Daniel Niesner

Trap states in lead iodide perovskites.

Recent discoveries of highly efficient solar cells based on lead iodide perovskites have led to a surge in research activity on understanding photo carrier generation in these materials, but little is known about trap states that may be detrimental to solar cell performance. Here we provide direct evidence for hole traps on the surfaces of three-dimensional (3D) CH3NH3PbI3 perovskite thin films and excitonic traps below the optical gaps in these materials. The excitonic traps possess weak optical transition strengths, can be populated from the relaxation of above gap excitations, and become more significant as dimensionality decreases from 3D CH3NH3PbI3 to two-dimensional (2D) (C4H9NH3I)2(CH3NH3I)(n-1)(PbI2)(n) (n = 1, 2, 3) perovskites and, within the 2D family, as n decreases from 3 to 1. We also show that the density of excitonic traps in CH3NH3PbI3 perovskite thin films grown in the presence of chloride is at least one-order of magnitude lower than that grown in the absence of chloride, thus explaining a widely known mystery on the much better solar cell performance of the former. The trap states are likely caused by electron-phonon coupling and are enhanced at surfaces/interfaces where the perovskite crystal structure is most susceptible to deformation.

Screening in crystalline liquids protects energetic carriers in hybrid perovskites

Hybrid lead halide perovskites exhibit carrier properties that resemble those of pristine nonpolar semiconductors despite static and dynamic disorder, but how carriers are protected from efficient scattering with charged defects and optical phonons is unknown. Here, we reveal the carrier protection mechanism by comparing three single-crystal lead bromide perovskites: CH3NH3PbBr3, CH(NH2)2PbBr3, and CsPbBr3. We observed hot fluorescence emission from energetic carriers with ~102-picosecond lifetimes in CH3NH3PbBr3 or CH(NH2)2PbBr3, but not in CsPbBr3. The hot fluorescence is correlated with liquid-like molecular reorientational motions, suggesting that dynamic screening protects energetic carriers via solvation or large polaron formation on time scales competitive with that of ultrafast cooling. Similar protections likely exist for band-edge carriers. The long-lived energetic carriers may enable hot-carrier solar cells with efficiencies exceeding the Shockley-Queisser limit.

Many-body interactions in photo-excited lead iodide perovskite

Lead halide perovskite is emerging as a promising semiconductor material for thin film solar cells. Despite a large number of recent photophysical studies, the nature of photo-excitation in lead halide perovskite remains a subject of debate. Here we use transient absorption spectroscopy to re-examine lead halide perovskite thin films that have been reported to give very high solar cell efficiencies. We focus on many-body interactions that are manifested in (1) the transient Stark effect exerted by hot carriers on subsequent e–h pair generation; and (2) the Auger recombination due to three-body carrier–carrier interactions. These observations establish the dominance of free carriers from above band-gap excitation in lead halide perovskite. We also suggest the effective dynamic screening of charge carriers, likely due to orientational freedom of methylammonium cations in the perovskite lattice, and the presence of negligible charge carrier trapping in lead halide perovskite thin films grown in the presence of chloride precursors.

Image-potential states and work function of graphene

Image-potential states of graphene on various substrates have been investigated by two-photon photoemission and scanning tunneling spectroscopy. They are used as a probe for the graphene-substrate interaction and resulting changes in the (local) work function. The latter is driven by the work function difference between graphene and the substrate. This results in a charge transfer which also contributes to core-level shifts in x-ray photoemission. In this review article, we give an overview over the theoretical models and the experimental data for image-potential states and work function of graphene on various substrates.

Unoccupied topological states on bismuth chalcogenides

The unoccupied part of the band structure of topological insulators Bi

Trapping surface electrons on graphene layers and islands

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Two-photon photoemission from image-potential states of epitaxial graphene

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Structural fluctuations cause spin-split states in tetragonal (CH3NH3)PbI3 as evidenced by the circular photogalvanic effect

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Correction to “Persistent Energetic Electrons in Methylammonium Lead Iodide Perovskite Thin Films”

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Giant Rashba Splitting in CH_{3}NH_{3}PbBr_{3} Organic-Inorganic Perovskite.

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