P-doping of organic semiconductors via transition metal oxides: Energetic vs. structural properties
Speaker: Michael Kröger, InnovationLab GmbH, Heidelberg, Germany
Series: Topical Seminars
Date/Time: Monday, December 5, 2011, 1:30 p.m. - 2:30 p.m.
Electrochemical doping of organic semiconductors is a viable path to overcome device limitations imposed by low conductivity and high charge injection barriers, which is often determined for intrinsic organic semiconductors. Stable p-doping was realized by co-evaporation of the respective organic material and a transition metal oxide, e.g. MoO3. Even wide band gap materials with high ionization potentials can be p-doped by TMOs. A case-study on p-doping of 4,4′-Bis(N-carbazolyl)-1,1′-biphenyl (CBP), which is very often used in highly-efficient OLEDs, is presented.
Charge transfer from the organic molecule to MoO3 dopants is enabled by very deep lying unoccupied states in the metal oxide as verified by photoemission spectroscopy. From the energy level alignment between MoO3 and CBP one would infer a dopant efficiency of 100%, meaning, that each dopant site (assumingly Mo3O9) is ionized. Nevertheless, results from infrared spectroscopy and further independent techniques indicate that the dopant activation is in the range of 1%, sometimes even below. Therefore, high molar doping ratios of up to 30% are commonly applied for the described or similar systems. The nanostructure of MoO3 agglomerates within the organic thin film was further studied via transmission electron microscopy, electron spectroscopic imaging and electron tomography. Instead of homogenously dispersed MoO3 dopants one finds MoO3 aggregates with a filamentous structure, which are preferentially oriented in the growth direction of the thin film. Charge doping can only occur at the surface of these nano-filaments, explaining the low doping efficiency.