Session P49: Focus Session: Organic Electronics and Photonics - Interfaces and Contacts

Charge transfer and polarization at interfaces with conjugated molecules

The function and efficiency of organic electronic devices is determined to a significant extent by the electronic properties of organic/organic heterojunctions and interfaces between electrodes and organic semiconductors. The energy level alignment between metal electrodes and active organic layers can be adjusted over wide ranges by employing interlayers of strong molecular acceptors and donors that undergo charge transfer reactions with the metal. It will be shown that such interlayers lead to lower charge injection barriers than pristine metals, even when the work function is the same. It is argued that the molecularly modified electrodes are electronically more rigid than their pristine metal counterparts, i.e., the electron spill-out at the organic-terminated surface is less pronounced compared to metal surfaces. The energy levels at organic/organic heterojunctions comprising donors and acceptors as used in organic photovoltaic cells are essentially independent of deposition sequence, as long as supporting electrodes to not induce energy level pinning. When a high work function electrode is used, the energy levels may become Fermi-level pinned and an electric field drops right at the heterojunction. This effect is exemplified for the donor diindenoperylene and the acceptor C60. The electric field distribution within an organic opto-electronic device may thus be adjusted locally by employing interfacial energy level pinning, even at weakly interacting organic/organic interfaces.   

Tuning Contact Recombination and Open-Circuit Voltage in Polymer Solar Cells via Self-Assembled Monolayer Adsorption

We adsorbed fluoro-alkyl and hydrogenated-alkyl phosphonic acid derivatives onto indium tin oxide (ITO) for forming self-assembled monolayers (FSAMs and HSAMs, respectively) to tune the open-circuit voltage (V$_{oc})$ of polymer solar cells. The adsorption of FSAM and HSAM alters the work function of ITO from 4.3 eV to 5.5 eV and 4.0 eV, respectively, as verified by ultraviolet photoemission spectroscopy. Polymer solar cells having FSAM-, HSAM-treated ITO, and bare ITO as anodes display V$_{oc}$ of 0.58V, 0.48V, and 0.31V, respectively. Yet, the hole injection barrier from the anode to the active layer is the same for all three devices. Inverse photoemission spectroscopy measurements indicate that the energy barrier for minority carrier transport to the anode is largest for solar cells comprising FSAM-treated ITO and lowest for devices with bare ITO as anode. Since a high energy barrier for minority carrier transport results in lower contact recombination at the anode, it is this energy barrier that is responsible for differences in the V$_{oc}$s observed in polymer solar cells having anodes that have been pre-treated with SAMs.   

Electronic Structure of Organic-Metal Interfaces

Organic self-assembled monolayers (SAMs) are important to the understanding of molecular electronics as well as the study of charge transfer in photovoltaic applications. We use two-photon photoemission (2PPE) to investigate the interfacial electronic structure of SAMs on metals. In particular we study the unoccupied states of thiolates and fluorinated thiolates on Cu(111) as a function of molecular coverage using both monochromatic and time-resolved bichromatic 2PPE. While accurate measurements have been made on mostly full monolayer systems, similar effort on detailed coverage-dependence has not yet been reported. We track the formation of the interfacial dipole layer as well as the emergence of intermediate and final states vs. coverage.   

Fermi level pinning by integer charge transfer at electrode-organic semiconductor interfaces

The atomic structure of interfaces between conducting electrodes and molecular organic materials varies considerably. Yet experiments show that pinning of the Fermi level, which is observed at such interfaces, does not depend upon the structural details. In this work [1], we develop a general model to explain Fermi level pinning, and formulate simple expressions for the pinning levels, based upon integer charge transfer between the conductor and the molecular layer. In particular, we show that DFT calculations give good values for the pinning levels. \\[4pt] [1] Appl. Phys. Lett 98, 113303 (2011).   

