Longer lifetimes for perovskite absorbers

An international team of scientists has improved greatly the stability of organic-inorganic lead halide perovskites. These materials have enormous potential for photovoltaic applications but still suffer from comparably moderate device lifetime. The scientists, led by researchers from the EPFL, Lausanne, Switzerland, incorporated a large organic cation – guanidinium - into the perovskite crystal structure, in part replacing the traditionally used methylammonium and formamidinium cations. Overall, the new material delivered average power conversion efficiencies over 19%, and stabilized performance for 1,000 h under continuous light illumination. This is a fundamental step within the perovskite field. These groundbreaking research results were recently published in Nature Energy. Among the authors is the member of IRIS Adlershof, Prof. Norbert Koch.
Large guanidinium cation mixed with methylammonium in lead iodide perovskites for 19% efficient solar cells
A.D. Jodlowski, C. Roldán-Carmona, G. Grancini, M. Salado, M. Ralaiarisoa, S. Ahmad, N. Koch, L. Camacho, G. de Miguel, and M.K. Nazeeruddin
Nature Energy 2 (2017) 972


Beating the thermodynamic limit with photo-activation of n-doping in organic semiconductors using “hyper-reductants”

Image by Jing Wang and Xin Lin
Doping of semiconductors is a key process for controlling the materials’ charge carrier density, which directly impacts the electrical conductivity. Electronic and optoelectronic devices used in information, communication, energy conversion, and energy storage technologies rely on precise and efficient doping, i.e., the admixture of a small amount of a doping agent into the semiconductor. However, n-type doping of organic semiconductors – electron transfer from the dopant to the semiconductor – is notoriously difficult as the molecular dopants employed presently are highly sensitive to ambient exposure, upon which they react with water and oxygen and are rendered inactive.
In an article that just appeared in Nature Materials, a team of researchers from the Georgia Institute of Technology, the Helmholtz-Zentrum Berlin, Humboldt-Universität zu Berlin, and Princeton University demonstrates a new approach towards n-doping of organic semiconductors, which allows bypassing the dopant sensitivity to the ambient and simultaneously enables doping organic electron transport materials that have been out of reach for n-doping so far. The first step of innovation lies in chemically connecting two organometallic molecular dopants in a dimer that is stable even in air, with reduced ability to dope organic electron transport semiconductors. Consequently, when mixing these into the organic semiconductor, nothing happens at first. The revolutionary step now involves illuminating the mixture with light. A dimer and a semiconductor molecule in immediate proximity absorb a photon, the dimer can dissociate and unfold the full doping power of each dopant in a multi-step process. “By this optical activation of dopants, we could enhance the conductivity of organic electron transport materials by five orders of magnitude. This boosts the efficiency of organic light emitting diodes and solar cells, using rather simple and technologically relevant processing.” says Prof. Antoine Kahn from Princeton University, who coordinated the project. The choice of the article’s title is explained by Prof. Seth Marder from Georgia Tech: “This doping is actually beyond the thermodynamic limit of what the dopant should be able to do, thus once the light is turned off one might naively expect the reverse reaction to occur (rapidly, within seconds perhaps) and the conductivity increase to disappear. However, this is not the case. The reason for this is that the doping process involves multiple steps, and the back-reaction to the starting system involves many uphill intermediate steps creating a kinetic barrier, thus the reverse reaction is extremely slow.” Indeed, no indications of a loss in conductivity upon light-activation after hundreds of hours were found. For these reasons, the compounds are referred to as “hyper-reductants”. The fact that the team demonstrated the beneficial effect of their doped electron transport semiconductors in highly efficient light emitting diodes underlines the huge potential of this approach in device applications. “We believe that our work enables simple processing of n-doped organic semiconductors in numerous device architectures, where the critical step - doping activation - can take place after standard device encapsulation. This will contribute substantially to improved device lifetime and in some case simplify device fabrication.” notes Prof. Norbert Koch from Humboldt-Universität, member of IRIS Adlershof. The work was part of a project within the strategic partnership program of Princeton University and Humboldt-Universität.
Beating the thermodynamic limit with photo-activation of n-doping in organic semiconductors using “hyper-reductants”
X. Lin,  B. Wegner,  K.M. Lee,  M.A. Fusella,  F. Zhang, K. Moudgil , B.P. Rand, S. Barlow, S.R. Marder, N. Koch, A. Kahn
Nature Materials 16 (2017)1209


