Simon King

Flexible Blade‐Coated Multicolor Polymer Light‐Emitting Diodes for Optoelectronic Sensors

A method to print two materials of different functionality during the same printing step is presented. In printed electronics, devices are built layer by layer and conventionally only one type of material is deposited in one pass. Here, the challenges involving printing of two emissive materials to form polymer light-emitting diodes (PLEDs) that emit light of different wavelengths without any significant changes in the device characteristics are described. The surface-energy-patterning technique is utilized to print materials in regions of interest. This technique proves beneficial in reducing the amount of ink used during blade coating and improving the reproducibility of printed films. A variety of colors (green, red, and near-infrared) are demonstrated and characterized. This is the first known attempt to print multiple materials by blade coating. These devices are further used in conjunction with a commercially available photodiode to perform blood oxygenation measurements on the wrist, where common accessories are worn. Prior to actual application, the threshold conditions for each color are discussed, in order to acquire a stable and reproducible photoplethysmogram (PPG) signal. Finally, based on the conditions, PPG and oxygenation measurements are successfully performed on the wrist with green and red PLEDs.

Fundamental processes governing operation and degradation in state of the art P-OLEDs

We present a theoretical and experimental analysis of operation and degradation of model fluorescent blue bilayer polymer organic light emitting diodes (P-OLED). Optical and electrical simulations of bilayer P-OLEDs are used to highlight the key material and device parameters required for efficient recombination and outcoupling of excitons. Mobility data for a model interlayer material poly (9,9-dioctylfluorene-N-(4-(2-butyl)phenyl)-diphenylamine) (TFB) and a model fluorescent blue light emitting material poly-(9,9- dioctylfluorene-co-bis-N, N-(4-butylphenyl)-bis-N,N- phenyl-1,4-phenylenediamine) (95:5 mol%) (F8-PFB random copoloymer), is shown to satisfy the key charge transport characteristics required to ensure exciton formation at the optimum location for efficient extraction of the light where μh (LEP) < μe (iL) < μe (LEP) < μh (iL). A method to measure the photon generation zone profile and dipole orientation is presented and shown to follow the expected behavior. The efficiency drop of P-OLEDs during device operation is a known issue, the understanding and prevention of which is key for the commercial success of P-OLED technology. We present a detailed degradation study of devices containing model materials, and highlight the generation of fluorescence quenching sites as the key factor limiting the operational stability. A striking feature of this degradation is its partial (~50%) reversibility upon baking above the LEP glass transition temperature. Some reversibility is also observed in the conductivity, suggesting a common origin to the optical and electrical degradation. We also show that the species responsible for the generation of the reversible PL quenching sites are the excitons themselves, and that optically excited excitons can also generate many of the features characteristic of electrical stressing. Finally we demonstrate that materials with a dramatically improved lifetime also suffer from a similar, although slowed down, degradation mechanism, where the reversible component is increased to almost all (>90%) of the quenching sites produced. This highlights the importance of understanding these reversible phenomena in improving P-OLED lifetime and commercial adoption of the technology.

Post-polymerization C–H Borylation of Donor–Acceptor Materials Gives Highly Efficient Solid State Near-Infrared Emitters for Near-IR-OLEDs and Effective Biological Imaging

Post-polymerization modification of the donor-acceptor polymer, poly(9,9-dioctylfluorene-alt-benzothiadiazole), PF8-BT, by electrophilic C-H borylation is a simple method to introduce controllable quantities of near-infrared (near-IR) emitting chromophore units into the backbone of a conjugated polymer. The highly stable borylated unit possesses a significantly lower LUMO energy than the pristine polymer resulting in a reduction in the band gap of the polymer by up to 0.63 eV and a red shift in emission of more than 150 nm. Extensively borylated polymers absorb strongly in the deep red/near-IR and are highly emissive in the near-IR region of the spectrum in solution and solid state. Photoluminescence quantum yield (PLQY) values are extremely high in the solid state for materials with emission maxima ≥ 700 nm with PLQY values of 44% at 700 nm and 11% at 757 nm for PF8-BT with different borylation levels. This high brightness enables efficient solution processed near-IR emitting OLEDs to be fabricated and highly emissive borylated polymer loaded conjugated polymer nanoparticles (CPNPs) to be prepared. The latter are bright, photostable, low toxicity bioimaging agents that in phantom mouse studies show higher signal to background ratios for emission at 820 nm than the ubiquitous near-IR emissive bioimaging agent indocyanine green. This methodology represents a general approach for the post-polymerization functionalization of donor-acceptor polymers to reduce the band gap as confirmed by the C-H borylation of poly((9,9-dioctylfluorene)-2,7-diyl-alt-[4,7-bis(3-hexylthien-5-yl)-2,1,3-benzothiadiazole]-2c,2cc-diyl) (PF8TBT) resulting in a red shift in emission of >150 nm, thereby shifting the emission maximum to 810 nm.

56.1: Invited Paper: Excited State Interactions in P-OLEDs: Implications For Efficiency And Lifetime

A fluorescent blue P-OLED has high densities of triplet excited states within the emissive layer. In this paper we will present an efficiency and degradation analysis of model P-OLED devices and materials, and demonstrate that control of triplet transfer and annihilation processes are of crucial importance in realising the efficiencies and device stabilities required for commercialisation of the technology.