A modelling approach for carrier transport in a Gaussin density of states is presented which takes into account that the
energetic distribution of carriers moving via Miller-Abrahams rates is affected by an electric field. This reorganisation of
the energetic carrier distribution can be described by virtual carrier heating to an effective temperature. We show that
combining this approach with an existing percolation model reproduces the field dependence found in computer studies
in literature for uncorrelated Gaussian disorder. Comparing to our experimental results, we also demonstrate that the
parameterizations from these publications do not hold at low temperatures.
We produced samples of 4,4′,4″-tris-(2-methylphenyl phenylamino)triphenylamine (m-MTDATA) doped into N,N'-
diphenyl-N,N'-bis(1-naphthylphenyl)-1,1'-biphenyl-4,4'-diamine (NPB) in the ratio 1:1 with four different thicknesses
from 50 nm to 400 nm and measured the IV curves in a temperature range from T=77 K to 346 K. We successfully
demonstrate a unified simulation of the samples’ IV characteristics over the entire temperature range under the
assumption of carrier heating, which is not possible with published models for correlated and uncorrelated Gaussian
disorder.
Organic and hybrid organic / inorganic semiconductor heterostructures offer great potential as key technology for
cost-efficient electro-photonic devices. To exploit their full potential, fundamental understanding of charge carrier
injection is essential. Therefore we use current-luminance-voltage (I-L-V) and capacitance-voltage (C-V) measurements
to analyze the injection characteristics of monochrome OLED test structures and hybrid organic / inorganic (HOI)
pentacene / n-GaN and Alq3 / n-GaN heterostructures processed by organic vapor phase deposition (OVPD) and metal
organic chemical vapor deposition (MOCVD), respectively.
In a first step, we fundamentally analyze the specific C-V characteristics of OLED test structures. The multilayer devices
show additional features in the C-V profile as compared to the bilayer OLED investigated by Brütting et al.1,2.
We attribute this behavior to the additional organic / organic interfaces and the resulting energetic barriers in multilayer
devices. In addition, we compared the C-V measurements of pristine and degraded OLED test structures. Here we
conclude that a deterioration of hole injection in degraded devices is dominant, whereas electron injection remains
largely unaffected. Furthermore, in pristine and degraded OLED, increased temperatures generally lead to improved
charge injection as well as a reduced impact of the additional barriers in multilayer stacks.
By analyzing I-V and C-V measurements of HOI heterostructures, we find ambipolar currents in pentacene-based diodes
with electrons from n-GaN being injected at lower bias than the onset of hole injection from the gold anode contacts.
Generally, I-V measurements of both types of HOI heterostructures show an onset of charge injection at very low bias
voltage. Accordingly, n-GaN offers superior electron injection characteristics recommending n-GaN as cathode contact
e.g. in fully transparent OLED microdisplays.
Florian Lindla, Manuel Boesing, Christoph Zimmermann, Philipp van Gemmern, Dietrich Bertram, Dietmar Keiper, Michael Heuken, Holger Kalisch, Rolf Jansen
Organic light-emitting diodes (OLEDs) are a key technology in solid state lighting. Without a long-lived phosphorescent blue emitter, a hybrid concept based on phosphorescent red and green emitters and a fluorescent blue emitter in a white OLED stack is a promising approach for pure-white emission. Several challenges such as exciton recombination on all emitters and triplet diffusion, as well as quenching, have to be overcome. To address these issues, a mixed-host phosphorescent emission layer is employed. The mixture ratio is locally varied in the emission layer. An interlayer separates the phosphorescent and fluorescent emission layer. Strategies to tune the color coordinates are presented. The lifetime and color stability versus luminance are investigated. At Commission Internationale de I'Eclairage color coordinates of 0.44/0.44, a current efficacy of 28.0 cd/A (at 1000 cd/m2), and a luminous efficacy of 20.6 lm/W can be measured.
Efficient white OLEDs are becoming attractive as large area light sources for illumination and in future also for general
lighting. We discuss device concepts for white OLEDs and their potential to achieve high efficacy and good lumen- and
color-maintenance at the same time. We focus on OLEDs using a combination of fluorescent blue and phosphorescent
red and green emitters (hybrid OLEDs). Hybrid OLEDs have high efficacy and lifetime in the white to warm white color
region (color points B and A on the black-body-curve). Near illuminant A efficacy values of 28-29 lm/W without optical
out-coupling can be achieved with a hybrid OLED. The external quantum efficiency (EQE) is 14%. A typical color
rendering index (CRI) is 84. Recent results for monochrome OLEDs and for hybrid OLED stacks are presented.
The electron and hole transport in a triple-barrier resonant tunneling diode are investigated using photoluminescence spectroscopy on a picosecond and nanosecond time scale. Time- resolved populations are created by exciting only the GaAs contact layers on either side of the tunneling structure but luminescence signals are detected from each of the two consisting quantum wells. Under external bias, several alignment conditions are investigated. First, the resonance of the ground state of the accumulation layer with either the narrow or the wide well depending on the bias direction. Second, the alignment of the accumulation layer with both quantum well subbands. For most external biases, the excess photocreated electron density is small with respect to the steady-state injected current density and the transient luminescence reflects the hole population. Both rise and decay of the transient photoluminescence are governed by the hole tunneling rate which increases with increasing bias. The sequential process of tunneling from the first to the second quantum well is apparent from a comparison of the signals in opposite bias direction. In the low current regime, time- resolved electron tunneling manifests, which adds a fast component to the transient luminescence signal.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.