Chinese Chemical Letters  2018, Vol. 29 Issue (1): 175-178   PDF    
Photoresponsive n-channel organic field-effect transistors based on a tri-component active layer
Li-Na Fua,b, Bing Lengb, Yong-Sheng Lia, Xi-Ke Gaob    
a Laboratory of Low-Dimensional Materials Chemistry, Key Laboratory for Ultrafine Materials of the Ministry of Education, School of Materials Science and Engineering, East China University of Science and Technology, Shanghai 200237, China;
b Key Laboratory of Synthetic and Self-Assembly Chemistry for Organic Functional Molecules, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, Shanghai 200032, China
Abstract: Introducing photochromic molecules into the active layer of organic field-effect transistors (OFETs) is a direct way to implement a photoresponse nature in OFETs. However, active layer blended photoresponsive transistors based on n-type semiconductors are challenging and rarely studied, which are crucial for multifunctional organic-based logic applications. Herein, we fabricated n-channel photoresponsive OFETs based on a tri-component active layer spin-coated from the mixed solution of an n-type semiconductor (NDI2OD-DTYM2), spiropyran and polystyrene with a weight ratio of 1:1:1. The morphology of the blended films was improved by the introduction of the polymer matrix. Photochromic spiropyran molecules dispersed in the semiconductor layer could switch between the closed-ring state and ionic open-ring state flexibly under the irradiation of different wavelengths of light, and thus change the channel conductivity reversibly and modulate the OFET characteristics. Therefore, under the irradiation of alternate UV and vis light, both the device carrier mobility and current on and off ratio successfully realized a reversible switch.
Key words: Organic field-effect transistors     Photoresponse     n-Type organic semiconductor     Spiropyran     Polystyrene    

During the past decade, the properties of organic field-effect transistors (OFETs) have been rapidly improved by the design and synthesis of novel organic semiconductors [1-6], the integration of multiple active layer components [7], the modification of device interfaces [8, 9], as well as the optimization of device fabrication conditions [10-12]. Performances for both p-channel and n-channel OFETs are now comparable to and even higher than that of traditional amorphous silicon FETs. Besides pursuing excellent device performance, integrating OFETs with functional molecular building blocks to fabricate multi-functional OFETs is highly desired for the application of organic electronic devices [13, 14]. Novel multi-functional OFET devices have been constructed for applications in detection, monitoring, signal storage, etc.[15, 16].

A class of photoresponsive OFETs incorporating photochromic molecules that can be reversibly modulated by different wavelengths of light, have attracted more and more attention recently due to their potential applications in light switches and sensors [17-20]. Three main types of photochromic molecules including spiropyran (SP), diarylethene (DAE) and azobenzene (AZO) are usually adopted to build photoresponsive OFETs because of their unique photochromic behavior, as well as the variety of their physical properties during the photoisomerization process [21, 22]. Several strategies are approved effective in achieving photoresponsive OFETs, such as blending photochromic molecules in semiconducting layers or in dielectric layers [23-26], self-assembling monolayer of photochromic molecules in active/dielectric interfaces or in active/electrodes interfaces [27, 28], as well as the use of neat photochromic channel materials [29, 30]. Among them, blending photochromic molecules into the OFET semiconducting layer via solution process was regarded as the most direct way, but it is also ce other hand, there is a strong tendency for the blended films to undergo phase separation. Up to now, most photochromic OFETs with blended photochromic molecules are based on p-type polymeric semiconducting matrices such as poly-(3-hexylthiophene) (P3HT) and poly(triarylamine) (PTAA) [31-33], while n-type semiconducting materials are rarely adopted, because n-channel OFETs are usually sensitive to ambient air, due to the trapping of electron by oxygen and water [34, 35]. Therefore, n-channel OFETs often suffer from poor device performance and stability, especially for the solution-processed bottom-gate top-contact devices [36-38]. Besides, in order to avoid phase separation or aggregation, small-molecule semiconductors are seldom used in blended active layers. Samori et al.[24] have tried incorporating photochromic system into small-molecule semiconducting matrices, but the phase separation between BTBT crystallites and DAE molecules was not avoided. To date, the realizing of active layer blended photoresponsive OFETs based on small-molecule n-type semiconductors still remains challenging.

As shown in Fig. 1, we fabricated n-channel photoresponsive OFETs based on a hybrid active layer of an n-type small-molecule semiconductor (NDI2OD-DTYM2), photochromic spiropyran (SP) and polystyrene (PS). NDI2OD-DTYM2 is a typical n-type organic semiconductor with easy solution processibility, excellent OFET performance and ambient device stability [39, 40] and it has been successfully applied in the complementary invertor [41], organic oscillators [42] and gas sensors [43]. SP is one of the most potentially suitable photochromic molecules for the fabrication of photoresponsive OFETs because its photoisomerization is accompanied by a change in the SP electric dipole moment (from 6.4 D for SP-closed to 13.9 D for SP-open) [44]. In addition, for a better distribution of small-molecule semiconductor and photochromic SP molecules, we choose PS as a hybrid matrix. Our results showed that the transfer and output characteristics of these n-channel active layer blended OFETs can been reversibly changed under the irradiation of UV and vis light. The photomodulation efficient of current on and off ratio was more than 100 times. Besides, compared with those fabricated from bi-component mixture of NDI2OD-DTYM2 and SP, these tri-component thin films based on a polymeric matrix exhibited a better morphology without large-scale phase separation.

