Injection- and space charge limited-currents in doped conducting organic materials

Abstract

Most conducting organic materials have a background p-type doping varying in the range 1015–1017 cm−3. We report results of a theoretical and experimental study of carrier transport in p-doped organic Schottky diodes. The theory given in this article shows that in a doped organic material with ohmic contacts the current is ohmic at low voltages. If the ohmic contact at the cathode is replaced by an Al Schottky contact the current varies exponentially with the applied voltage V. The current changes to space charge limited current (SCLC) at high voltages. The voltage at which the change takes place depends on the doping concentrations. In the SCLC regime the current varies according to the well-known V2 law if there are no traps and the mobility is independent of the electric field. If either trapping or effect of field on mobility is important, the current varies as Vm, where m>2. We have investigated experimentally the I–V characteristics of Schottky diodes fabricated using the PPV-based oligomer 2,5-di-n-octyloxy-1,4-bis (4′, 4″-bis-styryl) styrylbenzene (Ooct-OPV5) blended with polystyrene (PS) and the PPV-based polymer poly(2-methoxy-5-(3,7-dimethyloctyloxy)-p-phenylene vinylene) (OC1C10). As predicted by the theory, Al/Ooct-OPV5:PS/ITO (indium tin oxide) and Al/OC1C10/ITO Schottky diodes do show that the current varies exponentially with V at low voltages and as SCLC according to the Vm law (with m=3) at high voltages. The V3 variation of the current in the SCLC regime can be due to trapping or field dependent mobility. It is not possible to distinguish unambiguously between the two mechanisms using the experimental results. The voltage at which transition from the Shockley current to SCLC takes place can be used to determine the background doping concentration. The p-type background doping concentration in the Ooct-OPV5 is found to be ∼1017 cm−3. From the temperature variation of the hole current at low voltages, a value 0.53±0.1 eV is determined for the Schottky barrier height at the Al/Ooct-OPV5:PS contact. When image barrier lowering for 1017 cm−3 doping is taken into account, this value of the barrier height is in good agreement with the difference in the Al work function and highest occupied molecular orbital of the organic material. Finally we suggest that if the background doping concentration can be eliminated, the SCLC and light emission in the light-emitting diodes should occur at lower voltages.