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Dive into the research topics where Rainer Eichberger is active.

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Featured researches published by Rainer Eichberger.


ChemPhysChem | 2012

Epitaxial III–V Films and Surfaces for Photoelectrocatalysis

Henning Döscher; Oliver Supplie; Matthias M. May; Philipp Sippel; Christian Heine; A. G. Muñoz; Rainer Eichberger; Hans-Joachim Lewerenz; Thomas Hannappel

Efficient photoelectrochemical devices for water splitting benefit from the highest material quality and dedicated surface preparation achieved by epitaxial growth. InP(100)-based half-cells show significant solar-to-hydrogen efficiencies, but require a bias due to insufficient voltage. Tandem absorber structures may provide both adequate potential and efficient utilization of the solar spectrum. We propose epitaxial dilute nitride GaPNAs photocathodes on Si(100) substrates to combine close-to-optimum limiting efficiency, lattice-matched growth, and established surface preparation. Prior to a discussion of the challenging III-V/Si(100) heterojunction, we describe the closely related epitaxial preparation of InP(100) surfaces and its beneficial impact on photoelectrochemical water-splitting performance. Analogies and specific differences to GaP(100) surfaces are discussed based on in situ reflectance anisotropy and on two-photon photoemission results. Preliminary experiments regarding GaP/Si(100) photoelectrochemistry and dilute nitride GaPN heteroepitaxy on Si(100) confirm the potential of the GaPNAs/Si tandem absorber structure for future water-splitting devices.


Nano Letters | 2013

Two-Photon Photoemission Study of Competing Auger and Surface-Mediated Relaxation of Hot Electrons in CdSe Quantum Dot Solids

Philipp Sippel; Wiebke Albrecht; Dariusz Mitoraj; Rainer Eichberger; Thomas Hannappel; Daniel Vanmaekelbergh

Solids composed of colloidal quantum dots hold promise for third generation highly efficient thin-film photovoltaic cells. The presence of well-separated conduction electron states opens the possibility for an energy-selective collection of hot and equilibrated carriers, pushing the efficiency above the one-band gap limit. However, in order to reach this goal the decay of hot carriers within a band must be better understood and prevented, eventually. Here, we present a two-photon photoemission study of the 1Pe→1Se intraband relaxation dynamics in a CdSe quantum dot solid that mimics the active layer in a photovoltaic cell. We observe fast hot electron relaxation from the 1Pe to the 1Se state on a femtosecond-scale by Auger-type energy donation to the hole. However, if the oleic acid capping is exchanged for hexanedithiol capping, fast deep hole trapping competes efficiently with this relaxation pathway, blocking the Auger-type electron-hole energy exchange. A slower decay becomes then visible; we provide evidence that this is a multistep process involving the surface.


Applied Physics Letters | 2011

Morphology effects on charge generation and recombination dynamics at ZnPc:C60 bulk hetero-junctions using time-resolved terahertz spectroscopy

Andreas Bartelt; Christian Strothkämper; Wolfram Schindler; Konstantinos Fostiropoulos; Rainer Eichberger

The influence of growth temperature induced phase segregation and crystallinity in ZnPc:C60 blend films on the charge generation and recombination dynamics is investigated with optical-pump terahertz-probe spectroscopy. While an ultrafast photo-induced charge generation process is observed for all morphologies, a subsequent sub-nanosecond photoconductivity rise depends on crystallinity and phase segregation. For higher intensities, the signal is dominated by a morphology-dependent bimolecular recombination process. High local mobilities of minimal μ ∼ 0.3 cm2/Vs are found. The increase of photoconductivity with film growth temperature correlates with formerly observed device photocurrent improvements.


Nano Letters | 2015

Femtosecond cooling of hot electrons in CdSe quantum-well platelets.

