With the aim of pushing the knowledge and understanding on mixed films of organic semiconductors forward, blends of ZnPC and F8ZnPC in different ratios are manufactured. The films have a polycrystalline structure, as indicated by electron diffraction profiles and infrared-spectroscopy. Photoluminescence data show completely different spectra for the blends, compared to the pure materials, which can be ascribed to the suppressing of excimer formation and the appearance of a new charge-transfer excitation between the two different molecules in the blends. This new excitation can also be seen in optical absorption. Momentum dependent measurements of the electronic excitations by electron energy-loss spectroscopy confirm the localized character of the new charge-transfer excitation in the blends. Our experimental data help understand the important issue of donor/acceptor coupling in organic semiconductors.

Organic light‐emitting materials exhibiting thermally activated delayed fluorescence (TADF) show great promise for improving display applications. Recently, intermolecular effects between emitting molecules have been given more attention, revealing strong solid‐state solvation or aggregation induced changes of sample performance. Implications of this on device performance are not yet fully covered. In this work, a thorough investigation of a novel TADF emitter, methyl 2,3,4,5,6‐penta(carbazol‐9‐yl)benzoate (5CzCO2Me), is provided. Steady‐state emission spectra reveal a luminescence redshift with increasing emitter concentration in a small molecule host. In all investigated concentrations, the emission profile remains the same; thus, the redshift is attributed to the solid‐state solvation effect. The highest photoluminescence quantum yield (PLQY) is achieved in the 20 wt% sample, reaching 66%. The best organic light‐emitting diode (OLED) in terms of current–voltage–luminance and external quantum efficiency (EQE) parameters is the device with 60 wt% emitter concentration, reaching maximal EQE values of 7.5%. It is shown that the emitter transports holes and that charge‐carrier recombination does not take place on the bandgap of the host, but rather, a mixed host–guest concentration‐dependent recombination is seen. The hole‐transporting properties of 5CzCO2Me allow for a new dimension in tuning the device performance by controlling the emitter concentration.

Blending organic molecules to tune their energy levels is currently being investigated as an approach to engineer the bulk and interfacial optoelectronic properties of organic semiconductors. It has been proven that the ionization energy and electron affinity can be equally shifted in the same direction by electrostatic effects controlled by blending similar halogenated derivatives with different energetics. Here we show that the energy gap of organic semiconductors can also be tuned by blending. We use oligothiophenes with different numbers of thiophene rings as an example and investigate their structure and electronic properties. Photoelectron spectroscopy and inverse photoelectron spectroscopy show tunability of the single-particle gap, with the optical gaps showing similar, but smaller, effects. Theoretical analysis shows that this tuning is mainly caused by a change in the dielectric constant with blend ratio. Further studies will explore the practical impact of this energy-level engineering strategy for optoelectronic devices.

Solution-processed metal oxide (MO) thin films have been extensively studied for use in thin-film transistors (TFTs) due to their high optical transparency, simplicity of fabrication methods, and high electron mobility. Here, we report, for the first time, the improvement of the electronic properties of solution-processed indium oxide (InOx) films by the subsequent addition of an organic p-type semiconductor material, here 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-pentacene), yielding organic-inorganic hybrid TFTs. The addition of TIPS-pentacene not only improves the electron mobility by enhancing the charge carrier percolation pathways but also improves the electronic and temporal stability of the IDS(VG) characteristics as well as reduces the number of required spin-coating steps of the InOx precursor solution. Very interestingly, the introduction of 10 nm TIPS-pentacene films on top of 15 nm InOx layers allows the fabrication of either enhancement- or depletion-mode devices with only minimal changes to the fabrication process. Specifically, we find that when the TIPS-pentacene layer is added on top of the source/drain electrodes, resulting in devices with embedded source/drain electrodes [embedded electrode TFTs (EETFTs)], the devices exhibit an enhancement-mode behavior with an average mobility (μ) of 6.4 cm2 V-1 s-1, a source-drain current ratio (Ion/Ioff) of around 105, and a near-zero threshold voltage (VTH). When on the other hand the TIPS-pentacene layer is added before the source-drain electrodes, i.e., in top-contact electrode TFTs (TCETFTs), a very clear depletion mode behavior is observed with an average μ of 6.3 cm2 V-1 s-1, an Ion/Ioff ratio of over 105, and a VTH of -80.3 V. Furthermore, a logic inverter is fabricated combining the enhancement (EETFTs)- and depletion (TCETFTs)-mode transistors, which shows a potential for the construction of organic-inorganic hybrid electronics and circuits.

