n-Dopants play a crucial role in improving organic electronic devices through controlled doping of organic semiconductors. Benzimidazoline-based dopants have been reported as one of the best solution-processed n-type dopant precursors. In this study, two benzimidazoline-based dopants (BIBDTO and BBIBDTO) were prepared using benzo[1,2-b:4,5-b′]dithiophene as the 2-Ar unit, and their n-doping properties on the fullerene derivative PTEG-2 as the host material were carried out. For BIBDTO and BBIBDTO, respectively, the temperature at which 5% weight loss was achieved was 229 and 265°C. By comparing the ultraviolet-visible absorption spectroscopy, cyclic voltammetry, and density functional theory calculated data, it is found that BBIBDTO has a higher energy level, which is more favorable for charge transfer. Additionally, both the oxidative titration experiments and conductivity characterization of the dopants showed that BBIBDTO was more advantageous at low doping concentrations, and the BBIBDTO-doped PTEG-2 films obtained a conductivity of 0.15 S cm−1 at 10 mol% doping concentration. However, at high dopant concentrations, the dopant volume increases, potentially disrupting the microstructure. The highest conductivity of 0.29 S cm–1 was obtained at a BIBDTO doping concentration of 15 mol%. This study delves into the effect of benzimidazole functional groups on the doping performance of benzimidazoline-based dopant molecules, providing insight into designing novel efficient n-type dopant molecules and further selecting the type of dopant for various doping systems.
Organic semiconductor materials, particularly organic thermoelectric materials, are potential futuristic materials due to their lightweight, high flexibility, and ability to produce large-area devices via solution processing [1,2,3,4]. The comprehensive index for the thermoelectric characteristics of materials is defined by the dimensionless thermoelectric figure of merit ZT = S 2 σT/k, where S is the Seebeck coefficient, σ is the electrical conductivity, T is the absolute temperature, and k is the thermal conductivity [5,6]. The thermal conductivity of organic thermoelectric materials is typically lower than that of conventional inorganic thermoelectric materials. But the low electrical conductivity of organic materials in their intrinsic state is disadvantageous over inorganic thermoelectric materials, limiting their practical applications. Thus, controlling the electrical conductivity of organic semiconductors is crucial for their applicability. Chemical doping serves as a plausible solution for modulating the electrical conductivity of organic semiconductors, producing transportable carriers by either oxidizing (p-doping) or reducing (n-doping) the organic semiconductor [7,8,9].
Among p-type organic thermoelectric materials, poly [2,5-bis(3-alkylthiophene-2-yl) thiophene(3,2-b)] (PBTTT) doped with Lewis acid or F4TCNQ has resulted in increased conductivity to 250–1100 S cm–1 [10,11,12]. Either p-type or n-type thermoelectric materials are sufficient to construct thermoelectric devices ; however, to achieve high conversion efficiencies, the synergy of high-performance p-type and n-type materials is required. This signifies the need to develop high-performance n-type materials in constructing efficient organic thermoelectric devices. Nevertheless, the processability and miscibility of the dopant with the host material, dopant stability, and doping efficiency are the persistent complications in doping of n-type organic thermoelectric materials [7,14,15,16]. Metal complexes and inorganic salts, for example, can only be deposited by thermal evaporation, severely limiting their use in solution-processable organic semiconductors [17,18,19]. Solution-processable aromatic molecules, such as charge-transfer complexes, have also been used as n-type dopants; however, they are unstable in the air . Later, in 2010, a solution-processable and air-stable n-dopant, N-DMBI, was reported by Wei et al.  N-DMBI breaks the 2-site C–H bond during thermal annealing, producing radicals and transferring electrons to the host material for n-doping, or releasing hydrogen negative ions (H–) for alkali doping of the host material. It has also shown excellent doping capabilities toward a wide range of semiconductors (as shown in Figure 1) [22,23,24,25]. Although benzimidazoline-based small molecules are widely used as n-type dopants, the role of benzimidazole functional groups is still unclear.
We designed and synthesized benzimidazoline-based dopant molecular systems with mono/bi-benzimidazole functional groups and benzo[1,2-b:4,5-b′]dithiophene as the central unit, as well as alkyl side chains of appropriate lengths to ensure dopant solubility. The two types of dopants are BIBDTO (monofunctional group) and BBIBDTO (bifunctional group) (Figure 2). Fullerene derivative PTEG-2 with bis(triethylene)-glycol-type side chains was chosen as the host material owing to its polar side chains, which improve the miscibility of the dopant with the host material. Particularly in solution co-deposited films, the dopant is located in the side chain plane of PTEG-2, reducing the negative effect of dopant volume change on the π–π stacking of PTEG-2 [26,27,28]. The effects of benzimidazole functional groups on the molecular heat treatment temperature of benzimidazoline-based dopants, the single occupied molecular orbital (SOMO) energy level of imidazoline radicals, and their doping properties were revealed using electrochemical tests, ultraviolet (UV)-visible (vis) spectroscopy, and density functional theory (DFT) calculations. Hence, this study provides guidelines for the design of higher-performance benzimidazoline-based dopants.
