Ultrafast XAS tracks the fate of electrical charges in perovskites
Part of the XAS suite at Majed Chergui's lab © Alain Herzog/EPFL
In perovskite materials, absorption of light generates electrical charges, making them prime candidates for cheap solar cells. EPFL scientists can track the fate of these charges using an ultrafast X-ray spectroscopy technique, providing important information for solar energy conversion.
Perovskites are materials that have gained immense interest in solar energy conversion lately, as they are bringing down the cost of solar cells. When light shines on them, perovskites can produce electrical charge carriers: electrons (negative charges) and “holes” (positive charges). However, we do not completely understand what ultimately happens to these charges. This knowledge, especially on the level of individual atoms is key for developing and optimizing perovskite solar cells. EPFL scientists have now been able to track the charge carriers on perovskites by using a cutting-edge spectroscopic technique. The work is published in Structural Dynamics.
The lab of Majed Chergui at EPFL pioneered a powerful technique called time-resolved X-ray absorption spectroscopy (XAS), which can capture detailed information about the electronic structure of specific atoms in a solid material or a single molecule, e.g. a protein. In this study, they used XAS in time-steps of 80 picoseconds (80 trillionths of a second) to track the fate of charge carriers from inorganic perovskite nanocrystals that had been excited by light. Because this approach is element-selective, the researchers were able to look at specific atoms of the perovskite nanocrystals – namely, bromine (Br), lead (Pb), and cesium (Cs).
The study found that after absorbing light, the “holes” in the perovskite localize around Br atoms, forming small quasiparticles known as “polarons”. Meanwhile, electrons remained mobile in the perovskite nanocrystals. Finally, the Cs atoms showed no sign of being affected by the creation of either holes or electrons, indicating that they play no role in the transport of electrical charges in perovskites.
The findings directly impact our understanding of charge-carrier mobility in perovskites, and also suggest the involvement of another quasiparticle, the exciton, which exists in insulators and semiconductors and arises when electrons interact with the positively charged holes. On the other hand, the absence of changes around cesium atoms indicates that they play no role in transporting charges, at least not within 80-picosecond intervals.
Taken together, the data explains the generally modest charge-carrier mobility in perovskites, and can help overcome some of their limitations in solar conversion schemes.
This study included contributions from ETH Zürich and the Paul-Scherrer-Institut. It was funded by the SEFRI via the COST project C13.0062/CM1202 and the NRP70 “Energy Turnaround” project and the NCCR:Molecular Ultrafast Science and Technology (MUST) of the Swiss National Science Foundation (SNSF), and by the European Union’s FP7 and Horizon 2020 programs.
Reference
Fabio G. Santomauro, Jakob Grilj, Lars Mewes, Georgian Nedelcu, Sergii Yakunin, Thomas Rossi, Gloria Capano, André Al Haddad, James Budarz, Dominik Kinschel, Dario S. Ferreira, Giacomo Rossi, Mario Gutierrez Tovar, Daniel Grolimund, Valerie Samson, Maarten Nachtegaal, Grigory Smolentsev, Maksym V. Kovalenko, Majed Chergui. Localized holes and delocalized electrons in photoexcited inorganic perovskites: Watching each atomic actor by picosecond X-ray absorption spectroscopy.Structural Dynamics 4, 044002 (2017). DOI: 10.1063/1.4971999