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The structure of the interfacial layers of lead halide perovskites is expected to play a crucial role in carrier dynamics and in the performance of perovskite-based devices. However, little is known about the structure and dynamics of perovskite interfaces at the molecular level. In this study, we access molecular dynamics at perovskite interfaces by monitoring the alignment of cations in methylammonium (MA) lead bromide perovskite. For this we apply heterodyne-detected vibrational sum-frequency generation (HD-VSFG) spectroscopy with the help of DFT calculations to a range of thin-film perovskite active layers interfaced with several organic and oxide semiconducting materials. At interfaces with air, glass, TiO2 and NiO transport layers and phenyl-C61-butyric acid methyl ester (PCBM), we observed no signature of cation vibrations, indicating a randomized arrangement of cations. On the other hand, HD-VSFG spectra of 2,2′,7,7′-tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9′-spirobifluorene (Spiro-MeOTAD)/perovskite interfaces exhibited a clear peak corresponding to the ammonium antisymmetric bending mode, indicating that the MA's ammonium moiety is preferentially oriented towards the Spiro-MeOTAD layer with a large portion of MA cations are tilted away from the surface normal. The observed interfacial anisotropy is in contrast with the general picture accepted for bulk perovskite materials. We discuss the potential consequences of the observed phenomena in the context of the local field and carrier injection at the various interfaces within the perovskite solar cells.

A leading hypothesis for silver nanoparticle biosynthesis suggests that the enzyme nitrate reductase plays a key role in the process, assisted by the coenzyme nicotinamide adenine dinucleotide phos...

We report on the optoelectronic properties of GaN$(0001)$ and $(1\bar{1}00)$ surfaces after their functionalization with phosphonic acid derivatives. To analyze the possible correlation between the acid's electronegativity and the GaN surface band bending, two types of phosphonic acids, n-octylphosphonic acid (OPA) and 1H,1H,2H,2H-perfluorooctanephosphonic acid (PFOPA), are grafted on oxidized GaN$(0001)$ and GaN$(1\bar{1}00)$ layers as well as on GaN nanowires. The resulting hybrid inorganic/organic heterostructures are investigated by X-ray photoemission and photoluminescence spectroscopy. The GaN work function is changed significantly by the grafting of phosphonic acids, evidencing the formation of dense self-assembled monolayers. Regardless of the GaN surface orientation, both types of phosphonic acids significantly impact the GaN surface band bending. A dependence on the acids' electronegativity is, however, only observed for the oxidized GaN$(1\bar{1}00)$ surface, indicating a relatively low density of surface states and a favorable band alignment between the surface oxide and acids' electronic states. Regarding the optical properties, the covalent bonding of PFOPA and OPA on oxidized GaN layers and nanowires significantly affect their external quantum efficiency, especially in the nanowire case due to the large surface-to-volume ratio. The variation in the external quantum efficiency is related to the modication of both the internal electric fields and surface states. These results demonstrate the potential of phosphonate chemistry for the surface functionalization of GaN, which could be exploited for selective sensing applications.

Solution-processed nickel oxide (sNiO) is known to be an excellent charge-selective interlayer in optoelectronic devices. Its beneficial properties can be further enhanced by an oxygen plasma (OP) treatment. In order to elucidate the mechanism behind this improvement, we use infrared transmission and X-ray photoelectron spectroscopy to probe the bulk and surface properties of the sNiO. We find that increasing the annealing temperature of the sNiO not only increases the structural order of the material but also reduces the concentration of nickel hydroxide species present in the bulk and on the surface of the film. This results in a decrease of the work function, while an additional OP treatment raises the work function to between 5.5 and 5.6 eV. For all annealing temperatures investigated, the consequences of the OP treatment are identified as reactions of both NiO and β-Ni(OH)2 to form thin β-NiOOH phases in the first atomic layers. Our results emphasize the importance of understanding the correlation be...

The key challenge in solution-processing efficient OLEDs is to realize the multilayer device architecture. We demonstrate that slot die coating is applicable to deposit small-molecule (SM) layers on top of each other without dissolving the underlying layer. A stack for a blue SMOLED is chosen comprising slot die-coated PEDOT:PSS, an SM emissive layer (EML) as well as an SM electron transport layer (ETL). The devices are fabricated in a sheet-to-sheet coating process with a slot die table coater under ambient conditions. While keeping the processing parameters constant for PEDOT:PSS and the EML, the composition of the ETL is varied. The choice of solvent for coating the ETL is crucial regarding wetting and dissolution of the underlying layer, solubility, surface roughness, and device efficiencies. Average roughness values down to 0.38 nm and peak to valley values around 10 nm were measured. Comparing device efficiencies of devices with and without ETL, an increase in efficiency with a factor up to 42 was achieved. In total, we show 135 blue SMOLEDs to demonstrate reproducibility.

Studying the structure-property relations of tailored dipolar phenyl and biphenylphosphonic acids, we report self-assembled monolayers with a significant decrease in the work function (WF) of indium-tin oxide (ITO) electrodes. Whereas the strengths of the dipoles are varied through the different molecular lengths and the introduction of electron-withdrawing fluorine atoms, the surface energy is kept constant through the electron-donating N, N-dimethylamine head groups. The self-assembled monolayer formation and its modification of the electrodes are investigated via infrared reflection absorption spectroscopy, contact angle measurements, and photoelectron spectroscopy. The WF decrease in ITO correlates with increasing molecular dipoles. The lowest ever recorded WF of 3.7 eV is achieved with the fluorinated biphenylphosphonic acid.

