Novel hot-carrier solar cells convert sunlight to electricity more efficiently than conventional solar cells. These cells work by harnessing charge carriers—particles that carry an electric charge—before they lose their energy to heat. A key to keeping electric charges hot longer is to slow the quantum-scale vibrations, or phonons, that transport heat. Neutron scattering and other techniques show that thermal transport in hot-carrier solar cells can be reduced by replacing hydrogen atoms with heavier deuterium atoms. Swapping in a heavier atom dramatically improved performance. It delayed the cooling of a material used to make hot-carrier solar cells and extended the hot-carriers’ useful lifetime for energy conversion.
A system’s efficiency in transforming energy is controlled by the difference between its hottest and coolest temperatures. This means that preventing heat loss from the hot charge carriers has the potential to double the efficiency of solar cells. This study highlights a strategy for keeping charge carriers hot longer. The strategy may help novel hot-carrier solar cells achieve record solar-to-electric conversion efficiency. It may also guide materials design for applications beyond solar power. These applications could include optical sensors and communication devices.
Experts in materials synthesis, neutron scattering, laser spectroscopy, and condensed matter theory revealed a way to inhibit charge cooling in a photovoltaic perovskite. The finding established a new strategy for designing hot-carrier solar cells, which keep charge carriers hot longer to better harness energy before it is lost to heat. The researchers studied collective excitations transporting heat in methylammonium lead iodide, a crystalline solar absorber. A “hot phonon bottleneck” inhibits electrons from losing their energy to collective vibrations that transport heat. To amplify this effect, researchers synthesized crystals of methylammonium lead iodide in which a lighter isotope of hydrogen (protium) was substituted with a heavier one (deuterium). This substitution enhanced the moment of inertia by increasing the mass of the ends of the perovskite’s central organic molecule, methylammonium (MA). Triple-axis neutron scattering experiments mapped phonon dispersions, and neutron spectroscopy revealed vibrational energies of the molecule in the crystal. Thermal diffusivity measurements showed how heat moved in the protonated and deuterated crystals. Deuteration decreased thermal conductivity by 50 percent. With additional insight into complexities of phonon behavior provided by first-principles calculations, the researchers showed the greater moment of inertia of deuterated MA slows its swaying to become closer in frequency to that of the collective vibrations. The two motions start to interact and then strongly couple. The synced phonons slow, becoming less effective at removing heat. In fact, optical experiments showed that the charge-carrier cooling time doubled. The discovery suggests new routes for design of hot solar cell materials in which additives improve material and device performance.
Oak Ridge National Laboratory
This research was supported by the Department of Energy Office of Science, Materials Sciences and Engineering Division, by the DOE Office of Energy Efficiency and Renewable Energy, Vehicle Technology Officeand the U.S. Department of Homeland Security. This work used several DOE Office of Science user facilities: the High Flux Isotope Reactor, the Spallation Neutron Source, and the Center for Nanophase Materials Sciences.
Manley, M. E., et al., Giant isotope effect on phonon dispersion and thermal conductivity in methylammonium lead iodide. Science Advances 6(31), eaaz1842 (2020). [DOI: 10.1126/sciadv.aaz1842]
Blocking vibrations that remove heat could boost efficiency of next-gen solar cells [Oak Ridge National Laboratory press release]