A pre-formed Meisenheimer complex of an NDI withTBAF was obtained in a simple way by mixing dibrominated NTCDI and tetrabutylammonium fluoride (TBAF) in solution and used as dopant for n-type organic thermoelectrics. Two n-type polymers PNDIClTVT and PBDOPVTT were synthesized, n-doped, and characterized as conductive and thermoelectric materials. PNDIClTVT doped with the NDI-TBAF presents a high σ value of 0.20 S cm -1 , a Seebeck Coefficient, S, of -1854 μV K -1 and a power factor (PF) of 67 μW m -1 K -2 , among the highest reported PF in solution-1 processed conjugated n-type polymer thermoelectrics. Using N-DMBI and NDI-TBAF as co-dopants, PNDIClTVT has a PF >35 μW m -1 K -2 ; while for PBDOPVTT σ = 0.75 S cm -1 and PF = 58 μW m -1 K -2 . In this work, we found that an ionic adduct together with a neutral dopant improved the performance of n-type organic thermoelectrics leading to an enhanced power factor, and elucidated the role of such an adduct more generally in polymer doping.Received: ((will be filled in by the editorial staff))Revised: ((will be filled in by the editorial staff))
Achieving high electrical conductivity and thermoelectric power factor simultaneously for n‐type organic thermoelectrics is still challenging. By constructing two new acceptor‐acceptor n‐type conjugated polymers with different backbones and introducing the 3,4,5‐trimethoxyphenyl group to form the new n‐type dopant 1,3‐dimethyl‐2‐(3,4,5‐trimethoxyphenyl)‐2,3‐dihydro‐1H‐benzo[d]imidazole (TP‐DMBI), high electrical conductivity of 11 S cm−1 and power factor of 32 μW m−1 K−2 are achieved. Calculations using Density Functional Theory show that TP‐DMBI presents a higher singly occupied molecular orbital (SOMO) energy level of −1.94 eV than that of the common dopant 4‐(1, 3‐dimethyl‐2, 3‐dihydro‐1H‐benzoimidazol‐2‐yl) phenyl) dimethylamine (N‐DMBI) (−2.36 eV), which can result in a larger offset between the SOMO of dopant and lowest unoccupied molecular orbital (LUMO) of n‐type polymers, though that effect may not be dominant in the present work. The doped polymer films exhibit higher Seebeck coefficient and power factor than films using N‐DMBI at the same doping levels or similar electrical conductivity levels. Moreover, TP‐DMBI doped polymer films offer much higher electron mobility of up to 0.53 cm2 V−1 s−1 than films with N‐DMBI doping, demonstrating the potential of TP‐DMBI, and 3,4,5‐trialkoxy DMBIs more broadly, for high performance n‐type organic thermoelectrics.
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.
We illustrate the critical importance of the energetics of cation–solvent versus cation–iodoplumbate interactions in determining the stability of ABX3 perovskite precursors in a dimethylformamide (DMF) solvent medium. We have shown, through a complementary suite of nuclear magnetic resonance (NMR) and computational studies, that Cs+ exhibits significantly different solvent vs iodoplumbate interactions compared to organic A+-site cations such as CH3NH3 + (MA+). Two NMR studies were conducted: 133Cs NMR analysis shows that Cs+ and MA+ compete for coordination with PbI3 – in DMF. 207Pb NMR studies of PbI2 with cationic iodides show that perovskite-forming Cs+ (and, somewhat, Rb+) do not comport with the 207Pb chemical shift trend found for Li+, Na+, and K+. Three independent computational approaches (density functional theory (DFT), ab initio Molecular Dynamics (AIMD), and a polarizable force field within Molecular Dynamics) yielded strikingly similar results: Cs+ interacts more strongly with the PbI3 – iodoplumbate than does MA+ in a polar solvent environment like DMF. The stronger energy preference for PbI3 – coordination of Cs+ vs MA+ in DMF demonstrates that Cs+ is not simply a postcrystallization cation “fit” for the perovskite A+-site. Instead, it may facilitate preorganization of the framework precursor that eventually transforms into the crystalline perovskite structure.
Efficient doping of polymer semiconductors is required for high conductivity and efficient thermoelectric performance. Lewis acids, e.g., B(C6F5)3, have been widely employed as dopants, but the mechanism is not fully understood. 1:1 “Wheland type” or zwitterionic complexes of B(C6F5)3 are created with small conjugated molecules 3,6‐bis(5‐(7‐(5‐methylthiophen‐2‐yl)‐2,3‐dihydrothieno[3,4‐b][1,4]dioxin‐5‐yl)thiophen‐2‐yl)‐2,5‐dioctyl‐2,5‐dihydropyrrolo[3,4‐c]pyrrole‐1,4‐dione [oligo_DPP(EDOT)2] and 3,6‐bis(5''‐methyl‐[2,2':5',2''‐terthiophen]‐5‐yl)‐2,5‐dioctyl‐2,5‐dihydropyrrolo[3,4‐c]pyrrole‐1,4‐dione [oligo_DPP(Th)2]. Using a wide variety of experimental and computational approaches, the doping ability of these Wheland Complexes with B(C6F5)3 are characterized for five novel diketopyrrolopyrrole‐ethylenedioxythiophene (DPP‐EDOT)‐based conjugated polymers. The electrical properties are a strong function of the specific conjugated molecule constituting the adduct, rather than acidic protons generated via hydrolysis of B(C6F5)3, serving as the oxidant. It is highly probable that certain repeat units/segments form adduct structures in p‐type conjugated polymers which act as intermediates for conjugated polymer doping. Electronic and optical properties are consistent with the increase in hole‐donating ability of polymers with their cumulative donor strengths. The doped film of polymer (DPP(EDOT)2‐(EDOT)2) exhibits exceptionally good thermal and air‐storage stability. The highest conductivities, ≈300 and ≈200 S cm−1, are achieved for DPP(EDOT)2‐(EDOT)2 doped with B(C6F5)3 and its Wheland complexes.
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