Quantitative Fermi level tuning in amorphous organic semiconductor by molecular doping: Toward full understanding of the doping mechanism

The doping mechanism in organic-semiconductor films has been quantitatively studied via ultrahigh-sensitivity ultraviolet photoelectron spectroscopy of N,N-bis(1-naphthyl)-N,N-diphenyl-1,1-biphenyl-4,4-diamine (α-NPD) films doped with hexaazatriphenylene-hexacarbonitrile [HAT(CN)₆]. We observed that HOMO of α-NPD shifts to the Fermi level (E_F) in two different rates with the doping concentration of HAT(CN)₆, but HOMO distributions of both pristine and doped amorphous α-NPD films are excellently approximated with a same Gaussian distribution without exponential tail states over ∼5 × 10¹⁸ cm⁻³ eV⁻¹. From the theoretical simulation of the HAT(CN)₆-concentration dependence of the HOMO in doped films, we show that the passivation of Gaussian-distributed hole traps, which peak at 1.1 eV above the HOMO onset, occurs at ultralow doping [HAT(CN)₆ molecular ratio (MR) < 0.01], leading to a strong HOMO shift of ∼0.40 eV towards E_F, and MR dependence of HOMO changes abruptly at MR ∼ 0.01 to a weaker dependence for MR > 0.01 due to future of the dopant acceptor level.