The radiative generation and recombination of charge carriers in semiconductors control both photovoltaic and LED operation. Understanding of these processes in hybrid perovskites has advanced,1–3 but remains incomplete.
Using femtosecond transient absorption (TA) and photoluminescence (PL) experiments on the formation of emissive states at early times after photoexcitation. We find that the PL signal rises over 2 picoseconds while initially hot photo-generated carriers cool to the band edge. This shows that PL of hot carriers is slower than that of cold carriers, as expected from strongly-allowed radiative transitions. We conclude that electrons and holes show strong overlap in momentum space, despite the potential presence of a small band offset that we model to arise from a Rashba effect.4,5 We find that photon recycling processes further affect externally measured radiative recombination rates in hybrid perovskites. Taking into account photon recycling, we connect the externally measured radiative efficiencies with the actual internal values, and derive internal PLQEs exceeding 80%.
Fundamentally, radiative rates are controlled by the time-scales of carrier-carrier interactions, which we study with two-dimensional electronic spectroscopy with sub-10fs resolution. We report the dependence of carrier scattering rates on excess energy and carrier density and extract carrier thermalization times from 8 to 85 fs. These values allow for mobilities of up to at carrier densities lower than , and limit the electronic coherence times in lead-halide hybrid perovskites. I will discuss how the extracted characteristic carrier thermalization times are relevant for the application of hybrid perovskites in hot carrier photovoltaics.
(1) Johnston, M. B.; Herz, L. M. Hybrid Perovskites for Photovoltaics: Charge-Carrier Recombination, Diffusion, and Radiative Efficiencies. Acc. Chem. Res. 2016, 49, 146–154.
(2) Saba, M.; Cadelano, M.; Marongiu, D.; Chen, F.; Sarritzu, V.; Sestu, N.; Figus, C.; Aresti, M.; Piras, R.; Geddo Lehmann, A.; et al. Correlated Electron–hole Plasma in Organometal Perovskites. Nat. Commun. 2014, 5, 5049.
(3) Staub, F.; Hempel, H.; Hebig, J.-C.; Mock, J.; Paetzold, U. W.; Rau, U.; Unold, T.; Kirchartz, T. Beyond Bulk Lifetimes: Insights into Lead Halide Perovskite Films from Time-Resolved Photoluminescence. Phys. Rev. Appl. 2016, 6, 44017.
(4) Niesner, D.; Wilhelm, M.; Levchuk, I.; Osvet, A.; Shrestha, S.; Batentschuk, M.; Brabec, C.; Fauster, T. Giant Rashba Splitting in CH3NH3PbBr3 Organic-Inorganic Perovskite. Phys. Rev. Lett. 2016, 117, 1–6.
(5) Isarov, M.; Tan, L. Z.; Bodnarchuk, M. I.; Kovalenko, M. V; Rappe, A. M.; Lifshitz, E. Rashba Effect in a Single Colloidal CsPbBr 3 Perovskite Nanocrystal Detected by Magneto-Optical Measurements. Nano Lett. 2017, 17, 5020–5026.