2 Experimental section
2.1 Materials and characterization
Benzo[1,2-b:4,5-b′]dithiophene-4,8-dione was purchased from ENERGY CHEMICAL (98% purity). The complexes, N,N′-dimethyl-o-phenylenediamine , 4,8-diethoxybenzo[1,2-b:4,5-′]dithiophene , 4,8-bis((2-ethylhexyl)oxy)benzo[1,2-b:4,5-b′]dithiophene , PTEG-2  were prepared following the literature (for specific synthesis steps, see Supplementary materials).
DFT calculations were carried out for the reduced neutral state, neutral radical state, and oxidized cationic state of benzimidazoline-based dopants to obtain the highest occupied molecular orbital (HOMO), SOMO, and lowest unoccupied molecular orbital (LUMO), respectively. Calculations have been carried out at the (U)B3LYP/6–311++G(d,p) level of theory using the Gaussian16 program package (revision A.03) . The additional information about the instrumentation and characterization are presented in Supporting Information.
2.2 Synthesis of compounds
The molecular structures and synthetic routes of benzimidazoline-based dopants BIBDTO and BBIBDTO are shown in Figure 2.
2.2.1 4,8-Diethoxybenzo[1,2-b:4,5-b′]dithiophene-2-carbaldehyde (compound 2)
Accurately weighed 4,8-diethoxybenzo[1,2-b:4,5-b′]dithiophene (556.7 mg, 2 mmol) and 25 mL anhydrous tetrahydrofuran were taken into a 50 mL dry double-necked flask under nitrogen atmosphere. Then, at –78°C, 2.5 M n-butyllithium solution (2 mmol, 0.8 mL) was slowly added drop wise into the aforementioned mixture. After 1 h, 0.3 mL of anhydrous N,N-dimethylformamide was added, slowly ramped up to room temperature, poured the mixture into deionized water (50 mL), and extracted with dichloromethane. The organic phase was dried with anhydrous MgSO4, and the crude product was purified using silica gel column chromatography using petroleum ether:ethyl acetate (10/1, v/v) eluent. Finally, 551.5 mg of an orange solid product was obtained (89% yield): 1H NMR (500 MHz, CDCl3), δ (ppm): 10.10 (s, 1H), 8.20 (s, 1H), 7.50 (s, 2H), 4.46 (d, J = 7.0 Hz, 2H), 4.36 (d, J = 7.0 Hz, 2H), and 1.56–1.46 (m, 6H) (Figure S2).
2.2.2 4,8-Bis((2-ethylhexyl)oxy)benzo[1,2- b :4,5- b ′]dithiophene-2,6-dicarbaldehyde (compound 4)
Compound 4 was synthesized and purified following the same procedure as that for the preparation of compound 2. Accurately weighed 4,8-bis((2-ethylhexyl)oxy)benzo[1,2-b:4,5-b′]dithiophene(446.7 mg, 2 mmol), 2.5 M n-butyllithium solution (2 mmol, 0.8 mL) and 25 mL anhydrous tetrahydrofuran as solvent. After the reaction for 1 h, 0.3 mL of anhydrous N,N-dimethylformamide was added. An orange solid product was obtained (85% yield). 1H NMR (500 MHz, CDCl3), δ (ppm): 10.13 (s, 2H), 8.18 (s, 2H), 4.28 (dd, J = 5.5, 1.8 Hz, 4H), 1.84 (dd, J = 12.2, 6.0 Hz, 2H), 1.71–1.55 (m, 4H), 1.39 (dd, J = 7.5, 3.8 Hz, 12H), 1.02 (t, J = 7.5 Hz, 4H), and 0.97–0.91 (m, 4H) (Figure S3).
