Effect of multi-walled carbon nanotubes on electron injection and charge generation in AC field-induced polymer electroluminescence

We investigate the use of multi-walled carbon nanotubes (MWNTs) dispersed in an emissive layer of poly (N-vinylcarbazole) (PVK):fac-tris(2-phenylpyri-dine)iridium(III) [Ir(ppy)3] in alternating current (AC) field-induced polymer electroluminescence (FIPEL) devices. A symmetric device structure, with the polymer/MWNT composite between two dielectric layers, was used to study the effect of MWNTs on charge generation within the active layer. An asymmetric device structure, using one dielectric layer, was used to study band alignment effects of carbon nanotubes in charge injection from a contact. The presence of MWNTs within the emissive layer facilitates effective internal charge generation in the symmetric devices, as would be expected if they acted as a charge source. However, electron injection under AC-driven fields also increases in the asymmetric devices, suggesting a modification to band alignment. Increase in light emission of five times is achieved in composite devices compared to devices with the pure polymer. From the trends in behavior with nanotube loading, we suggest that the nanotubes effectively doped the polymer, modifying energy level alignment in the device and increasing field-induced polarization currents. The combined effects of electron injection and charge generation may pave the way for widespread use of MWNTs in high-performance FIPEL devices.