Session A25: Focus Session: Organic Electronics and Photonics - Electronic Processes at Interfaces

Interface Energetics and Chemical Doping of Organic Electronic Materials

The energetics of organic semiconductors and their interfaces are central to the performance of organic thin film devices. The relative positions of charge transport states across the many interfaces of multi-layer OLEDs, OPV cells and OFETs determine in great part the efficiency and lifetime of these devices. New experiments are presented here, that look in detail at the position of these transport states and associated gap states and electronic traps that tail into the energy gap of organic molecular (e.g. pentacene) or polymer (P3HT, PBDTTT-C) semiconductors, and which directly affect carrier mobility in these materials. Disorder, sometime caused by simple exposure to an inert gas, impurities and defects are at the origin of these electronic gap states. Recent efforts in chemical doping in organic semiconductors aimed at mitigating the impact of electronic gap states are described. An overview of the reducing or oxidizing power of several n- and p-type dopants for vacuum- or solution-processed films, and their effect on the electronic structure and conductivity of both vacuum- and solution-processed organic semiconductor films is given. Finally, the filling (compensation) of active gap states via doping is investigated on the electron-transport materials C$_{\mathrm{60}}$ and P(NDI$_{\mathrm{2}}$OD-T$_{\mathrm{2}})$, and the hole-transport polymer PBDTTT-C.   

Efficient Density Functional Approximation for Electronic Properties of Conjugated Systems

There is on-going discussion about reliable prediction of electronic properties of conjugated oligomers and polymers, such as ionization potential IP and energy gap. Several exchange-correlation (XC) functionals are being used by the density functional theory community, with different success for different properties. In this work we follow a recent proposal [1]: a fraction $\alpha$ of exact exchange is added to the semi-local PBE XC [2] aiming consistency, for a given property, with the results obtained by many-body perturbation theory within the G0W0 approximation. We focus the IP, taken as the negative of the highest occupied molecular orbital energy. We choose $\alpha$ from a study of the prototype family trans-acetylene, and apply this same $\alpha$ to a set of oligomers for which there is experimental data available (acenes, phenylenes and others). Our results indicate we can have excellent estimates, within 0,2eV mean ave. dev. from the experimental values, better than through complete $E_{N-1}-E_N$ calculations from the starting PBE functional. We also obtain good estimates for the electrical gap and orbital energies close to the band edge.\\[4pt] [1] V. Atalla, M. Yoon, F. Caruso, P. Rinke, M. Scheffler PRB 88, 165122 (2013).\\[0pt] [2] J.P. Perdew, K. Burke, M. Ernzerhof PRL 77, 3865 (1996).   

Surface-enhanced Raman spectroscopic studies of the Au-pentacene interface: a combined experimental and theoretical investigation

A large enhancement in the Raman intensity due to surface-enhanced Raman scattering (SERS) is observed from pentacene when probed through the Au contact in organic field-effect transistor (OFET) structures. The SERS spectrum is shown to exhibit a high sensitivity to disorder introduced in the pentacene film by Au atoms. The Raman signature of the metal-semiconductor interface in pentacene OFETs is calculated within density-functional theory by explicitly considering the Au-pentacene interaction. The observed enhancement in the 1380 cm$^{-1}$ and the 1560 cm $^{-1}$ regions of the experimental Raman spectrum of pentacene is successfully modeled by Au-pentacene complexes, giving insights into the nature of disorder in the pentacene sp$^2$ network. Raman maps across the pentacene-Au interface provide a powerful visualization tool for correlating the device performance, namely changes in the threshold voltages upon bias stress, to structural changes of the molecule. Unlike high-operating voltage OFETs, low-operating voltage OFETs show no change in the SERS spectra before and after the application of a bias stress, concurrent with no degradation in their threshold voltage.   

Subphthalocyanine on C$_{70}$ Contact Layer Structure and Properties from First Principles Calculations

Boron subphthalocyanine (SubPc) is a promising donor material for organic photovoltaics, having one of the highest reported open circuit voltages among bilayer OPVs when coupled with C$_{60}$. Recently, C$_{70}$ has attracted attention as a substitute for C$_{60}$, largely due to a broader optical absorption spectrum, which leads to a higher current at relatively high voltages. The structure and electronic properties of SubPc derivatives on C$_{70}$-fullerene were explored using density functional theory (DFT) calculations with added Van der Waals interactions. Total-energy calculations were used to elucidate the initial adsorption derivatives on low index surfaces of C$_{70}$. The dependence of the electronic and optical excitations on the interface morphology is studied within the Green's-function GW and Bethe-Salpeter approaches. Insights gained from these calculations, and how they can be used to improve device efficiency, are discussed.   

