INTRODUCTION
Phototransistors, an important type of optoelectronic device integrating signal amplification and optical detection to achieve high sensitivity and low noise, have attracted tremendous attention spanning from biomedical imaging to remote control and communication technologies.[] Organic semiconductors have unique advantages such as facile processing, good flexibility, potential large-area roll-to-roll manufacturing, ease of synthesis and modification with tailor-designed electrical and optical properties.[] With the rapid development of organic electronics, organic phototransistors (OPTs) have broad application prospects in the next generation of wearable and humanized electronic products. Recently, multifunctional organic semiconductors with high mobility and strong luminescent properties have been used to fabricate high-performance OPTs. However, it remains a huge challenge to achieve organic semiconductors integrating both high-mobility and strong emission properties. On one hand, excellent charge carrier mobility usually requires organic molecules to pack tightly and periodically in solid state, which would typically result in significant fluorescence quenching.[] On the other hand, organic molecules with strong fluorescence emission often lead to very low mobilities due to the distorted molecular configurations.[] Therefore, a delicate yet controllable molecular packing modes should be introduced into such systems. So far, only a few molecular systems managed to combine the excellent charge carrier mobilities and strong solid emissions for high-performance OPTs, such as acene compounds,[] oligothiophene derivatives,[] and thienothiophene compounds.[] Thienoacenes are another important class of fused-ring conjugated systems for organic photoelectric materials, because sulfur has a large atomic radius to lead to a stronger intermolecular interaction and high electron densities in the highest occupied molecular orbital (HOMO) energy level to give rise to an effective overlap between the HOMO energy levels of neighboring molecules in the solid state.[] At present, few multifunctional thienoacene fused-ring organic conjugated compounds with excellent light-sensitivity, good semiconductor properties, strong fluorescence emission are virtually nonexistent. Therefore, it is necessary to develop new high performance multi-functional thienoacene derivatives for comprehensively understanding the structure–property relationship of organic optoelectronic materials.
In this work, we have successfully designed and synthesized a new type of organic semiconductors based on triphenylamine-functionalized thienoacenes, that, 4,8-distriphenylamineethynylbenzo[1,2-b:4,5-b′]dithiophene (4,8-DTEBDT) and 2,6-distriphenylamineethynylbenzo[1,2-b:4,5-b′]dithiophene (2,6-DTEBDT), in which the large atomic radius of sulfur has promoted stronger intermolecular interactions, leading to an effective modulation of both electrical and optical properties. The obtained molecules feature high mobility and strong fluorescence emission, which lead to their superior optoelectronic properties. The single crystals for 4,8-DTEBDT and 2,6-DTEBDT exhibit maximum charge carrier mobilities up to 0.25 and 0.06 cm2 V−1 s−1, the photoluminescence quantum yields (PLQYs) of 51% and 45%, the small binding energies (EB) of 55.13 and 58.79 meV. The study on the photophysical processes reveals that the strong fluorescence emission of 4,8-DTEBDT and 2,6-DTEBDT single crystals originate from the effective suppression of the non-radiative recombination, which result in the fast radiative transition rates. OPTs based on 4,8-DTEBDT and 2,6-DTEBDT single crystals deliver high photoresponsivity of 9.60 × 105 and 6.43 × 104 A W−1, and detectivity exceeding 5.68 × 1017 and 2.99 × 1016 Jones, demonstrating the great potential of organic single crystals (OSCs) for future high-performance optoelectronics.
RESULTS AND DISCUSSION
In order to effectively expand the π-conjugation of thienoacene nucleus while maintaining its aromatic structure, two thienoacene derivatives with different structures (Figure ), “cruciform” molecule 4,8-distriphenylamineethynylbenzo[1,2-b:4,5-b']dithiophene (4,8-DTEBDT) and “linear” molecule 2,6-distriphenylamineethynylbenzo[1,2-b:4,5-b']dithiophene (2,6-DTEBDT), were designed and synthesized, as shown in . Compounds 4,8-DTEBDT and 2,6-DTEBDT are soluble in common organic solvents such as tetrahydrofuran, chloroform, and dichloromethane, and have good thermal stability. The corresponding temperatures at 5% weight loss from thermogravimetric analysis (TGA, Figure ) are 495°C for 4,8-DTEBDT and 490°C for 2,6-DTEBDT, respectively.
