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 Cs 2 AgBiBr 6 , in spite of considerable optimization efforts by several groups. Here, we present a detailed study of Cs 2 AgBiBr 6 thin films deposited on poly(methyl methacrylate) and mesoporous TiO 2 . Femtosecond UV−vis−NIR transient absorption experiments clearly identify the presence of excitons. In addition, strong electron−phonon coupling via Froḧlich 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 Cs 3 Bi 2 Br 9 and the parent compound BiBr 3 . 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.

Low-dimensional copper halides, such as CsCu 2 I 3 , have emerged as promising LED materials featuring strongly Stokes-shifted photoluminescence with high quantum yield. Previous calculations suggest an exciton self-trapping mechanism; however, direct experimental evidence for this process is still lacking. Here, we present femtosecond UV−vis transient absorption experiments of CsCu 2 I 3 thin films. The films were analyzed by SEM, XRD, and 133 Cs/ 63 Cu NMR for crystallinity and defects. Unique spectral dynamics is observed. The band gap absorption exhibits a characteristic double-peak structure arising from the 130 meV spin− orbit splitting of the copper d electrons. Emission at the direct band gap disappears because of the formation of the lowest-energy self-trapped exciton state. We determined the time constant of 12 ps for the trapping process of thermally relaxed free excitons, with an energy barrier of at least 60 meV. The data are successfully modeled by global kinetic analysis, providing also accurate time constants for charge carrier cooling processes.