Optimization of the efficacy and angle dependence of emission of top-emissive organic light-emitting diodes on metal foils

Abstract

Abstract. We have investigated how to optimize the efficacy and angle dependence of emission of top-emissive organic light-emitting diodes (OLEDs) based on metal foil substrates with the aim of creating efficient flexible devices for lighting and signage applications. By systematically varying the device architecture we were able to tune the optical microcavity which exists within the device structure and observe the change in performance. We paid particular attention to the effects of the metal foil roughness. We have seen that by changing the layer thickness of the poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) [PEDOT:PSS] in the device, and the substrate reflectivity and roughness we can obtain efficacies not too far from those achieved for standard bottom-emissive devices on glass substrates made with the same emitter. The angle dependence of luminance can be tuned from pointing in the forward direction via Lambertian to having a maximum at around 65• , and we have used optical modeling to help us find the optimum device structure. We conclude that (rough) metal foils are a realistic possibility for making flexible OLEDs and have demonstrated large area (up to 12 cm × 12 cm), thin film encapsulated, flexible devices. C 2011 Society of Photo-Optical Instrumentation Engineers (SPIE).

Tunable microcavities in organic light-emitting diodes by way of low-refractive-index polymer doping

“…Similar patterns have been reported previously and attributed to the microcavity effect [19]. The changes of the angle profiles were due to the shift of the reflectivity dip with the viewing angle both in wavelength and in magnitude corresponding to their overlaps with the emission spectrum of the organic emitters [19]. In addition, from the reflectivity spectra, we could also realized that the unusual pattern was due to the large mismatch between the resonance conditions and the intrinsic light-emitting wavelength of the polymers.…”

“…Interestingly, the emission pattern exhibited the highest intensity at an off-axis angle of 40-50°and a relatively weak intensity in the forward direction. Similar patterns have been reported previously and attributed to the microcavity effect [19]. The changes of the angle profiles were due to the shift of the reflectivity dip with the viewing angle both in wavelength and in magnitude corresponding to their overlaps with the emission spectrum of the organic emitters [19].…”

“…In principle, the presence of a dip in the reflectivity spectrum can be contributed to the interference effect and its location indicates the resonance wavelength. Further, because the weak microcavity of the PLEDs acts as a filter, the photons whose wavelength range falls outside of the dips are trapped in the cavity [19]. In our cases, the normal direction resonance wavelength k R (0°) was 590 nm for the control device and shifted to 570 and 560 nm for the device containing 0.1% and 0.2% PPFPMA, respectively (Fig.…”

“…As a result, the g out was enhanced along the normal emission direction after the nature of the weak microcavity was modified. In order to characterize the properties of the microcavity, the reflectivity spectroscopy was used [19,24]. Fig.…”

“…To the best of our knowledge, however, most studies adjust the optical cavity length (L) of the microcavities through altering the thickness (d) of the organic layers [17,18] emission layers. Adjusting the values of L through varying the thicknesses of the active layers, however, inevitably affects other electrical properties, such as the internal electrical field strength and the width and location of the recombination zone, resulting in a rather complicated situation for structure designs [19]. An alternative way is to change the refractive indices of the emission layers.…”