In organic thin‐film transistors (TFTs) fabricated in the inverted (bottom‐gate) device structure, the surface roughness of the gate dielectric onto which the organic‐semiconductor layer is deposited is expected to have a significant effect on the TFT characteristics. To quantitatively evaluate this effect, a method to tune the surface roughness of a gate dielectric consisting of a thin layer of aluminum oxide and an alkylphosphonic acid self‐assembled monolayer over a wide range by controlling a single process parameter, namely the substrate temperature during the deposition of the aluminum gate electrodes, is developed. All other process parameters remain constant in the experiments, so that any differences observed in the TFT performance can be confidently ascribed to effects related to the difference in the gate‐dielectric surface roughness. It is found that an increase in surface roughness leads to a significant decrease in the effective charge‐carrier mobility and an increase in the subthreshold swing. It is shown that a larger gate‐dielectric surface roughness leads to a larger density of grain boundaries in the semiconductor layer, which in turn produces a larger density of localized trap states in the semiconductor.

Molecular emitters that combine circularly polarized luminescence (CPL) and high radiative rate constants of the triplet exciton decay are highly attractive for electroluminescent devices (OLEDs) or next-generation photonic applications, such as spintronics, quantum computing, cryptography, or sensors. However, the design of such emitters is a major challenge because the criteria for enhancing these two properties are mutually exclusive. In this contribution, we show that enantiomerically pure {Cu­(CbzR)­[( S/R )-BINAP]} [R = H (1), 3,6-tBu (2)] are efficient thermally activated delayed fluorescence (TADF) emitters with high radiative rate constants of k TADF up to 3.1 × 105 s–1 from 1/3LLCT states according to our temperature-dependent time-resolved luminescence studies. The efficiency of the TADF process and emission wavelengths are highly sensitive to environmental hydrogen bonding of the ligands, which can be disrupted by grinding of the crystalline materials. The origin of this pronounced mechano-stimulus photophysical behavior is a thermal equilibrium between the 1/3LLCT states and a 3LC state of the BINAP ligand, which depends on the relative energetic order of the excited states and is prone to inter-ligand C–H···π interactions. The copper­(I) complexes are also efficient CPL emitters displaying exceptional dissymmetry values g lum of up to ±0.6 × 10–2 in THF solution and ±2.1 × 10–2 in the solid state. Importantly for application in electroluminescence devices, the C–H···π interactions can also be disrupted by employing sterically bulky matrices. Accordingly, we have investigated various matrix materials for successful implementation of the chiral copper­(I) TADF emitters in proof-of-concept CP-OLEDs.

We analyze the impact of the first order structural phase transition between the α- and β-phases in crystalline zinc-phthalocyanine thin films on their luminescent behavior. Upon optical excitation, two intensity maxima governing the dual fluorescence can be attributed to thin film excimer emission of the α-phase at 930 nm and bulk Frenkel exciton emission of the corresponding β-phase at 780 nm. Comprehensive temperature and time dependent studies on the structural phase transition reveal a controllable shift of spectral weight between these two maxima. Applying this approach to thin film devices, we demonstrate the preparation of dual luminescent organic light emitting diodes with tunable emission characteristics in the near infrared, which are of interest as electro-optical units in future hybrid photonic devices.

Determining noninvasively the microscopic optoelectronic parameters of molecular assemblies would constitute an important achievement in material as well as in life sciences. In this contribution, we cope with this challenge by utilizing fluorescent tetraphenyl­dibenzo­periflanthene (DBP) molecules as optically addressable sensors deterministically positioned in N,N′-di­(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine/tris­(8-hydroxy­quinolato)­aluminum (NPB/Alq3) organic light-emitting diodes (OLEDs) as a model system. Measuring in operando the variation of the fluorescence intensity allows for direct correlation with the respective charge carrier distribution and transport processes at spatial resolutions below 15 nm as confirmed by complementary drift-diffusion simulations. Moreover, the molecular sensing technique renders sensitive enough to detect the presence of mobile charge carriers already below the built-in voltage. Operating the OLEDs under reverse bias, the sensing molecules provide information about the internal interface polarization caused by the alignment of the dipolar Alq3 entities during sample preparation. Benchmarking the macroscopic device behavior by means of electrical impedance measurements confirms the accuracy and reliability of the microscopic data obtained by the molecular sensing approach. Furthermore, the comparison demonstrates the dynamic range of the optical sensors being susceptible to variations in the local charge carrier distribution over several orders of magnitude and to interfacial polarization occurring at molecular heterojunctions within the OLED matrix. By its universality the presented sensing concept is applicable to a variety of molecular systems and, thus, being of relevance not only for OLEDs but also for other organic thin film devices like transistors.

Molecular emitters that combine circularly polarized luminescence (CPL) and high radiative rate constants of the triplet exci-ton decay are highly attractive for electroluminescent devices (OLEDs) or next generation photonic applications, such as spintronics, quantum computing, cryptography or sensors. However, the design of such emitters is a major challenge because the criteria for enhancing these two properties are mutually exclusive. In this contribution, we show that enantiomerically pure [Cu(CbzR)((S/R)-BINAP)] (R = H (1), 3,6-tBu (2)) are efficient TADF emitters with high radiative rate constants of kTADF up to 3.1·105 s-1, and exceptional dissymmetry values of the emission glum of ±0.7·10-2 in THF solution and ±2.3·10-2 in the solid state are observed. Importantly for application in electroluminescence devices, the efficiency of the TADF pro-cess and emission wavelengths are highly sensitive to environmental hydrogen bonding of the ligands, which can be disrupt-ed either by grinding of the crystalline materials or by employing sterically bulky matrices. Accordingly, we have investigat-ed various matrix materials for successful implementation of the chiral copper(I) TADF emitters in proof-of-concept CP-OLEDs.