Publications

Combinatorial study of the long-term stability of organic thin-film solar cells

We present a setup that allows us to combinatorially study organic multilayer devices for durability and degradation mechanisms. Arrays of 8×8 thin-film organic devices are prepared by vacuum deposition and investigated under vacuum conditions. Measurements of nine types of organic solar cells are presented. Continuous solar-like illumination at 80 mW/cm2 and a temperature of 80 °C provide realistic testing conditions. The degradation of solar cells has been investigated as a function of layer combination and layer thickness over a period of 800 h. Different time scales of degradation are identified and attributed to the layer structure of the devices.

An Organic Optical Transistor Operated under Ambient Conditions

Aller guten Dinge sind drei: Die Hauptmerkmale eines optischen Transistors (Steuerung und Verstärkung) werden anhand der photophysikalischen Eigenschaften einer molekularen Triade gezeigt (siehe Bild). Zwei Bausteine der Triade sind hocheffiziente Fluorophore, wohingegen der dritte Baustein ein photochromes Molekül ist, das durch Licht reversibel von einer bistabilen Form in die andere umgewandelt werden kann. Detailed facts of importance to specialist readers are published as ”Supporting Information”. Such documents are peer-reviewed, but not copy-edited or typeset. They are made available as submitted by the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

Intricacies and Mechanism of p‐Doping Spiro‐MeOTAD Using Cu(TFSI)₂

Abstract Copper salts are a popular choice as p ‐dopants for organic semiconductors, particularly in N 2 ,N 2 ,N 2′ ,N 2′ ,N 7 ,N 7 ,N 7′ ,N 7′ ‐octakis(4‐methoxyphenyl)‐9,9′‐spirobi[9H‐fluoren]‐2,2′,7,7′‐tetramine (Spiro‐MeOTAD) hole transport material for solar cells. While being exceptionally effective, no scientific consensus about their doping mechanism has been established so far. This study describes the thermodynamic equilibria of involved species in copper(II) bis(trifluoromethanesulfonyl)imide (Cu(TFSI) 2 ) doped, co‐evaporated Spiro‐MeOTAD. A temperature‐independent formation of charge transfer states is found, followed by an endothermic release of free charge carriers. Impedance and electron paramagnetic resonance spectroscopy unravel low activation energies for hole release and hopping transport. As a result, (52.0 ± 6.4)% of the total Cu(TFSI) 2 molecules form free, dissociated holes at 10 mol% and room temperature. Cu I species arising out of doping are stabilized by formation of a [Cu I (TFSI) 2 ] ‐ cuprate, inhibiting elemental copper formation. This Cu I species presents a potent hole trap reducing their mobility, which can be averted by simple addition of a bathocuproine complexing agent. A nonlinear temperature‐dependent conductivity and mobility that contradicts current charge transport models is observed. This is attributed to a combination of trap‐ and charge transfer state freeze‐out. These insights may be adapted to other metal salts, providing guidelines for designing next‐generation ultra‐high efficiency dopants.

Low molecular weight and polymeric heterocyclics as electron transport/hole‐blocking materials in organic light‐emitting diodes

The use of low molecular weight, oligomeric and polymeric heterocyclics as electron transport/hole-blocking layers in organic light-emitting diodes is reviewed. The most widely applied materials are π-electron deficient heterocyclics carrying imine nitrogen atoms in the aromatic ring, such as 1,3,4-oxadiazoles, 1,2,4-triazoles, 1,3,5-triazines, and 1,4-quinoxalines. Properties such as redox potentials, ionization potential, electron affinity and charge transport mobility of the materials, if known, are taken into consideration to support the electron injection/transport and hole-blocking effectiveness. It can be generalized that heterocyclic moieties with high reduction potential reduce the interface barriers caused by the band offset between organic material and cathode and are most suitable materials for electron injection in organic electroluminescent devices. These materials are generally characterized by high ionization potential values that contribute towards the hole-blocking property. A general comparison of devices and materials is only possible with limitations owing to the variations in device structure, fabrication, electrode materials, emitter materials, etc. © 1998 John Wiley & Sons, Ltd.

Energy- and charge-transfer processes in flexible organic donor-acceptor dyads

Organic donor-bridge-acceptor dyads consisting of a triphenyldiamine donor that was linked to a perylenebisimide acceptor by a flexible nonconjugated bridge have been investigated by complementary spectroscopic techniques as a function of the length and the polarity of the linker. Time-resolved fluorescence spectroscopy revealed a quenching of the donor emission accompanied by a corresponding rise in the acceptor fluorescence, which indicates an efficient energy transfer between the donor and acceptor moieties. A second fluorescence quenching process that affects the acceptor emission is ascribed to a ground-state electron transfer from the donor to the acceptor. The lifetimes of the radicals that were determined by transient-absorption spectroscopy covered the range from 10 to 100 ms.

