Synthesis, characterization and OFET performance of A–D–A semiconducting small molecules functionalized with perylene diimide groups

Abstract

Organic semiconductor molecules have been utilized in commercial applications owing to their outstanding advantages, such as tunable absorption spectra and electronic energy levels, and superior thermal stability. Three perylene diimide (PDI)-based small molecules have been synthesized and their chemical–physical, electrochemical and organic field effect transistor (OFET) properties have been characterized. Three different central IDT electron donor cores symmetrically functionalized with PDI electron-acceptor moieties and flanked in the terminal positions with alkyl groups were purposely designed. All the molecules consist of a PDI group as the acceptor (A) unit and indacenodithiophene (IDT) as the donor (D) unit. The impact of the acceptor–donor–acceptor (A–D–A) structures on the photoelectric performance of the molecules was investigated. The conjugated molecules exhibit full coverage absorption in the visible light spectrum, implying that they could be utilized for more potential applications in optoelectronic devices. All the molecules exhibited good thermal stability and typical n-type semiconductor behavior with a highest electron mobility of up to 2.09 × 10⁻¹ cm² V⁻¹ s⁻¹ when I_on/I_off = 10⁴. S-PDI-IDT-2 exhibited ambipolar transport properties, with a hole transportation mobility on the order of 10⁻³.

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1. Introduction

Organic semiconductors (OSCs) are key materials that have been applied in various organic electronic devices, such as OFETs and organic photovoltaics (OPVs)^1–18 because of their low-cost, large-area fabrication, excellent mechanical flexibility and the ease with which their energy levels can be modulated through organic synthesis. In recent years, organic electronic products have received increasing attention in the fields of flexible displays and wearable devices. Among the many known electron acceptors, non-fullerene materials, such as PDI and its derivatives in particular have been researched extensively due to their low energy levels, high carrier mobility and other unique advantages associated with their charge carrier transport behaviors and photoelectric performance.^{5,8,10–17,19–43} However, the electron transfer pathways in PDI are restricted because of its self-aggregation behavior. Breaking this self-aggregation to achieve a twisted molecular geometry is an effective method to overcome this problem. Therefore, the molecular arrangement and stacking properties of PDI should be optimized by molecular design and investigation.^44–46 IDT has been used as an electron-donating unit because IDT has a well-conjugated fused ring structure and branched hexylphenyl groups, which suppress the strong aggregation between the molecules, whilst the strong electron-donating ability of IDT leads to stronger intramolecular charge transfer (ICT).^47–51

In this work, three molecules with branched alkyl chains were successfully synthesized via Stille coupling. The chemical structures are shown in Scheme 1. Three kinds of electron donor with IDT as the central core and symmetrically functionalized with PDI electron-acceptor moieties were purposely designed and a further extension of the molecular conjugation could be obtained via the use of cross coupling reactions in the preparation of the molecules. It was possible to obtain tailored tuning of the molecular orbitals energies and to achieve the optimization of the hole and electron mobilities of the OFETs by combining IDT units with PDI units. A–D–A structures were built and constructed as channels for efficient electron transport that allow electrons to be more delocalized, leading to a red shift and the extension of absorption bands.⁵² All the molecules were thoroughly characterized in terms of their thermal, structural, optical, and electrochemical properties. The molecules were finally processed as thin films and applied as active layers in OFETs to investigate their charge transport properties. The carrier mobility was tested in a glove box and all the examinations revealed that the three molecules exhibited n-type semiconductor transport capabilities. In the bottom gate/top contact (BGTC) OFET devices fabricated from the molecules, the molecules S-PDI-IDT-1, S-PDI-IDT-2 and S-PDI-IDT-3 demonstrated the highest charge mobilities of 4.59 × 10⁻² cm² V⁻¹ s⁻¹, 4.59 × 10⁻² cm² V⁻¹ s⁻¹ and 2.09 × 10⁻¹ cm² V⁻¹ s⁻¹ in the glove box. GIWAXS and AFM were conducted to explore the microstructure of the thin films.

Scheme 1 Synthesis of the three molecules.