Organic electronics is the technology enabling truly flexible electronic devices. However, despite continuous improvements in the charge-carrier mobility, devices used for digital circuits based on organic field-effect transistors (OFETs) have still not achieved a commercial breakthrough. A substantial hurdle to the realization of effective digital circuitry is the proper control of the threshold voltage V-th. Previous approaches include doping or self-assembled monolayers to provide the threshold voltage control. However, while self-assembled monolayers-modified OFETs often do not show the level of reproducibility which is required in digital circuit engineering, direct doping of the channel material results in a poor on/off ratio leading to unfavorable power dissipation. Furthermore, direct doping of the channel material in organic semiconductors could cause the formation of trap states impeding the charge-carrier transport. Employing the concept of modulation-doped field-effect transistors (MODFETs), which is well established in inorganic electronics, the semiconductor-dopant interaction is significantly reduced, thereby solving the above-described problems. Here, we present the concept of an organic semiconductor MODFET which is composed of an organic-organic heterostructure between a highly doped wide-energy-gap material and an undoped narrow-energy-gap material. The effectiveness of charge transfer across the interface is controlled by the doping concentration and thickness of an undoped buffer layer. A complete picture of the energy landscape of this heterostructure is drawn using impedance spectroscopy and ultraviolet photoelectron spectroscopy. Furthermore, we analyze the effect of the dopant density on the charge-carrier transport properties. The incorporation of these heterostructures into OFETs enables a precise adjustment of the threshold voltage by using the modulation doping concept.

In modern electronics, it is essential to adapt band structures by adjusting energy levels and band gaps. At first sight, this "band structure engineering" seems impossible in organic semiconductors, which usually exhibit localized electronic states instead of Bloch bands. However, the strong Coulomb interaction in organic semiconductors allows for a continuous shift of the ionization energy (IE) over a wide range by mixing molecules with halogenated derivatives that exhibit different quadrupole moments. Here, this effect of energy level engineering on blends of pentacene and two fluorinated derivatives, in which the position but not the number of fluorine atoms differ, is studied. Structural investigations confirm that pentacene forms intermixed phases in blends with the fluorinated species. The investigation of electronic properties and simulations reveals a much larger shift of the ionization energy (1.5 eV) than in previous studies, allowing to test this model in a range not investigated so far, and emphasizing the role of the position of the halogen atoms. The tuning effect is preserved in electronic devices such as field-effect transistors and significantly influences device characteristics.

p-Type molecular doping of organic materials with high ionization energies (IEs) of above 5.50 eV is still a challenge, limiting the use of doping in high-performance organic light-emitting diodes (OLEDs). Here, we investigate the molecular dopant hexacyano-trimethylene-cyclopropane (CN6-CP) with a high electron affinity of 5.87 eV as p-dopant in OLEDs. We show that CN6-CP can be used not only as a dopant in the traditional hole transport material 4,4'-cyclohexylidenebis[ N, N-bis(4-methylphenyl)benzenamine] (TAPC, IE = 5.50 eV) but also effectively dopes the host material tris(4-carbazoyl-9-ylphenyl)amine (TCTA, IE = 5.85 eV), reaching a conductivity of 1.86 × 10-4 S/cm at a molar ratio of 0.25. Using CN6-CP-doped TAPC as hole injection and transport layer, we achieve a low driving voltage of 2.92 V at the practical brightness of 1000 cd/m2 and 3.18 V at a current density of 10 mA/cm2 for a green phosphorescent OLED based on bis[2-(2-pyridinyl- N)phenyl- C](acetylacetonato)iridium(III) (Ir(ppy)2(acac)), together with a maximum external quantum efficiency of 18% and a luminous efficacy of 78 lm/W. The device also exhibits a very low efficiency roll-off at high luminance. Further, by directly adopting CN6-CP-doped TCTA as the injection/transport layer, the driving voltage drops to 2.78 V at 1000 cd/m2 and 2.93 V at 10 mA/cm2. Moreover, conductivity and absorption measurements suggest that CN6-CP could also dope CBP with an IE as high as 6.05 eV. The results show that CN6-CP is an excellent p-type dopant for efficient OLEDs and possesses great potential for future application in organic optoelectronic devices.

