Ultrafast Carrier Dynamics and Electrical Properties of the Palladium-Based Vacancy-Ordered Perovskite Cs2PdBr6

Phase-pure powders of the vacancy-ordered, metal-deficient perovskite cesium hexabromopalladate(IV), Cs2PdBr6, were synthesized and deposited as a dense nanocrystalline thin film by spin-coating a solution in DMSO on glass substrates. The thin films featured a panchromatic absorption spectrum with an extended wing reaching into the near-infrared region. Ultrafast broadband UV–vis–NIR pump–probe spectroscopy provided time scales for fast carrier trapping with a time constant of 1.4 ps followed by a biphasic recombination with a time constant of 31 ps and a much slower time constant, which was beyond our observation window of 1.5 ns. The recombination time constants were independent of the initial carrier number density over the range of 2.4 × 1017–2.9 × 1018 cm–3, suggesting a trap-mediated recombination mechanism with a fast and slow channel. The transient absorption kinetics identified pronounced coherent oscillations, and a Fourier transform analysis revealed a dominant wavenumber of 187 cm–1, which─according to accompanying DFT calculations─arose from a strongly Raman-active A1g phonon mode. This finding suggested considerable electron–phonon coupling in this material. Electrical measurements employing step-scan Fourier transform photocurrent spectroscopy (FTPS) of Cs2PdBr6 thin films deposited on interdigitated ITO electrodes found an extended electrical response of the film with a tail reaching 1400 nm, which is also in agreement with a substantial concentration of deep traps. Summarizing the above findings, although Cs2PdBr6 appears to be an appealing material in terms of its extended absorption spectrum, its indirect band gap, the presence of deep traps, and its strong electron–phonon coupling render it a far-from-ideal absorber material for photovoltaic applications.