Session S22: Focus Session: Organic Electronics: Contacts and Interfaces

Energetics of organic semiconductor interfaces: enhancing injection via chemical doping

Chemical doping of organic molecular films is a powerful way to improve charge injection and transport in organic devices, and to enhance device functionality. The formation of narrow depletion regions at doped organic-conductor interfaces facilitates injection via carrier tunneling through the barrier, and allows the use of moderate work function and non-reactive metals as efficient contacts. P-doping with the electronegative molecule, tetrafluoro-tetracyano-quinodimethane (F$_{4}$-TCNQ), has been used on a number of hole-transport materials. N-type doping is more challenging, often hindered by the energetic requirements of transferring an electron from the dopant HOMO to the host low lying LUMO. We recently demonstrated efficient n-doping of the electron transport material tris{2,5-bis(3,5-bis-trifluoromethyl-phenyhl)-thieno}[3,4-b,h,n]-1,4,5,8,9,12-hexaazatriphenylene (THAP), which has a 4.59 eV electron affinity (EA), with cobaltocene (CoCp, IE = 4.07 eV). We now introduce a stronger n-dopant, i.e. decamethylcobaltocene (CoCp*$_{2}$), and demonstrate n-doping of copper phthalocyanine (CuPc, EA = 3.25 eV). CoCp*$_{2}$ is found to have a remarkably low IE of 3.30 eV. N-doping is evidenced by a large upward swing of the Fermi-level in the gap of CuPc, and confirmed by current-voltage (I-V) measurements. A 10$^{4}$- to 10$^{7}$-fold increase in current density of the interface-doped device a compared to the undoped CuPc device is due to enhanced injection. An additional 10$^{3}$-fold increase in current density is observed for the uniformly doped device and is attributed to enhanced conductivity of the bulk film. The application of p- and n-doping of CuPc to an organic homojunction p-i-n diode with a 1.47 eV built-in potential is demonstrated.   

Electronic Structure of Interfaces and Heterojunction Ambipolar Organic Thin Film Transistor.

There has been a considerable interest in forming ambipolar organic thin film transistors (OTFTs) due to their advantageous for integrated circuits. Recently, Shi \textit{et al.} observed a substantial improvement for both the hole and the electron mobility in ambipolar OTFTs based on the heterojunction formed between copper-hexadecafluoro-phthalocyanine (F$_{16}$CuPc) and 2,5-bis(4-biphenylyl) bithiophene (BP2T). We examined the interface formation between F$_{16}$CuPc and BP2T using ultraviolet photoemission (UPS) and inverse photoemission spectroscopy (IPES). It is observed that in F$_{16}$CuPc/BP2T the heterojunction is characterized by band bending in both materials, while in BP2T/F$_{16}$CuPc the band bending is confined in BP2T only. For F$_{16}$CuPc/BP2T, the band bending of BP2T and F$_{16}$CuPc are 0.40 and 0.35 eV, respectively. The band bending region is about 15 nm in both materials, from which the Debye lengths of the materials can be deduced. The combination of the band bending and finite Debye lengths may provide an explanation to the observed ambipolar behavior and improved mobility of the OTFTs based on such heterojunctions.   

Sub-100 nm Contact Effects in Poly 3-hexylthiophene (P3HT)

Poly 3-hexylthiophene (P3HT) is a widely studied, versatile material used in organic electronics. Important to understanding the behavior of P3HT lies in its interaction with metal contacts. Contact effects between P3HT and metal electrodes can influence charge injection into P3HT, giving rise to a contact resistance and thereby influencing device performance. The origin of this contact resistance still remains poorly understood, but may result from changes in film morphology near the metal contact, charge transfer and band bending near the contact, or both. Understanding the detailed behavior of the interface between P3HT and metal electrodes will allow for optimization of device behavior. Based on previous work, the voltage drop at the P3HT-metal interface happens over 10-100 nm. We have fabricated devices on the tens of nanometer scale that directly probe this region, and present data on the effect of channel length versus device resistance as well as data on the temperature dependence of device resistance for gold and platinum electrodes. Implications for contact engineering will be discussed.   

Infrared study of charge injection in organic field-effect transistors

We present a systematic infrared (IR) spectroscopic study of charge injection in organic field-effect transistors (FET). These experiments have revealed new unexpected aspects of both polymers and molecular crystals. IR spectromicroscopy was employed to image the charges in poly(3-hexylthiophene) (P3HT) FETs. The charge density profile in the conducting channel uncovers a density-dependent mobility in P3HT due to disorder effects. Our IR studies of single crystal rubrene based FETs show that charge transport in these devices at room temperature is governed by light quasiparticles in molecular orbital bands. This result is at variance with the common beliefs of polaron formation in molecular solids. The above experiments have demonstrated the unique potential of IR spectroscopy for investigating physical phenomena at the nanoscale occurring at the semiconductor-insulator interface in FET devices. This work is in collaboration with G. M. Wang, D. Moses, A. J. Heeger (UCSB), V. Podzorov, M.E. Gershenson (Rutgers), Z. Hao, M. C. Martin (ALS), N. Sai, A. D. Meyertholen, M. M. Fogler, M. Di Ventra and D. N. Basov (UCSD).   