Spiro-Bridged Ladder-Type Oligo(para-phenylene)s:
Fine Tuning Solid State Structure and Optical Properties

In this recent research highlight the authors developed synthetic routes that allow to subsequently replace every pair of symmetry-equivalent alkyl groups in ladder-type quaterphenyl by a spiro-bifluorene group. With an increasing number of spiro groups, the optical gap for absorption and emission slightly decreases, which is disadvantageous with respect to resonant energy transfer with ZnO. Thus, a synthetic route to a para-linked ladder-type quaterphenyl carrying all bridging units on one side of the ribbon was developed, which results in an in-plane bending of the para-phenylene. The optival gap increased compared to the linear molecule, however, the absorption coefficient slightly decreased.

The authors analyzed the influence of different deposition techniques on the solid state structure by X-ray diffraction of single crystals obtained by crystallization from solution as well as sublimation. In the cases of L4P-sp2 and L4P-sp3, it could even be shown that sublimation and crystallization from solution result in different crystal structures, of which the ones from sublimation are obviously more relevant in view of the typically employed vacuum deposition and might be advantageous in terms of application in light-emitting devices.

An increasing number of spiro-bifluorene substituents was found to aid thin-film formation on oxide surfaces, such that the optical properties could be preserved in pure, nondiluted thin films.

Finally, promising spiro-L4P derivatives have been employed in energy-transfer devices, for which highly efficient energy transfer from an inorganic quantum well to the organic layer followed by efficient light emission could successfully be demonstrated.

Asymmetric units (left) and arrangement of molecules in the crystal (right) of different molecule types. Thermal ellipsoids drawn at 50% probability level, cell edges marked in a: red, b: blue and c: green. For more details please see the publication text.

Absorption (solid lines) and normalized PL (dotted) spectra of final products 10-6 - 10-5 mol L-1 in CH2Cl2. For more details please see the publication text.

Spiro-Bridged Ladder-Type Oligo(para-phenylene)s: Fine Tuning Solid State Structure and Optical Properties
B. Kobin, J. Schwarz, B. Braun-Cula, M. Eyer, A. Zykov, S. Kowarik, S. Blumstengel, and S. Hecht
Adv. Funct. Mater. 2017, 1704077 (2017)

Water makes the proton shake -
Ultrafast motions and fleeting geometries in proton hydration

Basic processes in chemistry and biology involve protons in a water environment. Water structures accommodating protons and their motions have so far remained elusive. Applying ultrafast vibrational spectroscopy, Dahms et al. map fluctuating proton transfer motions and provide direct evidence that protons in liquid water are predominantly shared by two water molecules. Femtosecond proton elongations within a hydration site are 10 to 50 times faster than proton hopping to a new site, the elementary proton transfer step in chemistry.

The proton, the positively charged nucleus H+ of a hydrogen atom and smallest chemical species, is a key player in chemistry and biology. Acids release protons into a liquid water environment where they are highly mobile and dominate the transport of electric charge. In biology, the gradient of proton concentration across cell membranes is the mechanism driving the respiration and energy storage of cells. Even after decades of research, however, the molecular geometries in which protons are accommodated in water, and the elementary steps of proton dynamics have remained highly controversial.

Protons in water are commonly described with the help of two limiting structures (Fig. 1A). In the Eigen complex (H9O4+) (left), the proton is part of the central H3O++ ion surrounded by three water molecules. In the Zundel cation (H5O2+) (right), the proton forms strong hydrogen bonds with two flanking water molecules. A description at the molecular level employs the potential energy surface of the proton (Fig. 1B) which is markedly different for the two limiting geometries. As shown in Fig. 1B, one expects an anharmonic single-minimum potential for the Eigen species and a double minimum potential for the Zundel species. In liquid water, such potentials are highly dynamic in nature and undergo very fast fluctuations due to thermal motions of surrounding water molecules and the proton.

