Ultrafast extreme ultraviolet absorption spectroscopy of methylammonium lead iodide perovskite

Methylammonium lead iodide (perovskite) is a leading candidate for use in next-generation solar cell devices. However, the photophysics responsible for its strong photovoltaic qualities are not fully understood. Ultrafast extreme ultraviolet (XUV) absorption was used to investigate electron and hole dynamics in perovskite by observing transitions from a common inner-shell level (I 4d) to the valence and conduction bands. Ultrashort (30 fs) pulses of XUV radiation with a broad spectrum (40-70 eV) were generated via high-harmonic generation using a tabletop instrument. Transient absorption measurements with visible pump and XUV probe directly observed the relaxation of charge carriers in perovskite after above-band excitation in the femtosecond and picosecond time ranges

RAPID HOLE COOLING AND SLOW ELECTRON COOLING IN METHYLAMMONIUM LEAD IODIDE PEROVSKITE

Methylammonium lead iodide perovskite is a promising candidate for next-generation photovoltaics. One application for this perovskite is in hot-carrier collection devices. In a standard cell any excess energy from absorbed photons is lost as heat, but a cell can be designed to extract carriers before they cool to increase its efficiency above the Shockley-Queisser limit. In order to achieve this, the cooling rate of carriers must be sufficiently slower than the extraction time. Perovskite may fit this criterion due to the presence of a hot-phonon bottleneck for carrier cooling. Time-resolved XUV absorption from the core I4d level to the valence band (45-50 eV) after optical excitation (3.1 eV) was used to probe the hole distribution of photoexcited perovskite. The holes were found to cool rapidly (cooling time shorter than 400 fs) at high carrier density (10$^{19}$ cm$^{-3}$). In comparison, time-resolved optical absorption (1.5-2.5 eV) was used to probe the electron distribution, which was found to cool slowly (cooling time longer than 5 ps) for the same excitation density. This indicates that a hot-carrier collection device using perovskite should be designed to only extract hot electrons, not hot holes

ULTRAFAST EXTREME ULTRAVIOLET SPECTROSCOPY OF METHYLAMMONIUM LEAD IODIDE PEROVSKITE FOR CARRIER SPECIFIC PHOTOPHYSICS

Methyl ammonium lead iodide (perovskite) is a leading candidate for next-generation solar cell devices. However, the fundamental photophysics responsible for its strong photovoltaic qualities are not fully understood. Ultrafast extreme ultraviolet (XUV) spectroscopy was used to investigate relaxation dynamics in perovskite with carrier specific signals arising from transitions from the common inner-shell level (I 4d) to the valence and conduction bands. Ultrashort (30 fs) pulses of XUV radiation in a broad spectrum (40-70 eV) were obtained using high-harmonic generation in a tabletop instrument. Transient absorption measurements with visible pump and XUV probe directly observed the dynamics of charge carriers after above-band and band-edge excitation

Bottleneck-Free Hot Hole Cooling in CH3NH3PbI3 Revealed by Femtosecond XUV Absorption

Femtosecond carrier cooling in the organohalide perovskite semiconductor CH3NH3PbI3 is measured using extreme ultraviolet (XUV) and optical transient absorption spectroscopy. XUV absorption between 44 eV and 58 eV measures transitions from the I 4d core to the valence and conduction bands and gives distinct signals for hole and electron dynamics. The core-to-valence-band signal directly maps the photoexcited hole distribution and provides a quantitative measurement of the hole temperature. The combination of XUV and optical probes reveals that upon excitation at 400 nm, the initial hole distribution is 3.5 times hotter than the electron distribution. At an initial carrier density of 1.4×1020 cm-3 both carriers are subject to a hot phonon bottleneck, but at 4.2×1019 cm-3 the holes cool to less than 1000 K within 400 fs. This result places significant constraints on the use of organohalide perovskites in hot-carrier photovoltaics.<br /

Shrinking the Synchrotron : Tabletop Extreme Ultraviolet Absorption of Transition-Metal Complexes

We show that the electronic structure of molecular first-row transition-metal complexes can be reliably measured using tabletop high-harmonic XANES at the metal M2,3 edge. Extreme ultraviolet photons in the 50-70 eV energy range probe 3p → 3d transitions, with the same selection rules as soft X-ray L2,3-edge absorption (2p → 3d excitation). Absorption spectra of model complexes are sensitive to the electronic structure of the metal center, and ligand field multiplet simulations match the shapes and peak-to-peak spacings of the experimental spectra. This work establishes high-harmonic spectroscopy as a powerful tool for studying the electronic structure of molecular inorganic, bioinorganic, and organometallic compounds

Tabletop Femtosecond M‑edge X‑ray Absorption Near-Edge Structure of FeTPPCl: Metalloporphyrin Photophysics from the Perspective of the Metal

Iron porphyrins are the active sites of many natural and artificial catalysts, and their photoinduced dynamics have been described as either relaxation into a vibrationally hot ground state or as a cascade through metal-centered states. In this work, we directly probe the metal center of iron­(III) tetraphenyl porphyrin chloride (FeTPPCl) using femtosecond M_2,3-edge X-ray absorption near-edge structure (XANES) spectroscopy. Photoexcitation at 400 nm produces a (π,π*) state that evolves in 70 fs to an iron­(II) ligand-to-metal charge transfer (LMCT) state. The LMCT state relaxes to a vibrationally hot ground state in 1.13 ps, without involvement of (d,d) intermediates. The tabletop extreme-ultraviolet probe, combined with semiempirical ligand field multiplet calculations, clearly distinguishes between metal-centered and ligand-centered excited states and resolves competing accounts of Fe­(III) porphyrin relaxation. This work introduces tabletop M-edge XANES as a valuable tool for measuring femtosecond dynamics of molecular transition metal complexes in the condensed phase

Shrinking the Synchrotron: Tabletop Extreme Ultraviolet Absorption of Transition-Metal Complexes

We show that the electronic structure of molecular first-row transition-metal complexes can be reliably measured using tabletop high-harmonic XANES at the metal M_2,3 edge. Extreme ultraviolet photons in the 50–70 eV energy range probe 3p → 3d transitions, with the same selection rules as soft X-ray L_2,3-edge absorption (2p → 3d excitation). Absorption spectra of model complexes are sensitive to the electronic structure of the metal center, and ligand field multiplet simulations match the shapes and peak-to-peak spacings of the experimental spectra. This work establishes high-harmonic spectroscopy as a powerful tool for studying the electronic structure of molecular inorganic, bioinorganic, and organometallic compounds

Carrier-Specific Femtosecond XUV Transient Absorption of PbI₂ Reveals Ultrafast Nonradiative Recombination

Femtosecond carrier recombination in PbI₂ is measured using tabletop high-harmonic extreme ultraviolet (XUV) transient absorption spectroscopy and ultrafast electron diffraction. XUV absorption from 45 to 62 eV measures transitions from the iodine 4d core level to the conduction-band density of states. Photoexcitation at 400 nm creates separate and distinct transient absorption signals for holes and electrons, separated in energy by the 2.4 eV band gap of the semiconductor. The shape of the conduction band, and therefore the XUV absorption spectrum, is temperature-dependent, and nonradiative recombination converts the initial electronic excitation into thermal excitation within picoseconds. Ultrafast electron diffraction (UED) is used to measure the lattice temperature and confirm the recombination mechanism. The XUV and UED results support a second-order recombination model with a rate constant of 2.5 × 10^–9 cm³/s