Analysis of interface states in blend of polythiophene and polyselenophene: experiments and theory

Organic photovoltaic devices are presently the subject of intense research since they could eventually propose solar energy solutions at a much reduced cost compared to inorganic devices. Presently, electron transport in organic photovoltaic devices is achieved with a fullerene derivative (PCBM) but this solution has some disadvantages. First, the ratio of PCBM to polymer has to be quite high to assume good electronic transport, and second, the relatively high cost of PCBM is not ideal with the goal to reduce the cost of the device. For these reasons, a replacement for PCBM is desirable and an all polymer device solution is viewed as the best avenue. Since polythiophene (P3HT) is ideal for hole transport, its isovalent polyselenophene (P3HS) where sulfur atoms are replaced with selenium atoms might offer an interesting alternative to PCBM. The physical processes in organic devices are quite different from those of inorganic devices. Charge separation in organic devices is achieved by forming an interface between two organic materials with type 2 level alignment. However, because of the large binging energies observed in organic compounds, there is often the presence of H-aggregate states at the junction between the two organic materials. In this presentation, we will report the results of interface states in a blend of polythiophene and polyselenophene. Photoluminescence spectra will be presented along with calculations of these states with the help of the time-dependent density-functional theory.   

Controlling Side Chain Density of Electron Donating Polymers for Improving $V_{OC}$ in Polymer Solar Cells

The ability to tune the LUMO/HOMO levels of electroactive materials in active layer of polymer solar cells is critical in controlling their optical and electrochemical properties because the HOMO and LUMO offsets between the polymer donor and the electron acceptor strongly affect charge separation and the open circuit voltage ($V_{OC})$ of a solar cell. Here, we developed two series of electroactive materials for improving $V_{OC}$ in polymer solar cells. First, we enable facile control over the number of solubilizing groups ultimately tethered to the fullerene by tuning the molar ratio between reactants from 1:1 to 1:3, thus producing $o$-xylenyl C$_{60}$ mono-, bis-, and tris-adducts (OXCMA, OXCBA, and OXCTA) as electron acceptors with different LUMO levels. As the number of solubilizing groups increased, $V_{OC}$ values of the P3HT-based BHJ solar cells increased from 0.63, 0.83, to 0.98 V. Second, we present a series of novel poly[3-(4-n-octyl)phenylthiophene] (POPT) derivatives (POPT, POPTT, and POTQT) as electron donors with different side-chain density. As a result of lower HOMO levels by decrease in the side-chain density of the polymers, the devices consisting of POPT, POPTT, and POPQT with PCBM showed increased $V_{OC}$ values of 0.58, 0.63, and 0.75 V, respectively.   

Charge Injection Mechanism at Carbon Nanotube-Organic Semiconductor Interface

One of the major challenges in the fabrication of high performance organic electronic devices is to overcome the inefficient charge injection from metal electrodes into organic semiconductors caused by large interfacial contact barrier. One potential key step of improving the charge injection is to employ single-walled carbon nanotubes as electrodes. Towards this end, we fabricated pentacene field effect transistor using densely aligned carbon nanotube array electrodes with open-ended and parallel tips. The room temperature electronic transport measurements of the devices show excellent transistor properties with field effect mobility of up to 0.65 cm$^{2}$/Vs and current on-off ratio of up to 1.7$\times $10$^{6}$, which are higher than that of the control devices fabricated with gold electrodes. The high-performance of the devices is attributed to lower charge injection barrier at carbon nanotubes and pentacene interface. In order to find the direct evidence of the low injection barrier, we carry out low temperature electron transport measurement of our devices. We will present the detailed analysis of the low temperature data.   

Does Organic Field Effect Transistors (OFETs) Device Performance using Single-walled Carbon Nanotubes (SWNTs) Depend on the Density of SWNT in the Electrode?

Carbon nanotubes as an electrode material for organic field effect transistors (OFETs) have attracted significant attention. One open question is that whether the density of the Single-walled carbon nanotubes (SWNTs) in the electrode has any influence in the device performance of OFETs. In order to address this issue, we fabricated OFETs using SWNT aligned array electrode, where we varied the linear density of the nanotubes in the array of the electrodes during dielectrophoretic assembly of high quality surfactant free and stable aqueous SWNT solution. The source and drain of SWNT electrodes have been formed by electron beam lithography (EBL) and oxygen plasma etching. The OFETs were fabricated by depositing a thin film of poly (3-hexylthiophene) on the SWNT electrodes. We will present detailed result of our study.   