2.2.3 2-(4,8-Diethoxybenzo[1,2-b:4,5-b′]dithiophen-2-yl)-1,3-dimethyl-2,3-dihydro-1 H -benzo[ d ]imidazole (BIBDTO)
To a 25 mL microwave reaction flask, N,N′-dimethyl-o-phenylenediamine (136.2 mg,1 mmol) and compound 2 (306.4 mg, 1 mmol) were added. Then, 2 mL of methanol was used as the solvent, and a drop of glacial acetic acid was added to the mixture. After microwave reaction at 60°C for 1 h, the precipitate was filtered and the pale-yellow crystalline product was obtained by recrystallization from acetone (40% yield). 1H NMR (500 MHz, CDCl3), δ (ppm): 7.64 (s, 1H), 7.50 (d, J = 5.4 Hz, 1H), 7.39 (d, J = 5.4 Hz, 1H), 6.83 6.70 (m, 2H), 6.55 6.46 (m, 2H), 5.22 (s, 1H), 4.42 (q, J = 7.0 Hz, 2H), 4.37 (q, J = 7.0 Hz, 2H), 2.74 (s, 6H), 1.54 (t, J = 7.0 Hz, 3H), and 1.48 (t, J = 7.0 Hz, 3H). 13C NMR (125 MHz, CDCl3), δ (ppm): 145.07, 144.53, 144.10, 141.44, 132.47, 131.04, 130.34, 129.99, 126.19, 120.99, 120.49, 119.69, 106.24, 89.72, 69.40, 69.32, 33.44, 16.14,16.08. MS for C23H24N2O2S2: calcd 424.13; and found 424.05 (M+) (Figures S4–S6).
2.2.4 2,2′-(4,8-Bis((2-ethylhexyl)oxy)benzo[1,2- b :4,5- b ′]dithiophene-2,6-diyl)bis(1,3-dimethyl-2,3-dihydro-1 H -benzo[ d ]imidazole) (BBIBDTO)
The synthesis was carried out using the same reaction procedure as that for the preparation of the dopant BIBDTO. Accurately weighted quantities of N,N′-dimethyl-o-phenylenediamine (272.4 mg, 2 mmol) and Compound 4 (502.7 mg, 1 mmol) were taken in a 25 mL microwave reaction flask and 2 mL of methanol was used as solvent with a drop of glacial acetic acid. After the reaction, the crude product was filtered and recrystallized using acetone to obtain a yellow crystalline product (38% yield). 1H NMR (500 MHz, CDCl3), δ (ppm): 7.61 (s, 2H), 6.78 (dd, J = 5.4, 3.2 Hz, 4H), 6.51 (dd, J = 5.4, 3.2 Hz, 4H), 5.21 (s, 2H), 4.23 (dd, J = 5.4, 4.2 Hz, 4H), 2.73 (s, 12H), 1.84–1.78 (m, 2H), 1.72–1.30 (m, 16H), 1.00 (t, J = 7.5 Hz, 6H), and 0.91 (t, J = 7.0 Hz, 6H). 13C NMR (125 MHz, CDCl3), δ (ppm): 145.07, 144.70, 141.49, 131.38, 129.47, 121.12, 119.69, 106.23, 89.81, 75.84, 40.68, 33.41, 30.37, 29.18, 23.83, 23.08, 14.12, 11.32. MS for C44H58N4O2S2: calcd 738.40; found 738.35 (M+) (Figures S7–S9).
3 Results and discussion
3.1 Thermal stability
The effect of the number of benzimidazole functional groups on the thermal stability of benzimidazole-like dopants was investigated using thermogravimetric analysis (TGA) (Figure 3). During the analysis, both dopants were placed in a nitrogen atmosphere to prevent their reaction with H2O and O2 in the air, affecting the reliability of the test results. The analysis was carried out at a temperature ranging from room temperature to 600°C, and the ramp rate was 10°C min−1. The obtained 5% weight loss temperature was 229 and 265°C for BIBDTO and BBIBDTO, respectively. Thus, the results indicate that the use of double benzimidazole functional groups in the design strategy has significantly improved the thermodynamic stability of benzimidazole derivatives containing the same core unit.
3.2 Energy level structure
To obtain the energy-level structures of BIBDTO and BBIBDTO, the two dopants were analyzed using UV-vis absorption spectroscopy and cyclic voltammetry (Table 1). As shown in Figure 4, the dopants exhibit an oxidation onset potential of 0.14 and 0.09 V for BIBDTO and BBIBDTO, respectively, using the reference electrode Ag/AgNO3 (–4.7 eV for the electrode potential E Ag/AgNO3 relative to vacuum). For BIBDTO, the first oxidation is quasi-reversible, and a minor peak at 0.2 V provides a negative reduction current. Compared with BIBDTO, there is no reduction peak in the voltammogram of BBIBDTO, showing irreversible oxidation characteristics. The HOMO energy levels of BIBDTO and BBIBDTO were calculated following the equation.