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The regular millimeter crystals of 4,8-DTEBDT and 2,6-DTEBDT are obtained by the slow solvent-evaporation method in ambient atmosphere (Table ), and their molecular structures and intermolecular arrangements were illustrated in Figure . As shown in Figure , there are two dihedral angles between thienoacene ring and benzene ring connected to the triple bond for the single crystal structures of 4,8-DTEBDT and 2,6-DTEBDT. There are two small dihedral angles (47.87° and 47.87°) for 4,8-DTEBDT, so that 4,8-DTEBDT has better molecular planarity, which effectively expands the π-conjugate length of 4,8-DTEBDT molecule. However, there are two large dihedral angles (52.88° and 52.88°) for 2,6-DTEBDT, so that 2,6-DTEBDT has poor molecular π-conjugation length compared with 4,8-DTEBDT. Moreover, the stacking of 4,8-DTEBDT molecules in the crystal has a tighter packing structure than that of 2,6-DTEBDT molecules (Figure ), which is more beneficial to the charge transport property for 4,8-DTEBDT. There are multiple intermolecular interactions such as intermolecular C–H···π (2.649–2.770 Å) interaction and intermolecular π–π (3.412–3.595 Å) interaction in the molecular stacking structure of 4,8-DTEBDT crystal (Figure ), and there are only a few intermolecular C−H···π (2.821–3.216 Å) interactions in the molecular stacking structure of 2,6-DTEBDT crystal (Figure ). The good molecular planarity of 4,8-DTEBDT and 2,6-DTEBDT is beneficial to the carrier transport, and the multiple interaction forces in 4,8-DTEBDT and 2,6-DTEBDT crystal are beneficial to carrier transport and luminescence.[] Compared with 2,6-DTEBDT crystal, 4,8-DTEBDT crystal has stronger intermolecular interaction and better molecular planarity, which is more conducive to obtain excellent optoelectronic properties for 4,8-DTEBDT crystals.
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In order to study the basic optoelectronic properties of the two thienoacene derivatives, their optical and electrochemical properties ( and ) were evaluated by UV–vis absorption spectra, fluorescence spectra, and cyclic voltammetry (CV). The HOMO energy levels of 4,8-DTEBDT and 2,6-DTEBDT were −5.29 and −5.32 eV, respectively, estimated from the CV curve (Figure ). The suitable HOMO energy levels suggested the potential application of 4,8-DTEBDT and 2,6-DTEBDT as p-type organic semiconductors (OSCs). According to the onset absorption of their films, the optical bandgaps of 4,8-DTEBDT and 2,6-DTEBDT were estimated to be 2.49 and 2.60 eV, respectively. The lowest unoccupied molecular orbital (LUMO) energy levels of 4,8-DTEBDT and 2,6-DTEBDT were −2.80 and −2.72 eV, respectively, which calculated by HOMO energy levels and bandgaps. The density functional theory (DFT) calculations were also performed to gain insight into the distribution of the electron clouds of 4,8-DTEBDT and 2,6-DTEBDT, as shown in Figures . The results indicate that the electron clouds of the two compounds are well delocalized on the HOMO energy levels, which is conducive to obtain excellent opto-electronic properties.
UV–vis absorption spectra (Figure ) shows that the solutions, thin films and crystals of 4,8-DTEBDT and 2,6-DTEBDT feature strong UV absorption, suggesting their potential applications in UV photodetectors. The absorption peaks of 4,8-DTEBDT and 2,6-DTEBDT crystals display significant redshift as compared with those of their solutions (Figure ), suggesting the formation of J-aggregation in their crystals,[] which is conducive to their charge transport and luminescent properties. The PLQYs of 4,8-DTEBDT and 2,6-DTEBDT single crystals obtained by slow solvent volatilization are 51% and 45% at room temperature, respectively. While the PLQYs of 4,8-DTEBDT and 2,6-DTEBDT films are 45% and 40% at room temperature, respectively. Moreover, the fast radiative transition rates calculated from the PLQYs and lifetimes of 4,8-DTEBDT and 2,6-DTEBDT single crystals are 1.87 × 108 and 4.55 × 108 s−1, and other optical properties are summarized in Table .
