Effect of Electron and Proton Irradiation on Structural and Electronic Properties of Carbon Nanowalls.

In this work, a complex experimental study of the effect of electron and proton ionizing radiation on the properties of carbon nanowalls (CNWs) is carried out using various state-of-the-art materials characterization techniques. CNW layers on quartz substrates were exposed to 5 MeV electron and 1.8 MeV proton irradiation with accumulated fluences of 7 × 1013 e/cm2 and 1012 p/cm2, respectively. It is found that depending on the type of irradiation (electron or proton), the morphology and structural properties of CNWs change; in particular, the wall density decreases, and the sp2 hybridization component increases. The morphological and structural changes in turn lead to changes in the electronic, optical, and electrical characteristics of the material, in particular, change in the work function, improvement in optical transmission, an increase in the surface resistance, and a decrease in the specific conductivity of the CNW films. Lastly, this study highlights the potential of CNWs as nanostructured functional materials for novel high-performance radiation-resistant electronic and optoelectronic devices.

Unraveling the electrochemical and spectroscopic properties of neutral and negatively charged perylene tetraethylesters.

A detailed investigation of the energy levels of perylene-3,4,9,10-tetracarboxylic tetraethylester as a representative compound for the whole family of perylene esters was performed. It was revealed via electrochemical measurements that one oxidation and two reductions take place. The bandgaps determined via the electrochemical approach are in good agreement with the optical bandgap obtained from the absorption spectra via a Tauc plot. In addition, absorption spectra in dependence of the electrochemical potential were the basis for extensive quantum-chemical calculations of the neutral, monoanionic, and dianionic molecules. For this purpose, calculations based on density functional theory were compared with post-Hartree-Fock methods and the CAM-B3LYP functional proved to be the most reliable choice for the calculation of absorption spectra. Furthermore, spectral features found experimentally could be reproduced with vibronic calculations and allowed to understand their origins. In particular, the two lowest energy absorption bands of the anion are not caused by absorption of two distinct electronic states, which might have been expected from vertical excitation calculations, but both states exhibit a strong vibronic progression resulting in contributions to both bands.

The Importance of Quantifying the Composition of the Amorphous Intermixed Phase in Organic Solar Cells.

The relation of phase morphology and solid-state microstructure with organic photovoltaic (OPV) device performance has intensely been investigated over the last twenty years. While it has been established that a combination of donor:acceptor intermixing and presence of relatively phase-pure donor and acceptor domains is needed to get an optimum compromise between charge generation and charge transport/charge extraction, a quantitative picture of how much intermixing is needed is still lacking. This is mainly due to the difficulty in quantitatively analyzing the intermixed phase, which generally is amorphous. Here, fast scanning calorimetry, which allows measurement of device-relevant thin films (<200 nm thickness), is exploited to deduce the precise composition of the intermixed phase in bulk-heterojunction structures. The power of fast scanning calorimetry is illustrated by considering two polymer:fullerene model systems. Somewhat surprisingly, it is found that a relatively small fraction (<15 wt%) of an acceptor in the intermixed amorphous phase leads to efficient charge generation. In contrast, charge transport can only be sustained in blends with a significant amount of the acceptor in the intermixed phase (in this case: ≈58 wt%). This example shows that fast scanning calorimetry is an important tool for establishing a complete compositional characterization of organic bulk heterojunctions. Hence, it will be critical in advancing quantitative morphology-function models that allow for the rational design of these devices, and in delivering insights in, for example, solar cell degradation mechanisms via phase separation, especially for more complex high-performing systems such as nonfullerene acceptor:polymer bulk heterojunctions.

Organic Electrochemical Transistors Based on the Conjugated Polyelectrolyte PCPDTBT-SO3 K (CPE-K).

