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

Most conducting organic materials have a background p-type doping varying i n the range 10(15)-10(17) cm(-3). We report results of a theoretical and ex perimental study of carrier transport in p-doped organic Schottky diodes. T he 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 exp onentially with the applied voltage V. The current changes to space charge limited current (SCLC) at high voltages. The voltage at which the change ta kes place depends on the doping concentrations. In the SCLC regime the curr ent varies according to the well-known V-2 law if there are no traps and th e mobility is independent of the electric field. If either trapping or effe ct of field on mobility is important, the current varies as V-m, where m >2 . We have investigated experimentally the I-V characteristics of Schottky d iodes 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) an d 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 (indiu m tin oxide) and Al/OC1C10/ITO Schottky diodes do show that the current var ies exponentially with V at low voltages and as SCLC according to the V-m l aw (with m=3) at high voltages. The V-3 variation of the current in the SCL C regime can be due to trapping or field dependent mobility. It is not poss ible to distinguish unambiguously between the two mechanisms using the expe rimental results. The voltage at which transition from the Shockley current to SCLC takes place can be used to determine the background doping concent ration. The p-type background doping concentration in the Ooct-OPV5 is foun d to be similar to 10(17) cm(-3). From the temperature variation of the hol e current at low voltages, a value 0.53 +/-0.1 eV is determined for the Sch ottky barrier height at the Al/Ooct-OPV5:PS contact. When image barrier low ering for 10(17) cm(-3) doping is taken into account, this value of the bar rier height is in good agreement with the difference in the Al work functio n and highest occupied molecular orbital of the organic material. Finally w e suggest that if the background doping concentration can be eliminated, th e SCLC and light emission in the light-emitting diodes should occur at lowe r voltages. (C) 2001 American Institute of Physics.