Nickel oxide (NiO) is a widely used material for efficient hole extraction in optoelectronic devices. However, its surface characteristics strongly depend on the processing history and exposure to adsorbates. To achieve controllability of the electronic and chemical properties of solution-processed nickel oxide (sNiO), we functionalize its surface with a self-assembled monolayer (SAM) of 4-cyanophenylphosphonic acid. A detailed analysis of infrared and photoelectron spectroscopy shows the chemisorption of the molecules with a nominal layer thickness of around one monolayer and gives an insight into the chemical composition of the SAM. Density functional theory calculations reveal the possible binding configurations. By the application of the SAM, we increase the sNiO work function by up to 0.8 eV. When incorporated in organic solar cells, the increase in work function and improved energy level alignment to the donor does not lead to a higher fill factor of these cells. Instead, we observe the formation of a transport barrier, which can be reduced by increasing the conductivity of the sNiO through doping with copper oxide. We conclude that the widespread assumption of maximizing the fill factor by only matching the work function of the oxide charge extraction layer with the energy levels in the active material is a too narrow approach. Successful implementation of interface modifiers is only possible with a sufficiently high charge carrier concentration in the oxide interlayer to support efficient charge transfer across the interface.

Stable electrical doping of organic semiconductors is fundamental for the functionality of high performance devices. It is known that dopants can be subjected to strong diffusion in certain organic semiconductors. This work studies the impact of operating conditions on thin films of the polymer poly(3-hexylthiophene) (P3HT) and the small molecule Spiro-MeOTAD, doped with two differently sized p-type dopants. The negatively charged dopants can drift upon application of an electric field in thin films of doped P3HT over surprisingly large distances. This drift is not observed in the small molecule Spiro-MeOTAD. Upon the dopants' directional movement in P3HT, a dedoped region forms at the negatively biased electrode, increasing the overall resistance of the thin film. In addition to electrical measurements, optical microscopy, spatially resolved infrared spectroscopy, and scanning Kelvin probe microscopy are used to investigate the drift of dopants. Dopant mobilities of 10-9 to 10-8 cm2 V-1 s-1 are estimated. This drift over several micrometers is reversible and can be controlled. Furthermore, this study presents a novel memory device to illustrate the applicability of this effect. The results emphasize the importance of dynamic processes under operating conditions that must be considered even for single doped layers.

We report on the electronic properties of GaN ( 1 1 ¯ 00 ) and (0001) surfaces after three different and subsequent device processing compatible cleaning steps: HCl etching, annealing at 400 °C in N2 atmosphere, and O2 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 1 ¯ 00 ) and (0001) surfaces into pre-defined states.

Direct optical visualization of electrochemical processes at the nanoscale is an exciting field of research. It became more and more feasible through the development of enclosed liquid cells for transmission electron microscopes over the past years [1]. The liquid cell itself is built by stacking two silicon chips with electron transparent silicon nitride windows, which are then sealed by a gasket against the surrounding high vacuum in the microscope column. Microfabrication of electrode structures onto the chips opened up the opportunity to control the electric potential and current within the cell in order to perform in-situ potentiometric measurements and impedance spectroscopy. This unique feature enables the correlation of real-time optical imaging at the nanoscale and electrochemistry. So far, most of the experiments have focused on batteries and corrosion studies [2,3]. While there have been numerous reports on the diffusion and agglomeration of nanoparticles [4], the visualization of nanoparticle dynamics coupled to voltammetry in the vicinity of an electrode surface has received less attention. Herein, we investigate the diffusion, deposition and stripping of nanoparticles at an electrode under an applied electric bias. We give insight into how a nanoparticle collision event quantitatively transfers into an electrochemical current signal and disentangle electron beam effects. These results might pave the way towards observation and understanding of reactions catalyzed by nanoparticles in general. References: [1] N. de Jonge and F. M. Ross, Electron microscopy of specimens in liquid. Nat. Nanotechnol., 6 (2011), 695-704 [2] B. L. Mehdi, J. Qian, E. Nasybulin et al., Observation and quantification of nanoscale processes in lithium batteries by operando electrochemical (S)TEM. Nano Lett., 15 (2015), 2168-2173 [3] S. W. Chee, S. H. Pratt, K. Hattar et al., Studying localized corrosion using liquid cell transmission electron microscopy. Chem. Commun., 51 (2015), 168-171 [4] H.-G. Liao, K. Niu and H. Zheng, Observation of growth of metal nanoparticles. Chem. Commun., 49 (2013), 11720-11727

The ability of biomolecules to reduce metal salts to nanoparticles has been repeatedly demonstrated. However, the underlying mechanism of the occurring redox reaction is still vague. We present our recent results, demonstrating that NADPH, a major redox shuttle in organisms, can solely serve as a reducing agent, leading to the formation of monodispersed silver nanoparticles. We further show that this process does not depend on enzymatic catalysis which is the currently accepted route for biological synthesis of nanoparticles. We support our results with UV-vis spectroscopy, TEM images and electrochemical measurements at the ensemble and single entity level.