2. Results and discussion

2.1. Synthesis and characterization

N,N′-Bis(2-octyldodecyl)-1-bromoperylene,3,4,9,10-tetracarboxylic diimide (PDI-Br), (4,4,9,9-tetrahexyl-4,9-dihydro-s-indaceno[1,2-b:5,6-b′]-dithiophene-2,7-diyl)bis(trimethylstannane) (IDT-1), 1,1′-[6,6,12,12-tetrakis(4-hexylphenyl)-6,12-dihydrodithieno[2,3-d:2′,3′-d′]-s-indaceno[1,2-b:5,6-b′]dithiophene-2,8-diyl]bis[1,1,1-trimethylstannane] (IDT-2), and (4,4,9,9-tetrakis(4-hexylphenyl)-4,9-dihydro-s-indaceno[1,2-b:5,6-b′]dithiophene-2,7-diyl)bis(trimethylstannane) (IDT-3) were obtained through direct purchase from Derthon Optoelectronic Materials Science Technology Co. Ltd, and were used without further purification. PDI-Br was reacted with IDT-1, IDT-2, and IDT-3via a Stille coupling reaction in nitrogen for 48 h in toluene solvent using 2 mL of dimethylformamide (DMF) and Pd(PPh₃)₄ (24 mg, 0.02 mmol). The molecules were dissolved in chloroform with minimum solvent and then methanol was added dropwise to the solution for precipitation. The crude products were further refined via column chromatography with a mixture of petroleum ether and dichloromethane (1 : 1–1 : 1.5) as the eluent, yielding the target molecules S-PDI-IDT-1, S-PDI-IDT-2, and S-PDI-IDT-3. The structures of the molecules are displayed in Scheme 1 (for detailed ¹H NMR data for each molecule see Fig. S4, S10, and S16 (ESI†). For detailed ¹³C NMR data for each molecule see Fig. S5, S11 and S17 (ESI†). For detailed high resolution mass spectrometry data for each molecule see Fig. S6, S12 and S18, ESI†).

Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were carried out to characterize the thermal properties under a nitrogen atmosphere. Fig. 1(a) shows that all the molecules exhibit excellent thermal stability with decomposition temperatures (T_d, 5% weight loss) of 459.82 °C, 472.31 °C, and 476.88 °C, which were all above 300 °C, thus demonstrating that the three molecules could be used for device preparation (for detailed data for each molecule see Fig. S2, S8 and S14, ESI†). The DSC curves in Fig. 1(b) of S-PDI-IDT-1 and S-PDI-IDT-3 show that there are no significant melting or crystallization transition peaks in the temperature range of 30–300 °C. An endothermic peak was observed in the DSC curve of S-PDI-IDT-2 at a temperature of 273 °C. For detailed data for each molecule see Fig. S3, S9 and S15 (ESI†). It is suggested that the molecules were nearly amorphous with no crystalline regions.

Fig. 1 (a) TGA and (b) DSC analysis of the molecules measured at scan rate of 10 °C min⁻¹ under an N₂ atmosphere.

2.2. Optical and electrochemical properties and theoretical calculations

The UV-Vis-NIR absorption spectra of the molecules are shown in Fig. 2, and detailed data are shown in Table 1. The spectral absorption at 300–450 nm was attributed to the π–π* transition. The main absorption peaks that fall within the range of 300–450 nm for S-PDI-IDT-1, S-PDI-IDT-2, and S-PDI-IDT-3 were 389 nm, 428 nm and 389 nm in solution and 387 nm, 427 nm and 394 nm in the film state, respectively. A–D–A push–pull interactions between the A unit and D unit of the molecules contribute to a broad absorption in the range of 600–900 nm (for detailed data for each molecule see Fig. S1, S7 and S13, ESI†). The maximum absorption peaks (λ_max) in the low-energy region were 684, 686, and 657 nm in solution and 704, 724, and 689 nm in the film state for S-PDI-IDT-1, S-PDI-IDT-2, and S-PDI-IDT-3, respectively. The film state exhibited a red-shift within the range of 20–30 nm. The absorption observed at these high wavelengths was attributed to the unique molecular architecture of the molecules as characterized by their tailored A–D–A groups. In another published manuscript⁵³ in which the same monomers were polymerized and examined as n-type OFET semiconductors, the λ_max of the polymer in the published manuscript was redshifted compared with the molecules due to the conjugation length of the polymer increasing. The absorption wavelengths exhibited by the molecules were distinctly modulated by the conjugated structure of the A–D–A groups. Therefore, an enhancement in the donor cores of IDT results in a shift of the absorption spectrum towards a higher λ_max value. Observation of the structure of the three molecules shows that the conjugated plane of the IDT in S-PDI-IDT-1 is relatively small when compared to the other two molecules, and its possesses a higher λ_onset value in the absorption spectrum, which may be related to the regularity of its spatial molecular conformation.

