Time Dependent Parallel Resistance in an Organic Schottky Contact
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Time dependent parallel resistance in an organic Schottky contact Arash Takshi1, John D. Madden1, Chi Wah Eddie Fok1, Mya Warren2 1
Electrical and Computer Engineering, The University of British Columbia (UBC) Vancouver, BC V6T 1Z4, Canada 2 Department of Physics and Astronomy, The University of British Columbia (UBC) Vancouver, BC V6T 1Z1, Canada
ABSTRACT The DC characteristics of a Schottky contact between regioregular poly (3-hexylthiophene) and aluminum are studied in forward and reverse bias regimes. Current-voltage curves of the junction in reverse bias show a resistive path in parallel with the expected Schottky contact. This is the sign of a nonuniform junction between the metal and semiconductor that exhibits ohmic behavior in some regions. Reduction of this parallel resistance and degradation of the Schottky junction are observed over a period of two weeks. Accumulation of undesired ions in the polymer or diffusion of aluminum atoms into the semiconductor are two possible mechanisms which may explain the time dependent behavior of these Schottky junctions. INTRODUCTION Since Schottky diode structures are used to build various organic electronic devices such as organic light-emitting diodes (OLEDs), their electrical characteristics are widely studied [1,2,3]. The main concern in an OLED is the forward bias characteristic, which makes the device emit light. However, the reverse bias characteristic of a Schottky contact can theoretically be used to obtain the barrier potential. Also the variation of reverse current with temperature reveals the carrier transport mechanism through the junction [4]. Such information can be obtained only if the device shows nearly ideal junction characteristics. In this paper we discuss some of the important factors that make a junction nonideal. Also the voltage-current characteristics of a Schottky contact between a metal and a conducting polymer are presented and aging effect on the DC characteristics is studied. BACKGROUND The carrier density in an undoped organic semiconductor is so low that a junction between a metal and an organic semiconductor is usually modeled as a metal-insulator junction [3]. On the other hand, the dopant density in an organic semiconductor which has been exposed to air is usually so high [5] that band bending occurs at the metal-semiconductor interface and creates a Schottky junction [4]. From thermionic emission theory the current in a Schottky diode in the forward bias regime is expressed by [4]: qV I F = I S exp nkT
(for
V>0 )
(1)
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where IF is the current passing through the diode, IS is the saturation current, q is the unit charge, V is the applied voltage, k is the Boltzman constant, T is the temperature and n is an empirical constant known as the ideality factor. The reverse current, IR, in an ideal Schottky diode is constant (equal to IS) and depends only on the physical parameters of the Schottky junction at a given temperature: qϕ I R = I S = I S 0 exp − b kT
(for
V>RS, the device would
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