Intermolecular Hybridization Governs Molecular Electrical Doping

Current models for molecular electrical doping of organic semiconductors are found to be at odds with other well-established concepts in that field, like polaron formation. Addressing these inconsistencies for prototypical systems, we present experimental and theoretical evidence for intermolecular hybridization of organic semiconductor and dopant frontier molecular orbitals. Common doping-related observations are attributed to this phenomenon, and controlling the degree of hybridization emerges as a strategy for overcoming the present limitations in the yield of doping-induced charge carriers.

Figure 1

(a) Schematic of the generally assumed integer electron transfer from OSC (PEN) to $p$ dopant (F4-TCNQ); $E_{\mathrm{vac}}$ denotes the vacuum level, and $H$ and $L$ the HOMO and the LUMO levels, respectively, drawn to scale [23, 24, 37]. (b) Commonly observed energy-level shift upon increasing $p$-dopant concentration, which is seen to saturate at a critical doping concentration; a constant IE is retained. $H$ has never been observed to cross $E_F$, suggesting pinning at (singly occupied) positive polaron states (${\mathrm{P}}^{+}$). (c) Two different propositions for the energy levels in this saturation regime are shown as solid [13, 14] and dashed boxes [8], respectively (center); neutral compounds are shown left and right.

Figure 2

(a) Left: Synchrotron UPS results ($h\nu =35\mathrm{eV}$) in the range of $E_F$ for 20 nm PEN on ${\mathrm{SiO}}_x$ (black curve), $10:1$ doped (blue curve), and $1:1$ mixed (red curve) PEN:F4-TCNQ films; binding energy is given with respect to $E_{\mathrm{vac}}$. Right: Corresponding ionization energies. (b) Synchrotron specular and grazing-incidence x-ray diffraction results; ${\mathbf{q}}_z$ and ${\mathbf{q}}_{||}$ denote the scattering vector components perpendicular and parallel to the substrate, respectively. All spectra are vertically offset for the sake of clarity.

Figure 3

(a) Schematic of frontier-orbital hybridization and orbital isosurface plots for a PEN–F4-TCNQ complex. Circled a, b, and c indicate optical transitions assigned in (c); experimental energy values (exp.) taken from Fig. 2a, Ref. 7 (F4-TCNQ), and Ref. 37 (PEN LUMO), and that for the empty antibonding hybrid orbital estimated via the optical gap assuming an exciton binding energy equal to that of PEN; DFT values (cal.) given in parentheses. (b) Top: Individual PEN HOMO and F4-TCNQ LUMO orbitals; bottom: representative local-minimum mutual orientation. (c) Experimental UV-VIS absorption spectra; transitions assigned to pristine PEN (circled a), the PEN–F4-TCNQ complex (circled b) in films of equal PEN content and to pristine F4-TCNQ (circled c) for a drop-cast reference film. (d) Corresponding cwEPR spectra normalized to number of molecules in the respective films.

Figure 4

(a) Schematic energy-level diagram for molecular electrical $p$ doping via OSC-dopant frontier-orbital hybridization. $H_{\mathrm{hyb}}$ and $L_{\mathrm{hyb}}$ denote the bonding and antibonding supramolecular hybrid orbital, respectively, and IE′ the increased hybrid IE observed here in the $1:1$ blend of OSC and dopant. (b) $n$-type doping proceeds in full analogy to (a); right: chemical structures and calculated bonding hybrid orbitals for the prototypical material pair 1,4,5,8-naphthalene-tetracaboxylic-dianhydride and bis(ethylenedithio)-tetrathiafulvalene.