Exciton Dynamics and Electron-Phonon Coupling Affect the Photovoltaic Performance of the Cs2AgBiBr6 Double Perovskite

Lead-free double perovskites have been proposed as promising nontoxic photovoltaic materials for the replacement of lead perovskites. While the latter ones reach remarkably high power conversion efficiencies (PCEs) above 23% in small lab devices, the lead-free double perovskites so far have severely underperformed, with PCEs below 3% for the prototypical system Cs2AgBiBr6, in spite of considerable optimization efforts by several groups. Here, we present a detailed study of Cs2AgBiBr6 thin films deposited on poly(methyl methacrylate) and mesoporous TiO2. Femtosecond UV-vis-NIR transient absorption experiments clearly identify the presence of excitons. In addition, strong electron-phonon coupling via Frohlich interactions is observed in terms of pronounced coherent oscillation of a strong A(1g) optical phonon mode of the double perovskite at 177 cm(-1). Similar behavior is also found for the related vacancy-ordered perovskite Cs3Bi2Br9 and the parent compound BiBr3. Excitonic effects and electron phonon coupling are known to induce unwanted electron hole recombination and hamper carrier transport. New strategies will thus be required for efficient carrier extraction at the interfaces of the double perovskite with electron and hole transport layers.