Philipp Sippel; Wiebke Albrecht; Johanna C. van der Bok; Relinde Moes; Thomas Hannappel; Rainer Eichberger; Daniel Vanmaekelbergh

Semiconductor quantum wells are ubiquitous in high-performance optoelectronic devices such as solar cells and lasers. Understanding and controlling of the (hot) carrier dynamics is essential to optimize their performance. Here, we study hot electron cooling in colloidal CdSe quantum-well nanoplatelets using ultrafast two-photon photoemission spectroscopy at low excitation intensities, resulting typically in 1-5 hot electrons per platelet. We observe initial electron cooling in the femtosecond time domain that slows down with decreasing electron energy and is finished within 2 ps. The cooling is considerably faster at cryogenic temperatures than at room temperature, and at least for the systems that we studied, independent of the thickness of the platelets (here 3-5 CdSe units) and the presence of a CdS shell. The cooling rates that we observe are orders of magnitude faster than reported for similar CdSe platelets under strong excitation. Our results are understood by a classic cooling mechanism with emission of longitudinal optical phonons without a significant influence of the surface.


Journal of Applied Physics | 2016

Intragrain charge transport in kesterite thin films-Limits arising from carrier localization

Hannes Hempel; Alex Redinger; Ingrid Repins; Camille Moisan; Gerardo Larramona; Gilles Dennler; Martin Handwerg; Saskia F. Fischer; Rainer Eichberger; Thomas Unold

Intragrain charge carrier mobilities measured by time-resolved terahertz spectroscopy in state of the art Cu2ZnSn(S,Se)4 kesterite thin films are found to increase from 32 to 140 cm2 V−1 s−1 with increasing Se content. The mobilities are limited by carrier localization on the nanometer-scale, which takes place within the first 2 ps after carrier excitation. The localization strength obtained from the Drude-Smith model is found to be independent of the excited photocarrier density. This is in accordance with bandgap fluctuations as a cause of the localized transport. Charge carrier localization is a general issue in the probed kesterite thin films, which were deposited by coevaporation, colloidal inks, and sputtering followed by annealing with varying Se/S contents and yield 4.9%–10.0% efficiency in the completed device.


Journal of Photonics for Energy | 2012

Charge separation dynamics at inorganic/organic nanostructured hybrid photovoltaic interfaces

Rainer Eichberger

A new Forster resonance energy transfer (FRET) concept for a multichromophoric organic sensitizer in a dye-sensitized solar cell is presented based on a phenyl base body that accommodates the separately linked donor and acceptor moieties. The whole assembly is attached to the surface of a ZnO nanorod electrode via a carboxylic anchor group. FRET activity was demonstrated with UV-VIS measurements, and the charge separation dynamics at the inorganic/organic interface were analyzed with fs transient absorption and terahertz pump/probe for the precursors and fully assembled FRET units.


Journal of Applied Physics | 2015

Excitation correlation photoluminescence in the presence of Shockley-Read-Hall recombination

M. Borgwardt; Philipp Sippel; Rainer Eichberger; M. P. Semtsiv; W. T. Masselink; Klaus Schwarzburg

Excitation correlation photoluminescence (ECPL) measurements are often analyzed in the approximation of a cross correlation of charge carrier populations generated by the two delayed pulses. In semiconductors, this approach is valid for a linear non-radiative recombination path, but not for a non-linear recombination rate as in the general Shockley-Read-Hall recombination scenario. Here, the evolution of the ECPL signal was studied for deep trap recombination following Shockley-Read-Hall statistics. Analytic solutions can be obtained for a fast minority trapping regime and steady state recombination. For the steady state case, our results show that the quadratic radiative term plays only a minor role, and that the shape of the measured signal is mostly determined by the non-linearity of the recombination itself. We find that measurements with unbalanced intense pump and probe pulses can directly provide information about the dominant non-radiative recombination mechanism. The signal traces follow the charge carrier concentrations, despite the complex origins of the signal, thus showing that ECPL can be applied to study charge carrier dynamics in semiconductors without requiring elaborate calculations. The model is compared with measurements on a reference sample with alternating layers of InGaAs/InAlAs that were additionally cross-checked with time resolved optical pump terahertz probe measurements and found to be in excellent agreement.


