Functionalized tetrapodal diazatriptycenes for electrostatic dipole engineering in n-type organic thin film transistors

V.R., F.S.B., S.H., M.M., M.-M.B., S.H., J.F., W.K., W.J., A.K., A.P., U.H.F.B., and K.M. acknowledge the German Federal Ministry of Education and Research (BMBF) for financial support within the INTERPHASE project (nos. 13N13656, 13N13657, 13N13658, 13N13659). V.R. thanks the German Research Foundation for financial support within the SFB1249 project and the Heidelberg Graduate School of Fundamental research.The authors also appreciate financial support by the German Research Foundation (grant ZH 63/39-1) and by the DAAD-ACEH Scholarship of Excellence (A.A.).A diazatriptycene‐based tetrapodal scaffold with thiol anchors enforces a nearly upright orientation of functional groups, introduced to its quinoxaline subunit, with respect to the substrate upon formation of self‐assembled monolayers (SAMs). Substitution with electron‐withdrawing fluorine and cyano as well as electron‐rich dimethylamino substituents allows tuning of the molecular dipole and, consequently, of the work function of gold over a range of 1.0 eV (from 3.9 to 4.9 eV). The properties of the SAMs are comprehensively investigated by infrared reflection absorption spectroscopy, near edge X‐ray absorption fine structure spectroscopy, and X‐ray photoelectron spectroscopy. As prototypical examples for the high potential of the presented SAMs in devices, organic thin‐film transistors are fabricated.Publisher PDFPeer reviewe

n-type doping of organic semiconductors : immobilization via covalent anchoring

We gratefully acknowledge the German Federal Ministry of Education and Research (BMBF) for financial support within the InterPhase project (FKZ 13N13659, 13N13656, 13N13657, and 13N13658).Electrical doping is an important tool in the design of organic devices to modify charge carrier concentration in and Fermi level position of organic layers. The undesired diffusion of dopant molecules within common transport materials adversely affects both lifetime and device performance. To overcome this drawback, we developed a strategy to achieve immobilization of dopants through their covalent attachment to the semiconductor host molecules. Derivatization of the commonly employed n-type dopant 2-(2-methoxyphenyl)-1,3-dimethyl-2,3-dihydro-1H-benzoimidazole (ο-MeO-DMBI) with a phenylazide enables the resulting o-AzBnO-DMBI to photochemically generate a reactive nitrene, which subsequently binds covalently to the host material, 6,6-phenyl-C61-butyric acid methyl ester (PCBM). Both the activation and addition reactions are monitored by mass spectrometry as well as optical and photoelectron spectroscopy. A suppression of desorption and a decrease in volatility of the DMBI derivative in ultrahigh vacuum were observed after activation of a bilayer structure of PCBM and o-AzBnO-DMBI. Electrical measurements demonstrate that the immobilized o-AzBnO-DMBI can (i) dope the PCBM at conductivities comparable to values reported for o-MeO-DMBI in the literature and (ii) yield improved electrical stability measured in a lateral two terminal device geometry. Our immobilization strategy is not limited to the specific system presented herein but should also be applicable to other organic semiconductor–dopant combinations.Publisher PDFPeer reviewe

Crosslinking Super Yellow to produce super OLEDs Crosslinking with azides enables improved performance

An increasing number of organic light-emitting diodes (OLEDs) is nowadays based on the use of polymers as the emissive material. For this material class in particular, solution-processing of the OLEDs has gained traction in both research and industry. However, in order to access multilayer material systems, orthogonal solvents must be used to prevent dissolution of previously prepared layers. The use of crosslinkers can facilitate this production method by reducing the number of orthogonal solvents needed since insoluble networks are generated. In this work, a novel bisazide crosslinker is employed to insolubilize Super Yellow, a polyphenylene-vinylene emitter. This allows the use of an additional poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine electron blocking layer (EBL) from the same solvent. Devices including the blocking layer show improved efficacies compared to reference devices without the additional EBL, while also maintaining the emission spectrum. Using the upscalable technique of doctor blading, OLEDs were fabricated which showed a particularly noticeable effect of the blocking layer with a nearly twofold increase in luminance and a 56% increase in current efficacy.Peer Reviewe

