Organic light emitting diodes fabricated with single wall carbon nanotubes dispersed in a hole conducting buffer: the role of carbon nanotubes in a hole conducting polymer

In order to investigate the role of single wall carbon nanotubes (SWNTs) in a hole conducting polymer, organic light emitting diodes (OLEDs) were fabricated with a conjugated emissive copolymer, poly(3,6-N-2-ethylhexyl carbazolyl cyanoterephthalidence) (PECCP) and SWNTs dispersed in a hole conducting buffer polymer, polyethylene dioxythiophene (PEDOT). Devices made with SWNTs dispersed in PEDOT and devices made without SWNTs in the PEDOT emit green light at 2.37 eV as expected for PECCP. However, we observed that the device made with SWNTs in the buffer layer shows a significant decrease in the electroluminescence (EL) as compared to that of the device without the SWNTs. In contrast, the photoluminescence (PL) from the same organic layer combination, excited from the PECCP side and measured through the PEDOT and the indium tin oxide glass, shows very little difference between the films with and without the SWNTs. The current–voltage (I–V) characteristics of OLEDs with SWNTs show a lower I–V power dependence (I–V2) near 1–2 V than that of the device without SWNTs which has a power dependence of I–V5. The EL and the I–V data together with the PL suggest an electronic interaction between the SWNTs and the host polymeric material, PEDOT. We propose that this electronic interaction originates from the hole trapping nature of SWNTs in a hole conducting polymer.