Highly Efficient Red-Emitting Hybrid Polymer Light-Emitting Diodes via Fö rster Resonance Energy Transfer Based on Homogeneous Polymer Blends with the Same Polyfluorene Backbone Bo Ram Lee, †,‡ Wonho Lee, § Thanh Luan Nguyen, § Ji Sun Park, ⊥ Ji-Seon Kim, ∥,○ Jin Young Kim, ∇ Han Young Woo,* ,§ and Myoung Hoon Song* ,†,‡ † School of Mechanical and Advanced Materials Engineering/Low Dimensional Carbon Materials Center, Ulsan National Institute of Science and Technology (UNIST), Banyeon-ri 100, Ulsan 689-798, Republic of Korea ‡ KIST-UNIST Ulsan Center for Convergent Materials, Ulsan National Institute of Science and Technology (UNIST), Banyeon-ri 100, Ulsan 689-798, Republic of Korea § Department of Cogno-Mechatronics Engineering (WCU), Pusan National University, Miryang 627-706, South Korea ⊥ Energy Nano Materials Research Center, Korea Electronics Technology Institute (KETI), 68 Yatap-dong, Bundang-gu, Seongnam-si, Gyeonggi-do 463-816, Republic of Korea ∥ Department of Physics and Centre for Plastic Electronics, Imperial College London, Prince Consort Road, London, SW7 2AZ, United Kingdom ∇ Interdisciplinary School of Green Energy, Ulsan National Institute of Science and Technology (UNIST), Banyeon-ri 100, Ulsan 689-798, Republic of Korea ○ Department of Materials Science and Engineering, KAIST, Daejeon 305-701, South Korea * S Supporting Information ABSTRACT: Highly efficient inverted-type red-emitting hybrid polymeric light-emitting diodes (HyPLEDs) were successfully demonstrated via Fö rster resonance energy transfer (FRET) and interfacial engineering of metal oxide with a cationic conjugated polyelectrolyte (CPE). Similarly structured green- and red-emissive polyfluorene copolymers, F8BT and F8TBT, were homogeneously blended as a FRET donor (host) and acceptor (dopant). A cationic polyfluorene- based CPE was also used as an interfacial layer for optimizing the charge injection/transport and improving the contact problem between the hydrophilic ZnO and hydrophobic polymer layer. A long Fö rster radius (R 0 = 5.32 nm) and high FRET efficiency (∼80%) was calculated due to the almost-perfect spectral overlap between the emission of F8BT and the absorption of F8TBT. A HyPLED containing 2 wt % F8TBT showed a pure red emission (λ max = 640 nm) with a CIE coordinate of (0.62, 0.38), a maximum luminance of 26 400 cd/m 2 (at 12.8 V), a luminous efficiency of 7.14 cd/A (at 12.8 V), and a power efficiency of 1.75 lm/W (at 12.8 V). Our FRET-based HyPLED realized the one of the highest luminous efficiency values for pure red-emitting fluorescent polymeric light-emitting diodes reported so far. KEYWORDS: energy transfer, hybrid polymer light-emitting diodes (HyPLEDs), red emission, F8BT, F8TBT ■ INTRODUCTION Over the past two decades, conjugated polymer-based light- emitting diodes (PLEDs) have been extensively exploited for full-color flat-panel displays, solid-state lighting, and flexible optoelectronics, because of their low cost, facile color tunability by chemical structure modification, solution processability, large area fabrication, and mechanical flexiblility. 1−4 The balanced device efficiency and lifetime of R-G-B light emissions are required to realize full-color display devices as a commercial product. However, both color purity and device efficiency of red-emissive PLEDs are still far behind those of green-emissive PLEDs. Several approaches have been suggested to realize red-light emission. A main strategy contains a chemical synthesis by combining a low-band-gap red-emitting moiety (such as 2,1,3-benzothiadiazole 5,6 and 2,1,3-benzosele- nadiazole derivatives, 6,7 etc.) into a polymeric main chain, side chains, or end groups. In particular, the optical and electrical properties of polyfluorene (PFO)-based polymers can be easily controlled by the modification of chemical structure and red emission can be realized through introduction of comonomers into the PFO backbone, such as 4,7-bis(2-thienyl)-2,1,3- benzothiadiazole (TBT). 8 However, the red-emitting structures Received: March 26, 2013 Accepted: May 23, 2013 Published: May 23, 2013 Research Article www.acsami.org © 2013 American Chemical Society 5690 dx.doi.org/10.1021/am401090m | ACS Appl. Mater. Interfaces 2013, 5, 5690−5695