Efficient blue and white polymer light emitting diodes based on a well charge balanced, core modified polyfluorene derivative

Abstract

Fabrication of efficient blue and white polymer light-emitting diodes (PLEDs) using a well charge balanced, core modified polyfluorene derivative, poly[2,7-(9,9′-dioctylfluorene)-co-N-phenyl-1,8-naphthalimide (99:01)] (PFONPN01), is presented. The excellent film forming properties as observed from the morphological study and the enhanced electron transport properties due to the inclusion of the NPN unit in the PFO main chain resulted in improved device properties. Bright blue light was observed from single layer PLEDs with PFONPN01 as an emissive layer (EML) as well as from double layer PLEDs using tris-(8-hydroxyquinoline) aluminum (Alq₃) as an electron transporting layer (ETL) and LiF/Al as a cathode. The effect of ETL thickness on the device performance was studied by varying the Alq₃ thickness (5 nm, 10 nm and 20 nm) and the device with an ETL thickness of 20 nm was found to exhibit the maximum brightness value of 11 662 cd m⁻² with a maximum luminous efficiency of 4.87 cd A⁻¹. Further, by using this highly electroluminescent blue PFONPN01 as a host and a narrow band gap, yellow emitting small molecule, dithiophene benzothiadiazole (DBT), as a guest at three different concentrations (0.2%, 0.4% and 0.6%), WPLEDs with the ITO/PEDOT:PSS/emissive layer/Alq₃(20 nm)/LiF/Al configuration were fabricated and maximum brightness values of 8025 cd m⁻², 9565 cd m⁻² and 10 180 cd m⁻² were achieved respectively. 0.4% DBT in PFONPN01 was found to give white light with Commission International de l'Echairage (CIE) coordinates of (0.31, 0.38), a maximum luminous efficiency of 6.54 cd A⁻¹ and a color-rendering index (CRI) value of 70.