Work function recovery of air exposed molybdenum oxide thin films with vacuum annealing

We report substantial work function (WF) recovery of air exposed molybdenum oxide thin films with vacuum annealing. The high WF ($\sim $6.8 eV) of thermally evaporated MoO$_{x}$ thin film was observed to decrease sharply to $\sim $5.6 eV with an air exposure of one hour. The drop in the WF was accompanied with a very thin layer of oxygen rich adsorbate on the MoO$_{x}$ film. The WF of the exposed MoO$_{x}$ film started to gradually recover with increasing annealing temperature in a vacuum chamber having base pressure of 8 x 10$^{-11}$ torr. The saturation in the WF recovery was observed around 460 $^{\circ}$C, with WF $\sim $6.4 eV. The adsorb layer was found to be removed after the vacuum annealing. We further studied the interface formation between the annealed MoO$_{x}$ and copper pthalocynine (CuPc). The highest occupied molecular orbital (HOMO) level of CuPc was observed to be almost pinned to the Fermi level, strongly suggesting an efficient hole injection through the vacuum annealed MoO$_{x}$ film.   

Ab-inito calculation of energy level alignment and vacuum level shift at CuPc/C60 interfaces

The alignment of the donor and acceptor enegy levels is of crucial importance for organic photovotaic performance. We investigate the interfaical electronic structure and energy level alignment of copper phthalocyanine (CuPc)/fullerene (C60) using ab-inito density functional theory calculations including van der Waals interactions and hybrid density functionals. We show that energy level alignment critically depends on the standing-up and lying-down orientation of the CuPc molecules relative to C60 at the interface. We calculate the magnitude of the interface dipole at different molecular orientations and compare them to the vacuum level shift observed in photoemission spectroscopy. The validity of existing theoretical models which invoke charge transfer on this organic interface will be discussed in light of our predictions. Sandia National Laboratories is a multiprogram laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Deparment of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.   

Solution based cathode deposition for polymer light emitting devices

We report on a method for deposition of cathodes for polymer light-emitting devices (PLEDs), which is fundamentally different than the widely used thermal evaporation of metals. The thermal evaporation is well established in the industry but is very different than the solution processing of the rest of the PLEDs. It requires much more sophisticated equipment and much longer processing time than the solution processing. The metal evaporation requires temperatures around 1000$^{\circ}$C with all following requirements for materials handling. Proposing a method alternative to the already well-established thermal evaporation technique is a challenging task, and we are demonstrating only the principal feasibility of a solution based deposition of the cathodes. It is based on electroless deposition of silver. The process is compatible with the solution processing of the rest of the device and allows to finalize the entire device using solution based processes. We demonstrate the most representative current-voltage characteristics, emission spectra, stability tests, and microstructure of the newly developed electrode. These initial experiments demonstrate the feasibility of the proposed method and point avenues for further improvement.   

Organic solar cells: How can the theory guide the experience?

Research in organic photovoltaic applications are receiving a great interest in the last few years as they offer an environmentally clean and low-cost solution to the world's rising energy needs. One of the main problems limiting the efficiency of an organic solar cell device is the strong binding energy of the excitons, typically of a few hundreds of meV, which is ten to one hundred times more than in inorganic devices. Another limiting factor, persistent in P3HT:PCBM devices, can be the misalignment of the the HOMO (Highest Occupied Molecular Orbital) and the LUMO (Lowest Unoccupied Molecular Orbital) energy levels of the different components of the solar cell. Scharber's model (Scharber, M.C., Adv. Mater. 18, 789) is a simple yet interesting approach for predicting the efficiency of those devices, mainly based on the values of the HOMO and the LUMO and reasonable assumptions for the exciton binding energy. In this presentation, we will discuss how theoretical calculations based on density-functional theory can provide a guide to find promising polymers for photovoltaic cells. The accuracy, limits and possible expansions of Scharber's model will be examined, and a number of interesting polymer candidates to reach and perhaps break the well-known 10 {\%} efficiency will be presented.