photovoltaic specialists conference | 2014

Charge carrier mobilities and dynamics in thin film compound semiconductor materials from transient Thz absorption

Thomas Unold; H. Hempel; Ch. Strothkämper; Christian A. Kaufmann; Rainer Eichberger; A. Bartelt

We use optical pump Thz probe spectroscopy to access the microscopic mobilities and fast charge carrier dynamics processes in polycrystalline chalcopyrite and kesterite thin films grown by coevaporation. In order to avoid complicating effects from the presence of Ga-gradients, ternary CuInSe2 samples were used as a model system. Significantly different DC mobilities were found for stoichiometric and Cu-poor samples, respectively. While the stoichiometric samples exhibit Drude-like free carrier mobilities with DC mobilities up to 1200cm2/Vs at room temperature, the Cu-poor samples show non-Drude behavior, with much lower DC mobilities, indicative of carrier localization. Kesterite materials are found to show even stronger signatures of carrier localization than the chalcopyrites.


Applied Physics Letters | 2014

Solvent-induced surface state passivation reduces recombination in semisquarylium dye-sensitized solar cells

Andreas Bartelt; Robert Schütz; Christian Strothkämper; Ivo Kastl; Stephan Janzen; Dennis Friedrich; Wolfram Calvet; Gerda Fuhrmann; David Danner; Lars-Peter Scheller; Gabriele Nelles; Rainer Eichberger

The semisquarylium dye SY1T that is strongly bound to the surface of nanocrystalline TiO2 experiences very fast back-electron transfer of injected electrons to the SY1T cation, when the TiO2/SY1T interface is surrounded by ultrahigh vacuum. However, when located in methoxypropionitrile (MPN), which is frequently used as electrolyte solvent in dye-sensitized solar cells, the back-electron transfer is significantly retarded. Results are obtained both for picosecond and microsecond time scales using transient absorption spectroscopy. As solvent-induced interfacial energy level shifts can be excluded as possible cause, the role of TiO2 surface states in the beneficial retardation process is investigated. Highly surface sensitive synchrotron-induced photoelectron spectroscopy exhibits high densities of surface states on the pristine nanocrystalline TiO2 (nc-TiO2) surfaces. While SY1T dye-sensitization from a SY1T solution in tetrahydrofuran saturates about 30% of the surface states, the subsequent in-situ adso...


Optics Express | 2017

Measurement of charge carrier mobilities in thin films on metal substrates by reflection time resolved terahertz spectroscopy

Hannes Hempel; Thomas Unold; Rainer Eichberger

We show that charge carrier mobilities can be measured by reflection time resolved THz spectroscopy (R-TRTS) even for thin films on metal contacts, such as polycrystalline Cu2SnZnSe4 grown on molybdenum. In the measurement a reduced THz reflection upon photo-excitation is observed in contrast to increased THz reflection commonly observed on insulating substrates, and which excludes standard analytic R-TRTS analyses. Instead, a numerical transfer matrix method is used to model the THz reflection from which we derive carrier mobilities of 100 cm2/Vs consistent with literature. We show that R-TRTS on metal substrates is ~100x less sensitive compared to measurements on insulating substrates. These sensitivity of these R-TRTS measurements can be increased by using lower substrate refractive indices, lower substrate conductivities, thicker sample layers or higher THz probe frequencies.

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Hannes Hempel

Helmholtz-Zentrum Berlin

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Thomas Hannappel

Technische Universität Ilmenau

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Thomas Unold

Helmholtz-Zentrum Berlin

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Andreas Bartelt

Free University of Berlin

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Philipp Sippel

Helmholtz-Zentrum Berlin

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Robert Schütz

Helmholtz-Zentrum Berlin

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Sönke Müller

Helmholtz-Zentrum Berlin

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