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Towards Efficient and Metal-Free Emissive Devices: A SolutionProcessed Host-Guest Light-Emitting Electrochemical Cell Featuring Thermally Activated Delayed Fluorescence Petter Lundberg, E. Mattias Lindh, Shi Tang, and Ludvig Edman ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b07826 • Publication Date (Web): 01 Aug 2017 Downloaded from http://pubs.acs.org on August 5, 2017
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ACS Applied Materials & Interfaces
Towards Efficient and Metal-Free Emissive Devices: A Solution-Processed Host-Guest Light-Emitting Electrochemical Cell Featuring Thermally Activated Delayed Fluorescence Petter Lundberg, E. Mattias Lindh, Shi Tang and Ludvig Edman* The Organic Photonics and Electronics Group, Umeå University, SE-901 87 Umeå, Sweden. Keywords: Light-emitting electrochemical cell, Thermally activated delayed fluorescence, Lowcost solution processing, Efficient and metal-free emitter, Sustainable illumination technology
ABSTRACT:
The next generation of emissive devices should preferably be efficient, low cost and
environmentally sustainable, and as such utilize all electrically generated excitons (both singlets and triplets) for the light emission, while being free from rare metals such as iridium. Here, we report on a step towards this vision through the design, fabrication, and operation of a host-guest light-emitting electrochemical cell (LEC) featuring an organic thermally activated delayed fluorescence (TADF) guest that harvests both singlet and triplet excitons for the emission. The rare-metal-free active material also comprises a polymeric electrolyte and a polymeric compatibilizer for the facilitation of a cost-efficient and scalable solution-based fabrication, and for the use of air-stabile electrodes. We report that such TADF-LEC devices can deliver uniform
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green light emission with a maximum luminance of 228 cd m-2 when driven by a constant-current density of 770 A m-2, and 760 cd m-2 during a voltage ramp, which represents a one-order-ofmagnitude improvement in comparison to previous TADF-emitting LECs.
INTRODUCTION Advances in the field of artificial lighting have resulted in the emergence of light-emission technologies that can be thin, light-weight, flexible, and which can emit from large-area surfaces—notably organic light-emitting diodes (OLEDs) and light-emitting electrochemical cells (LECs). It has recently been demonstrated that LEC devices can be directly deposited onto, or integrated into, a wide variety of ubiquitous substrates, e.g. plastic,1-3 paper,4 and textile,5-6 and they are as such promising for a wide variety of new or improved applications in medicine,7 packaging,8-9 and wearables5-6. Importantly, in order for this vision to turn into reality, it is fundamental that the devices are efficient in transforming the input electricity into light and that they can be fabricated and recycled with cost-efficient means using sustainable materials.10-15 The LEC is distinguished from the OLED by the existence of mobile ions in the active material, and it is the redistribution of these ions that facilitates the characteristic turn-on process during which the active material is electrochemically doped and a p-n junction doping structure is formed in-situ.16-17 This unique operation renders the LEC highly robust to variations in the thickness of the active material and to the work function of the electrode materials,18 which opens up for the employment of low-cost and scalable solution-based fabrication processes, such as slot-die19 and spray20 coating, and ink-jet21 and gravure22 printing, and for the utilization of metal-free materials for both electrodes13, 23. The vast majority of today’s LEC devices either com-prise a conjugated polymer (CP)24-25 or an ionic transition metal complex (iTMC)26-27 as the emitter. This is problematic since CPs only
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ACS Applied Materials & Interfaces
harvest singlet excitons for the light-emission,28-29 and 75 % of the electrically generated excitons are formed in the triplet state; and while iTMCs can utilize both singlet excitons (via an initial singlet-to-triplet intersystem crossing) and triplet excitons (on the merit of a strong spin-orbit coupling) for the emission,30-31 these compounds are based on rare metals, such as iridium and ruthenium, and thereby directly associated with undesired cost and environmental issues14-15. In this context the recent introduction of organic and copper-based compounds that harvest both the singlets and triplets for light emission by Adachi et al. was a breakthrough.32-33 The emission process is termed thermally activated delayed fluorescence (TADF), and TADF emitters have been introduced in both OLED and LEC devices. But while TADF-OLEDs already have reached a very impressive performance, as exemplified by a recent report of green luminance of 3,000 cd m-2 at high efficiency,34 LECs based on organic TADF emitters are lagging behind with the highest luminance to date being a modest 26 cd m-2.35-37 In consideration of the intrinsic cost and environmental opportunities offered by the LEC technology, it is clearly motivated to develop a practical rare-metal-free TADF-LEC that can deliver strong and efficient luminance. Here, we report a step towards this important goal via the design and tuning of an active material in which undesired triplet quenching and self-absorption are suppressed and solution-processing facilitated. The active-material ink comprised 4,4’-di(9Hcarbazol-9-yl)biphenyl (p-CBP) as the wide-gap organic host with balanced p-type and n-type doping capacity, 2,4,5,6-tetra(9H-carbazol-9-yl)isophthalonitrile (4CzIPN) as the organic guest featuring TADF emission, poly(ethylene oxide) (PEO):KCF3SO3 as the polymeric solid electrolyte, and high-molecular-weight polystyrene as a compatibilizer. We demonstrate that such host-guest TADF-LECs devices, with a solution-processed and composition-optimized active material sandwiched between two air-stabile electrodes, can deliver uniform green area emission
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with a record-high luminance of 228 cd m-2 during constant-current driving, and 760 cd m-2 during a voltage ramp.
EXPERIMENTAL The 4,4’-di(9H-carbazol-9-yl)biphenyl (p-CBP, Sigma-Aldrich) host, the 2,4,5,6-tetra(9Hcarbazol-9-yl)isophthalonitrile (4CzIPN, LumTec) guest, and the polystyrene (MW ~ 9·105 g mol1
, Sigma-Aldrich) compatibilizer were used as received, while the poly(ethylene oxide) (PEO,
MW ~ 5·106 g mol-1, Sigma-Aldrich) ion-transporter and the KCF3SO3 salt (Sigma-Aldrich) were dried under vacuum at 50 and 190 ºC, respectively, before use. The compounds were separately dissolved in cyclohexanone (≥99.5 %, Sigma-Aldrich) under stirring at 50 °C in a concentration of 20 mg ml-1, with the exception of PEO that was dissolved in a concentration of 10 mg ml-1. The optimized active-material ink was prepared by blending the master solutions in a mass ratio of {p-CBP:4CzIPN:PEO:KCF3SO3:polystyrene} = {10:3:2.6:0.78:1.81}. The active-material ink was stirred on a magnetic hot plate at 50 °C for >4 h before further processing. The indium-tin-oxide (ITO) coated glass substrates (20 Ω □-1, Thin Film Devices) were cleaned by sequential 20 min ultrasonic treatment in detergent (Extran® MA 01, Merck), distilled water, acetone (VWR), and isopropanol (VWR), before being dried at 120 ºC for >4 h, and exposed to a UV-ozone treatment for 20 min. Poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS, Clevios PVP AI 4083, Heraeus) was spin-coated (4000 rpm, 1000 rpm s-1, 60 s) onto the ITO, and the resulting film was dried in air at 120 ºC for >5 h. The active-material ink was spin-coated (2000 rpm, 2000 rpm s-1, 60 s) on top of the ~30 nm thick layer of dry PEDOT:PSS, and thereafter dried at 70 ºC for >4 h. The active material featured a dry thickness of ~160 nm. Finally, a 100 nm thick Al cathode was deposited on the active material by thermal evaporation (p