PCPDTBT-SO3 K (CPE-K), a conjugated polyelectrolyte, is presented as a mixed conductor material that can be used to fabricate high transconductance accumulation mode organic electrochemical transistors (OECTs). OECTs are utilized in a wide range of applications such as analyte detection, neural interfacing, impedance sensing, and neuromorphic computing. The use of interdigitated contacts to enable high transconductance in a relatively small device area in comparison to standard contacts is demonstrated. Such characteristics are highly desired in applications such as neural-activity sensing, where the device area must be minimized to reduce invasiveness. The physical and electrical properties of CPE-K are fully characterized to allow a direct comparison to other top performing OECT materials. CPE-K demonstrates an electrical performance that is among the best reported in the literature for OECT materials. In addition, CPE-K OECTs operate in the accumulation mode, which allows for much lower energy consumption in comparison to commonly used depletion mode devices.

A High-Performance Solution-Processed Organic Photodetector for Near-Infrared Sensing.

Sensitive detection of near-infrared (NIR) light enables many important applications in both research and industry. Current organic photodetectors suffer from low NIR sensitivity typically due to early absorption cutoff, low responsivity, and/or large dark/noise current under bias. Herein, organic photodetectors based on a novel ultranarrow-bandgap nonfullerene acceptor, CO1-4Cl, are presented, showcasing a remarkable responsivity over 0.5 A W-1 in the NIR spectral region (920-960 nm), which is the highest among organic photodiodes. By effectively delaying the onset of the space charge limited current and suppressing the shunt leakage current, the optimized devices show a large specific detectivity around 1012 Jones for NIR spectral region up to 1010 nm, close to that of a commercial Si photodiode. The presented photodetectors can also be integrated in photoplethysmography for real-time heart-rate monitoring, suggesting its potential for practical applications.

Understanding the High Performance of over 15% Efficiency in Single-Junction Bulk Heterojunction Organic Solar Cells.

The highly efficient single-junction bulk-heterojunction (BHJ) PM6:Y6 system can achieve high open-circuit voltages (VOC ) while maintaining exceptional fill-factor (FF) and short-circuit current (JSC ) values. With a low energetic offset, the blend system is found to exhibit radiative and non-radiative recombination losses that are among the lower reported values in the literature. Recombination and extraction dynamic studies reveal that the device shows moderate non-geminate recombination coupled with exceptional extraction throughout the relevant operating conditions. Several surface and bulk characterization techniques are employed to understand the phase separation, long-range ordering, as well as donor:acceptor (D:A) inter- and intramolecular interactions at an atomic-level resolution. This is achieved using photo-conductive atomic force microscopy, grazing-incidence wide-angle X-ray scattering, and solid-state 19 F magic-angle-spinning NMR spectroscopy. The synergy of multifaceted characterization and device physics is used to uncover key insights, for the first time, on the structure-property relationships of this high-performing BHJ blend. Detailed information about atomically resolved D:A interactions and packing reveals that the high performance of over 15% efficiency in this blend can be correlated to a beneficial morphology that allows high JSC and FF to be retained despite the low energetic offset.

Electroluminescent and Optoelectronic Properties of OLEDs with Bay-Extended, Distorted Perylene Esters as Emitter Materials.

Three esters with a perylene, a unilaterally, and a bilaterally extended perylene core, respectively, were used as emitter materials for organic light-emitting diodes. The electroluminescent properties of these devices were studied. Different spectral shifts were found, which can be attributed to the formation of excited dimers (excimers) in the nanofilms of the emitter materials. Thermal treatment of the unilaterally extended derivative resulted in a red-shift of the electroluminescence owing to the formation of a denser nanofilm. The luminance and efficiency of optoelectronic devices employing the extended perylene esters exceed those of devices using an emitter layer comprised of the perylene ester. Different deposition methods, limitations in the deposition process, and the role of hole-transporting materials are compared.

Polycyclic aromatic hydrocarbons obtained by lateral core extension of mesogenic perylenes: absorption and optoelectronic properties.

Bilaterally extended perylenes were synthesized, characterized, and used to create organic light-emitting devices. A detailed investigation of the electronic and optical properties, and a comparison of perylene derivatives and compounds with unilaterally and bilaterally extended aromatic cores, reveal unexpected changes of the absorption spectrum, which are in agreement with simulations based on DFT.