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.
We present calculated values of effective masses, bandgap reduction, and Fermi energy of p-doped Si and strained p-doped SiGe layers. The calculations have been made for Ge concentrations in the range 0 to 30% and boron concentration in the range 10 18 cm -3 to 3×10 20 cm -3 . Empirical expressions for the effective masses are given. These expressions and calculated values of the other parameters are convenient for use in computer codes for modeling device processing and performance. To validate the calculated values, we have compared them with the available experimental results. Good agreement between the calculated and the experimental values is found.
We have calculated J-V characteristics of an organic conducting sample (containing traps) including the Poole-Frenkel Effect (PFE). Both shallow and exponentially distributed traps are considered. We show that our approach is equivalent to combining the effect of trapping and using the field dependent mobility in one unified model. For shallow single level or shallow Gaussian traps, inclusion of PFE or using the (well-known) field dependent mobility gives the same dependence of current on voltage at a given temperature. However the value of zero field mobility µ 0 comes out to be different. We have fabricated and measured the J-V curves of the ITO/MEH-OPV5/Al diodes. An extremely fast rise with voltage V is observed at small voltages, which can be interpreted either by the Schottky contact limited Shockley like current or by bulk space charge limited current with PFE. The correct mechanism can be determined by making J-V measurements at different temperatures.
A survey of the literature shows that reverse bias current IL of an illuminated conducting polymer Schottky diode increases with voltage. We suggest that this increase in IL with applied reverse bias is due to a combination of two factors: (1) increase of mobility, and (2) dissociation of excitons. The experimental results agree with the values of IL calculated using either of the two mechanisms. Therefore it is difficult to determine the relative importance of the two mechanisms. The relative importance can be determined only if reliable values of material parameters are available. We have fabricated Schottky diodes and FETs using 5-ring n-octyloxy-substituted oligo[p-phenylene vinylene](Ooct-OPV5) and C60. The mobility of the oligomer derived from the measured characteristics of the diode is 3.29×10−7cm2/Vs and from the FET data, 3.24 × 10−4 cm2/Vs. These results show that the mobility (and other material parameters) depend strongly on the structure of the device. Therefore for interpreting the IL data it is important to measure the material parameters on the same structure on which IL measurements are made.
IJAIP fosters the exchange and dissemination of applications and case studies in the area of advanced intelligence paradigms among education and research professionals. The thrust of the journal is to publish papers dealing with the design, development, testing, implementation and management of advanced intelligent systems, and to provide guidelines in the development/management of these systems. IJAIP publishes archival articles and assessments of current trends, providing a medium for exchanging scientific research and technological achievements accomplished by the international community.
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