In order to elucidate the reason for the PL enhancement of 4,8-DTEBDT and 2,6-DTEBDT single crystals compared with their thin films at room temperature, we measured the time-resolved PL spectra of the two compounds (Figure and Figure ). We observed that the average lifetimes of 4,8-DTEBDT and 2,6-DTEBDT single crystals were significantly longer than those of thin films (Table ). The slow PL decay of the crystal form indicates that there is a suppressed non-radiative decay pathway in crystals,[] which is contributed to the presence of more ordered stacking structure in crystals than in films, resulting in the high PLQYs of 4,8-DTEBDT and 2,6-DTEBDT single crystals. Furthermore, we compared the steady-state PL and time-resolved PL of 4,8-DTEBDT and 2,6-DTEBDT single crystals at 77 K and room temperature (Figure ). Both 4,8-DTEBDT and 2,6-DTEBDT single crystals exhibit slightly stronger PL intensity and slower PL decay at 77 K, indicating that the radiation recombination probabilities at 77 K are only a bit higher than that at room temperature.
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The temperature-dependent fluorescence spectra of 4,8-DTEBDT and 2,6-DTEBDT single crystals from 108 to 288 K are shown in Figure . With the increase of temperature, the fluorescence intensity decreases, suggesting temperature-activated exciton dissociation, which is beneficial for the photogenerated carrier production at room temperature. We extract the maximum fluorescence intensity of each temperature from the temperature-dependent fluorescence spectra of compounds 4,8-DTEBDT and 2,6-DTEBDT single crystals, and draw the temperature dependent free-exciton emission intensity diagram of 4,8-DTEBDT and 2,6-DTEBDT by Equation (S5) fitting.[] The small EB of 4,8-DTEBDT and 2,6-DTEBDT single crystals obtained from fitting were 55.13 and 58.79 meV, respectively.
The above photophysical analysis shows that the non-radiative recombination in 4,8-DTEBDT and 2,6-DTEBDT single crystals can be effectively suppressed. Meanwhile, the exciton binding energy is small and the radiative transition rate is relatively fast. These results all suggest the great potential of 4,8-DTEBDT and 2,6-DTEBDT single crystals in photodetection.
To investigate the carrier transport properties of 4,8-DTEBDT and 2,6-DTEBDT crystals, we fabricated bottom-gate top-contact (BGTC) single-crystal organic field-effect transistors (SC-OFETs) on OTS modified silicon wafers by the slow solvent-evaporation method, and the source and drain electrodes are prepared with gold by the “Au stripe mask” method.[] The ribbon crystals of 4,8-DTEBDT and 2,6-DTEBDT were successfully prepared on SiO2 substrates though drop-casting CHCl3/n-hexane solutions, respectively. We measured the carrier transport properties of 4,8-DTEBDT and 2,6-DTEBDT ribbon crystals by employing the SC-OFETs′ configuration. Figure showed the transfer and output curves of 4,8-DTEBDT and 2,6-DTEBDT SC-OFETs, and the maximum mobilities are up to 0.25 and 0.06 cm2 V−1 s−1, and high Ion/Ioff ratios are up to 106 and 104, respectively (Table ). Such performance maintains a high level as compared to the reported OSCs with high mobility and strong fluorescence emission.