Fluorination in thieno[3,4-c]pyrrole-4,6-dione copolymers leading to electron transport, high crystallinity and end-on alignment

GIWAXS measurements confirm end-on alignment for TPD-copolymers obtained from Stille polycondensation using fluorinated and non-fluorinated comonomers.

Morphology, Crystal Structure and Charge Transport in Donor–Acceptor Block Copolymer Thin Films

We studied structure and charge transport properties of thin films of donor-acceptor block copolymers, poly(3-hexylthiophene-block-perylene bisimide acrylate), using a combination of X-ray scattering, AFM and vertical charge transport measurements in diode devices. Block copolymer self-assembly and crystallization of the individual components are interrelated and different structural states of the films could be prepared by varying preparation conditions and thermal history. Generally the well-defined microphase structures found previously in bulk could also be prepared in thin films, in addition alignment induced by interfacial interactions was observed. Microphase separated block copolymers sustain ambipolar charge transport, but the exact values of electron and hole mobilities depend strongly on orientation and connectivity of the microdomains as well as the molecular order within the domains.

Heteroleptic ruthenium complex containing substituted triphenylamine hole-transport unit as sensitizer for stable dye-sensitized solar cell

Polymer Thermoelectrics: Opportunities and Challenges

The inherent low thermal conductivity of polymeric materials along with their capability for nonconventional processability, light weight, and flexibility makes them potential candidates for low-power thermoelectrical applications. However, it requires the improvement of both Seebeck coefficient and electrical conductivity simultaneously. This Perspective addresses some of the concepts in the literature to achieve this by considering the intricacies and interconnectivities of different thermoelectric parameters. Especially, the applicability of the Wiedemann–Franz rule as well as DOS engineering by blending two semiconductors, valid in inorganics, is reviewed for polymeric systems. One of the biggest advantages of polymeric semiconductors is that their thermal conductivity is mostly phononic (lattice thermal conductivity) and remains low with increasing charge carrier concentrations, as needed for TE applications. The momentum in polymer synthetic strategies has to be coupled with the studies on doping efficiency and doping mechanisms to overcome the bottlenecks in polymeric thermoelectrics. We recommend for a paradigm shift toward low dopant ratios with high doping efficiencies and for DOS engineering to separate the Fermi level and transport level to achieve high absolute Seebeck coefficients in polymers.

Tuning of composition and morphology of LiFePO4 cathode for applications in all solid-state lithium metal batteries

All solid-state rechargeable lithium metal batteries (SS-LMBs) are gaining more and more importance because of their higher safety and higher energy densities in comparison to their liquid-based counterparts. In spite of this potential, their low discharge capacities and poor rate performances limit them to be used as state-of-the-art SS-LMBs. This arise due to the low intrinsic ionic and electronic transport pathways within the solid components in the cathode during the fast charge/discharge processes. Therefore, it is necessary to have a cathode with good electron conducting channels to increase the active material utilization without blocking the movement of lithium ions. Since SS-LMBs require a different morphology and composition of the cathode, we selected LiFePO₄ (LFP) as a prototype and, we have systematically studied the influence of the cathode composition by varying the contents of active material LFP, conductive additives (super C65 conductive carbon black and conductive graphite), ion conducting components (PEO and LiTFSI) in order to elucidate the best ion as well as electron conduction morphology in the cathode. In addition, a comparative study on different cathode slurry preparation methods was made, wherein ball milling was found to reduce the particle size and increase the homogeneity of LFP which further aids fast Li ion transport throughout the electrode. The SEM analysis of the resulting calendered electrode shows the formation of non-porous and crack-free structures with the presence of conductive graphite throughout the electrode. As a result, the optimum LFP cathode composition with solid polymer nanocomposite electrolyte (SPNE) delivered higher initial discharge capacities of 114 mAh g^-1 at 0.2C rate at 30 °C and 141 mAh g^-1 at 1C rate at 70 °C. When the current rate was increased to 2C, the electrode still delivered high discharge capacity of 82 mAh g^-1 even after 500 cycle, which indicates that the optimum cathode formulation is one of the important parameters in building high rate and long cycle performing SS-LMBs.

Trapping on demand: External regulation of excitation energy transfer in a photoswitchable smart matrix

A thin film of polystyrene has been doped with small amounts of dithienylcyclopentene (DCP) based molecular switches and perylene bisimide (PBI) chromophores to obtain a photoswitchable smart matrix. The photochromic DCP can be converted by light between two bistable conformations and thereby changes the energetic position of its lowest excited singlet state. We exploit this feature to regulate the transfer of excitation energy between PBI and DCP as a function of the externally controllable illumination conditions in such a blend.

