Fermi level shift and doping efficiency inp-doped small molecule organic semiconductors: A photoelectron spectroscopy and theoretical study

We study the mechanism of molecular doping of the organic small molecule N,N,N${}^{\ensuremath{'}}$,N${}^{\ensuremath{'}}$-tetrakis(4-methoxyphenyl)-benzidine (MeO-TPD) doped with the fluorinated fullerene ${\mathrm{C}}_{60}{\mathrm{F}}_{36}$ or the acceptor molecule 2,2${}^{\ensuremath{'}}$-(perfluoronaphthalene-2,6-diylidene)dimalononitrile (F6-TCNNQ). Varying the doping concentration, photoemission spectroscopy measurements show a comparable Fermi level shift for both dopants. The doping efficiency, defined as the ratio of free charge carriers (holes) to acceptors, is estimated from the depletion layer thickness in metal/intrinsic/$p$-doped structures. For low concentrations, we observe rather high doping efficiencies of up to 36$%$ for ${\mathrm{C}}_{60}{\mathrm{F}}_{36}$, whereas for both dopants the doping efficiency strongly decreases with increasing doping concentration down to less than 10$%$. By numerically solving the charge neutrality equation using a classical semiconductor physics approach and comparing the results to the ultraviolet photoelectron spectroscopy data, we show that for very low concentrations doping is hindered by deep intragap states. In particular, the calculations can statistically explain the strong decrease of the doping efficiency for high doping concentrations.