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In order to unveil the difference of charge transport properties between 4,8-DTEBDT and 2,6-DTEBDT, their crystals were characterized by atomic force microscopy (AFM) and X-ray diffraction (XRD). AFM images and optical images showed that the crystals were ribbon-shaped (Figure ), which correspond to the shape of the 4,8-DTEBDT and 2,6-DTEBDT single crystals. Furthermore, the AFM images (Figure ) showed that 4,8-DTEBDT single crystal had a smoother surface and a smaller root-mean-square (RMS) value than 2,6-DTEBDT single crystal (RMS value of about 2.802 nm for 4,8-DTEBDT and 4.387 nm for 2,6-DTEBDT), which indicated that 4,8-DTEBDT has formed higher quality crystal than 2,6-DTEBDT. As shown in Figure , the diffraction peaks of 4,8-DTEBDT and 2,6-DTEBDT micro/nano single crystals correspond to the diffraction peaks of (0 0 h) and (h 0 0) facet reflections of its single crystals, respectively. The d-spacings of 4,8-DTEBDT and 2,6-DTEBDT micro/nano single crystals calculated from (0 0 4) and (2 0 0) peaks were 29.11 and 15.15 Å, which are close to the d-spacing (29.38 and 15.51 Å) of 4,8-DTEBDT and 2,6-DTEBDT single crystals, respectively. The mobility of 4,8-DTEBDT single crystal is four times higher than that of 2,6-DTEBDT single crystal, because “cruciform” 4,8-DTEBDT molecules in single crystal have stronger intermolecular interactions, tighter molecular stacking and smaller dihedral angles, and greater delocalization of electron clouds on HOMO energy levels compared with “linear” 2,6-DTEBDT molecules in single crystal, which is easier to realize three-dimensional transmission of “cruciform” 4,8-DTEBDT molecules in single crystal. Such precise modulation of the carrier mobility through the controllable variation of molecular structures and packing modes shed light on the in-depth understanding of the structure–property relationship of OSCs.
The 4,8-DTEBDT and 2,6-DTEBDT single crystals have manifested the unique combination of distinguished charge transport properties and excellent optical properties, which ensure their potential applications in optoelectronic devices, especially phototransistors. By varying the illumination intensity and wavelength of the light, we explored the photo-response properties of SC-OFETs based on 4,8-DTEBDT and 2,6-DTEBDT. First, we studied the correlation between light intensity and the performance of 4,8-DTEBDT and 2,6-DTEBDT single-crystal OPTs. A series of white lights with different light intensity (0.0345 to 0.6320 mW cm−2) were used to illuminate the channel region of single-crystal OPTs (Table ). As shown in Figures and , the “transfer” and “output” characteristic curves of 4,8-DTEBDT and 2,6-DTEBDT single-crystal OPTs show that the threshold voltage shifts to the positive voltage direction and the source–drain current increases with the increase of white light intensity. Furthermore, we studied the effect of light wavelength on the performance of 4,8-DTEBDT and 2,6-DTEBDT single-crystal OPTs, and used a series of 0.0020 mW cm−2 light wavelengths (370–600 nm) to illuminate the channel region of their devices (Table ). From the “transfer” characteristic curves of 4,8-DTEBDT and 2,6-DTEBDT single-crystal OPTs (Figure ), we could be seen that the threshold voltage shifts to the positive voltage direction and the source–drain current increases by alternating from higher to lower wavelength under the illumination of 0.0020 mW cm−2, which may be attributed to the lower wavelength light with the higher photon energy is more conducive to produce more photogenerated carriers and reduce trap density. Moreover, we obtained 4,8-DTEBDT and 2,6-DTEBDT ultrasensitive single-crystal OPTs by using 0.0002 mW cm−2 370 nm light to illuminate their devices (Figure ). According to Equations (S2)–(S4),[] we calculated photosensitivity (P), photoresponsivity (R), and detectivity (D*) of 4,8-DTEBDT and 2,6-DTEBDT single-crystal OPTs under 0.0002 mW cm−2 370 nm light illumination (Table ), and the P up to 7.36 × 104 and 3.87 × 103, the R up to 9.60 × 105 and 6.43 × 104 A W−1, and the ultrahigh D* up to 5.68 × 1017 and 2.99 × 1016 Jones for 4,8-DTEBDT and 2,6-DTEBDT ultrasensitive single-crystal OPTs were obtained, respectively, which outperformed most of the reported OPTs (Table ).