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Fig. 1. (a) The bottom-gate top-contact OFET device structure; (b) Chemical structure of NDI2OD-DTYM2; (c) Reversible isomerization of SP between SP-closed and SP-open; (d) Chemical structure of polystyrene.

Bottom-gate top-contact OFETs were fabricated on a Si wafer with a 300 nm thick SiO2 layer. Here, the Si wafer and SiO2 layer respectively serve as gate electrode and dielectric layer. A tri-component semiconductor layer was spin-coated from the mixed solution of NDI2OD-DTYM2, SP and PS (weight ratio: 1:1:1) in CHCl3. The thickness of the blended active layers was 60–80 nm measured by the Dektak 150 step profiler. The source and drain electrodes are 50 nm Au films deposited through a shadow mask. Device geometry is shown in Fig. 1a and the chemical structures of NDI2OD-DTYM2, PS, as well as the photochromism reaction of SP, are provided in Figs. 1b–d.

We firstly fabricated OFETs by spin-coating from pristine NDI2OD-DTYM2 dissolved in CHCl3 solution (5 mg/mL) and annealed the thin films of NDI2OD-DTYM2 at different temperatures (80 ℃, 120 ℃, 160 ℃ and 200 ℃). The device performances are listed in Table S1 in Supporting information. As shown in Fig. S1 in Supporting information, the transfer and output curves of OFETs based on NDI2OD-DTYM2 demonstrated typical n-channel semiconducting characteristics with well linear and saturated regimes. AFM images (Fig. S2 in Supporting information) showed a gradually increase of NDI2OD-DTYM2 grain size and the diffraction peaks strengthened in the XRD patterns (Fig. S3 in Supporting information) upon thermal annealing.

NDI2OD-DTYM2 and SP were blended at a weight ratio of 1:1 in CHCl3, but obvious phase separation can be seen from the AFM images (Fig. 2b). A drastic increase of thin film surface roughness (RMS values) was observed from pristine NDI2OD-DTYM2 films (RMS = 3.87 nm, Fig. 2a) to NDI2OD-DTYM2/SP blended films (RMS = 26.2 nm, Fig. 2b). As shown in Figs. 2c and d, a PS matrix was introduced to the blended films containing 33 wt% and 50 wt% of SP, respectively, leading to the much decreased RMS values about 1 nm of the tri-component thin films (NDI2OD-DTYM2:SP:PS = 1:1:1 and 1:2:1), relative to the bi-component thin films (NDI2OD-DTYM2:SP = 1:1, RMS = 26.2 nm). XRD patterns of the blended thin films (Figs. S4c, 4d in Supporting information) exhibited much weaker diffraction peaks compared with that of pristine NDI2OD-DTYM2 thin film (Fig. S4a), indicating a poor crystallization of the tri-component thin films. We suppose that the small-molecule NDI2OD-DTYM2 and SP dispersed in the uniform polymer matrix, thus producing good quality films. Another possible reason is that the SP molecules may acted as obstacles and limited the crystallization of NDI2OD-DTYM2 [24].

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Fig. 2. AFM images of thin films spin-coated from (a) Pristine NDI2OD-DTYM2 (5 mg/mL); (b) NDI2OD-DTYM2 (5 mg/mL) + SP (5 mg/mL) with a weight ratio of 1:1; (c) NDI2OD-DTYM2 (5 mg/mL) + SP (5 mg/mL) + PS (5 mg/mL) with a weight ratio of 1:1:1; (d) NDI2OD-DTYM2 (5 mg/mL) + SP (10 mg/mL) + PS (5 mg/mL) with a weight ratio of 1:2:1.

We then examined the device performance of OFETs incorporating the tri-component semiconductor layers in the dark. As shown in Table S2 and Fig. S5 in Supporting information, all the OFET devices exhibit well defined linear and saturation regimes similar to OFETs based on pristine NDI2OD-DTYM2. However, the average carrier mobility calculated from about 20 as-spun devices was found decreased with the integration of SP and PS (~0.01 cm2 V−1 s−1) compared with that of as-spun devices based on pristine NDI2OD-DTYM2 (~0.015 cm2 V−1 s−1). This might because that there are more traps and grain boundaries in the blended films [45-47], as well as the poor crystallization of NDI2OD-DTYM2 in blended films. During the following photoresponsing process, the weight ratio of the three components are fixed at 1:1:1 (5 mg/mL, 5 mg/mL and 5 mg/mL for NDI2OD-DTYM2, SP and PS) to get a balance between high enough OFET performance and photoresponsive performance.

