Experimental determination of the charge neutrality level of poly[2-methoxy-5-(2'-ethylhexyloxy)-p-phenylene...

The charge neutrality level (CNL) of poly[2-methoxy-5-(2'-ethylhexyloxy)-p-phenylene vinylene] (MEH-PPV) was determined. This was achieved by measuring the hole injection barrier at the MEH-PPV/Al interface with X-ray and ultraviolet photoemission spectroscopies in combination with in situ multistep electrospray-based thin film deposition. This deposition technique allows the deposition of polymers into a vacuum environment from solution, preventing significant contamination of the interface. Hence, the hole injection barrier energy at the interface between MEH-PPV and Al could be determined without the presence of significant amounts of ambient contamination. The result of this measurement was combined with published barrier energies for Au and Ag measured with the same technique. This enabled the correlation between barrier heights and substrate work functions as prescribed by the "induced density of states" (IDIS) model. This yielded a CNL of MEH-PPV of 3.76 eV relative to the vacuum level. The corresponding "screening factor" was calculated to be 0.21. The results are discussed in light of the IDIS, which was initially developed for small molecular materials, as well as the integer charge transfer model (ICT). The ICT model was determined for contaminated polymer interfaces featuring only weak interaction.