Insight into doping efficiency of organic semiconductors from the analysis of the density of states in n-doped C60 and ZnPc

Abstract

Doping plays a crucial role in semiconductor physics, with n-doping being controlled by the ionization energy of the impurity relative to the conduction band edge. In organic semiconductors, efficient doping is dominated by various effects that are currently not well understood. Here, we simulate and experimentally measure, with direct and inverse photoemission spectroscopy, the density of states and the Fermi level position of the prototypical materials C and zinc phthalocyanine n-doped with highly efficient benzimidazoline radicals (2-Cyc-DMBI). We study the role of doping-induced gap states, and, in particular, of the difference Δ between the electron affinity of the undoped material and the ionization potential of its doped counterpart. We show that this parameter is critical for the generation of free carriers and influences the conductivity of the doped films. Tuning of Δ may provide alternative strategies to optimize the electronic properties of organic semiconductors.

Charge‐Transfer‐Controlled Growth of Organic Semiconductor Crystals on Graphene

“…The models successfully replicated the experimental values (Figure 1b,c). Moreover, the charge transfer modifies the density of states of C 60 so that the LUMO level of charged C 60 molecules is split into an “occupied” LUMO level (L1) that is shifted downward and an unoccupied LUMO level (L2) that is shifted upward (Figure 1d, right) 16. This downshift of the LUMO level upon charge transfer can substantially stabilize C 60 adsorbates on graphene 17…”

Excited State Dynamics of a Self‐Doped Conjugated Polyelectrolyte

“…In Figure a, the higher doping concentration after addition of H 2 SO 4 to CPE‐K is obvious when comparing the ratio between the absorption bands for the N 1 and P 2 transitions, which clearly increases in CPE‐K/H 2 SO 4 . We have shown above that the concentration of doped monomers increases from 17% to 27% with the addition of acid, as also confirmed via EPR and conductivity measurements . The doping level is relatively high for both CPE‐K and CPE‐K/H 2 SO 4 films, with practically all chains containing doped sites and being close to polarons of other chains due to chain packing in the films (insets of Figure a).…”

“…Achieving high conductivity in doped organic semiconductors requires high mobility and a high yield of free charges that do not remain bound (electrostatically or by orbital hybridization) to the ionized dopant 3,5,6c. Here, via spectroscopic elucidation of the optoelectronic properties of a self‐doped system, we find characteristics of the polaron that might help its dissociation from interfacial bound states.…”

“…Significant efforts have thus been invested to understand and to improve the electrical and transport properties of doped organic semiconductors through understanding of the elementary processes associated with different levels and types of doping . Experimental and theoretical research on doped organic semiconductors has been intensified over the past few years as a consequence of recent synthetic advances that enable better control over the bulk distribution and stability of charges . One drawback in the field is that upon doping of organic semiconductors, even for systems where integer charge transfer occurs between the dopant and the host, only a small fraction (typically 5%) of the charges are mobile and contribute to the conductivity, while most of them remain bound to the ionized dopant .…”

Molecular Semiconductors for Logic Operations: Dead‐End or Bright Future?

“…Surprisingly, the η Dop is on the order of 10% for most systems. Several causes have been invoked of which clustering of dopant molecules, charge carrier traps, large dopant activation energy caused by large Coulomb interaction, or strong hybridization of dopant and matrix molecules, and impurity reserve regime . At a molecular level, two extreme situations can occur when an electron donor and an electron acceptor encounter: either a full charge transfer takes place giving rise to the formation of an ion‐pair or hybrid states are created, as shown in Figure a,b, respectively .…”