The functionality of organic semiconductor devices crucially depends on molecular energies, namely the ionisation energy and the electron affinity. Ionisation energy and electron affinity values of thin films are, however, sensitive to film morphology and composition, making their prediction challenging. In a combined experimental and simulation study on zinc-phthalocyanine and its fluorinated derivatives, we show that changes in ionisation energy as a function of molecular orientation in neat films or mixing ratio in blends are proportional to the molecular quadrupole component along the π-π-stacking direction. We apply these findings to organic solar cells and demonstrate how the electrostatic interactions can be tuned to optimise the energy of the charge-transfer state at the donor−acceptor interface and the dissociation barrier for free charge carrier generation. The confirmation of the correlation between interfacial energies and quadrupole moments for other materials indicates its relevance for small molecules and polymers.

In modern electronics, it is essential to create almost arbitrary band structures by adjusting the energy bands and the band gap. Until now, band structure engineering in organic semiconductors has not been possible, since they usually exhibit localized electronic states instead of energy bands. In a recent publication [1], we showed that it is possible to continuously shift the ionization energy (IE) of organic semiconductors over a wide range by mixing them with halogenated derivatives. This tuning mechanism is based on interactions of excess charges with the mean quadrupole field in the thin film. In this work, we raise the question whether the band structure engineering concept can be generalized to other organic semiconductor materials and even be used to tune the size of the band gap. As a model system we study oligothiophenes and in particular we address questions not only about the energy landscape, but also about the micro-structure and the molecular mixing in the film. For this purpose, we analyze optical measurements as well as photoelectron spectroscopy measurements of single and blended layers. Reference: [1] M. Schwarze et al., Science 352, 1446 (2016)

Increasing the amount of charge carriers by molecular doping is important to improve the function of several organic electronic devices. In this work, we use highly fluorinated fullerene (C60F48) to p-type dope common amorphous molecular host materials. We observe a general relation between the material's electrical conductivity and Seebeck coefficient, both strongly depending on the energy level offset between the amorphous host and the dopant. This suggests that the doping efficiency at similar doping levels is mainly determined by the electron transfer yield from host to dopant. Indeed, the dopant anion and host cation absorption strength correlate with the ionization energy (IE) of the host material. Host materials with an IE significantly below the electron affinity of the dopant yield the highest doping efficiency. Surprisingly, the doping efficiency reduces only by about 1 order of magnitude when the IE of the host material is increased by 0.55 eV, which we attribute to the disordered nature of the host materials.

Optical sensing based on plasmonic nanorod arrays witnesses an increased interest for device applications due to a manifold of benefits of these structures, such as broadband optical tunability and low-cost bottom-up fabrication. Key to this success is the assembly of nanorod antenna arrays mediated through high-quality anodized aluminum oxide (AAO) matrices. The present work reports the greatly improved fabrication of thin-film AAO matrices based on aluminum thin-films deposited by magnetron sputtering under controlled argon/oxygen atmosphere, and investigates the influence of oxygen on the aluminum morphology and hence AAO pore formation upon anodization. Such optimized templates then subsequently allow the fast and reproducible fabrication of nanorod arrays with a nearly 100% pore-filling degree, which is favorable in plasmonic applications, with the plasmonic properties greatly benefiting from homogeneous distances between neighboring nanorods. For an optimal oxygen content of 10–22 at.%, we find the long-axis plasmon resonance peak to show the least optical losses, confirming the optimized nanostructure fabrication and performance.

A key breakthrough in modern electronics was the introduction of band structure engineering, the design of almost arbitrary electronic potential structures by alloying different semiconductors to continuously tune the band gap and band-edge energies. Implementation of this approach in organic semiconductors has been hindered by strong localization of the electronic states in these materials. We show that the influence of so far largely ignored long-range Coulomb interactions provides a workaround. Photoelectron spectroscopy confirms that the ionization energies of crystalline organic semiconductors can be continuously tuned over a wide range by blending them with their halogenated derivatives. Correspondingly, the photovoltaic gap and open-circuit voltage of organic solar cells can be continuously tuned by the blending ratio of these donors.