Led by Thomas Elsässer, member of  IRIS Adlershof, researchers from the Max Born Institute in Berlin, Germany, and the Ben Gurion University of the Negev in Beer-Sheva, Israel, have now elucidated the ultrafast motions and structural characteristics of protons in water under ambient conditions. They report experimental and theoretical results in Science which identify the Zundel cation as a predominant species in liquid water. The femtosecond (1 fs = 10-15 s) dynamics of proton motions were mapped via vibrational transitions between proton quantum states (red and blue arrows in Fig. 1B). The sophisticated method of two-dimensional vibrational spectroscopy provides the yellow-red and blue contours in Fig. 2A which mark the energy range covered by the two transitions. The blue contour occurs at higher detection frequencies than the red, giving the first direct evidence for the double-minimum character of the proton potential in the native aqueous environment. In contrast, the blue contour is expected to appear at smaller detection frequencies than the red one.

The orientation of the two contours parallel to the vertical frequency axis demonstrates that the two vibrational transitions explore a huge frequency range within less than 100 fs, a hallmark of ultrafast modulations of the shape of proton potential. In other words, the proton explores all locations between the two water molecules within less than 100 fs and very quickly loses the memory of where it has been before. The modulation of the proton potential is caused by the strong electric field imposed by the water molecules in the environment. Their fast thermal motion results in strong field fluctuations and, thus, potential energy modulations on a sub-100 fs time scale. This picture is supported by benchmark experiments with Zundel cations selectively prepared in another solvent and by detailed theoretical simulations of proton dynamics (Fig. 2B).

A specific Zundel cation in water transforms into new proton accommodating geometries by the breaking and reformation of hydrogen bonds. Such processes are much slower than the dithering proton motion and occur on a time scale of a few picoseconds. This new picture of proton dynamics is highly relevant for proton transport by the infamous von Grotthuss mechanism, and for proton translocation mechanisms in biological systems.

Figure 1: Chemical structure of hydrated protons in liquid water.

A Schematic of the Eigen cation H9O4+ (left) and the Zundel cation H5O2++ (right). The arrows indicate the O-H bond coordinate r and the (O...H+...O) proton transfer coordinate z. In the Eigen cation a covalent O-H bond localizes the proton whereas in the Zundel cation the proton is delocalized between two water molecules.
B Anharmonic vibrational potential (left) and double minimum potential of the Zundel cation along z (right, red. Distortions by the solvent surrounding impose a modulation of the double minimum potential (right, dotted line). Red and blue arrows indicate transitions between particular quantum states of the proton motion , i.e., the ground-state-to-first-excited-state transition (red) and the first-excited-state-to-second-excited-state transition (blue). The modulation of the potentials leads to spectral shifts of the vibrational transitions which are mapped by two-dimensional infrared spectroscopy.

Fig 2: Femtosecond dynamics of proton motions (1 fs = 10-15 s).

A Two-dimensional vibrational spectra with the ground-state-to-first-excited-state transition (red) at lower detection frequency than the first-excited-state-to-second-excited-state transition (blue). The orientation of both contours parallel to the excitation frequency axis is due to ultrafast frequency fluctuations and the loss of memory in the proton position.
B Simulated real-time dynamics of the proton motions in the Zundel cation. Within less than 100 fs, the proton displays large amplitude excursions along z, the coordinate linking the two water molecules in the Zundel cation. Due to the ultrafast modulation of the shape of proton potential by surrounding solvent molecules, the proton explores all locations between the two water molecules.

Fig 3: Cartoon picture of proton hydration dynamics, visualized with the help of classical physics.

The proton Smiley is sitting in the middle of a sofa with two seats. When shaking the sofa with a mechanical force, the shape of the seating changes and the proton moves forth and back on the sofa. Such motions occur on a time scale shorter than 100 fs (10-13 s). After an average time of 1 ps = 1000 fs = 10-12 s, the sofa breaks and the proton moves to a new site/sofa, including the red halve on the right.

