Charge Carrier Injection Into A Disordered Organic Dielectric
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B6.7.1
Charge Carrier Injection Into A Disordered Organic Dielectric Vladimir I. Arkhipov1, Heinz Bässler2, and Evgenia V. Emelianova3 1 IMEC, B-3001 Heverlee-Leuven, Belgium. 2 Institute of Physical, Nuclear and Macromolecular Chemistry and Material Science Center, Philipps University of Marburg, Hans-Meerwein-Strasse, D-35032 Marburg, Germany. 3 Semiconductor Physics Laboratory, University of Leuven, B-3001 Heverlee-Leuven, Belgium. ABSTRACT Based upon the concept of charge carrier hopping within a disordered organic solid a formalism for the temperature and field assisted charge injection from an electrode into an organic dielectric has been developed. Both back flow of charges towards the injecting electrode and the dependence of the injection efficiency upon the charge carrier mobility has implicitly been taken care of. Experimental results for electrode limited injection are in good agreement with the theory.
OUTLINE OF THE PROBLEM In wide band-gap semiconductors charge carriers have to be injected or supplied via donor/acceptor doping. Although the latter strategy has been applied successfully to organic semiconductors [1,2] the vast majority of organic light-emitting diodes (OLEDs) employ the injection of electrons and holes from metallic or semimetallic electrodes such as an indium-tin oxide (ITO) contact [3-5]. Depending on the concentration of charge carriers in the bulk the current flowing in the diode is controlled by either the internal space charge or injection. The condition for the space-charge limited current (SCLC) flow is that the contact is able to supply more charge carriers per unit time than can be transported across the bulk implying that the electric field at the contact is zero, i.e. a virtual anode/cathode is established close to the interface between the electrode and dielectric [6]. Alternatively, the rate-limiting step is the transfer of a charge carrier from the electrode to the dielectric, the relevant material parameters being the magnitude of the zero-field energy barrier ∆ at the electrode/dielectric interface and the charge carrier mobility µ . In practice, the tolerable injection barrier for unipolar SCLC in a dielectric with a mobility of 10-6 cm2/Vs is about 0.3 eV [7]. The classic concepts for charge injection are the thermionic emission, also known as the Richardson-Schottly (RS) approach, and tunneling described by the Fowler-Nordheim (FN) model. These concepts yield the following expressions for the injection-current density j [8]:
∆ − e 3 F / 4πε 0ε
kT
j RS = AT 2 exp −
,
(1)
,
(2)
and
j FN = BF 2 exp −
8π 2meff ∆3 / 3he F
B6.7.2
where T is the temperature, F the field, e the elementary charge, meff the effective carrier mass, ε0 the permittivity, ε the dielectric constant, and A, B, k, and h are the RS, NF, Boltzmann, and Planck constants, respectively. In both cases the dependence of the current on the applied electric field is solely related to the injection event itself regardless of the transport velocity of the inje
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