p-type doping efficiency of MoO3 in organic hole transport materials

We report on the p-type doping efficiency of molybdenum trioxide (MoO3) in the ambipolar organic charge transport material 4,4(')-Bis(carbazol-9-yl)-biphenyl (CBP). Kelvin probe analysis is used to study the work function with increasing thickness of doped CBP layers with varied MoO3 concentration deposited on indium tin oxide (ITO). Based on the model of a one-sided abrupt (n(+)p) junction between ITO and the MoO3 doped CBP layer, the density of free holes has been determined. A surprisingly low p-type doping efficiency of less than 2% has been derived. Segregation and clustering of the MoO3 dopant could explain these results.