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.