The formation of conductive polymer chains from Biphenyl-4,4'-dithiol (BPDT) molecules on rough Ag surfaces

Single-molecular electronics, which exploits novel physical and chemical properties of organic molecules, has attracted much attention in the last decade. Many experimental and theoretical efforts have been made in manipulating high-quality organic polymers, understanding electron transport properties and developing electronic devices. Our experiments show that self-assembled monolayer (SAM) of Biphenyl-4,4'-dithiol (BPDT) can readily form on roughened surfaces of elemental silver, instead of a flat surface. To understand ``why so,'' we performed systematic density functional studies on the structural, energetic and electronic characteristics of both isolated BPDT molecules and BPDT on Ag(111) surfaces, with the inclusion of van der Waals correction in DFT. The formation of S-Ag-S linkage makes the molecule chain metallic, different from the insulating feature of S-S linkage. The adsorption of BDPT on roughened Ag surface is energetically preferred compared to that on flat surface. Moreover, the Ag adatom makes BPDT molecules attractive to each other on Ag(111) surface, crucial for the formation of polymer chains. Our joint theoretical and experimental results indicate the feasibility of fabricating conductive organo-silver polymer sheets.   

Current focussing in organic semiconductors due to high local field, inhomogeneous trap distributions, and fibrous morphologies

Charge transport in organic devices is a key factor controlling device performance and as a means for characterizing devices. We have developed a fully three dimensional device simulation tool enabling treatment of inhomogeneous systems including c-AFM tip geometry, spatially varying trap distributions, and fibrous morphologies. The model and simulation procedures will be described and current focussing in three cases will be presented (i) high voltage at a c-AFM tip, (ii) inhomogeneous trap distributions and (iii) fibrous morphologies. Inhomogeneous trap distributions contribute to current focusing in both device and tip geometries and in both cases transport preferentially follow low trap pathways. In fibrous systems where the fibers have a low trap density, current flow concentrates on pathways where the low trap fibers occupy a higher fraction of the total path length.   

Graphene-Based Polymer Bulk Heterojunction Solar Cells

We propose and demonstrate BHJs that utilize pristine graphene in order to facilitate exciton dissociation and charge transfer in polymeric solar cells. Devices based on P3HT:PCBM:graphene were fabricated on patterned ITO glass, and the effect of graphene on performance was investigated. Various device parameters including short-circuit current density, open-circuit voltage, fill factor, power conversion efficiency, and external quantum efficiency are compared with traditional BHJs. Results are discussed in the context of the morphology of the active layer, and the distribution and orientation of graphene platelets, as characterized by GIXRD, and neutron reflectometry.   

Ferromagnetic-organic interfacial states detected by transient conductivity and their role on low voltage current injection in organic spinvalves

Recently, there has been an increasing interest in utilising organic materials as spin transport layers as they have long spin-coherence times due to low spin-orbit and hyperfine coupling present in these materials [1]. Whilst there has been considerable research into organic spinvalves, there is a fundamental unsolved problem of how spin injection occurs. All organic spinvalves have been found to operate best at very low voltages, in the order of millivolts, where there should be no carrier injection. In this work we investigate the role of hybrid interface states (HINTS) between a ferromagnetic contact (FM) and an organic semiconductor (OSC). Using transient conductivity measurements on a variety of devices, the presence of these HINTS in a real device but only in the presence of a FM contact. We then consider the consequences that these filled HINTS will have on the electrical properties of devices. We argue that the filling of these HINTS introduces a large electric field at the FM-OSC interface, which causes an effect analogous to ``band-bending'' in conventional semiconductors. This explains the Ohmic injection seen in organic spinvalves which results in hole injection even at low (mV) applied voltages. \\[4pt] [1] Dediu, V. A., Nat. Mat. 8, 707-716 (2009).   

Controlling organic magnetoresistance via interface engineering

We present the results of experiments in which we manipulate organic magnetoresistance (OMAR) in devices based on Alq3 (tris-(8-hydroxyquinoline) aluminum) and TPD (N,N$\prime $-Bis(3-methylphenyl)-N,N$\prime $-diphenylbenzidine) by adding a self-assembled monolayer (SAM). The results of OMAR measurements on this OLED-like architecture are correlated with impedance spectroscopy results to elucidate charge carrier transport and accumulation. We observe competing OMAR mechanisms in these devices, the relative strength of which can be tuned by adding SAMs at electrode interfaces. To determine how the interfacial and structural properties of these organic devices effect the OMAR, we obtained a complete picture of the interfacial, topological, and crystalline properties of these devices by performing UPS (Ultraviolet Photoelectron Spectroscopy), XPS (X-ray PS), XRD (X-ray diffraction), and AFM (atomic force microscopy). To verify our understanding of how interfacial changes affect OMAR, we characterized simple Alq3-only devices: one with a SAM and one without it. Despite having the same current density at room temperature, the latter shows a negative MR while the former displays a positive MR.   