Fig. 2 Normalized UV-Vis-NIR absorption spectra of the molecules in chloroform (a) and in thin-film states spin-cast from chloroform solutions on quartz substrates (b).

Table 1 Molecular parameters and electrochemical properties

Molecules	λ max [nm]		λ onset [nm]	E ^optg ^b [eV]	λ max [nm]
	Solution	Film			Solution	Film
a The maximum absorption peaks in the low-energy region. b E ^optg = 1240/λonset (in the film). c The maximum absorption peaks in the range 300–450 nm.
S-PDI-IDT-1	684	704	853	1.45	389	387
S-PDI-IDT-2	686	724	822	1.51	428	427
S-PDI-IDT-3	657	689	826	1.50	389	394

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The conjugated molecules exhibit absorption profiles that cover the full visible light spectrum. However, the optimal conformation of IDT-PDI was also highly twisted, and the dihedral angle between the IDT and PDI planes was ca. 50°, which results in molecular absorptions of less than 1000 nm.⁵¹ The information on the geometry, electronic structure, molecular conformation and molecular orbitals (HOMO and LUMO) of the three molecules was analyzed using density-functional theory with the B3LYP hybrid function and 6-311G(d,p) basis set. Gaussian 16 was applied to perform the theoretical molecular computations. Fig. 3 firmly confirms that the calculated dihedral angles between the IDT and PDI planes are 51.17°, 52.71° and 51.44°.

Fig. 3 Optimal conformations of S-PDI-IDT-1 (a), S-PDI-IDT-2 (b), and S-PDI-IDT-3 (c), in which the alkyl groups are simplified to methyl groups.

From the calculated images (see Fig. 4) of the HOMO and LUMO of the molecules, it was seen that the HOMO energy levels were distributed on the central aromatic ring because the central IDT core remains electron-rich. The theoretical band gaps were 1.81 eV, 1.66 eV, and 1.79 eV for S-PDI-IDT-1, S-PDI-IDT-2, and S-PDI-IDT-3, respectively, which was slightly higher than the optical bandgap. Fig. 4 shows that the theoretically calculated HOMO levels were found to be −5.29 eV, −5.11 eV and −5.25 eV and the LUMO levels were found to be −3.48 eV, −3.45 eV and −3.46 eV for S-PDI-IDT-1, S-PDI-IDT-2, and S-PDI-IDT-3, respectively. From the perspective of the molecular structure, S-PDI-IDT-2 and S-PDI-IDT-3 had more side chains with benzene rings on the middle IDT plane compared to S-PDI-IDT-1, resulting in an increase in the steric hindrance of the molecules. Furthermore, the length of the middle IDT planes for S-PDI-IDT-3 was significantly smaller than that of S-PDI-IDT-2, resulting in a larger steric hindrance of the endcap PDI group. Thus, the distribution of the LUMO energy levels in the molecule was uneven, with a preferential distribution towards one side of the PDI. It also can be seen that the calculated LUMO for S-PDI-IDT-2 has its electron density localized less symmetrically than S-PDI-IDT-1. This is the reason that the steric hindrance of the molecule plays a vital role in its behavior.

Fig. 4 Theoretically calculated HOMO and LUMO electron density distributions of (a) S-PDI-IDT-1, (b) S-PDI-IDT-2 and (c) S-PDI-IDT-3.

