Publications

The dependence of zero field charge carrier mobilities obtained from time-of-flight (TOF) measurements for a series of doped triphenyldiamines on trap depth and concentration were determined. The influence of these mobilities on the solar cell characteristics have been studied by measuring I–V characteristics and IPCE values using the same doped hole conductors in solid-state dye-sensitized TiO2 solar cells. The traps exhibit only a weak influence on the solar cell characteristics, because the traps which are responsible for low charge transport mobilities in the TOF experiment are filled under steady state illumination conditions. In consequence the effective mobility is raised almost to that of the pristine hole conductor, thus allowing the use of doped hole conductor systems in solar cells without any adverse effects.

In an endeavor to correlate the optoelectronic properties of π-conjugated polymers with their structural properties, we investigated the aggregation of P3HT in THF solution within a temperature range from 300 to 5 K. By detailed steady-state, site-selective, and time-resolved fluorescence spectroscopy combined with Franck–Condon analyses, we show that below a certain transition temperature (265 K) aggregates are formed that prevail in different polymorphs. At 5 K, we can spectroscopically identify two H-type aggregates with planar polymer backbones yet different degree of order regarding their side chains. Upon heating, the H-character of the aggregates becomes gradually eroded, until just below the transition temperature the prevailing “aggregate” structure is that of still phase-separated, yet disordered main and side chains. These conclusions are derived by analyzing the vibrational structure of the spectra and from comparing the solution spectra with those obtained from thin films that were cooled slowly from the melting temperature to room temperature and that had been analyzed previously by various X-ray techniques. In addition, site selectively recorded fluorescence spectra show that there is—dependent on temperature—energy transfer from higher energy to lower energy aggregates. This suggests that they must form clusters with dimensions of the exciton diffusion length, i.e., several nanometers in diameter.

A flexible organic dyad consisting of a perylene bisimide antenna covalently linked to a [60]fullerene has been synthesized and studied by electrochemistry, steady-state spectroscopy, and time-resolved spectroscopy. We found that the energy absorbed by the perylene bisimide is transferred to the fullerene with an efficiency close to 100%. The fullerene in turn undergoes intersystem crossing followed by triplet energy transfer back to the perylene bisimide with an efficiency of at least 20%. Hence the perylene bisimide unit acts as an antenna for the fullerene, i.e., effectively extending the fullerene absorption far into the visible spectral range, while at the same time the fullerene acts as a triplet sensitizer for the perylene bisimide. This has severe consequences for the exploitation of the dye antenna-fullerene concept for light harvesting in solar cells.

A series of donor–acceptor diblock copolymers with varying molecular weight are studied in thin film and compared with an ‘equivalent’ blend formed from donor and acceptor homopolymers. Steady-state and transient spectroscopies are used to demonstrate a correlation between low molecular weight block copolymers and increased photoluminescence quenching (up to 99%) leading to higher yields of long-lived free charges. Such block copolymers are shown, by electron microscopy, to exhibit phase-segregated micrdomains whose size and periodicity are determined by their molecular weight. Photovoltaic devices made using these materials show a peak efficiency of 0.11% and correlate with our spectroscopic results, subject to a trade-off between charge generation and tranpsort/collection.

A PC₇₁BM-grafted donor–acceptor block copolymer with enhanced absorption showing a periodic nanostructure of 37 nm both in bulk and in thin films was synthesized by combining KCTP and CRP methods.

The degree of fluorination in π-extended polydiketopyrrolopyrroles is correlated with semiconductor properties in transistors and an improved molecular alignment in thin films using depth-dependent grazing incidence X-ray scattering.

The question of designing high electron mobility polymers by increasing the planarization using diffusive nonbonding heteroatom interactions in diketopyrrolopyrrole polymers is addressed in this. For this, three different diketopyrrolo[3,4‐ c ]pyrrole (DPP) derivatives with thienyl‐, 2‐pyridinyl‐, and phenyl‐flanked cores are copolymerized with an electron‐rich thiophene unit as well as an electron‐deficient 3,4‐difluorothiophene unit as comonomer to obtain diverse polymeric DPPs which vary systematically in their structures. The crystallinity differs significantly with clear trends on varying both flanking unit and comonomer. The optical gap and energy levels depend more on the nature of the flanking aryl units rather than on fluorination. Additionally, the charge transport properties are compared using different methods to differentiate between interface or orientation effects and bulk charge carrier transport. In organic field effect transistor devices with very high electron as well as hole mobilities (up to 0.6 cm 2 V −1 s −1 ) are obtained and fluorination leads to a more pronounced n‐type nature in all polymers, resulting in ambipolar behavior in otherwise p‐type materials. In contrast, space‐charge limited current measurements show a strong influence of the flanking units only on electron mobilities. Especially, the elegant synthetic strategy of combining pyridyl flanking units with difluorothiophene as the comonomer culminates in a record bulk electron mobility of 4.3 × 10 −3 cm 2 V −1 s −1 in polymers.