|Dopant||λ opt onset (nm)||E opt g(eV)||CV E (V)||LUMO (eV)||CVHOMO (eV)||DFTLUMO (eV)||DFTSOMO-1 (eV)||DFTSOMO-2 (eV)||DFTHOMO (eV)|
The obtained HOMO energy levels of BIBDTO and BBIBDTO are –4.84 and –4.79 eV, respectively, suggesting easy oxidation characteristics with similar values. The UV-vis absorption spectra of the dopants in dichloromethane solution (concentration of 1 × 10–5 M) are shown in Figure 6. The absorption profile is concentrated in the UV region for the dopants with the starting absorption wavelength λ opt onset of 369 and 383 nm for BIBDTO and BBIBDTO, respectively. Additionally, the optical band gaps were 3.36 and 3.23 eV (E optg = 12.40/λ opt onset) for BIBDTO and BBIBDTO, respectively. The increase in the number of benzimidazole functional groups has resulted in narrowing the bandgap. The LUMO energy levels of BIBDTO and BBIBDTO were obtained as –1.48 and –1.56 eV for BIBDTO and BBIBDTO, respectively, from the following equation.
The frontier molecular orbitals of BIBDTO, BBIBDTO, and their imidazoline radicals were calculated using DFT at the (U)B3LYP/6–311++G(d,p) level to reveal the electronic structures of the dopants, especially the SOMO energy levels of their corresponding imidazoline radicals that are difficult to be obtained experimentally (Figure 5). The results show that the HOMO electron cloud distribution of BIBDTO and BBIBDTO is mostly influenced by the benzimidazole functional group. The obtained HOMO energy levels are –4.72 and –4.53 eV for BIBDTO and BBIBDTO, respectively, indicating the presence of the double benzimidazole functional group can increase the HOMO energy level. The LUMO electron cloud distribution of the dopants is primarily affected by the core 2-Ar unit, and the LUMO energy levels were found to be –1.16 and –1.35 eV, respectively, showing a decreasing trend. The calculated HOMO and LUMO energy levels obtained are in good agreement with the experimental results. Furthermore, no spontaneous electron transfer is expected between the dopants and the host material PTEG-2 because the dopants’ HOMO energy levels are lower than the LUMO energy level of PTEG-2 (−4.06 eV; Figure S10) . Typically, the benzimidazole derivatives can produce high-energy imidazoline radicals as a single-electron donor by thermal annealing [21,35]. In this study, the distribution of SOMO electron clouds and their orbital energy levels were obtained using DFT calculations. As the C–H bond at dopant position 2 is broken, producing imidazoline radical, the hybridization state of 2-C changes from sp3 to sp2. The optimized configuration of the imidazoline radical has a planar structure, facilitating the effective conjugation between the core 2-Ar unit and the benzimidazole unit. Therefore, the distribution of the SOMO electron cloud of the two dopant imidazoline radicals can be attributed to the synergistic effect of the core 2-Ar unit and benzimidazole unit. The SOMO energy levels of the BIBDTO single imidazoline radical, BBIBDTO single radical, and BBIBDTO double radical were found to be –1.54, –163, and –1.07 eV, respectively. Among all, the bi-radical exhibits a higher SOMO energy level. As shown in Figure 1, higher SOMO energy levels will provide higher δE ET, facilitating spontaneous electron transfer of the radicals to the host material and eventually forming the corresponding cationic products.
3.3 Oxidative titration UV-vis absorption spectra
The UV-vis absorption spectra of the dopant cations were measured by adding the oxidant tris (4-bromophenyl) ammonium hexachloroantimonate (TBAH) stepwise to a dichloromethane solution of each benzimidazole derivative (final solution concentration of 1 × 10–5 M). Figure 6a and c show the changes in UV-vis absorption of BIBDTO after dropwise addition of 1 eq. and 2 eq. TBAH, respectively. As can be observed from Figure 6c, the absorption peak gradually enhanced with time after the addition of TBAH, reaching a maximum at 14 min. Further increase in time showed no obvious change in the absorption peak, indicating that BBIBDTO had been completely oxidized at this time. Meanwhile, no visible change was observed even after 30 min in the absorption curve, indicating that the dopant was oxidized to form cations that are stable in the air for a long time. After the oxidation of BIBDTO by 1 eq. TBAH, the UV-vis absorption peak showed the same trend in change as was observed for the time. However, the small change in absorption intensity after 14 min is due to the increase in the concentration of low boiling dichloromethane caused by the volatilization. It can be observed from Figure 6b that the BIBDTO with the mono-benzimidazole functional group gradually increases with the amount of TBAH, and the neutral state absorption peak at 362 nm gradually disappears and is accompanied by a new absorption band between 375 and 480 nm. The new peak can be ascribed to the formation of the C–T complex between the donor and the acceptor. The absorption between 375 and 480 nm is saturated when TBAH reached 1 eq. and showed only a slight enhancement after it. Unlike the mono-benzimidazole functional group BIBDTO, the absorption of the bifunctional BBIBDTO radical cation at 375–480 nm reaches a maximum after the amount of TBAH reaches 2 eq. As a result, the bifunctional dopant molecule can give two electrons at the same time and has a stronger n-doping ability than the monofunctional dopant (Figure 6d).