The energy levels were determined via cyclic voltammetry (CV) measurements in deoxygenated dichloromethane. The CV plots and the energy levels are shown in Fig. 5. The optical band gaps (E^opt_g, eV) of the three molecules were calculated from the boundary absorption values using the formula given in Table 1. The calculated E^opt_g values of the molecules were 1.45 eV for S-PDI-IDT-1, 1.51 eV for S-PDI-IDT-2, and 1.50 eV for S-PDI-IDT-3. Generally, greater conjugation and more rigid planes are beneficial for building electron transport channels. However, a comparison of S-PDI-IDT-2 with S-PDI-IDT-3 shows that the λ_onset values were almost the same, which indicates that the highly twisted conformation dominated the molecular absorption. The values of the HOMO and LUMO energy levels are summarized in Table 2. According to the CV plots in Fig. 5a, S-PDI-IDT-1, S-PDI-IDT-2, and S-PDI-IDT-3 displayed onset reduction potentials (E^onset_red) of −0.83 eV, −0.72 eV, and −0.74 eV, respectively. The LUMO energy levels of all the molecules were −3.57 eV, −3.68 eV and −3.57 eV for S-PDI-IDT-1, S-PDI-IDT-2 and S-PDI-IDT-3, respectively. All values were probably around −3.6 eV. Unfortunately, no peaks were observed in the oxidation scans of the molecules, except for the case of S-PDI-IDT-3. The HOMO energy values were extracted from the optical bandgap. The calculated HOMO energy levels were −5.02 eV, −5.19 eV and −5.16 eV. The energy level diagram of the molecules is depicted in Fig. 5b. The calculated HOMO values are quite similar, suggesting that the primary contribution to the HOMO comes from the central molecular core. Among the three molecules, S-PDI-IDT-1 retained the lowest band gap resulting in behavior where the charge transport in the OFET devices is dominated by electronic transmission.^54,55

Fig. 5 CVs (a) and energy level diagrams (b) of the three molecules.

Table 2 Molecules parameters and electrochemical properties

Molecule	E LUMO (eV)	E HOMO (eV)	E ^onsetox (eV)	E ^onsetred (eV)	E LUMO (eV)	E HOMO (eV)	E g (eV)
a E LUMO = − (4.4 + E^onsetred) eV.^56 b E HOMO = (ELUMO − E^optg) eV. c Calculated using DFT.
S-PDI-IDT-1	−3.57	−5.02	—	−0.83	−3.48	−5.29	1.81
S-PDI-IDT-2	−3.68	−5.19	—	−0.72	−3.45	−5.11	1.66
S-PDI-IDT-3	−3.66	−5.16	0.94	−0.74	−3.46	−5.25	1.79

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2.3. Charge transport characteristics of OFETs fabricated from the three molecules

To investigate the carrier transport behaviors, OFET devices featuring a bottom-gate/top-contact (BGTC) configuration (Fig. 6) were fabricated and assembled (for the detailed process see the ESI†). The charge mobility (μ) was calculated using the equation I_d = (W/2L) × C_i × μ × (V_g − V_th)², where I_d denotes the drain current, C_i represents the dielectric oxide capacitance per unit area, V_g is the gate-source voltage, and V_th is the threshold voltage. The output and transfer characteristics of the OFET devices are shown in Fig. 7, and the testing data are summarized in Table 3. It was revealed that the S-PDI-IDT-3 molecule has the most promising OFET performance. It was characterized by a strongly dominant n-type character with a maximum electron mobility of more than 0.02 cm² V⁻¹ s⁻¹.

Fig. 6 The structure of the OFET devices.

Fig. 7 Transfer (left) and output (right) characteristics of the OFETs devices annealed at 150 °C for 10 min. n-Type: (a) and (e) S-PDI-IDT-1, (b) and (f) S-PDI-IDT-2, (c) and (g) and S-PDI-IDT-3. p-Type: (d) and (h) S-PDI-IDT-2.