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We further investigated the switching stability of 4,8-DTEBDT and 2,6-DTEBDT single-crystal OPTs, and performed 5 or 10 on/off cycles for 4,8-DTEBDT and 2,6-DTEBDT single-crystal OPTs (Figures –), which demonstrated their good operation stability. The effect of VGS modulation on R, P, D* for 4,8-DTEBDT and 2,6-DTEBDT single-crystal OPTs are shown in Figures . We believe that 4,8-DTEBDT and 2,6-DTEBDT single-crystal OPTs have excellent device performance, mainly due to high mobilities, fast radiative transition rates, small exciton binding energy and strong fluorescence emission suppressed non-radiative decay pathway for their crystals. The difference between the performance of 4,8-DTEBDT and 2,6-DTEBDT single-crystal OPTs is due to the higher mobility and higher Ion/Ioff ratio of 4,8-DTEBDT devices.
CONCLUSIONS
In conclusion, we adopted an effective strategy to synthesize two new thienoacene derivatives, “cruciform” molecule 4,8-DTEBDT and “linear” molecule 2,6-DTEBDT, realizing new class of p-type organic semiconductors with high mobility and strong fluorescence emission for the ultrasensitive UV phototransistors. The photophysical analysis for 4,8-DTEBDT and 2,6-DTEBDT single crystals indicated that the high PLQYs of 51% and 45% for their single crystals could originate from the effective suppression of the non-radiative recombination, leading to the fast radiative transition rates of 1.87 × 108 and 4.55 × 108 s−1, and their single crystals had the smaller EB of 55.13 and 58.79 meV, respectively. The single crystals of 4,8-DTEBDT and 2,6-DTEBDT have the highest charge carrier mobilities up to 0.25 and 0.06 cm2 V−1 s−1, ultrahigh photoresponsivity of 9.60 × 105 and 6.43 × 104 A W−1, and detectivity exceeding 5.68 × 1017 and 2.99 × 1016 Jones. Our results suggest that the single crystals with combined high mobility and strong emission are promising candidates in the field of organic optoelectronics.
[CCDC 2175286 (4,8-DTEBDT), 2175287 (2,6-DTEBDT) contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via .]
CONFLICT OF INTEREST
The authors declare that there is no conflict of interest that could be perceived as prejudicing the impartiality of the research reported.
DATA AVAILABILITY STATEMENT
Research data are not shared.
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Abstract
Organic single crystals (OSCs) offer a unique combination of both individual and collective properties of the employed molecules, but it remains highly challenging to achieve OSCs with both high mobilities and strong fluorescence emissions for their potential applications in multifunctional optoelectronics. Herein, we demonstrate the design and synthesis of two novel triphenylamine‐functionalized thienoacenes‐based organic semiconductors, 4,8‐distriphenylamineethynylbenzo[1,2‐b:4,5‐b′]dithiophene (4,8‐DTEBDT) and 2,6‐distriphenylamineethynylbenzo[1,2‐b:4,5‐b′]dithiophene (2,6‐DTEBDT), with high‐mobility and strong fluorescence emission. The two compounds show the maximum mobilities up to 0.25 and 0.06 cm2 V−1 s−1, the photoluminescence quantum yields (PLQYs) of 51% and 45%, and the small binding energies down to 55.13 and 58.79 meV. The excellent electrical and optical properties ensured the application of 4,8‐DTEBDT and 2,6‐DTEBDT single crystals in ultrasensitive UV phototransistors, achieving high photoresponsivity of 9.60 × 105 and 6.43 × 104 A W−1, and detectivity exceeding 5.68 × 1017 and 2.99 × 1016 Jones.
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Details
1 State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, China
2 State Key Laboratory of Supramolecular Structure and Materials, Jilin University, Changchun, China
3 Beijing National Laboratory for Molecular Sciences, Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing, China
4 China–Australia Institute for Advanced Materials and Manufacturing, Jiaxing University, Jiaxing, China