Electronic and chemical structure of conjugated polymer surfaces and interfaces: Implications for polymer-based electronic devices

Electronic and chemical structure of conjugated polymer surfaces and interfaces: Implications for polymer-based electronic devices

ELSEVIER Synthetic Metals 85 (1997) 1219-1220 Electronic and Chemical Structure of Conjugated Polymer Surfaces and Interfaces: Implications Polyme...

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ELSEVIER

Synthetic

Metals 85 (1997)

1219-1220

Electronic and Chemical Structure of Conjugated Polymer Surfaces and Interfaces: Implications Polymer-Based Electronic Devices W. R. Salaneck,

M.

Liigdlund,

J. Birgersson,

and P. Bartal,

R.Lazzaroni’,

for

and J. L. Bredas

Department of Physics,Linkb>ing Uinversity,S-58183Linktiping, Sweden ’ Faculty ofphysics andNuclear Techniques,UniversityofMining andMetallurgy, al. Mickiewicza 30,PI 30-059Krakbw, Poland 2Sewicede ChimiedesMaGriaux Nouveau, Universitt?deMons-ainaut,B-7000Mons (Belgium) Abstract It is of critical importance to understand the nature of the electronic structure of the polymer surface and the interface with other polymers, semiconductors and metals, in order to control the chemistry of the interface under device fabrication and operation. Details of the early stages of interface formation with metals lead to several important implications for polymer-based LED devices. Applying certain knowledge obtained within the surface and interface studies, several optimized light-emitting devices have been fabricated. Key words:

Organic/inorganic

interfaces;

Photoelectron

spectroscopy;

Light sources

1. Introduction Interest in the surfaces and interfaces of semiconducting mnjugated polymers [l, 21 anises from the influence of the electrical contacts when determining electrical properties. Since the discovery at the University of Cambridge [3] that poly(p-phenylenevinylene), or PPV, and related polymers, may be used as the active media in polymer-based lightemitting devices, or “polymer-LEDs”, it has become obvious that much of device performance is determined directly by the chemistry of the relevant interfaces [4]. One of the major factors determining the quantum yield for luminescence is the efficiency of injection of electrons and holes at the respective interfaces [5]. Typically, iridium-tin-oxide (ITO), is used as the hole injecting contact. Electron injection is usually accomplished using a metal with low work function, which is applied by physical vapor deposition (PVD). The injection efficiency of the minority carrier may be one major limiting factor in over-all device efficiency. It has been shown over the past decade that the concerted combined experiment-theory approach of the University of Mons-Hainaut and LinkBping University to the study of polymer surfaces and interfaces, when applied to a variety of n-conjugated polymers and model molecules, as well as to the eady stages of metal-interface formation, provides more information than possible from either experimental or the theoretical studies alone [4].at the metal-on-polymer contact in most studies of polymer-LEDs. The polymer materials studied have been chosen from among those relevant to polymer-based LEDs, including PPV, poly(2, 5-diheptyl-1, 4phenylenevinylene), or DHPPV, a cyano-substituted PPV, denoted CNPPV, poly(2-methoxy-S-(2’-ethyl-hexoxy)-1,4-phenylenevinylene), denoted MEH-PPV, and several alkyl-substituted polythiophenes, as well as certain molecules which are models for conjugated polymers. The metal atoms studied include aluminum, sodium, calcium, potassium, rubidium, and lithium [4].

