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The relaxation of high-energy "hot" carriers in semiconductors is known to involve the redistribution of energy between (i) hot and cold carriers and (ii) hot carriers and phonons. Over the past few years, these two processes have been identified in lead-halide perovskites (LHPs) using ultrafast pump-probe experiments, but the interplay between these processes is not fully understood. Here we present a comprehensive kinetic model to elucidate the individual effects of the hot and cold carriers in bulk and nanocrystal $CsPbBr_{3}$ films obtained from "pump-push-probe" measurements. In accordance with our previous work, we observe that the cooling dynamics in the materials decelerate as the number of hot carriers increases, which we explain through a "hot-phonon bottleneck" mechanism. On the other hand, as the number of cold carriers increases, we observe an acceleration of the cooling kinetics in the samples. We describe the interplay of these opposing effects using our model, and by using series of natural approximations, reduce this model to a simple form containing terms for the carrier-carrier and carrier-phonon interactions. The model can be instrumental for evaluating the details of carrier cooling and electron-phonon couplings in a broad range of LHP optoelectronic materials., The following article has been submitted to The Journal of Chemical Physics. After it is published, it will be found at https://aip.scitation.org/journal/jcp

We report on the electronic properties of GaN$(1\bar{1}00)$ and $(0001)$ surfaces after three different and subsequent device processing compatible cleaning steps: HCl etching, annealing at $400$ $^\circ$C in N$_2$ atmosphere, and O$_2$ plasma exposure. The surface electronic properties are quantified, in the dark and under ultraviolet illumination, using X-ray photoelectron spectroscopy and a Kelvin probe. We find that the cleaning steps largely affect the work function and the band bending of both GaN orientations. These modifications are attributed to the presence of different surface states as well as to the formation of adsorbates building up distinct surface dipoles. Besides these results, we detect that under ultraviolet illumination the work function of the surfaces exposed to HCl decreases by at least $0.2$ eV without screening of the band bending. We thus attribute the observed surface photovoltage to a photo-induced modification of the surface dipole. Overall, these results emphasize the strong dependence of the electronic properties of air-exposed GaN surfaces on adsorbates. As a result, we advocate the use of the common cleaning steps analyzed here to re-initialize at will GaN$(1\bar{1}00)$ and $(0001)$ surfaces into pre-defined states., This is the accepted manuscript version of an article that appeared in Applied Surface Science copyright (C) Elsevier. The CC BY-NC-ND 4.0 license applies, see http://creativecommons.org/licenses/by-nc-nd/4.0/ . To access the final edited and published work, see https://doi.org/10.1016/j.apsusc.2019.07.256

Small organic molecules of the push-pull architecture are rapidly gaining their status in the organic electronics applications. In densely packed molecular films, both intra- and intermolecular interactions play an essential role for the device performance. Here we study two different molecules, a highly symmetric star-shaped one and its newly synthesized single arm analogue, for their photophysical properties. Both chromophores were dissolved in a solid matrix at different concentrations to vary their separation and therefore intermolecular coupling. We show that in both molecules the population relaxation accelerates by more than a factor of 10 at shorter intermolecular distances due to self-quenching thereby reducing the exciton survival time. The transient anisotropy dynamics are also quite similar, with their substantial acceleration at shorter interchromophore distances due to exciton diffusion caused by the Förster-like resonance energy transfer. However, the anisotropy values are noticeably lower for the star-shaped molecule because of intramolecular mixing of different polarization states. Finally, a model is presented that accounts for the observed results.

Filme weniger Nanometer Dicke aus Chrom und Aluminium wurden im Ultrahochvakuum auf einer (1 0 0)-Diamantoberfläche hergestellt. Während des Wachstums wurde ihre dynamische Leitfähigkeit in-situ im infraroten (IR) Spektralbereich mittels Transmissionsspektroskopie gemessen. Um ein Verständnis der die Leitfähigkeit bestimmenden Prozesse zu erlangen, wurden die gemessenen Spektren mit Hilfe des Drude- und des Drude-Smith-Modells interpretiert. Es ist möglich, verschiedene Phasen des Filmwachstums zu unterscheiden. Für Aluminium wurde Vollmer-Weber-Wachstum beobachtet. Es konnte gezeigt werden, dass die mittlere Filmdicke am Perkolationsübergang erwartungsgemäß stark von der Substrattemperatur abhängt. Das Verhalten der Chrom-Filme muss als ungewöhnlich bezeichnet werden: Unterhalb einer kritischen Dicke von etwa 2,5nm können die IR-Spektren nicht mit dem Drude-Modell beschrieben werden. Aus den Messungen bei verschiedenen Substrattemperaturen kann aber sowohl ein amorphes Wachstum, als auch Vollmer-Weber-Wachstum ausgeschlossen werden. Als ursächlich wird ein aus der Literatur bekannter Phasenübergang von fcc- zu bcc-Chrom angenommen. Die IR-Spektren der nanokristallinen fcc-Phase können mit dem Drude-Smith-Modell beschrieben werden, wobei der Verlauf der erhaltenen Parameter die vorgestellte Interpretation stützt.

The relaxation of high-energy “hot” carriers in semiconductors is known to involve the redistribution of energy between hot and cold carriers, as well as the transfer of energy from hot carriers to phonons. Over the past few years, these two processes have been identified in lead-halide perovskites (LHPs) using ultrafast pump–probe experiments, but their interplay is not fully understood. Here we present a practical and intuitive kinetic model that accounts for the effects of both hot and cold carriers on carrier relaxation in LHPs. We apply this model to describe the dynamics of hot carriers in bulk and nanocrystalline CsPbBr3 as observed by multi-pulse “pump–push–probe” spectroscopy. The model captures the slowing of the relaxation dynamics in the materials as the number of hot carriers increases, which has previously been explained by a “hot-phonon bottleneck” mechanism. The model also correctly predicts an acceleration of the relaxation kinetics as the number of cold carriers in the samples is increased. Using a series of natural approximations, we reduce our model to a simple form containing terms for the carrier–carrier and carrier–phonon interactions. The model can be instrumental for evaluating the details of carrier relaxation and carrier–phonon couplings in LHPs and other soft optoelectronic materials., Physical Chemistry Chemical Physics, 22 (31), ISSN:1463-9084, ISSN:1463-9076