Studies of Au/SAMs/PEDOT-PSS/Au tunnel junctions

We report on tunneling through thin organic films. Junctions of the form: Au/SAMs/Polymer/Au were prepared on electronic-grade Si substrates with Self-Assembled Monolayers (SAMs) including octanedithiol (HS-C$_{8}$H$_{16}$-SH) and mercaptohexadecanoic (HS-C$_{15}$H$_{30}$-COOH). A transitional conducting polymer film PEDOT-PSS was spun on to the SAMs layer, and junctions were completed with a gold film. X-ray photoelectron spectroscopy (XPS) was employed to monitor the quality of the SAMs films. The electron tunneling properties including dI/dV and d$^{2}$I/dV$^{2}$ versus bias for the SAMs are discussed.   

Electronic functionalization of organic semiconductors with self-assembled monolayers

Self-assembled monolayers (SAM) are widely used in a variety of emerging applications for surface modification of metals and oxides. Here, we demonstrate a new type of molecular self-assembly: the growth of organosilane SAMs at the surface of organic semiconductors. Remarkably, SAM growth results in a pronounced increase of surface conductivity of organic materials, which can be very large for SAMs with a strong electron withdrawing ability. For example, the conductivity induced by perfluorinated alkyl silanes in organic molecular crystals approaches 10\^{}-5 S per square, two orders of magnitude greater than the maximum conductivity typically achieved in organic field-effect transistors (OFETs). The observed large electronic effect opens new opportunities for nanoscale surface functionalization of organic semiconductors with molecular self-assembly. In particular, SAM-induced conductivity exhibits sensitivity to different molecular species present in the environment, which makes this system very attractive for chemical sensing applications [1]. [1]. M. F. Calhoun, J. Sanchez, D. Olaya, M. E. Gershenson and V. Podzorov, ``Electronic functionalization of the surface of organic semiconductors with self-assembled monolayers'', Nature Materials, Nov. 18, (2007).   

Impedance Spectroscopy of Organic Thin Film Transistors and Contacts

We have developed a novel method of characterizing organic thin films using a 2- or 3-contact transmission line configuration, in which an AC voltage is applied to the thin film and the phase and magnitude of the current are measured. This simple method can shed light not only on the bulk properties of the semiconductor film, but also on the contacts, by varying the effective length scale probed in the sample. We present the results of transmission line measurements of pentacene thin films, with several types of contacts including thin films of carbon nanotubes.   

Cross-sectional Imaging of Organic Optoelectronic Devices and Molecularly Assembled Nanostructures

As the science of organic optoelectronic devices continuously matures, performance often improves at the expense of molecular and architectural complexity. One widespread approach toward optimization is the use of several polymers and hybrid materials, either as blends or in multiple layers. Tools to provide a thorough understanding of interfacial structure are lacking. Therefore, we employed a dual beam scanning electron microscope/focused-ion beam (SEM/FIB) to create device cross-sections that we subsequently investigated by transmission electron microscopy (TEM). High resolution images of an assortment of devices will be presented, including: interfaces of polymer-electrode, polymer-polymer, polymer-nanoparticles, and oligomer-nanoparticles in fully fabricated devices and optical cavities. We directly observed a variety of polymer-polymer interfaces depending upon solvent casting conditions, annealing treatments, and molecular functionality.   

Trapping carriers in organic field-effect transistors by metal nanoparticles

A thin layer of metallic Au nanoparticles was coated on substrates that were used for organic field-effect transistors, in order to study how the motion of carriers in the organic was affected by metal/organic coupling. A huge reduction of mobility was observed, due to the increase of the characteristic activation energy. We speculate that this follows from the polaronic motion of carriers resulted from the organic/metal coupling, similar to the organic/dielectric coupling. Further experiments demonstrate that the performance of those devices can be adjusted by changing the configurations of nanoparticles.   

Charge-retraction time-of-flight technique for mobility measurements in organic materials

This presentation will explore a recently reported, all-electrical technique, charge-retraction time-of-flight (CR-TOF), for the measurement of charge carrier mobility through an organic layer. Carriers are injected and accumulated at a blocking interface, then retracted. The retraction current transient is nearly indistinguishable from a traditional time-of-flight photocurrent. The CR-TOF technique is validated by measurement of the hole mobility of two well-known compounds using a common hole-blocking layer. An advantage of the technique is the applicability to sample layers less than 300 nm in thickness. This method also offers new opportunities such as catching charges in the middle of the sample layer and an alternate determination of the transition voltage of organic-organic interfaces.