Influence of Interactions between Excited States on Magnetic Field Effects in Organic Semiconducting Materials

The magnetic field effects in organic semiconducting materials are essentially determined by spin-exchange interaction and hyperfine interaction within individual intermolecular excited states. Intermolecular excited states can inevitably experience interactions between them due to their spatially extended wavefunctions. This interaction can be involved in the development of magnetic field effects, but this important issue has not been discussed. We study the influence of interactions between intermolecular excited states on magnetic field effects by using magneto-photoluminescence based on well-controlled organic composite containing N,N-dimethylaniline and pyrene in liquid state. We find that the interactions between intermolecular excited states can cause a line-shape narrowing in magneto-photoluminescence. The line-shape narrowing indicates that the interactions between the intermolecular excited states can decrease the force-constant of magnetic field-dependent singlet-triplet intersystem crossing within individual intermolecular excited states. Our studies show that the interactions between the excited states can occur through three different regimes, namely long-range Coulomb interaction, mid-range spin-orbital interaction, and short-range spin interaction, and consequently influence the spin-conserving and spin-dephasing processes within individual intermolecular excited states in the development of magnetic field effects in organic semiconducting materials.   

Engineering hybrid polymer/metal-oxide interfaces by self-assembled molecular interlayers

Hybrid organic heterojunctions are of great technological interest as both optically active layers as well as hole blocking interfaces in organic or hybrid solar cells. Despite the potential of combining processable organic polymers with inorganic components, they have not yet demonstrated high efficiencies. promising approach towards more efficient systems consists in engineering the interface by self-assembled molecular interlayers that can selectively affect the interactions of the donor and acceptor components. a combination of molecular dynamics and electronic structure calculations [1] we study thermodynamic and optoelectronic properties of polymer/metaloxide interfaces in presence of several molecular interlayers such as metal-organic macrocyclic complexes [2,4] or pyridine derivatives [1]. The theoretical results are tested on specifically designed hybrid solar cells providing evidence of impressive enhancement of interface properties.\\[4pt] [1] M. I. Saba, et al. J. Phys. Chem. C 115, 9651--9655 (2011).\\[0pt] [2] C. Melis et al. ACS Nano 5 9639 (2011).\\[0pt] [3] E. Canesi et al.,Energy Environ. Sci. 5 9068 (2012).\\[0pt] [4] G. Mattioli et al. Submitted (2013)/   

Molecular Ordering of Poly(3-hexylthiophene) on Self-Assembled Monolayers

The molecular ordering of semiconducting polymers such as Poly(3-hexylthiophene) (P3HT) at surfaces and interfaces has significant influence on the performance of organic solar cell devices. The charge-carrier transport and the charge collection at the electrodes strongly depend on the molecular ordering of P3HT at interfaces. Molecular ordering of P3HT can be tuned by varying the substrate surface chemistry and film processing conditions. Using all-atom molecular dynamics simulations and validated force field parameters, we have investigated the molecular ordering of P3HT on self-assembled monolayers (SAMs) of n-alkanethiols with varying end-functional groups and spacer length. In this study we elucidate the dependence of the molecular ordering of P3HT (edge-on or face-on conformation) on the surface chemistry of SAMs. Moreover, we investigated the effect of solvent on the molecular ordering of P3HT on SAMs surfaces. Understanding the correlation between P3HT morphology and surface chemistry will help in designing P3HT-based solar devices with better efficiency.   

Charge injection across a metal-organic interface suppressed by thermal diffusion

Considerable progress has been made in developing metallophthalocyanine devices, although details of the underlying mechanisms in electrical transport are not fully understood. More importantly, few studies have explored their performance at realistic working temperatures, well above room temperature. In this work we explore the performance of Co-phthalocyanine (CoPc) vertical capacitive devices up to 460K. We find that the ohmic conductance is irreversibly suppressed by orders of magnitude when the devices are heated above 340 K. Detailed structural and transport studies imply that the changes in the conductance are due to diffusion of the top Pd electrode into the CoPc layer. This leads to a decrease in Pd electrode work function, which increases the potential barrier for hole injection. These results have a direct impact on technological applications since the instabilities of metallic-organic capacitive devices occur at operational temperatures typical for electronic (350K to 400K).