5 May 2000 Ž . Chemical Physics Letters 321 2000 379–384 www.elsevier.nlrlocatercplett Electronic structure of hybrid interfaces of ž / poly 9,9-dioctylfluorene G. Greczynski ) , M. Fahlman, W.R. Salaneck Department of Physics, Linkoping UniÕersity, S-58183 Linkoping, Sweden ¨ ¨ Received 29 November 1999; in final form 2 March 2000 Abstract In this Letter we report the results of a study of the energy level alignment at interfaces between ultra-thin films of Ž . Ž . poly 9,9-dioctylfluorene and Al O , SiO or gold substrates, prepared under ambient air conditions. In all the cases, x y 2 vacuum level alignment occurs, and the work function tracks that of the substrates. There is no evidence of band bending in ˚ the pristine polymer layers up to 1100 A in film thickness. Upon increasing the electrical conductivity of the polymer films by continuous doping with sodium under ultra-high-vacuum conditions, the energy levels of the polymer film gradually shift towards higher binding energies and finally become substrate independent. q 2000 Elsevier Science B.V. All rights reserved. 1. Introduction The process of carrier injection from a metal contact to a polymer layer is a crucial issue for operation of electronic devices based on organic Ž . materials, e.g., in light emitting devices LED . The details of energy level alignment at interfaces be- tween organic layers and metal electrodes, a funda- mental issue for designing better devices, still remain w x controversial 1,2 . The current status can be found wx in a review by Ishii et al. 2 . The major tool used for studying energy level alignment is ultraviolet photo- Ž . electron spectroscopy UPS . However, some work employing the Kelvin probe method has also been w x carried out 3–5 . In almost all cases reported, ‘ideal’ interfaces have been prepared under ultra-high ) Corresponding author. Fax: q 46-13-288-969; e-mail: grzgr@ifm.liu.se Ž . vacuum UHV conditions by means of vapor depo- sition of an organic layer onto ultra-clean metallic substrate, thereby minimizing possible contamina- tion. The fundamental question here is whether these ‘ideal’ interfaces follow the Schottky–Mott model, i.e. whether the barriers for carrier injection are given by the difference between the metal work Ž . Ž . function f , and the electron affinity EA of the M organic layer in the case of electron injection, or by the difference between f and ionization potential M Ž . IP for injection of holes. Surprisingly, recent stud- w x ies 1,2 show substantial deviations from this classi- cal rule and emphasize the importance of the influ- ence of several factors, including charge transfer through the interface, interfacial states, chemical in- teraction, image forces, etc., that may lead to the formation of interfacial dipole layers or other com- plications at the interfaces. Interfaces involving polymers cannot easily be prepared by vapor deposition under UHV conditions. 0009-2614r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. Ž . PII: S0009-2614 00 00338-9