Structure–Property Relationship of Phenylene-Based Self-Assembled Monolayers for Record Low Work Function of Indium Tin Oxide

Studying the structure–property relations of tailored dipolar phenyl and biphenylphosphonic acids, we report self-assembled monolayers with a significant decrease in the work function (WF) of indium–tin oxide (ITO) electrodes. Whereas the strengths of the dipoles are varied through the different molecular lengths and the introduction of electron-withdrawing fluorine atoms, the surface energy is kept constant through the electron-donating <i>N</i>,<i>N-</i>dimethylamine head groups. The self-assembled monolayer formation and its modification of the electrodes are investigated via infrared reflection absorption spectroscopy, contact angle measurements, and photoelectron spectroscopy. The WF decrease in ITO correlates with increasing molecular dipoles. The lowest ever recorded WF of 3.7 eV is achieved with the fluorinated biphenylphosphonic acid

Compensation of Oxygen Doping in p‐Type Organic Field‐Effect Transistors Utilizing Immobilized n‐Dopants

Poly(3‐hexyl‐thiophene‐2,5‐diyl) (P3HT) is one of the most commonly used materials in organic electronics, yet it is considered to be rather unattractive for organic field‐effect transistors (OFETs) due to its tendency to oxidize under aerobic conditions. Strong p‐doping of P3HT by oxygen causes high off‐currents in such devices opposing the desired high on/off‐ratios. Herein, a new application‐oriented method involving the recently developed immobilizable organic n‐dopant 2‐(2‐((4‐azidobenzyl)oxy)phenyl)‐1,3‐dimethyl‐2,3‐dihydro‐1H‐benzoimidazol (o‐AzBnO‐DMBI) is presented allowing to process and operate P3HT OFETs in air. The n‐dopants compensate oxygen doping by trapping generated free holes, thereby rediminishing OFET off‐currents by approximately two orders of magnitude. At the same time, field‐effect mobilities remain high in the order of up to 0.19 cm² V⁻¹ s⁻¹. Due to the covalent attachment of the dopants to the host matrix after photochemical activation, a drift of the otherwise mobile ions within the device is prevented even at high operating voltages and, thus, hysteresis in the corresponding transfer characteristics is kept low. In this manner, the air instability of P3HT OFETs is successfully resolved paving an auspicious way toward OFET mass production. As the immobilization process employed here is nonspecific with respect to the host material, this strategy is transferable to other p‐type semiconductors

Tetrapodal Diazatriptycene Enforces Orthogonal Orientation in Self-Assembled Monolayers

none17Conformationally rigid multipodal molecules should control the orientation and packing density of functional head groups upon self-assembly on solid supports. Common tripods frequently fail in this regard because of inhomogeneous bonding configuration and stochastic orientation. These issues are circumvented by a suitable tetrapodal diazatriptycene moiety, bearing four thiol-anchoring groups, as demonstrated in the present study. Such molecules form well-defined self-assembled monolayers (SAMs) on Au(111) substrates, whereby the tetrapodal scaffold enforces a nearly upright orientation of the terminal head group with respect to the substrate, with at least three of the four anchoring groups providing thiolate-like covalent attachment to the surface. Functionalization by condensation chemistry allows a large variety of functional head groups to be introduced to the tetrapod, paving the path toward advanced surface engineering and sensor fabrication.NEXUSnoneBenneckendorf, Frank S; Rohnacher, Valentina; Sauter, Eric; Hillebrandt, Sabina; Münch, Maybritt; Wang, Can; Casalini, Stefano; Ihrig, Katharina; Beck, Sebastian; Jänsch, Daniel; Freudenberg, Jan; Jaegermann, Wolfram; Samorì, Paolo; Pucci, Annemarie; Bunz, Uwe H F; Zharnikov, Michael; Müllen, KlausBenneckendorf, Frank S; Rohnacher, Valentina; Sauter, Eric; Hillebrandt, Sabina; Münch, Maybritt; Wang, Can; Casalini, Stefano; Ihrig, Katharina; Beck, Sebastian; Jänsch, Daniel; Freudenberg, Jan; Jaegermann, Wolfram; Samorì, Paolo; Pucci, Annemarie; Bunz, Uwe H F; Zharnikov, Michael; Müllen, Klau