Polymeric Light-Emitting Diodes Based on Poly(p-phenylene ethynylene), Poly(triphenyldiamine), and Spiroquinoxaline

In this paper polymeric light-emitting diodes (LEDs) based on alkoxy-substituted poly(p-phenylene ethynylene) EHO-OPPE as emitter material in combination with poly(triphenyldiamine) as hole transport material are demonstrated. Different device configurations such as single-layer devices, two-layer devices, and blend devices were investigated. Device improvement and optimization were obtained through careful design of the device structure and composition. Furthermore, the influence of an additional electron transporting and hole blocking layer (ETHBL), spiroquinoxaline (spiro-qux), on top of the optimized blend device was investigated using a combinatorial method, which allows the preparation of a number of devices characterized by different layer thicknesses in one deposition step. The maximum brightness of the investigated devices increased from 4 cd/m2 for a device of pure EHO-OPPE to 260 cd/m2 in a device with 25 % EHO-OPPE + 75 % poly(N,N′-diphenylbenzidine diphenylether) (poly-TPD) as the emitting/hole-transporting layer and an additional electron-transport/hole-blocking spiro-qux layer of 48 nm thickness.

Efficient screening of electron transport material in multilayer organic light-emitting diodes by combinatorial methods

A combinatorial approach combining vapor deposition of organic molecules and a mask technique was used to prepare on one substrate a matrix of 49 organic light emitting diodes (OLEDs) with different configuration and layer thickness. A landscape library with two orthogonal, linear gradients of an emitter and a hole blocking electron transport material on top of a hole transport layer of constant thickness was prepared. The aim of this experiment was to investigate the influence of an additional electron transport material on the efficiency. Using a semi-automated measurement set-up, the device parameters for each of the 49 OLEDs were evaluated. The existence of an optimum Alq<SUB>3</SUB> layer thickness for two-layer devices ITO/TPD/Alq<SUB>3</SUB>/Al is confirmed and such an optimized two-layer structure could not be improved by adding an additional hole blocking layer to the optimum Alq<SUB>3</SUB> layer. But an improvement of photometric efficiency can be obtained by replacing the optimum Alq<SUB>3</SUB> layer thickness by certain combinations of Alq<SUB>3</SUB>/spiro-Quinoxaline layers.

Plasmonic nanomeshes: their ambivalent role as transparent electrodes in organic solar cells

In this contribution, the optical losses and gains attributed to periodic nanohole array electrodes in polymer solar cells are systematically studied. For this, thin gold nanomeshes with hexagonally ordered holes and periodicities (P) ranging from 202 nm to 2560 nm are prepared by colloidal lithography. In combination with two different active layer materials (P3HT:PC₆₁BM and PTB7:PC₇₁BM), the optical properties are correlated with the power conversion efficiency (PCE) of the solar cells. A cavity mode is identified at the absorption edge of the active layer material. The resonance wavelength of this cavity mode is hardly defined by the nanomesh periodicity but rather by the absorption of the photoactive layer. This constitutes a fundamental dilemma when using nanomeshes as ITO replacement. The highest plasmonic enhancement requires small periodicities. This is accompanied by an overall low transmittance and high parasitic absorption losses. Consequently, larger periodicities with a less efficient cavity mode, yet lower absorptive losses were found to yield the highest PCE. Nevertheless, ITO-free solar cells reaching ~77% PCE compared to ITO reference devices are fabricated. Concomitantly, the benefits and drawbacks of this transparent nanomesh electrode are identified, which is of high relevance for future ITO replacement strategies.

Solid polymer electrolytes from polyesters with diester sidechains for lithium metal batteries

Seven novel polyesters with sidechain diester groups are synthesized, electrochemically characterized, and compared to polybutylacrylate and polycaprolactone. As “beyond PEO” SPEs, they show high capacity retention even at high C rate and low T.

Surface induced orientation and vertically layered morphology in thin films of poly(3-hexylthiophene) crystallized from the melt

Infrared Sensitizers in Titania‐Based Dye‐Sensitized Solar Cells using a Dimethylferrocene Electrolyte

Vide infra: Two boron-dipyrromethene (BODIPY)-type infrared sensitizers are investigated for use in titania-based dye-sensitized solar cells (DSCs). The DSCs use a solution of dimethylferrocene (Me2 Fc) in nitromethane as electrolyte. The titania conduction band edge is fine-tuned by adding lithium ions from LiTFSI, while the choice of dimethylferrocene as redox shuttle provides the ideal driving force for dye regeneration, allowing the BODIPY-based DSCs to reach unprecedented quantum efficiencies in the near-infrared.

Dual-functional materials for interface modifications in solid-state dye-sensitised TiO2 solar cells