Recovering cavity effects in corrugated organic light emitting diodes

Abstract

Cavity effects play an important role in determining the out-coupling efficiency of an OLED. By fabricating OLEDs on corrugated substrates, the waveguide and SPP modes can be extracted by diffraction. However, corrugation does not always lead to an enhancement in out-coupling efficiency due to the reduction of the electrode reflectance and hence the cavity effects. Based on the results of our rigorous couple-wave analysis (RCWA) simulation, we found that the cavity effects can be partially recovered using a low index Teflon layer inserted between the ITO anode and the substrate due to the enhancement of the reflectance of the corrugated electrodes. To verify the simulation results, we fabricated corrugated OLEDs having a low-index Teflon interlayer with an EQE of 36%, which is 29% higher than an optimized planar OLED. By experimentally measuring the OLED air mode dispersion, we confirm the cavity emission of a corrugated OLED is enhanced by the low index layer.

Light extraction from organic light emitting diodes (OLEDs)

“…We note that in comparing the EQE of an OLED with a light-outcoupling structure to the control device, to determine the enhancement factor, one needs to be aware of cavity effects in the control device, and possibly in the devices with the outcoupling structures as well. This issue was recently treated in detail by Fu et al [95].…”

“…Non-planar corrugated structures provide insight into the role of optics and photonics (e.g., cavity effects, TM TE, and SPP modes) in OLEDs and, as shown below, result in enhanced EQEs by minimizing the SPP loss and internally waveguided light [93][94][95][96][97].…”

“…Various experimental and theoretical studies of patterned structures were reported [116,145,146,[160][161][162][163][164] indicating reduced internal waveguiding and SPP losses. The outcoupling enhancement is typically attributed to diffraction and scattering in the periodic patterns [4,34,95,165], and to avoid directional EL, quasi random nanopatterns were suggested [138,140]. MLAs have the same effect on the angular profile, resulting in a nearly Lambertian EL.…”

Curved Mirror Arrays for Light Extraction in Top-Emitting Organic Light-Emitting Diodes

“…This is because the microcavity effect is more dominant in the TEOLED, and when external light extraction techniques are added, some of the microcavity enhancement is disrupted. In a BEOLED, 20% of the light is trapped in the substrate mode, and it can be extracted using microlens arrays − and a half-ball lens. − To extract waveguide and SPP modes, scattering layers − such as embedded nanoparticles , and corrugated substrates have been used. , Because the light extraction methods have a little effect on the cavity mode in a BEOLED, they can improve the device outcoupling efficiency without severely reducing the cavity emission. − In a TEOLED, the optical structure is very different, and both the top and bottom electrodes are metallic, leading to a stronger cavity effect and a larger SPP loss. While corrugation can be used to extract the trapped modes, it also reduces the reflectivity of the metallic mirror and therefore reduces the cavity effect, resulting in a reduction in outcoupling efficiency .…”

“…In a BEOLED, 20% of the light is trapped in the substrate mode, and it can be extracted using microlens arrays − and a half-ball lens. − To extract waveguide and SPP modes, scattering layers − such as embedded nanoparticles , and corrugated substrates have been used. , Because the light extraction methods have a little effect on the cavity mode in a BEOLED, they can improve the device outcoupling efficiency without severely reducing the cavity emission. − In a TEOLED, the optical structure is very different, and both the top and bottom electrodes are metallic, leading to a stronger cavity effect and a larger SPP loss. While corrugation can be used to extract the trapped modes, it also reduces the reflectivity of the metallic mirror and therefore reduces the cavity effect, resulting in a reduction in outcoupling efficiency . While adding a scattering layer or microlens array above the top electrode can extract the waveguide modes, a significant portion of the light is still lost to the SPP mode where the electric field is confined at the metal surface. , …”