Evidence of Preformed Lewis Acid–Base and Wheland-Type Complexes Acting as Dopants for p-Type Conjugated Polymers

Abstract

Efficient doping of polymer semiconductors is essential for their development as conductors. Although Lewis acids such as B(C 6 F 5 ) 3 have shown promise as dopants for polymers, their doping mechanism is not fully understood. We created 1:1 zwitterionic (including "Wheland-type") complexes of B(C 6 F 5 ) 3 with conjugated molecules difluorobis(triethylsilylethynyl)anthradithiophene (diF-TES-ADT) and didodecylthienothiophene (DTT-12) and characterized them with 1 H NMR, UV−vis spectroscopy, EPR spectroscopy, optical and scanning electron microscopy energy-dispersive X-ray spectroscopy (SEM-EDS), and X-ray diffraction. We employed these complexes as p-dopants for three conjugated polymers and established their doping abilities by conductivity measurements, Seebeck studies, field effect transistor (FET), and remote-gate sensing (RG-FET) measurements. Conductivity changes were dependent on the conjugated molecule adduct component, consistent with the adduct itself serving as the oxidant. The adduct complexes were capable of inducing changes in the surface potential of spun polymer films similar to the behavior shown by conventional dopants. Charge carrier density calculations by remote gate sensing revealed that these adducts can generate holes. We also studied the effect of adding the B(C 6 F 5 ) 3 first, followed by addition of the neat conjugated molecules; the observation of behavior that was different from that using preformed adducts was consistent with the adducts remaining intact during doping. When B(C 6 F 5 ) 3 was added to the polymers, followed by uncomplexed conjugated molecules, the generated hole carrier density is lower than that generated by the B(C 6 F 5 ) 3 dopant but often greater than that generated by the Wheland complexes, indicating a high probability of adduct formation in this case. Density functional theory calculations show that adduct formation between boranes and the conjugated molecules and segments of the polymers is energetically favorable and that some charge transfer between adducts and neutral polymers is plausible if Coulombic and entropic effects are taken into consideration. Thus, such adducts can be considered as possible doping sites for conjugated polymers.

Stable High‐Conductivity Ethylenedioxythiophene Polymers via Borane‐Adduct Doping

“…In the absence of the oligomer adducts, that is, when the polymer is doped with B(C 6 F 5 ) 3 ; it is possible that the polymer:B(C 6 F 5 ) 3 adducts may be a more common B(C 6 F 5 ) 3 transporting medium in the doping process as seen in our previous study, too. [45] The doping mechanism is believed to be similar to our previous study. [45] However, a ≈1.3 eV discrepancy still between the ionization potential of a lone polymer segment and the electron affinity of the oligomer:B(C 6 F 5 ) 3 complexes (Figure S83, Supporting Information).…”

“…[45] The doping mechanism is believed to be similar to our previous study. [45] However, a ≈1.3 eV discrepancy still between the ionization potential of a lone polymer segment and the electron affinity of the oligomer:B(C 6 F 5 ) 3 complexes (Figure S83, Supporting Information). Physically, this means that the energy required to cleave an electron (ionization potential) from any of the polymers explored in this work is not commensurate with the energy released when the dopant complex accepts an electron (electron affinity).…”

“…[50] In our previous study; two synthesized "Wheland complexes", in the form of Lewis acid-base complexes, were shown to be efficient and stable p-dopants and were established to be a more common species than the Meisenheimer complexes in doped polymers. [45] Electrical measurements revealed that our adducts are capable of generating hole carrier densities around 10 22 cm −3 , which is of the same order of magnitude as generated by the Lewis acid dopant B(C 6 F 5 ) 3 itself. DPP(EDOT) 2 -(EDOT) 2 (Figure 1) exhibited a conductivity value of ≈300 cm 2 V −1 s −1 on being doped with B(C 6 F 5 ) 3 and a value of ≈200 cm 2 V −1 s −1 on being doped with our Wheland complex.…”

“…[40][41][42][43][44] In this work a) we synthesized novel D-A copolymers based on DPP and EDOT in varying proportions (Figure 1) to tailor solid-state microstructures and electronic structures to achieve high electrical conductivity, excellent thermal, electrical, and air stability and to overcome the bottlenecks associated poor processability as discussed earlier, and b) we carried out a systematic investigation of the ability a Lewis-acid dopant B(C 6 F 5 ) 3 and its pre-formed adducts of the type "conjugated molecule": B(C 6 F 5 ) 3 to serve as dopants for our novel conductive polymers, as an extension of our previous study. [45] Here, the conjugated molecule is an oligomer resembling the repeat unit of the polymer (Figure 2) as opposed to our previous work where the "oligomer" was more arbitrarily selected. Thus, the present study more closely models the likely interactions of B(C 6 F 5 ) 3 with the polymers themselves and generates dopant complexes that would fit better with the polymer morphologies and generate less energetic disorder.…”

“…[ 50 ] In our previous study; two synthesized “Wheland complexes”, in the form of Lewis acid‐base complexes, were shown to be efficient and stable p ‐dopants and were established to be a more common species than the Meisenheimer complexes in doped polymers. [ 45 ]…”