3.4 Electrical characterization
As discussed in the previous sections, benzimidazole derivatives need to be thermally activated for the formation of a high-energy radical state. Therefore, the optimal temperature for dopant activation was investigated to test the change in conductivity of the doped state films at different doping concentrations. The same core 2-Ar unit structure was used for the mono- and bi-benzimidazole functional group dopants, which can affect the optimum activation temperature. Figure S12 shows the variation curve of conductivity with increasing annealing temperature at the same doping concentration. The doping concentration was 20 mol%, the heating duration used for different annealing temperatures was also maintained at one hour, and the thermal annealing was carried out under nitrogen protection. At a 20 mol% doping concentration, the conductivity of PTEG-2 doped with dopants showed an increasing trend as the annealing temperature increased from 75 to 125°C. This was related to the thermal activation process of the benzimidazole derivatives and the local spatial alignment between the host material and the dopant under thermal stress. As the annealing temperature rises further, the conductivity shows a downward trend due to the different polarities of the solution-mixed dopants and the fullerene derivatives. The optimum annealing temperature for both mono- and bi-benzimidazole functional group dopants was found to be 125°C.
Figure 7 shows the room temperature conductivity curves of doped PTEG-2 films with different doping concentrations after annealing at 125°C for one hour. According to the electrical conductivity equation σ = qnμ (q, carrier charge; n, charge carrier density; and μ, charge carrier mobility), great charge carrier density and efficient carrier transport are essential for high conductivity. The conductivity of the films doped with the two dopants shows an increase followed by a decreasing trend. The increase can be attributed to the added dopants, which introduce extra carriers into the host material. The highest conductivity of 0.29 S cm–1 was obtained at a BIBDTO doping concentration of 15 mol%, while the highest conductivity of 0.15 S cm–1 was obtained at a BBIBDTO doping concentration of 10 mol%. The carrier transport properties of organic semiconductor materials are critically related to the multiscale ordering, including their microscopic stacking and morphology. In many circumstances, it is expected that doping by combining molecular dopants into conjugated semiconductor matrices at a high dopant loading will significantly induce structural disorder . Upon further increase in doping ratio, the decreasing trend in conductivity can be ascribed to the miscibility between the dopant and the host material and damage to the microstructure by the dopant. The BBIBDTO-doped films have the highest conductivity at doping concentrations of less than 7 mol%. This reflects the contribution of the bifunctional dopant to increasing the carrier concentration that dominates at low doping concentrations, as well as the stronger doping ability of BBIBDTO than the monofunctional group BIBDTO. On the other hand, when the doping concentration is further increased, the large volume of bifunctional dopants gradually prevails, destroying the microstructure and is detrimental to the enhancement of conductivity.
In this work, we successfully designed and synthesized two benzimidazole dopants, BIBDTO with a mono-benzimidazole functional group and BBIBDTO with bi-benzimidazole functional groups using benzo[1,2-b:4,5-b′]dithiophene as the 2-Ar. The excellent thermal stability and stronger electron-donating ability of BBIBDTO are confirmed by TGA and UV-vis absorption spectra. Further, the electrochemical analysis and theoretical simulations suggested that benzimidazole functional groups primarily influence the HOMO electron cloud distribution of benzimidazoline-based dopants, LUMO is distributed on 2-Ar, and SOMO has both HOMO and LUMO characteristics. The strategy of the bi-benzimidazole functional group can increase the energy level of dopant and imidazoline radicals, favoring n-doping. The conductivity studies implied that the bi-benzimidazole functional group dopants have greater doping capacity at low doping concentrations. At high doping concentrations, the monofunctional BIBDTO is preferable with substantial conductivity. These results can provide guidelines for designing new-generation n-dopants and selecting the type of dopant in different doping systems.
Funding information: This research was funded by the National Key Research and Development Plan (grant No. Q2019YFE-010720).
Author contributions: All data were obtained by authors equally.
Conflict of interest: The authors state no conflict of interest.
Ethical approval: The conducted research is not related to either human or animal use.
Data availability statement: All data generated or analyzed during this study are included in this published article and its supplementary material files.
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