Small contact resistance and high-frequency operation of flexible low-voltage inverted coplanar organic transistors.

The contact resistance in organic thin-film transistors (TFTs) is the limiting factor in the development of high-frequency organic TFTs. In devices fabricated in the inverted (bottom-gate) device architecture, staggered (top-contact) organic TFTs have usually shown or are predicted to show lower contact resistance than coplanar (bottom-contact) organic TFTs. However, through comparison of organic TFTs with different gate-dielectric thicknesses based on the small-molecule organic semiconductor 2,9-diphenyl-dinaphtho[2,3-b:2',3'-f]thieno[3,2-b]thiophene, we show the potential for bottom-contact TFTs to have lower contact resistance than top-contact TFTs, provided the gate dielectric is sufficiently thin and an interface layer such as pentafluorobenzenethiol is used to treat the surface of the source and drain contacts. We demonstrate bottom-contact TFTs fabricated on flexible plastic substrates with record-low contact resistance (29 Ωcm), record subthreshold swing (62 mV/decade), and signal-propagation delays in 11-stage unipolar ring oscillators as short as 138 ns per stage, all at operating voltages of about 3 V.

Tunneling Probability Increases with Distance in Junctions Comprising Self-Assembled Monolayers of Oligothiophenes.

Molecular tunneling junctions should enable the tailoring of charge-transport at the quantum level through synthetic chemistry but are hindered by the dominance of the electrodes. We show that the frontier orbitals of molecules can be decoupled from the electrodes, preserving their relative energies in self-assembled monolayers even when a top-contact is applied. This decoupling leads to the remarkable observation of tunneling probabilities that increase with distance in a series of oligothiophenes, which we explain using a two-barrier tunneling model. This model is generalizable to any conjugated oligomers for which the frontier orbital gap can be determined and predicts that the molecular orbitals that dominate tunneling charge-transport can be positioned via molecular design rather than by domination of Fermi-level pinning arising from strong hybridization. The ability to preserve the electronic structure of molecules in tunneling junctions facilitates the application of well-established synthetic design rules to tailor the properties of molecular-electronic devices.

Area dependent behavior of bathocuproine (BCP) as cathode interfacial layers in organic photovoltaic cells.

Standard and inverted configuration small molecule OPV cells incorporating bathocuproine (BCP) as electron transport and exciton blocking layer is investigated, demonstrating that 2 mm2 standard and inverted cells display a maximum performance for BCP thicknesses of 10 nm and 1.5 nm, respectively. The reason for the different optimum BCP thicknesses for the two device configurations is the BCP-metal complex formed between the Ag electrode and the BCP layer in the standard configuration OPV devices. Interestingly, at optimum BCP thicknesses, the inverted OPV cells outperform the standard devices. Upon up-scaling of the device area of the cells from 2 mm2 to 10 and 100 mm2, device failure becomes prominent for the inverted OPV cells, due to aggregation of the evaporated BCP layer on the ITO surface. This demonstrates that although BCP can be adopted for efficient ETL in inverted configuration OPV devices on small scale, it is not suitable for device up-scaling due to severely decreasing device yields. In this work, a possible solution where an ultrathin layer of C70 is evaporated between the ITO and BCP layer is proposed. It is demonstrated that the proposed solution holds a strong potential to minimize the device failures of the BCP based inverted OPV cells to a significant extent, while maintaining good device performances.

Publisher Correction: Elementary steps in electrical doping of organic semiconductors.

The original version of this Article contained an error in Equation 1. A factor of 'c' was included in the right-hand term. This has been corrected in the PDF and HTML versions of the Article.

Elementary steps in electrical doping of organic semiconductors.

Fermi level control by doping is established since decades in inorganic semiconductors and has been successfully introduced in organic semiconductors. Despite its commercial success in the multi-billion OLED display business, molecular doping is little understood, with its elementary steps controversially discussed and mostly-empirical-materials design. Particularly puzzling is the efficient carrier release, despite a presumably large Coulomb barrier. Here we quantitatively investigate doping as a two-step process, involving single-electron transfer from donor to acceptor molecules and subsequent dissociation of the ground-state integer-charge transfer complex (ICTC). We show that carrier release by ICTC dissociation has an activation energy of only a few tens of meV, despite a Coulomb binding of several 100 meV. We resolve this discrepancy by taking energetic disorder into account. The overall doping process is explained by an extended semiconductor model in which occupation of ICTCs causes the classically known reserve regime at device-relevant doping concentrations.

Switching from weakly to strongly limited injection in self-aligned, nano-patterned organic transistors.

