We report on the enhanced light outcoupling efficiency of monochrome top-emitting organic light-emitting diodes (OLEDs). These OLEDs incorporate a hole transport layer (HTL) material with a substantially lower refractive index (∼ 1.5) than the emitter material or the standard HTL material (∼ 1.8) of a reference device. This low-index HTL is situated between the opaque bottom metal contact (anode) and the active emission layer. Compared to an HTL with common refractive index, the dispersion relation of the surface plasmon polariton (SPP) mode from the opaque metal contact is shifted to smaller in-plane wavenumbers. This shift enhances the outcoupling efficiency as it reduces the total dissipated power of the emitter. Furthermore, the excitation of the coupled SPPs at the thin transparent metal top contact (cathode) is avoided by using an ultrathin top electrode. Hence, the coupling of the electroluminescence from the emitter molecules to all non-radiative evanescent modes, with respect to the emitter material, is reduced by at least a factor of two, additionally increasing the outcoupling efficiency. Furthermore, for sufficiently high refractive index contrast the shift of the SPP at the anode/organic interface can lead to in-plane wavenumbers smaller than the wavenumber within the organic emitter layer and outcoupling of all excited modes by high index light extraction structures, e.g. microlens, seems feasible. In accordance to optical simulations, the external quantum efficiency is enhanced by about 20 % for monochrome green emitting OLEDs with low refractive index HTL compared to a reference sample.
Bragg scattering by one dimensional periodic structures is investigated in order to enhance the outcoupling effciency of optically optimized planar top-emitting OLEDs. Using a soft imprint process, we fabricate extremely homogeneous gratings with sub- m period. These gratings are integrated beneath the bottom contact of topemitting OLEDs, without affecting the electrical device performance. The reflective contacts of the top emission geometry introduce pronounced micro-cavity effects for directly outcoupled and internally trapped light modes. Bragg scattering of the trapped waveguided and surface plasmon modes into the air cone, i.e. the forward direction, leads to interference with the directly outcoupled mode. As a result, constructive and destructive interference of the modes is detected and analyzed. Overall, we find that the introduction of shallow one dimensional sub- m periodic grating structures underneath top-emitting OLEDs leads to an EQE and luminous efficacy enhancement by up to 42%.
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.