Unraveling the Mechanism of Molecular Doping in Organic Semiconductors

Abstract

The mechanism by which molecular dopants donate free charge carriers to the host organic semiconductor is investigated and is found to be quite different from the one in inorganic semiconductors. In organics, a strong correlation between the doping concentration and its charge donation efficiency is demonstrated. Moreover, there is a threshold doping level below which doping simply has no electrical effect.

Energetics at Doped Conjugated Polymer/Electrode Interfaces

“…That this holds for the 1:10 films (~5% doping level) is surprising given the work by Heremans et al, [18] on F4TCNQ-doped pentacene crystals that to us would suggest a transition to the high doping (free charges) regime occurring at closer to a couple of percent of doping.…”

“…2. The lack of mobile charges despite significant doping is not unexpected for organic semiconductors at low to intermediate doping levels, however, due to coulomb interaction between the localized charges on the donor and acceptor molecules forming the CT complexes, [18] F4TCNQ -:rr-P3HT + in this case. If no significant density of free charges are available to flow across the interface, instead the most easily oxidized states at the interface (neutral undoped sites of rr-P3HT or TQ1) will be accessed to create the charge transfer necessary to equilibrate the Fermi level and the regular ICT org/sub vs org dependence will occur, which indeed is found to be the case.…”

“…Several studies claim that effective p-type doping occurs when the dopant lowest unoccupied molecular orbital (LUMO) level is deeper than the host highest occupied molecular orbital (HOMO) level in order to realize integer charge transfer. [9,17,18] 2,3,5,6-Tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) with its LUMO located at ~ 5.24 eV [19] should then be a strong p-type dopant upon mixing with the host polymers [9,17] (crystalline) regioregular poly(3-hexylthiophene) (rr-P3HT, HOMO ~ 4.65 eV) and (amorphous) poly [2,3- Several studies find that energy level alignment at organic/electrode and organic/organic interfaces plays a critical role in multilayer stack device, nearly dominating the performance. [20][21][22][23][24] Typically, the energy level alignment of such interfaces follows the trends predicted by the so-called integer charge transfer (ICT) model, i.e.…”

“…Furthermore, they do not have the same energy as bulk polarons, mainly due to Coulombic interaction with the opposite charge across the interface and variations in molecular order. Several different approaches have recently been developed to quantitatively treat the effects of screening and molecular order on the E ICT+,-energies in the ICT model [30][31][32][33][34][35] , and the abruptness of the integer charge transfer region also is being intensively researched and may be material dependent, as Neher and coworkers show a region extending beyond 50 nm for two types of polymers [18] whereas Frisch and coworkers found that the integer charge transfer only involved the first monolayer for rr-P3HT. [36] Here, the aim of our study is to explore the formation mechanism of the molecule-doped conjugated polymer/electrode interface by photoemission spectroscopy, and conclusively establish its universal energy level alignment.…”

“…Analysis is carried out under the assumption that doping leads to charge transfer and not hybridization, as recent investigations by Pingel et al [37] revealed an ionized dopant and free charge formation in F4TCNQ doped polymer rr-P3HT and ruled out intermolecular hybridization [38] in this system. Considering that the doping efficiency is significantly dependent on the doping concentration and there is a threshold above which doping significantly affects electrical conductivity corresponding to a concentration where a sufficient percentage of the bound charge pairs (~5%) overcome the Coulomb dissociation barrier into free charges, [18,37] we chose doping concentrations of 3:1000, 3:100, 1:10 and 2:10 w/w (dopant to polymer weight ratio) in F4TCNQ:rr-P3HT, to span the range of low-intermediate-high doping (Fig.1c). To ascertain that if the polycrystalline structure of rr-P3HT affects the interface behavior, amorphous F4TCNQ:TQ1 is also studied.…”