Large-amplitude transfer motion of hydrated excess protons mapped by ultrafast 2D IR spectroscopy
F. Dahms, B.P. Fingerhut, E.T.J. Nibbering, E., and T. Elsaesser
Science, 357 (2017) 491
DOI: 10.1126/science.aan5144

X-ray "movie" provides insights into the formation of molecular layers

Thin-film technologies that promise control on the atomic and molecular scale have attracted increasing interest in recent years as traditional manufacturing processes reach their fundamental limits. A team from the Department of Physics at the Humboldt-Universität zu Berlin, led by Anton Zykov, Stefan Kowarik and Jürgen P. Rabe (member of  IRIS Adlershof) in collaboration with colleagues from the PETRA III Synchrotron at DESY Hamburg has now studied the non-equilibrium growth of molecular layers using innovative, time-resolved X-ray scattering. The movie sequence of the X-ray scattering during the molecular beam deposition was chosen as the cover image of a special topic issue of the Journal of Chemical Physics on "Atomic and molecular layer processing".

Semiconducting organic molecules have significant potential for future applications such as organic light-emitting diodes (OLED), camera sensors or memory devices. Many of these components are based on ultra-thin layers of functional molecular materials. Their preparation by deposition of molecules from the gas phase is a complex process involving molecular adsorption on a substrate, molecular diffusion and self-assembly. Since many of these processes do not proceed under conditions of local thermodynamic equilibrium, these processes and their velocities are still not well understood.
By means of innovative X-ray measurements of diffuse scattering at the P03 Beamline of the PETRA III synchrotron, the researchers were able to record "movies" of the growth processes on the nanoscale. The measurement makes it possible to follow the nucleation, island growth and the roughness evolution of the layer. The researchers show that the results of the new X-ray technique are consistent with established scanning probe techniques and time-resolved measurements are possible without disturbing the growth. In the study, a significant improvement in the diffusivity of molecules between the first and the subsequent molecular layers was found and the nucleation energy was determined within the framework of recent growth theories. The application of the new X-ray scattering technique will help to take our understanding beyond a recipe-based perspective to that of sound fundamental understanding of molecular growth.

Diffusion and nucleation in multilayer growth of PTCDI-C8 studied with in situ X-ray growth oscillations and real-time small angle X-ray scattering
A. Zykov, S. Bommel, C. Wolf, L. Pithan, C. Weber, P. Beyer, G. Santoro, J.P. Rabe, and S.Kowarik
J. Chem. Phys. 146, 052803 (2017)

GLAD makes new organic memory devices possible

Giovanni Ligorio, Marco Vittorio Nardi, and Norbert Koch, member of IRIS Adlershof, have invented a new technique for constructing novel memory devices. The results have now been published in Nano Letters.

Author Dr. Giovanni Ligorio explains: “Novel non-volatile memory devices are currently investigated to overcome the limitation of traditional memory technologies. New materials such as organic semiconductors and new architectures are now considered to address high-density, high-speed, low-fabrication costs and low power-consumption.
Usually nano-devices (traditionally based on inorganic semiconductors) are fabricated via lithography techniques. Here, we show the fabrication of devices with nanometric footprint using a different technique: Glancing Angle Deposition (GLAD).
This technique allows the tailoring of nanostructured morphologies through physical vapor deposition (CVD) via controlling the substrate orientation with respect to the vapor source direction. When thin films are deposited onto stationary substrates under condition of oblique deposition, meaning that the vapor flux is non-perpendicular to the substrate surface, an inclined columnar nanostructured is produced.

(a) Herstellung der Nanosäulen via CVD (b) AFM-Aufnahme der Säulen-columns (c) Skizze der Ansteuerung (d) Skizze eine Säule mit Filament ©G.Ligorio
Upon proper bias applied between the two electrodes of the memory device, it is possible to form a conductive path (or filament). The filament shorts the electrodes and drastically changes the resistivity characteristic of the device. Forcing a high current in the device, the filament can be distrust. This programs the device in the original high resistivity state. Since the process can be repeated consecutively we can program the device in a high or low resistive state (i.e. ON or OFF).
We aim for the fabrication of devices in structured arrays (in this publication the nano devices are not ordered in array, but they are randomly distributed.) This allows for connecting via cross bar electrodes, which can be fabricated via printing.
This allows fabricating memory devices with a density of roughly 1 GB/cm² employing novel material for electronics, i.e. organic semiconductors.”
Lithography-Free Miniaturization of Resistive Nonvolatile Memory Devices to the 100 nm Scale by Glancing Angle Deposition
G. Ligorio, M. Vittorio Nardi, and N. Koch
Nano Lett. 17 (2017) 1149