Plastic substrate design for enhanced light outcoupling from Organic Light Emitting Diodes (OLEDs)

Enhancing light extraction from Organic Light Emitting Diodes (OLEDs) is an ongoing scientific and industrial challenge that is particularly important for lighting applications. Light extraction, or the outcoupling factor out of conventional OLEDs devoid of any extraction means is limited to ~20%. This limit stems mainly from three photon loss processes: (i) so-called external waveguiding in the substrate, (ii) internal waveguiding in the high refractive index anode and organic layers, and (iii) excitation of surface plasmon polaritons at the metal cathode/organic interface. The ~30% external waveguided light can be extracted via a microlens array or a hemispherical lens at the air-side of the OLEDs\u27 substrate. However, mitigating the internal waveguided light and surface plasmon excitation losses, which amount to ~50% of the lost photons, in a cost-effective approach remains a challenge. Substrate corrugation is one of the innovative approaches used for addressing this issue. In this work, corrugated plastic substrates of polycarbonate and polyethylene terephthalate/cellulose acetate butyrate of various designs, such as different patterns\u27 height and periodicity, were evaluated. Detailed substrates\u27 design is a crucial metric for device performance; hence it requires in-depth analysis. Tapping mode atomic force microcopy (AFM) was used for probing the geometry, uniformity, and smoothness of the various plastic substrates. The essence of the work performed in this dissertation is combining promising substrate designs with carefully stacked green and white OLEDs that resulted in ~2x enhancements in out due to the patterns only, i.e., without additional means for extracting the externally waveguided light. In addition to broad optoelectronic characterization of the OLEDs, analyses of device stack conformality and top surface structure were performed via focused ion beam, SEM and AFM techniques

Benzobisoxazole cruciforms: a tunable, cross-conjugated platform for the generation of deep blue OLED materials

Four new cross-conjugated small molecules based on a central benzo[1,2-d:4,5-d′]bisoxazole moiety possessing semi-independently tunable HOMO and LUMO levels were synthesized and the properties of these materials were evaluated experimentally and theoretically. The molecules were thermally stable with 5% weight loss occurring well above 350 °C. The cruciforms all exhibited blue emission in solution ranging from 433–450 nm. Host–guest OLEDs fabricated from various concentrations of these materials using the small molecule host 4,4′-bis(9-carbazolyl)-biphenyl (CBP) exhibited deep blue-emission with Commission Internationale de L'Eclairage (CIE) coordinates of (0.15 ≤ x ≤ 0.17, 0.05 ≤ y ≤ 0.11), and maximum luminance efficiencies as high as ∼2 cd A−1. These results demonstrate the potential of benzobisoxazole cruciforms as emitters for developing high-performance deep blue OLEDs.We would like to thank Dr Sarah Cady, Dr Kamel Harrata and Mr Steven Veysey of Iowa State University (ISU) Chemical Instrumentation Facility for compound analysis. We thank Eeshita Manna for technical assistance. We also thank the National Science Foundation (CHE-1413173) for financial support of this work. RK and JS were partially supported by Basic Energy Sciences, Division of Materials Science and Engineering, USDOE. Ames Laboratory is operated by Iowa State University for the US Department of Energy (USDOE) under Contract No. DE-AC 02-07CH11358. Computational resources were provided in part by the MERCURY consortium (http://mercuryconsortium.org/) under NSF grants CHE-0116435, CHE-0521063, CHE-0849677, and CHE-1229354. (CHE-1413173 - National Science Foundation; Basic Energy Sciences, Division of Materials Science and Engineering, USDOE; DE-AC 02-07CH11358 - Iowa State University for the US Department of Energy (USDOE); CHE-0116435 - MERCURY consortium under NSF; CHE-0521063 - MERCURY consortium under NSF; CHE-0849677 - MERCURY consortium under NSF; CHE-1229354 - MERCURY consortium under NSF)http://pubs.rsc.org/en/Content/ArticleLanding/2016/TC/C5TC03622D#!divAbstractPublished versio

