A novel ambipolar polymer: from organic thin-film transistors to enhanced air-stable blue light emitting diodes

Abstract

The search for developing a deep blue long-lasting polymer light-emitting diode (PLED) has focused attention on polyfluorene (PFO) keto defect suppression with physical and chemical modifications. This study presents the synthesis and characterization of a new donor–acceptor (DA) polymer, based on naphthalene diimide (NDI) as a strong acceptor and n-phenyl-dithieno[3,2-b:2′,3′-d]pyrrole (DTP) as a strong donor. This polymer exhibits ambipolar behavior with stronger n-type properties, 0.07 cm² V⁻¹ s⁻¹, versus p-type 0.006 cm² V⁻¹ s⁻¹. By blending PNDIT-alt-DTP into PFO with hole and electron trapping sites, significantly enhanced electroluminescence (EL) efficiency in a simplified polymer light-emitting diode (PLED) was achieved. Further improvement is also achieved by introducing graphene oxide (GO) into the hole injection layer of PEDOT:PSS. A pure and strong long-lasting blue-color in a wide range of bias voltages can be acquired upon introducing 5.0 wt% PNDIT-alt-DTP and 0.01 wt% GO. New blend devices have lower turn-on voltages of 3.5 V, compared to 7.0–8.0 V for pure PFO PLEDs, high current efficiency up to 3.8 cd A⁻¹ and maximum luminance exceeding 5500 cd m⁻² which are at least 20 and 60 times greater than a pure PFO/0.01GO device, respectively. The enhanced efficiency can be related to improved hole injection and electron blocking nature of the GO doped PEDOT:PSS layer as well as reducing singlet exciton quenching at the interface. Additionally, the spatial confinement effect of the ambipolar polymer efficiently enhances the thermal stability of the binary blend and facilitates charge carrier balance and more efficient radiative recombination in the devices. These values are among the highest ever reported for PFO devices doped with an ambipolar polymer working in air.