Led by the Department of Energy’s Oak Ridge National Laboratory and the University of Tennessee, Knoxville, research into a solar energy material with a bright future revealed a way to slow down phonons, the waves that carry heat. The discovery could improve novel hot-carrier solar cells that convert sunlight into electricity more efficiently than conventional solar cells by using photo-generated charge carriers before they lose energy to heat.
“We have shown that the heat transfer and the cooling time of the charge carrier can be influenced by changing the mass of the hydrogen atoms in a photovoltaic material,” said Michael Manley of ORNL. “This avenue to extend the life of charge carriers opens up new strategies for achieving record-breaking conversion efficiency from solar to electricity in novel hot carrier solar cells.”
Mahshid Ahmadi of UT noted, “Tuning the dynamics of organic molecules can enable control of phonons, which are important for thermal conductivity in organometallic perovskites.” These semiconducting materials hold promise for photovoltaic applications.
Manley and Ahmadi designed and administered the study published in Science Advances. Experts in materials synthesis, neutron scattering, laser spectroscopy, and condensed matter theory have found a way to prevent wasteful charge cooling by swapping a lighter isotope for a heavier one in an organometallic perovskite.
When sunlight hits a solar cell, photons generate charge carriers – electrons and holes – in an absorber material. Hot carrier solar cells quickly convert the energy of the charge carriers into electricity before it is lost as waste heat. Avoiding heat loss is a major challenge for these solar cells, which may be twice as efficient as traditional solar cells.
The conversion efficiency of conventional perovskite solar cells has improved from 3% in 2009 to over 25% in 2020. A well-designed hot carrier device could achieve a theoretical conversion efficiency of close to 66%.
The researchers examined methylammonium-lead iodide, a perovskite absorber material. Collective excitations of atoms create vibrations in their lattice. Vibrations that move synchronously with each other are acoustic phonons, while vibrations that move out of sync are optical phonons.
“As a rule, charge carriers first lose their heat to optical phonons, which propagate more slowly than acoustic phonons,” explained Raphael Hermann, co-author of ORNL. “Later, optical phonons interact with acoustic phonons, which carry this energy away.”
However, in a region known as the “hot phonon bottleneck”, exotic physics prevents electrons from losing their energy through collective vibrations that carry heat. To amplify this effect in a photovoltaic perovskite, the researchers used inertia, the tendency of an object to do what it does, be it at rest or in motion.
“We basically slowed down how fast the molecules can fluctuate, much like a spinning ice skater by putting weights in her hands,” said Hermann.
To do this in an ordered atomic lattice, ORNL’s Ahmadi and Kunlun Hong led the synthesis of methylammonium lead iodide crystals at the Center for Nanophase Materials Sciences, a user facility of the DOE Office of Science at ORNL. They replaced a lighter isotope of hydrogen, normally occurring protium, which does not contain neutrons, with a heavier, deuterium, which contains a neutron, in the central organic molecule of perovskite, methylammonium or MA. Isotopes are chemically identical atoms that only differ in mass due to the different number of neutrons.
Next, Manley and Hermann, along with ORNL’s Songxue Chi, performed three-axis neutron scattering experiments at the High Flux Isotope Reactor, a user facility of the DOE Office of Science at ORNL, to map phonon dispersion in protonated and deuterated crystals. Seeing a disagreement between their measurements and published data from inelastic X-ray measurements, they took additional measurements at the Neutron Source spallation, another user facility of the DOE Office of Science at the ORNL. There, Luke Daemen from ORNL used the VISION vibration spectrometer to uncover all possible vibrational energies. The combined results showed that longitudinal acoustic modes with short wavelengths propagate more slowly in the deuterated sample, suggesting that the thermal conductivity may be reduced.
ORNL’s Hsin Wang took measurements of thermal diffusivity to study how heat moved in the crystals. “These measurements showed us that deuteration reduces the already low thermal conductivity by 50%,” said Manley. “At the time, we realized that this finding could have an impact on things that are important to the manufacturers of solar devices – especially on keeping charge carriers hot.”
The study provided an unprecedented understanding of the effect of increasing atomic mass on heat transfer.
“Many vibrations, such as stretching modes for the hydrogen atoms, have frequencies so high that they do not normally interact with the low-energy vibrations of the crystal,” said Daemen. The lower energy modes include rocking molecules.
The oscillation frequency of the organic molecule MA is slightly higher than the frequency of the collective oscillations. However, when a deuterium atom replaces a lighter isotope of hydrogen, its greater mass slows the fluctuation of MA. It fluctuates at a frequency closer to that of the collective vibrations and the two begin to interact and then couple strongly. The synchronized phonons slow down and become less effective at dissipating heat.
Hermann compared the influence of frequency to the various actions of a boy when his father was pushing him on a swing. “The protonated case is like the boy moving his legs too fast to be in sync with the father’s pressure. He won’t go any higher. But when he starts moving his legs at about the same frequency as the swing “It’s like the deuterated case. The boy has just slowed his legs down enough that he begins to synchronize with the pushed swing, which gives him momentum. He can swing higher because the two movements are coupled.”
The ORNL measurements showed an effect that was far above what was expected from a change in the hydrogen mass: the deuteration slowed down the heat transport so much that the cooling time of the charge carrier doubled.
To confirm this finding, ORNL co-author Chengyun Hua used pump-probe laser experiments to measure the energy dissipation of electrons in the deuterated and protonated perovskites over tiny timescales of billiard seconds.
“These measurements confirmed that the huge changes in phonons and thermal conductivity induced by the heavy isotope lead to a slower relaxation time for photoexcited electrons,” said Hua. “This is an important factor in improving the photovoltaic properties.”
The two authors Yao Cai and Mark Asta from the University of California at Berkeley, who also work with the DOE’s Lawrence Berkeley National Laboratory, performed theoretical calculations to gain insights into the complexity of phonon behavior.
The discovery made in the study carried out by ORNL-UT could represent a ray of hope for future manufacturers of solar cells with hot carriers.
“Phonons look like a pretty effective knob, and we know how to turn the knob,” said Manley. “If you want to improve the materials, you can add a molecule, methylammonium, or something else. The result can influence the decisions made by developers about how they grow their crystals.”
Ahmadi added, “This knowledge can be used to guide material design for non-photovoltaic applications such as optical sensors and communication devices.”