High carrier mobility and strong fluorescence emission: Toward highly sensitive single‐crystal organic phototransistors

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.


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. [1][2][3][4][5][6][7] Organic semiconductors have unique advantages such as facile processing, good flexibility, potential large-area rollto-roll manufacturing, ease of synthesis and modification with tailor-designed electrical and optical properties. [8][9][10][11][12][13][14] 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. [15,16] On the other hand, organic molecules with strong fluorescence emission often lead to very low mobilities due to the distorted molecular configurations. [17,18] 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, [19][20][21][22] oligothiophene derivatives, [23,24] and thienothiophene compounds. [25] 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. [26][27][28] 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 order to study the basic optoelectronic properties of the two thienoacene derivatives, their optical and electrochemical properties (Tables S2 and S3) 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 S2). 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 S3. 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 1) 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 1), suggesting the formation of J-aggregation in their crystals, [15,20] 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 × 10 8 and 4.55 × 10 8 s −1 , and other optical properties are summarized in Table S3. 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 3 and Figure S4). 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 S3). The slow PL decay of the crystal form indicates that there is a suppressed non-radiative decay pathway in crystals, [29] 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 3). 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.
The temperature-dependent fluorescence spectra of 4,8-DTEBDT and 2,6-DTEBDT single crystals from 108 to 288 K are shown in Figure S5. With the increase of temperature, the fluorescence intensity decreases, suggesting temperatureactivated 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. [29][30][31] The small E B 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 nonradiative 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 sil- icon wafers by the slow solvent-evaporation method, and the source and drain electrodes are prepared with gold by the "Au stripe mask" method. [32] The ribbon crystals of 4,8-DTEBDT and 2,6-DTEBDT were successfully prepared on SiO 2 substrates though drop-casting CHCl 3 /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 4 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 cm 2 V −1 s −1 , and high I on /I off ratios are up to 10 6 and 10 4 , respectively (Table S4) 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 4), which correspond to the shape of the 4,8-DTEBDT and 2,6-DTEBDT single crystals. Furthermore, the AFM images ( Figure S7) 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 4G (Table S5). As shown in Figures S8 and S9, 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 S6). From the "transfer" characteristic curves of 4,8-DTEBDT and 2,6-DTEBDT single-crystal OPTs ( Figure 5), 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  S17). According to Equations (S2)-(S4), [33,34] 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 S7), and the P up to 7.36 × 10 4 and 3.87 × 10 3 , the R up to 9.60 × 10 5 and 6.43 × 10 4 A W −1 , and the ultrahigh D* up to 5.68 × 10 17 and 2.99 × 10 16 Jones for 4,8-DTEBDT and 2,6-DTEBDT ultrasensitive single-crystal OPTs were obtained, respectively, which outperformed most of the reported OPTs (Table S8).
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 S8-S10), which demonstrated their good operation stability. The effect of V GS modulation on R, P, D* for 4,8-DTEBDT and 2,6-DTEBDT single-crystal OPTs are shown in Figures S11-S16. 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 I on /I off 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 × 10 8