2. Experimental

details

The experimental studies were carried out in UHV using polymer samples for which careful determinations of the electronic band structure had been determined previously. Details of sample preparation, the spectroscopy, and data analysis are to be found else where [4, 61. In all cases studied, the ideal, simple, abrupt metal-on-polymer interface has never been observed. There always exists an interfacial layer between the polymer surface and the vapor-deposited metallic contact. The nature of

0379-6779/97/317.00 0 1997 Elsevier PII SO379-6779(96)04330-5

Science S.A. AlI rights reserved

this interfacial layer is determined by the chemistry induced by the independent metal atoms during the PVD process. The presence of this interfacial layer seems to have been neglected in analyzing the results of electronic charge injection metal. 3. Some issues arising

form the polymer-metal

interface

studies

The case of calcium vapor-deposited on PPV is of particular interest. Calcium is a common an electron injecting metallic contact on PPV-based LEDs. As observed first on clean surfaces of model condensed molecular solids in IJHV, and subsequently on clean surfacessf DHPPV, calcium diffuses into the near surface region, forms Ca -ions, and donates electrons to the nsystem of the polymer. The interfacial region between the calcium contact and the polymer has an approximate scale in the range of 20 to 40 8, (a scale similar to the case of Al-atoms on PPV). If there is a large number of oxygen-containing species on the surface [7], however, an interfacial layer, consisting essentially of an oxide of calcium, is formed initially upon the deposition of calcium atoms in UHV. After the oxygencontaining species have be-en consumed by the initial calcium atoms, subsequent deposition of calcium results in the deposition of calcium (insulating) layer also is on the order of 20 to 30 A, depending upon the details of the surface contamination, chemical impurity of the polymer, and/or the vapor-deposition environment. The calcium-on-PPV results are summarised [4] in Fig. 1, where either a doped conducting polymer layer is formed on the oxygen-free surface of PPV (middle panel), or a calcium oxide layer is formed (lower panel [7]) on the surface of PPV containing a large amounts of oxygen-containing species. 4.Opportm3ities

for appkations

It is important to point out that work on the fundamental properties of materials often leads to oppottunities arise where in the detailed information obtained may be “applied” directly, in an opportunistic way, to perceived device areas, even though “applications” may not be the primary focus of the research. Here, a few such applications are described very briefly. 4.1 “Dirty

calcium”

Starting with oxygen-free surfaces of PPV or CN-PPV, the vapor deposition of calcium, to form electrodes for polymer-based LEDs, has been studied in the presence of a variable background pressure of 0,. It was found that an optimum figure-of-merit, defined loosly as a fun&n of device yield, turn on voltage, luminescence intensity, and device life-time,

l%R. Salaneck et al. /SyntheticMetals 85 (1997) 1219-1220

1220

is achieved when the pressure of 0 is about lo6 mbar, where an oxidized metallic contact is formed [8]. Th& work shows that not only is UHV not necessary for the fabrication of polymer-based LEDs, but that UIHV may actually be detrimental, if really clean surfaces are employed.

VACUUM LEVEL

narrow band of wavelengths from a broad electro-luminescence spectrum without the use of the more common dual metal electrodes or the use of a dielectric mirror [ll]. Proto-typical devices have been constructed using regioregular poly(4, 4’didecyl-2, 2’-bithiophene), or PDDBT, known as “yellowpolythiophene; so called since the regiomgularity leads to increased torsion angles between monomeric dimer units, resulting in a larger bandgap than conventional alkyl-substituted polythiophenes. Devices have been fabricated from a “dirty calcium” electrode, on a spincoated polymer layer, on an IT0 layer, on a SiOa buffer layer, on a glass substrate. Since the indicies of refraction alternate through the polymerITO-SiOs-glass structure as “low-high-low-high”, for a selected set of thicknesses, a narrow spectrum is emitted at 450 nm with a FWHM of less than 30 nm [ll]. 4.4

IT0

“PPV”

