Polymer band alignment at the interface with indium tin oxide: consequences for light emitting devices

The influence of the ITO work function on the band alignment in semiconducting polymer over-layers was studied using UPS. Ultra-thin films of poly(bis-(2-dimethyloctylsilyl)-1,4-phenylenevinylene) were studied directly on ITO, as well as with an intermediate layer of an electrically conducting polymer (poly(3,4-ethylenedioxythiophene) doped with poly(4-styrene sulfonate)). For the polymer spin-coated directly on ITO the vacuum levels are aligned. Correspondingly, with the intermediate conducting polymer, the bands in the semiconducting polymer align to the vacuum level of the conducting polymer layer. Thus, the barrier to hole injection is determined by the work function of the conducting polymer instead of the ITO.