Organic thin-film transistors for high frequency applications require large transconductances in combination with minimal parasitic capacitances. Techniques aiming at eliminating parasitic capacitances are prone to produce a mismatch between electrodes, in particular gaps between the gate and the interlayer electrodes. While such mismatches are typically undesirable, we demonstrate that, in fact, device structures with a small single-sided interlayer electrode gap directly probe the detrimental contact resistance arising from the presence of an injection barrier. By employing a self-alignment nanoimprint lithography technique, asymmetric coplanar organic transistors with an intentional gap of varying size (< 0.2 µm) between gate and one interlayer electrode are fabricated. An electrode overlap exceeding 1 µm with the other interlayer has been kept. Gaps, be them source or drain-sided, do not preclude transistor operation. The operation of the device with a source-gate gap reveals a current reduction up to two orders of magnitude compared to a source-sided overlap. Drift-diffusion based simulations reveal that this marked reduction is a consequence of a weakened gate-induced field at the contact which strongly inhibits injection.

Role of the Charge-Transfer State in Reduced Langevin Recombination in Organic Solar Cells: A Theoretical Study.

Reduced Langevin recombination has been observed in organic solar cells (OSCs) for many years, but its origin is still unclear. A recent work by Burke et al. (Adv. Energy Mater.2015, 5, 1500123-1) was inspired by this reduced Langevin recombination, and they proposed an equilibrium model of charge-transfer (CT) states that correlates the open-circuit voltage of OSCs with experimentally available device parameters. In this work, we extend Burke et al.'s CT model further and for the first time directly correlate the reduced Langevin recombination with the energetic and dynamic behavior of the CT state. Recombination through CT states leads in a straightforward manner to a decrease in the Langevin reduction factor with increasing temperature, without explicit consideration of the temperature dependence of the mobility. To verify the correlation between the CT states and reduced Langevin recombination, we incorporated this CT model and the reduced Langevin model into drift-diffusion simulations of a bilayer OSC. The simulations not only successfully reproduced realistic current-voltage (J-V) characteristics of the bilayer OSC, but also demonstrate that the two models consistently lead to same value of the apparent Langevin reduction factor.

Influence of morphology and polymer:nanoparticle ratio on device performance of hybrid solar cells-an approach in experiment and simulation.

We present a thorough study on the various impacts of polymer:nanoparticle ratios on morphology, charge generation and device performance in hybrid solar cells, comprising active layers consisting of a conjugated polymer and in situ prepared copper indium sulfide (CIS) nanoparticles. We conducted morphological studies through transmission electron microscopy and transient absorption measurements to study charge generation in absorber layers with polymer:nanoparticle weight ratios ranging from 1:3 to 1:15. These data are correlated to the characteristic parameters of the prepared solar cells. To gain a deeper understanding of our experimental findings, three-dimensional drift-diffusion-based simulations were performed. Based on elaborate descriptions of the contributions of polymer and nanoparticle phase to device performances, our results suggest that a polymer:CIS volume ratio of 1:2 (weight ratio 1:9) is necessary to obtain a balanced hole and electron percolation. Also at higher CIS loadings the photocurrent remains surprisingly high due to the contribution of the CIS phase to the charge carrier generation.

Heteroleptic platinum(II) complexes of 8-quinolinolates bearing electron withdrawing groups in 5-position.

A series of novel luminescent platinum(II) complexes bearing orthometalated 2-phenylpyridine ligands (C N), namely 2-phenylpyridine (4) and 3-hexyloxy-2-phenylpyridine (5), and several 5-substituted quinolinolate ligands (5-X-Q), where X = NO2 (a), X = CHO (b), X = Cl (bearing another Cl in 7-position of the Q-ligand) (c) and X = H (d) have been synthesized, characterized and their photophysical properties were studied. All complexes were obtained as a single isomer with N atoms of the C N and Q ligands trans-coordinated to the platinum center as evidenced using single-crystal X-ray crystallography and NMR spectroscopy. Absorbance, luminescence as well as lifetime measurements in solution and in the solid state have been performed to establish a qualitative relationship between structure and luminescence properties. The compounds under investigation absorb intensively via an intraligand charge transfer (ILCT) in the visible range (460-480 nm) and emit from fluid solution and in the solid state at room temperature at 600-630 nm. The complexes show quantum yields up to 25% and lifetimes in the range of 20-30 micros in deoxygenated organic solvents at room temperature. The emitting state can be best described as a triplet intraligand charge-transfer state localized mainly on the quinolinolate ligand. In these complexes the phenylpyridine ligand can be essentially regarded as an ancillary ligand. Density functional theory (DFT) calculations were carried out on both the ground (singlet) and excited (triplet) states of these complexes and revealed the influence of the substitution of the quinolinolate ligand on the HOMO/LUMO energies and the oscillator strengths. Substitution on 3-position of the phenylpyridine ligand does not impact on the transition energies, and is thus suited to introduce other functional moieties, such as a solubilizing hexyloxy group.