Application of Conducting Polymers for Optical Out-Coupling of OLEDs
Journal
Materials and Energy
Journal Volume
12
ISBN
978-981-12-4041-6
978-981-12-4032-4
Date Issued
2022-01-01
Author(s)
Jiao, Min
Abstract
Conducing polymer PEDOT:PSS possesses outstanding merits of excellent mechanical flexibility, solution processing capability, excellent thermal stability, high transmittance in visable range, tunable conductivity, suitable energy level for hole injection, and potentially low production cost. Intriguingly, in addition to the various advantages above, PEDOT:PSS also possesses a significantly low refractive index (n) of ~1.5. One of main limiting factors for light extraction of OLEDs is the high refractive index of the commonly used transparent electrode ITO (n ~ 1.9–2.1) and organic layers (n ~ 1.7–1.9), which lead to significant trapping of internally generated emission inside OLED active layers as waveguided modes. In view of these, special optical propeties of the low-index PEDOT:PSS can be adopted as hole-injection layers or transparent electrodes of OLEDs for optical out-coupling enhancement. By appropriate layer thicknesses and judicious combination of the low-index PEDOT:PSS hole-injection layer and high-index ITO electrode, out-coupling efficiency into air can be significantly enhanced compared with a typical ITO device. On the other hand, optical simulation and experiments reveal that by replacing high-index ITO with such a low-index PEDOT:PSS transparent electrode, although optimal out-coupling efficiency into air of PEDOT:PSS device is lower than conventional ITO device, the guided modes trapped within the organic/ITO layers in conventional OLEDs can be substantially suppressed, leading to more light coupled into the substrate than the conventional ITO device. In addition, due to its various advantages, PEDOT:PSS is an attractive candidate to combine with internal extraction micro-/ nano-structures for both current conduction and benefiting OLED out-coupling efficiency. Making use of very different refractive indices among various function/active layers (such as electrode, PEDOT:PSS, micro-/nano-structures, and organic layers), such conductive internal structures combine characteristics of optical micro-/ nano-structures and surface corrugation, it can effectively enhance coupling of internal radiation into the substrate or into the air, in addition to functioning as the current conductor/injector. By combining internal extraction structure and the external extraction scheme, very high EQEs can be achieved.
Type
book part