Eric Mankel

Role of the Selective Contacts in the Performance of Lead Halide Perovskite Solar Cells

The effect of electron- and hole-selective contacts in the final cell performance of hybrid lead halide perovskite, CH3NH3PbI3, solar cells has been systematically analyzed by impedance spectroscopy. Complete cells with compact TiO2 and spiro-OMeTAD as electron- and hole-selective contacts have been compared with incomplete cells without one or both selective contacts to highlight the specific role of each contact. It has been described how selective contacts contribute to enhance the cell FF and how the hole-selective contact is mainly responsible for the high Voc in this kind of device. We have determined that the recombination rate is mainly governed by the selective contacts. This fact has important implication for the future optimization of perovskite solar cells. Finally, we have developed a method to analyze the results obtained, and it has been applied for three different electron-selecting materials: TiO2, ZnO, and CdS.

Efficient Planar Heterojunction Perovskite Solar Cells Based on Formamidinium Lead Bromide

The development of medium-bandgap solar cell absorber materials is of interest for the design of devices such as tandem solar cells and building-integrated photovoltaics. The recently developed perovskite solar cells can be suitable candidates for these applications. At present, wide bandgap alkylammonium lead bromide perovskite absorbers require a high-temperature sintered mesoporous TiO2 photoanode in order to function efficiently, which makes them unsuitable for some of the above applications. Here, we present for the first time highly efficient wide bandgap planar heterojunction solar cells based on the structurally related formamidinium lead bromide. We show that this material exhibits much longer diffusion lengths of the photoexcited species than its methylammonium counterpart. This results in planar heterojunction solar cells exhibiting power conversion efficiencies approaching 7%. Hence, formamidinium lead bromide is a strong candidate as a wide bandgap absorber in perovskite solar cells.

Investigation of Solution-Processed Ultrathin Electron Injection Layers for Organic Light-Emitting Diodes

We study two types of water/alcohol-soluble aliphatic amines, polyethylenimine (PEI) and polyethylenimine-ethoxylated (PEIE), for their suitability as electron injection layers in solution-processed blue fluorescent organic light-emitting diodes (OLEDs). X-ray photoelectron spectroscopy is used to determine the nominal thickness of the polymer layers while ultraviolet photoelectron spectroscopy is carried out to determine the induced work-function change of the silver cathode. The determined work-function shifts are as high as 1.5 eV for PEI and 1.3 eV for PEIE. Furthermore, atomic force microscopy images reveal that homogeneous PEI and PEIE layers are present at nominal thicknesses of about 11 nm. Finally, we solution prepare blue emitting polymer-based OLEDs using PEI/PEIE in combination with Ag as cathode layers. Luminous efficiency reaches 3 and 2.2 cd A(-1), whereas maximum luminance values are as high as 8000 and 3000 cd m(-2) for PEI and PEIE injection layers, respectively. The prepared devices show a comparable performance to Ca/Ag OLEDs and an improved shelf lifetime.

Conformal and highly luminescent monolayers of Alq3 prepared by gas-phase molecular layer deposition.

The gas-phase molecular layer deposition (MLD) of conformal and highly luminescent monolayers of tris(8-hydroxyquinolinato)aluminum (Alq3) is reported. The controlled formation of Alq3 monolayers is achieved for the first time by functionalization of the substrate with amino groups, which serve as initial docking sites for trimethyl aluminum (TMA) molecules binding datively to the amine. Thereby, upon exposure to 8-hydroxyquinoline (8-HQ), the self-limiting formation of highly luminescent Alq3 monolayers is afforded. The growth process and monolayer formation were studied and verified by in situ quartz crystal monitoring, optical emission and absorption spectroscopy, and X-ray photoelectron spectroscopy. The nature of the MLD process provides an avenue to coat arbitrarily shaped 3D surfaces and porous structures with high surface areas, as demonstrated in this work for silica aerogels. The concept presented here paves the way to highly sensitive luminescent sensors and dye-sensitized metal oxides for future applications (e.g., in photocatalysis and solar cells).

The role of Ca traces in the passivation of silicon dioxide dielectrics for electron transport in pentacene organic field effect transistors

Recently, n-type transport in organic field effect transistors (OFETs) incorporating pentacene on a silicon dioxide (SiO2) dielectric has been demonstrated by Ahles et al. [Appl. Phys. Lett. 85, 4499 (2004)]. The electron transport was made possible by modifying the dielectric/semiconductor interface using traces of Ca. While the facilitation of electron current in pentacene remained unclear at that point, an interface near filling of electron trap states in the transistor channel or on the SiO2 dielectric could be suggested as a possible explanation. In the following the influence of the Ca interlayer on the n-type transport in pentacene based OFETs will be correlated with an x-ray photoelectron spectroscopy analysis of the SiO2/Ca interface, in dependence of the Ca layer thickness. It is demonstrated that for low thicknesses an oxidized Ca insulator is formed on the SiO2 dielectric, allowing for the observed pentacene electron transport. The formation of the oxide is suggested to compensate available el...

