Phase separation in polyfluorene-based conjugated polymer blends: Lateral and vertical analysis of blend spin-cast thin films

We have studied phase separation in polyfluorene-based electroluminescent polymer blends consisting of poly(2,7-(9,9-di-n-octylfluorene-alt-benzothiadiazole) (F8BT) and poly(2,7-(9,9-di-n-octylfluorene)-alt-(1,4-phenylene-((4-sec-butylphenyl)imino)-1,4-phenylene)) (TFB). We present detailed studies on spin-cast thin-film morphology of the blends, using micro-Roman spectroscopy and X-ray photoelectron spectroscopy. Micron-scale lateral phase separation is observed in these blend thin films. However, these phase-separated domains are not pure at the submicron length scale, and a nanoscale vertical phase segregation occurs with enrichment of the lower surface energy component (TFB) at both air and substrate interfaces. Imaging of the spatial uniformity of electroluminescence emission on the microscopic scale indicates spatially localised charge-carrier recombination in light-emitting diodes (LEDs) fabricated with these blends. On the basis of these studies, we propose a model for the development of thin-film phase-separated structure in spin-cast F8BT:TFB blends. LEDs without an additional hole-injection/transporting layer such as a poly(styrenesulfonate)-doped poly(3,4-ethylenedioxythiophene) (PEDT:PSS) have also been characterized to examine the self-organized vertical phase structure found in these blends. The results show that initial performance equivalent to devices with a PEDT:PSS layer is achievable in LEDs which contain only a single layer of F8BT:TFB blend material, confirming that the TFB wetting layer acts efficiently as a hole-accepting/transporting layer.