We studied the carrier injection and transporting properties of N,N'-diphenyl-N,N'-bis(1-naphthyl)(1,1'-biphenyl)-
4,4'diamine (NPB), a common hole transporter for organic light-emitting diodes (OLEDs). NPB was found to possess
significant electron mobility from time-of-flight (TOF) measurement. With bipolar transporting ability, NPB was used to
fabricate single-layer devices with a configuration of ITO/ PEDOT:PSS/ NPB/ Ca/ Ag. PEDOT:PSS was demonstrated
to form a quasi-Ohmic contact to NPB by admittance spectroscopy (AS) and dark-injection space-charge-limited current
(DISCLC) measurements. From current-voltage (JV) characteristics, single-layer NPB devices exhibited a bulk-limited
hole current in low-voltage region. Electron injection was clearly observed at a turn-on voltage of about 4V, which
coincided with the luminance-voltage measurement. In order to confine the recombination zone, dye-doped NPB layer
was inserted into single-layer devices. This intentional doping technique made a notable improvement in current
efficiency. The mechanisms of the doped devices were also addressed.
We demonstrate that poly(3,4-ethylenedioxythiophene) doped with polystrenesulphonic acid (PEDOT:PSS) can act as an
excellent hole injection material for small organic charge transporters. With PEDOT:PSS as a conducting anode, it is
possible to achieve nearly Ohmic hole injection contacts to phenlyamine-based materials with HOMO values of up to
5.5 eV. In current-voltage experiment, the PEDOT:PSS anode can achieve nearly Ohmic hole injection to NPB (N,N'-
diphenyl-N,N'-bis(1-naphthyl)(1,1'-biphenyl)-4,4'diamine), and TPD (N,N'-diphenyl-N,N'-bis(3-methylphenyl) (1,1'-
biphenyl)-4,4'diamine). Meanwhile, dark-injection space charge limited current (DI-SCLC) transients are clearly
observed and are used to evaluate the charge-carrier mobility of these phenylamine compounds. The carrier mobilities
extracted by DI-SCLC are in excellent agreement with independent time-of-flight (TOF) technique. It is conceivable that
PEDOT:PSS can be used as a general conducting anode for the electrical characterizations of organic materials that
require Ohmic hole contacts.
We show that admittance spectroscopy (AS) can be used to determine charge carrier mobilities and transport parameters in materials relevant to organic light-emitting diodes (OLEDs). Via computer simulation, we found that a plot of the negative differential susceptance vs frequency yields a maximum at a frequency τr-1. The position of the maximum τr-1 is related to the average carrier transit time τdc by τdc = 0.56 τr. Thus knowledge of τr can be used to determine the carrier mobility in the material. Devices with the structure anode/phenylamines/Ag have been designed to evaluate their mobilities. The extracted hole mobility data from AS in pristine and doped material systems are in excellent agreement with those independently extracted from time-of-flight (TOF) technique. In addition, materials with different energy levels of highest occupied molecular orbital (HOMO), are further examined in order to study the effects of injection barrier on the extracted mobility by AS. In the case of an Ohmic hole contact (e.g. ITO or Au /m-MTDATA), the mobility data is good agreement with TOF results. However, for a non-Ohmic contact, the extracted mobility appears to be smaller. Thus AS can be used a means of evaluating the quality of electric contact between the injection electrode and the organic material.