Impact of processing on the chemical and electronic properties of phenyl-C61-butyric acid methyl ester

For the comparison of solution-processed to evaporated materials in organic optoelectronic devices, phenyl-C61-butyric acid methyl ester (PCBM) has been claimed to be a suitable material. However, we ascertained differences between spin-coated and vacuum sublimed thin films. In this contribution, we thoroughly investigate the effects of thermal evaporation of PCBM in a strongly interdisciplinary approach, applying physical characterization techniques such as photoelectron (PES) and infrared (IR) spectroscopy in combination with further chemical analysis using thermal gravimetric analysis (TGA), differential scanning calorimetry (DSC) and ultra-performance liquid chromatography-coupled mass spectrometry (UPLC-MS), as well as proton nuclear magnetic resonance spectroscopy (1H-NMR). The methods were applied to thin films prepared by solution-based deposition techniques and by thermal evaporation. Additionally, comparison to an evaporated C60 film and the crucible residues is carried out. Changes in the IR spectrum of the PCBM films already indicate a change in the molecular structure of PCBM. The UPLC chromatogram of the redissolved organic film proves the formation of several molecular species, including bare C60. However, the effect of degradation on the electronic properties was found to be limited, as an almost unchanged ionization potential of 6.1 eV was determined with UPS for both the solution processed as well as the evaporated films. Also bulk-heterojunction (BHJ) solar cells fabricated using pure and thermally treated PCBM showed the same J–V characteristics under illumination.

Interface properties of a Li3PO4/Al cathode in organic light emitting diodes

Recently Li3PO4/Al has been introduced as an alternative cathode for the commonly used LiF/Al system for organic light emitting diodes (OLEDs) due to its competitive electron injection properties. In the present article the interfaces of the organic semiconductor with the Li3PO4/Al bilayer cathode are investigated using photoelectron spectroscopy to elucidate the origin behind the efficient electron injection. Therefore, a thick Li3PO4 layer was vacuum deposited onto an indium tin oxide substrate and characterized in order to learn about the stoichiometry of evaporated Li3PO4. During evaporation Li3PO4 decomposes, forming a layer consisting of P2O5 and LiPO3. In a second step the interface between Li3PO4 and Alq3 [tris(8-hydroxyquinoline) aluminum] was investigated, whereupon Li3PO4 coverage Alq3 molecules decompose, forming aluminum oxide or aluminum phosphate leaving 8-quinolinol molecules behind. A similar reaction occurs at the Li3PO4/Al interface where again an oxidation of the metallic aluminum poin...

Processing Follows Function: Pushing the Formation of Self- Assembled Monolayers to High-Throughput Compatible Time Scales

Self-assembled monolayers (SAMs) of organic molecules can be used to tune interface energetics and thereby improve charge carrier injection at metal-semiconductor contacts. We investigate the compatibility of SAM formation with high-throughput processing techniques. Therefore, we examine the quality of SAMs, in terms of work function shift and chemical composition as measured with photoelectron and infrared spectroscopy and in dependency on molecular exposure during SAM formation. The functionality of the SAMs is determined by the performance increase of organic field-effect transistors upon SAM treatment of the source/drain contacts. This combined analytical and device-based approach enables us to minimize the necessary formation times via an optimization of the deposition conditions. Our findings demonstrate that SAMs composed of partially fluorinated alkanethiols can be prepared in ambient atmosphere from ethanol solution using immersion times as short as 5 s and still exhibit almost full charge injection functionality if process parameters are chosen carefully. This renders solution-processed SAMs compatible with high-throughput solution-based deposition techniques.

Double-Walled Ag–Pt Nanotubes Fabricated by Galvanic Replacement and Dealloying: Effect of Composition on the Methanol Oxidation Activity

The synthesis of bimetallic nanostructures using galvanic replacement displays a versatile route toward efficient catalysts for fuel cell reactions. We show that electrolessly plated Ag nanotubes (NTs) are a unique template for the synthesis of double-walled Ag–Pt NTs. After replacement reaction, different dealloying protocols are applied to adjust the residual Ag content. The structures were thoroughly characterized by scanning electron microscopy, transmission electron microscopy, X-ray diffraction and X-ray photoelectron spectroscopy, providing evidence of a hollow tube structure composed of Ag–Pt alloy. Experiments under harsh conditions reveal, that a significant amount of Ag remain in the NTs, which strongly affects the methanol oxidation performance. With optimized Ag–Pt ratio, the specific activity of Pt/C catalysts can be outperformed. From the obtained results, we emphasize that each effort using galvanic replacement should be accompanied by detailed compositional analysis.

A doping mechanism for organic semiconductors derived from SXPS measurements on co-evaporated films of CuPc and TCNQ and on a TCNQ/CuPc interface

Synchrotron induced photoelectron spectroscopy on in situ co-evaporated blends of CuPc and TCNQ and on TCNQ/CuPc interfaces is applied for monitoring the electronic interaction indicated in systematic shifts of the CuPc and TCNQ HOMO and core orbitals. These shifts correspond to a movement of the Fermi level within the HOMO LUMO energy gap. The shifts in CuPc induced by the interaction with TCNQ are similar in the composites and in the interface model experiment. At the interface an additional induced dipole potential can be measured. The interface-dipole plus the Fermile level shift add up to the work function difference of pure CuPc and TCNQ. We conclude that TCNQ forms a separate phase in CuPc rather than single isolated acceptor molecules. Charge transfer at the bulk hetero-junction induces the Fermi level variations, which may be called doping.