Table 3 Performance of the molecules in OFET devices

Molecule	Type	Temperature (°C)	μ e/h,max (cm^2 V^−1 s^−1)	μ e/h,avg (cm^2 V^−1 s^−1)	V th (V)	I on/Ioff^d
a The highest carrier mobility: n-type corresponds to μe, p-type corresponds to μh. b Average carrier mobility in nitrogen. c Threshold voltage. d Current ratio corresponding to the highest carrier mobility. e Parameters for the p-type version of S-PDI-IDT-2.
S-PDI-IDT-1	N	150	4.59 × 10^−2	3.67 × 10^−2	31.89	1.65 × 10^4
S-PDI-IDT-2	N	150	3.44 × 10^−2	2.39 × 10^−2	17.12	1.86 × 10^4
S-PDI-IDT-3	N	150	2.09 × 10^−1	1.58 × 10^−1	32.75	2.66 × 10^4
S-PDI-IDT-2	N	150	1.60 × 10^−3	1.19 × 10^−3	−17.79	1.57 × 10^4

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The n-type transmission characteristics of the OFET devices were first evaluated in a glove box. The n-type transfer and output characteristics of the OFETs devices are shown in Fig. 7. All the molecules exhibited n-type charge transport behaviors owing to their deep-lying LUMO levels. S-PDI-IDT-1 and S-PDI-IDT-3 exhibited unipolar n-type charge transport behaviors. The two molecules showed an optimized electron mobility of 4.59 × 10⁻² cm² V⁻¹ s⁻¹ and 2.09 × 10⁻¹ cm² V⁻¹ s⁻¹, with an I_on/I_off ratio of over 10⁴ after thermal annealing. S-PDI-IDT-2 showed unbalanced ambipolar transport with an optimized electron mobility of 3.44 × 10⁻² cm² V⁻¹ s⁻¹ and an optimized hole mobility of 1.60 × 10⁻³ cm² V⁻¹ s⁻¹ when the I_on/I_off ratio was over 10⁴. Significantly, the hole mobility value was about one order of magnitude lower than the corresponding electron mobility value. The output curves of S-PDI-IDT-2 were characterized by a superposition of the standard saturated behavior for one carrier at high V_g and a super-linear current increase at low V_g and high V_ds due to injection of the opposite carrier. A comparison of the molecules to their polymers shows that the conjugation length of the polymer molecules increased significantly, which was more conducive to the transport of charge carriers. However, there were differences in molecular size, molecular weight, and spatial conformation regularity in the polymers, which can also affect the transport of charge carriers in polymer semiconductors.

2.4. Microstructure characterization

Charge transport behavior in organic semiconductors depends not only on the molecular structure but also on the microstructure and morphology of the final solid-state assemblies. Contact angle testing was applied to assess the formation conditions of the film layers. As shown in Fig. 8, the contact angles formed by dripping deionized water droplets onto the surface of the annealed films were over 100° indicating relatively strong hydrophobicity. The surface energies of the films were calculated based on Yang's equation, . The relationship between the surface energy of a solid (σ_s), the interfacial tension between a liquid and a solid (σ_sl), the surface tension of the liquid (σ_l), and the contact angle (θ) was examined. The surface energies of the films were 19.44, 19.16, and 19.13 mN m⁻¹ for S-PDI-IDT-1, S-PDI-IDT-2 and S-PDI-IDT-3, respectively, indicating that the three films possessed relatively strong hydrophobicity. The experimental results confirmed an improvement in the spreading ability of the molecules, thereby enabling the successful deposition of the film layers.

Fig. 8 Images of the thin-film contact angles of the three films after annealing.

Grazing incidence wide angle X-ray scattering (GIWAXS) was applied to investigate the molecular packing and crystalline features of S-PDI-IDT-1, S-PDI-IDT-2 and S-PDI-IDT-3. All the molecules were prepared by spin-coating on Si/SiO₂ substrates and annealing at 150 °C for 10 min. Fig. 9 shows the GIWAXS diffraction patterns of the films before and after the annealing treatment. The results revealed a weak diffraction peak in the out-of-plane (h00) directions and that there were no (0h0) diffraction peaks before annealing, thus indicating a disordered and random arrangement of stacking in both the lamellar and π–π orientations.^57–61 However, after annealing distinct (100) diffraction peaks could be observed in the out-of-plane (OP) direction. The (100) peaks of S-PDI-IDT-1, S-PDI-IDT-2, and S-PDI-IDT-3 with the highest intensity at q_z were found at 0.26, 0.26, and 0.255 Å⁻¹ corresponding to lamellar stacking distances of 24.15, 24.15, and 24.6 Å, respectively. It was clearly shown that the annealing treatment increased the scattering intensities of the (100) peaks and that all the molecules were lamellar stacked and exhibited a face-on orientation. In the in-plane (IP) direction, it was challenging to quantify the variations in molecular orientation precisely because no (010) peaks could be observed, and a random orientation was exhibited in the edge-on orientation. As the morphological structure of the film is significant to the carrier mobility, these findings indicated that annealing effectively enhanced the orientation of the film layer and improved the performance of the devices. The corresponding in-plane (IP) and out-of-plane (OP) direction line cuts of the GIWAXS profiles are depicted in Fig. S19 and S20 (ESI†).