Carbon-proton dipole-dipole couplings between bonded atoms represent a popular probe of molecular dynamics in soft materials or biomolecules. Their site-resolved determination, for example, by using the popular DIPSHIFT experiment, can be challenged by spectral overlap with nonbonded carbon atoms. The problem can be solved by using very short cross-polarization (CP) contact times, however, the measured modulation curves then deviate strongly from the theoretically predicted shape, which is caused by the dependence of the CP efficiency on the orientation of the CH vector, leading to an anisotropic magnetization distribution even for isotropic samples. Herein, we present a detailed demonstration and explanation of this problem, as well as providing a solution. We combine DIPSHIFT experiments with the rotor-directed exchange of orientations (RODEO) method, and modifications of it, to redistribute the magnetization and obtain undistorted modulation curves. Our strategy is general in that it can also be applied to other types of experiments for heteronuclear dipole-dipole coupling determinations that rely on dipolar polarization transfer. It is demonstrated with perylene-bisimide-based organic semiconductor materials, as an example, in which measurements of dynamic order parameters reveal correlations of the molecular dynamics with the phase structure and functional properties.

In article number 1700264, Sven Hüttner, Pablo Docampo, and co-workers give an overview of hybrid metal halide perovskites for solar cell applications. The focus is on the series of challenges that have been overcome and those still remaining to be solved. In particular, the authors lay out their understanding of the perovskite crystallization process and how this knowledge can be harnessed to enable better performing devices; how to overcome reproducibility and hysteresis issues; and the long-term prospects of the technology in terms of stability and sustainability. Image by Criss Hohmann (Nanosystems Initiative Munich).

Abstract Electron transporting ( n ‐type) polymers are the coveted complementary counterpart to more thoroughly studied hole transporting ( p ‐type) semiconducting polymers. Besides intrinsic stability issues of the doped form of n ‐type polymer toward ubiquitous oxidizing agents (H 2 O and O 2 ), the choice of suitable n ‐dopants and underlying mechanism of doping is an open research field. Using a low LUMO, n ‐type unipolar acceptor 1 ‐acceptor 2 copolymer poly(DPP‐TPD) in conjunction with bulk n ‐doping using Cs 2 CO 3 these issues can be addressed. A solid‐state acid‐base interaction between polymer and basic carbonate increases the backbone electron density by deprotonation of the thiophene comonomer while forming bicarbonate, as revealed by NMR and optical spectroscopy. Comparable to N‐DMBI hydride/electron transfer, Cs 2 CO 3 proton abstraction doping shifts the poly(DPP‐TPD) work function toward the LUMO. Thereby, the anionic doped state is resilient against O 2 but is susceptible toward H 2 O. Based on GIWAXS, Cs 2 CO 3 is mostly incorporated into the amorphous regions of poly(DPP‐TPD) with the help of hydrophilic side chains and has minor impact on the short‐range order of the polymer. Cs 2 CO 3 proton abstraction doping and the acceptor 1 ‐acceptor 2 copolymer architecture creates a synergistic n ‐doped system with promising properties for thermoelectric energy conversion, as evidenced by a remarkable power factor of (5.59 ± 0.39) × µW m −1 K −2 .

Solvent vapour annealing (SVA) is demonstrated as an attractive method to anneal polymer blend and block copolymer thin films at low temperatures. It is especially suitable for organic electronics, where sensitive materials with strong intermolecular interactions are used. We demonstrate the effect of solvent vapour exposure on the film properties of a perylene bisimide acrylate (PPerAcr) side-chain polymer with strong crystallinity at the perylene bisimide moieties. We record the film thickness, light absorption and fluorescence as a function of the relative solvent vapour pressure. At a certain threshold of relative solvent vapour pressure, we observe a disruption of the π–π stacking, which is responsible for perylene bisimide crystallisation. This leads to an increase in the polymer-chain mobility and therefore to changes in the film morphology. The results are applied to a film of a donor–acceptor block copolymer carrying PPerAcr segments, and the influence of solvent annealing on the nanoscale morphology is demonstrated.

Abstract A comprehensive investigation of four polydiketopyrrolopyrroles (PDPPs) with increasing ethylene glycol (EG) content and varying nature of comonomer is presented, and guidelines for the design of efficient mixed ion‐electron conductors (MIECs) are deduced. The studies in NaCl electrolyte‐gated organic electrochemical transistors (OECTs) reveal that a high amount of EG on the DPP moiety is essential for MIEC. The PDPP containing 52 wt% EG exhibits a high volumetric capacitance of 338 F cm −3 (at 0.8 V), a high hole mobility in aqueous medium (0.13 cm 2 V −1 s −1 ), and a μC* product of 45 F cm −1 V −1 s −1 . OECTs using this polymer retain 97% of the initial drain‐current after 1200 cycles (90 min of continuous operation). In a cell growth medium, the OECT‐performance is fully maintained as in the NaCl electrolyte. In vitro cytotoxicity and cell viability assays reveal the excellent cell compatibility of these novel systems, showing no toxicity after 24 h of culture. Due to the excellent OECT performance with a considerable cycling stability for 1200 cycles and an outstanding cell compatibility, these PDPPs render themselves viable for in vitro and in vivo bioelectronics.