Ca

BIPOLARON BANDS DOPED POLYMER REGION

1: The ITO-“PPV”-Calcium

interface

Exposure of PPV films to air, in the dark, leads to inclusion of water molecules in three different types of site [12]. Hz0 in one type of site leads to increases in torsion angles in such a way as to reduce the conjugation and increase the HOMO-LUMO gap. The effects of the geometrical changes, and the associated electronic structural changes, are clearly seen in the UPS. Effects seen in the optical absorption spectra are consistent with the occurrence of water-induced changes in the frontier electronic stmctum. The effects reported are completely reversible, disappearing upon heat treatment of PPV in UHV. 5. Acknowledgements Research on conjugated polymers in Lit-&ping is supported by grants from the Swedish Natural Sciences Research Council (NFR), the European Commission ESPRIT program (project 8013, LEDFOS), Philips Research, NL (within Brite/EURAM PolyLED, 0592), and Hoechst AG, FRG. The work in Mons is partly supported by the Belgien Federai government office of Science Policy, SSTC (Phle d’Attraction Intemniversitaim en Chimie Supramol6culaire et Catalyse), FNRQFRFC, and an IBM Academic Joint Study.

INSULATOR REGION

Figure

Water and PPV

[2].

References [l] W. R. Salaneck, I. Lundstrom, and B. R&by, “Nobel Symposium in Chemistry: Conjugated Polymers and Related Materials; The Interconnection of Chemical and Electronic Structure,” . Oxford: Oxford University Press, 1993. [2] W. R. Salaneck and J. L. Br&das, Solid State Communications,

Special Issue on “Highlights in Condensed Matter Physics and Materials Science”, 92,31, (1994). [3] J. H. Burro&es, D. D. C. Bradley, A. R. Brown, Mackay, R. H. Friend, P. L. Bum, and A. B. Holmes,

(1990). 4.2 Blue LEDs An interest in the electronic stmcture of the copolymer, poly(2, 5diheptyl-1, 4-phenylene-alt-2, 5-thienylene), or PDHPT, lead to the fabrication of a single-layer polymer LED consisting of “ditty calcium” on PDHPT on ITO/glass [9]. In order to increase the brightness, devices based upon polymer blends of PDHFT in a soluble substituted-PPP, namely poly(2, 5-diieptyl-2, 5’-dipentoxy-dipamphenylene), or PDDPP, were studied. At a concentration of 10 % PDHPI in PDDPP, a maximum in the aiernol quantum efficiency of 2 % was observed {IO]. The key feature of the electronic stmcture of the polymer blend is that the HOMOLUMO gap of the guest polymer (PDHPT) is bracketed by the HOMOLUMO gap of the host polymer. This relationship among the frontier electronic states leads to trapping sites for electrons and holes, such that the probability of electron-hole recombination is increased.

4.3 Inteference-filterLEDEDs Through selection of the combination of thicknesses and refractive indicies of materials for a polymer-based LED it is possible to select a

R. N. Marks,

K

Nature, 347, 539,

and J. L. Br&las, ConjugatedPolymer University Press, Cambridge, 1996). [5] D. D. C. Bradley and T. Tsutsui, “Characterization of the Interfaces between Low Work Function Metals and Conjugated Polymers in Light Emitting Diodes,” in Organic Electroluminescence. Cambridge: Cambridge University Press, 1995. [6] J. L. Br&das,Adv.Mar., 7,263, (1995). [7] Y. Gao, K. T. Park, and B. R. Hsieh, J. Chem. Phys., 97, 6991, (1992). [8] P. Brlims, J. Birgersson, N. Johnsson, M. Logdlund, and W. R. Salaneck, Synfb.Met., 74,179, (1995). [9] M. Fahlman, J. Birgerson, K Kaeriyama, and W. R. Salaneck, Synth. [4] W. R. Salaneck,

S. Stafstrom,

Su&ces andInterfaces (Cambridge

Met., (1995). [lO]J. Birgemon, K. Kaeriyama, P. Bar& P. Btims, M. Fahlman, T. Gnuthmd, and W. R. Salaneck,Submitted, (1996). [ll]P. Barta, J. Birgersson, S. Guo, W. R. Salaneck, and M. Zag&ska, Submitted, (1396). [123K 2. Xing, M. Fahlman, M. Liigdlund, D. A. dos Santos, V. Parent&, R. Lazzaroni, J. L. Btidas, R. W. Gymer, and W. R. Salaneck, Adv. Mat, 8,971 (1996).