Seeing the Invisible Plasma with Transient Phonons in Cuprous Oxide: Deducing Phonon Dynamics using the Phonon-to-Exciton Induced Absorption Triggered by Electron-Hole Plasma Relaxation in Cuprous Oxide*

In cuprous oxide (Cu₂O) there is no luminescence from electron-hole plasma. The emission of phonons from electron-hole plasma is the primary limit on the efficiency of photovoltaic devices operating above the bandgap. Therefore, we searched for optical phonons emitted by energetic charge carriers using phonon-to-exciton upconversion transitions. We found 14 meV phonons with a lifetime of 0.916 ± 0.008 ps and 79 meV phonons that are longer lived and overrepresented. It is surprising that the higher energy phonon has a longer lifetime.

Cuprous oxide is a semiconductor with a high theoretical photovoltaic/photocatalytic efficiency, composed of abundant and nontoxic elements. Charge carrier/phonon inelastic scattering can be examined by measuring carriers or by measuring phonons. However, in cuprous oxide, both the 3d-like valence and the 4s-like conduction band have positive parity. Therefore electron-hole plasma luminescence is forbidden and not observed. This led us to develop a bulk, femtosecond scale phonon measurement approach.

https://doi.org/10.1039/C6CP06532E