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

Abstract

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 (EF) 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 EF, 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.

Elementary steps in electrical doping of organic semiconductors

“…4a–c for typical parameters ( E CT b = 0.64 eV, σ CT = σ V = 1…150 meV). Despite complete ICT, N A − = N A , a decreasing doping efficiency p / N A with rising N A is obtained due to enhanced population of ICTCs with holes, N CT + , reflecting experimental trends 9 – 11 , 30 – 34 . The Fermi level E F is pinned below E CT b , just as for the previously assumed E A in Eq.…”

“…Most importantly, it was argued that the organic systems are forced into the classical reserve regime due to Fermi level pinning at the acceptor level E A , explaining commonly observed low doping efficiencies of < 10%. 9 , 30 – 34 .…”

Photoelectron spectroscopy on single crystals of organic semiconductors: experimental electronic band structure for optoelectronic properties

“…For this simple case, we approximate w if in eqn (14) by the following relation by using E R f,s = E R i,j-th in eqn (15a) (DE corr,s = 0), allowing very sharp theoretical spectral peaks represented with d functions for relevant energy bands/levels (j…”

“…As an emerging technology in the 21st century, organic semiconductor devices which utilize solids consisting of molecules with widely conjugated p-orbitals instead of atoms of inorganic elements are currently being pervasive into our daily life thanks to a number of technological innovations. [4][5][6][7][8] As summarized in a review report, 9 reasons for such rapid progress of organic devices are that they are believed to have unique functions that cannot be realized by inorganic semiconductors: (i) the organic semiconductor has a limitless-like diversity of tailoring new functional molecules because of freedom of molecular design and synthesis, and the continuous tuning of original energy levels [10][11][12][13] and the Fermi level within the energy gap between the highest occupied molecular orbitals (HOMO) and the lowest fully unoccupied molecular orbitals (LUMO) [10][11][12][13][14] by mixing/doping different molecules; (ii) the closed-shell-like structure of the HOMO and LUMO of these molecules, and weak intermolecular interaction, enable retention of their intrinsic molecular electronic properties in solidstate structures, ranging from randomly-mixed molecular structures to single crystals for both of single-and multi-component systems; and (iii) in many cases the organic semiconductor is 'insensitive' to crystal-imperfection and impurity. These offer significant advantages in the production of organic devices compared with the production of its inorganic counterpart.…”

Disorder compensation controls doping efficiency in organic semiconductors

“…To understand this unusual behavior, we investigated the modification of the density of states, Fermi level shift and the conductivity in a unified theoretical approach, where carriers and their interactions are treated individually with molecular resolution 17 . Our work goes beyond existing approaches and instead of postulating phenomenological modification of the density of states upon doping 9,13,14,18,19 : we extract DOS, Fermi level shift and conductivity from the simulation of the charge carriers dynamics in the actual three-dimensional energy landscape using kinetic Monte-Carlo method 17,20–24 (kMC). Here we analyze the role of energetic disorder in doped organic materials and elucidate the effect of the disorder compensation, which explains the superlinear increase of the conductivity and peculiarities of the Fermi level shift upon doping 16,25–28 .…”