technologies, such as silicon and perovskite-based solar cells, are constrained by efficiency limits and rapid charge carrier recombination. This project addresses these challenges by exploring the potential of ferroelectric materials, which possess intrinsic electric fields that enable highly efficient photo-voltage generation beyond the conventional bandgap limits.
The core objective of this research is to investigate the ultrafast photo-excited carrier dynamics in ferroelectric solar-energy converters using state-of-the-art ultrafast electron microscopy. By employing femtosecond and picometer-scale imaging, the project aims to capture the real-space dynamics of photo-generated carriers, providing fundamental insights into charge separation mechanisms and to probe the possibility of advancements in high-efficiency solar energy conversion.
Ferroelectric materials have unique ability to generate an internal electric field, which helps the charge carrier separation and minimizes recombination losses. Unlike conventional semiconductor materials, where charge separation is governed by external junctions, ferroelectric solar cells exploit spontaneous polarization, making them a promising candidate for next-generation photovoltaics. However, the underlying mechanisms of charge transport and separation in these materials remain poorly understood, limiting their practical application. This project aims to unfold the unknowns by visualizing the charge carrier dynamics in real time using ultrafast electron diffraction.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

The project was structured around three major work packages:
- Fabrication of Ferroelectric Thin Films: High-quality epitaxial ferroelectric, BaTiO¬3 (BTO) thin films were fabricated on Si(100) membranes using optimized pulsed deposition techniques. The BTO films were characterization through X-ray diffraction followed by the ferroelectric domain switching is recorded using the piezo-force microscopy. The structural and electrical characterization confirm the production of high-quality thin film for the project. Additionally, we collected free-standing BTO film to investigate the dynamics further in the absence of film and surface interface.
- Advancement of the ultrafast electron diffraction (UED) Techniques: A laser-driven ultrafast electron diffraction system was implemented to achieve femtosecond time resolution via the pump-probe technique. Femtosecond electron probe pulses are generated by two-photon photoemission from a gold cathode and subsequent acceleration to a kinetic energy of 70 keV. For a high-precision tracking of charge carrier motion in ferroelectric materials, the electron pulses are compressed in time by terahertz radiation. We have a wavelength of 343 nm pump pulses for the photoexcitation. The electron diffraction is probed via Bragg diffraction as a function of pump delay.
- Analysis of Photo-Excited Carrier Dynamics: The project successfully captured space-time imaging sequences revealing the ultrafast charge transport mechanisms in ferroelectric materials via intensity drop in the Bragg diffractions. Before, probing the dynamics in BTO, we investigate the dynamics and relaxation in Si(100) membrane. We found that in Si, the hot electron gas quenches the scattering into phonons in a temperature-dependent way.
- In case of BTO/Si heterostructure, the percentage drop in intensity and the rate constant is extracted from the sequence images to probe the photo-excited ultrafast dynamics. The obtained rate constant is 5 ps and the maximum intensity drop of ~1 % at (200) Bragg spots. From the measured experimental values, we have obtained the mobility of the charge carriers, an important parameter to understand or to play with for the improvement of the efficiency of the solar converts.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

This project has provided novel insights into the fundamental mechanisms governing charge separation in ferroelectric solar cells, pushing the boundaries of existing photovoltaic research. Key breakthroughs include:
• The direct visualization of ultrafast charge carrier motion in ferroelectric BTO at atomic resolution.
• Identification of the dominant transport mechanisms contributing to high-efficiency charge separation, including the interplay of ferroelectric domain wall motion and quasi-ballistic carrier transport followed by measuring the carrier mobility.
• Establishment of a new methodology for ultrafast electron microscopy in photovoltaic applications, setting a precedent for further research in the field. Open the idea to probe and visualize other solar materials and their dynamics to improve the efficiency further.
These findings pave the way for practical applications in next-generation solar cell designs, with the potential to significantly improve energy conversion efficiencies. Further research and industrial collaboration will be required to translate these insights into commercial photovoltaic technologies.