The hot-phonon bottleneck effect in lead-halide perovskites (APbX3) prolongs the cooling period of hot charge carriers, an effect that could be used in the next-generation photovoltaics devices. Using ultrafast optical characterization and first-principle calculations, four kinds of lead-halide perovskites (A=FA+/MA+/Cs+, X=I−/Br−) are compared in this study to reveal the carrier-phonon dynamics within. Here we show a stronger phonon bottleneck effect in hybrid perovskites than in their inorganic counterparts. Compared with the caesium-based system, a 10 times slower carrier-phonon relaxation rate is observed in FAPbI3. The up-conversion of low-energy phonons is proposed to be responsible for the bottleneck effect. The presence of organic cations introduces overlapping phonon branches and facilitates the up-transition of low-energy modes. The blocking of phonon propagation associated with an ultralow thermal conductivity of the material also increases the overall up-conversion efficiency. This result also suggests a new and general method for achieving long-lived hot carriers in materials.

We theoretically investigated the features of hot carrier solar cells, from which photogenerated carriers are extracted before they are completely thermalized. There are three channels of energy dissipation from photogenerated carriers that lowers the conversion efficiency: thermalization in the absorber, emission from the absorber, and thermodynamically unavoidable heat flux to the ambient. The emission increases with increasing carrier density in the absorber, whereas the heat flux decreases. Previous calculations of the conversion efficiency have been carried out under the supposition of no thermalization of carriers. In this case, the dominant process of energy dissipation is the emission, like conventional solar cells represented by the Shockley and Queisser formula. In practice, the carriers should be extracted to external circuits immediately after photogeneration because they are partially thermalized. This restriction leads to a much smaller carrier density and consequently more significant energy dissipation by heat flux, whereas the influence of the emission is negligible. As a result, the conversion efficiency is considerably lower than the values under the supposition of no thermalization. To suppress the heat flux to improve conversion efficiency, a smaller effective electron mass and a higher carrier temperature are required, as well as more intense irradiation. When the effective electron mass is much smaller than that of holes, the thermalization of holes has little influence on lowering the conversion efficiency.

No abstract