Enhanced Light Extraction from OLEDs Fabricated on Patterned Plastic Substrates

A key scientific and technological challenge in organic light-emitting diodes (OLEDs) is enhancing the light outcoupling factor ηout, which is typically \u3c20%. This paper reports experimental and modeling results of a promising approach to strongly increase ηout by fabricating OLEDs on novel flexible nanopatterned substrates that result in a \u3e2× enhancement in green phosphorescent OLEDs (PhOLEDs) fabricated on corrugated polycarbonate (PC). The external quantum efficiency (EQE) reaches 50% (meaning ηout ≥50%); it increases 2.6x relative to a glass/ITO device and 2× relative to devices on glass/poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) or flat PC/PEDOT:PSS. A significant enhancement is also observed for blue PhOLEDs with EQE 1.7× relative to flat PC. The corrugated PC substrates are fabricated efficiently and cost-effectively by direct room-temperature molding. These substrates successfully reduce photon losses due to trapping/waveguiding in the organic+anode layers and possibly substrate, and losses to plasmons at the metal cathode. Focused ion beam gauged the conformality of the OLEDs. Dome-shaped convex nanopatterns with height of ∼280–400 nm and pitch ∼750–800 nm were found to be optimal. Substrate design and layer thickness simulations, reported first for patterned devices, agree with the experimental results that present a promising method to mitigate photon loss paths in OLEDs

Plastic substrate design for enhanced light outcoupling from Organic Light Emitting Diodes (OLEDs)

Enhancing light extraction from Organic Light Emitting Diodes (OLEDs) is an ongoing scientific and industrial challenge that is particularly important for lighting applications. Light extraction, or the outcoupling factor out of conventional OLEDs devoid of any extraction means is limited to ~20%. This limit stems mainly from three photon loss processes: (i) so-called "external" waveguiding in the substrate, (ii) internal waveguiding in the high refractive index anode and organic layers, and (iii) excitation of surface plasmon polaritons at the metal cathode/organic interface. The ~30% external waveguided light can be extracted via a microlens array or a hemispherical lens at the air-side of the OLEDs' substrate. However, mitigating the internal waveguided light and surface plasmon excitation losses, which amount to ~50% of the lost photons, in a cost-effective approach remains a challenge. Substrate corrugation is one of the innovative approaches used for addressing this issue. In this work, corrugated plastic substrates of polycarbonate and polyethylene terephthalate/cellulose acetate butyrate of various designs, such as different patterns' height and periodicity, were evaluated. Detailed substrates' design is a crucial metric for device performance; hence it requires in-depth analysis. Tapping mode atomic force microcopy (AFM) was used for probing the geometry, uniformity, and smoothness of the various plastic substrates. The essence of the work performed in this dissertation is combining promising substrate designs with carefully stacked green and white OLEDs that resulted in ~2x enhancements in out due to the patterns only, i.e., without additional means for extracting the externally waveguided light. In addition to broad optoelectronic characterization of the OLEDs, analyses of device stack conformality and top surface structure were performed via focused ion beam, SEM and AFM techniques.</p

Enhanced Light Extraction from OLEDs Fabricated on Patterned Plastic Substrates

A key scientific and technological challenge in organic light-emitting diodes (OLEDs) is enhancing the light outcoupling factor ηout, which is typically 2× enhancement in green phosphorescent OLEDs (PhOLEDs) fabricated on corrugated polycarbonate (PC). The external quantum efficiency (EQE) reaches 50% (meaning ηout ≥50%); it increases 2.6x relative to a glass/ITO device and 2× relative to devices on glass/poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) or flat PC/PEDOT:PSS. A significant enhancement is also observed for blue PhOLEDs with EQE 1.7× relative to flat PC. The corrugated PC substrates are fabricated efficiently and cost-effectively by direct room-temperature molding. These substrates successfully reduce photon losses due to trapping/waveguiding in the organic+anode layers and possibly substrate, and losses to plasmons at the metal cathode. Focused ion beam gauged the conformality of the OLEDs. Dome-shaped convex nanopatterns with height of ∼280–400 nm and pitch ∼750–800 nm were found to be optimal. Substrate design and layer thickness simulations, reported first for patterned devices, agree with the experimental results that present a promising method to mitigate photon loss paths in OLEDs.</p