Fig. 9 GIWAXS patterns of the thin films on Si/SiO₂ substrates. as-cast (a–c) and as-annealed (d–f) for the molecules of S-PDI-IDT-1 (a, d), S-PDI-IDT-2 (b, e), and S-PDI-IDT-3 (c, f)

Atomic force microscopy (AFM) was applied to analyze the surface morphology of the thin films (see Fig. 10). The thin films of S-PDI-IDT-1, S-PDI-IDT-2, and S-PDI-IDT-3 exhibited root-mean-square (RMS) roughness values of 0.596 nm, 1.83 nm, and 0.579 nm, respectively. After being annealed at 150 °C for 10 min, the thin films have a smooth surface with smaller R_RMS values of 0.285 nm, 1.05 nm, and 0.278 nm for S-PDI-IDT-1, S-PDI-IDT-2, and S-PDI-IDT-3, respectively. After annealing, the surface became more compact and relatively ordered. The AFM analysis revealed that the S-PDI-IDT-2 film exhibited the highest RMS values and that the S-PDI-IDT-3 film exhibited the lowest RMS values. S-PDI-IDT-3 showed a more marked structuring with a compact surface. When annealed, the conjugated chains became more compact and relatively ordered, leading to a pronounced enhancement in the charge transport performance and a substantial reduction in charge hopping barriers. The annealed thin films demonstrated superior homogeneity relative to their as-deposited counterparts, suggesting the establishment of more conducive channels for charge carrier transport. It was concluded that the films of S-PDI-IDT-1 and S-PDI-IDT-3, characterized by their pronounced improvement in crystallinity, exhibited higher electron mobility than that of S-PDI-IDT-2. This outcome correlated well with the observed device efficiency and performance metrics.

Fig. 10 AFM height images of the thin films on Si/SiO₂ substrates of the as-cast (a)–(c) and as-annealed (d)–(f) molecules of S-PDI-IDT-1 (a) and (d), S-PDI-IDT-2 (b) and (e) and S-PDI-IDT-3 (c) and (f).

3. Conclusion

Three conjugated molecules based on a common molecular fragment coupled with a PDI group and tailored IDT-based electron-rich units, were successfully synthesized, and their optical, electrochemical, crystalline, morphological, and OFET charge transport properties were investigated. All the molecules absorbed at high wavelengths as a consequence of their alternating A–D–A molecular structures, and exhibited full absorption coverage in the visible light spectrum. The molecules showed good thermal stabilities and had n-type dominant carrier transport behavior, as optical analysis performed on the thin films displayed. As thin films, the molecules exhibited narrow optical bandgaps ranging from 1.45 to 1.51 eV. All the molecules present typical n-type charge transport characteristics. The best performance was that of S-PDI-IDT-3 which has both a very exciting electron mobility of up to 2.09 × 10⁻¹ cm² V⁻¹ s⁻¹ and good membrane morphology. S-PDI-IDT-2 was found to be p-type semiconductor with hole mobility values of up to 1.60 × 10⁻³ cm² V⁻¹ s⁻¹.

Data availability

The data supporting this article have been included as part of the ESI.† Data are available on request to the authors.

Conflicts of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by the program of the Beijing Municipal Education Commission (KM202110015002) and the College Scientific Research and Innovation Team Project (Ea202202).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5tc00610d

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