Electrical transport characteristics of single-layer organic devices from theory and experiment

An electrical model based on drift diffusion is described. We have explored systematically how the shape of the current density-voltage (J-V) curves is determined by the input parameters, information that isessential when deducing values of these parameters by fitting to experimental data for an ITO/PPV/Al organic light-emitting device (OLED), where ITO is shorthand for indium tin oxide and PPV is poly(phenylene vinylene). Our conclusion is that it is often possible to obtain a unique fit even with several parameters to fit. Our results allowing for a tunneling current show remarkable resemblance to experimental data before and after the contacts are conditioned. We have demonstrated our model on single-layer devices with ITO/PFO/Au and ITO/PEDOT/PFO/Au at room temperature and ITO/TPD/Al over temperatures from 130to290K. PFO is shorthand for poly(9,9′-dialkyl-fluorene-2,7-dyl) and TPD is shorthand for N,N′-diphenyl-N,N′-bis(3-methylphenyl)1-1′-biphenyl-4,4′-diamine. Good fits to experimental data have been obtained, but in the case of the TPD device, only if a larger value for the relative permittivity ϵs than would be expected is used. We infer that a layer of dipoles at the ITO/TPD interface could be responsible for the observed J-V characteristics by locally causing changes in ϵs. The strong temperature dependence of the hole barrier height from fitting J-V characteristics to the experimental data may indicate that the temperature dependence of the thermionic emission model is incorrect.