Neutron scattering from photovoltaic methylammonium lead iodide reveals a giant effect of isotopic substitution on phonons due to coupling with molecule dynamics that results in enhanced thermal resistivity and increased hot-carrier cooling times1. This route for keeping charge carriers hot longer bares new strategies for achieving record solar-to-electric conversion efficiency in novel hot carrier solar cells.
Organometallic halide perovskites burst on to the research scene owing to advances they bring to photovoltaics. Beyond standard solar cells, they show promise for hot-carrier devices. Harnessing charge carriers before they lose energy to heat could dramatically increase the maximum solar cell conversion efficiency. A big challenge is finding absorber materials from which charge carriers can be extracted before they have time to cool. Halide perovskites show a unique slow cooling of charge carriers owing to a “phonon bottleneck” that jams cooling. Now, neutron scattering measurements of phonons and molecular modes in protonated and deuterated methylammonium lead iodide reveal that a coupling of phonons to molecular modes leads to a giant effect: Heat transport slows and the charge-carrier cooling time doubles with deuteration – highlighting a route for enhancing hot-carrier solar cell efficiency.