In the irradiation of alternate UV and vis light, these n-channel OFETs based on a tri-component semiconductor layer showed reversibly photoresponsive property. Nine devices were marked and tested after each illumination. The irradiation times of UV light was fixed at 30 min and that of vis light was 60 min. The transfer characteristics of these OFET devices are illustrated in Fig. 3, which shows that the electron mobility and current on and off ratio (Ion/off) switched with alternating UV and vis irradiation. The Ion/off switches dramatically between 103–105 (in initial state and after vis irradiation) and 10–103 (after UV irradiation), with a 10–100 times modulation efficiency. However, as for the average electron carrier mobility, after finishing the first complete circle (from ~0.01 cm2 V−1 s−1 to ~0.02 cm2 V−1 s−1, and then recovered to the beginning value about 0.01 cm2 V−1 s−1), it declined gradually ascribed to the damping of SP and NDI2OD-DTYM2 under UV light irradiation [48, 49]. It should be noted that each device performed similar photoresponse process, though only the switches of average values are illustrated by filled circles connected by a line.

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Fig. 3. Photoresponsive changes of OFET characteristics: (a) carrier mobility and (b) current on and off ratio by alternating UV and vis irradiation on the tri-component blended thin films. The values of nine devices are represented by triangles, while the circles represent the average values. After light irradiation, the devices are transferred to glovebox and tested in darkness.

To explore the photoresponse mechanism, firstly, we prepared OFET devices based on pristine NDI2OD-DTYM2 films and tested their characteristics after the irradiation of UV and vis light. The devices were found relatively stable under light irradiation (Table S3 in Supporting information). Although the carrier mobility decreased slightly in the irradiation of UV light, probably due to the photodegradation [49]. For OFETs based on pristine NDI2OD-DTYM2, even after several times of UV light irradiation, the Ion/off was kept between 104–106 and the devices are stable with well linear and saturated regimes in the OFET transfer and output curves (Fig. S6 in Supporting information). This permits us to evaluate the photoresponsive characteristics of the active layer blended OFETs based on NDI2OD-DTYM2, SP and PS. In the in situ experiment, the device off state current was found increased as high as 300 times, from 1 × 10−9 A to 3 × 10−7 A (Fig. 4) under the UV irradiation, leading to the decrease of Ion/off and the disappeared saturated regime in both the OFET transfer and output curves (Fig. S7b in Supporting information). When the gate voltage (VGS) was fixed at 10 V, the drain/source current (IDS) almost increased linearly as the drain/source voltage (VDS) increased. After 60 min vis light irradiation, the OFET characteristics restored with a slight degradation (Fig. S7c in Supporting information). The photochromic SP dispersed in the active layer would play the most important role in the photoresponse process, since they can switch flexibly between a closed-ring state and an open-ring state as illustrated in Fig. 1c. In addition, the open-ring state of SP molecule is ionic after UV irradiation, so the isomerization of SP could repeatedly change the electrostatic environment of OFET semiconductor layer [44]. It should be noted that OFET transfer characteristics also showed reversible process, when shortened the irradiation time of UV light to 15 min and the vis light to 30 min, but the device off state current increased about 100 times after UV light irradiation (Fig. S8 in Supporting information), which is less than the increase of device off state current (about 300 times) under aforementioned long irradiation time (Fig. 4).

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Fig. 4. In situ measured transfer characteristics for an OFET device in initial state (triangles), after UV light irradiation (squares) and then after vis light irradiation (circles).

The study of SP photochromism process was carried out. As shown in Fig. S9 in Supporting information, a switchable variation of the absorption band between 450–700 nm of SP in CHCl3 solution was observed. In addition, SP molecules in spin-coated film on quartz can also switch between SP-closed and SP-open spontaneously. As shown in Fig. 5a, in the irradiation of UV light from 1 min to 30 min, a new absorption band between 450 and 700 nm appeared and strengthened, demonstrating the transformation of SP-closed to SP-open. After UV light irradiation, the films were irradiated by natural light for 30 min, but only a slight absorption recovery was gotten (Fig. 5b). Then we further imposed vis light (λ > 400 nm) on the films and their absorption band almost realized a complete recovery after 30 min (Fig. 5b). This proves that SP in thin films can also perform the reversible photoisomerization reaction. Since the strong absorption of NDI2OD-DTYM2 films between 500–650 nm, that overlaps the absorption band of SP, we did not get the absorption variation of the thin films spin-coated from the tri-component solution, except for that of the diluted mixed solution of NDI2OD-DTYM2, SP and PS (Fig. S10 in Supporting information). To gain a further insight into the effect of light irradiation on OFET characteristics, we examined the morphology variety of the tri-component blended thin films. AFM images of thin films spin-coated from the tri-component blended solution showed smooth film morphology without much difference (Fig. S11 in Supporting information) no matter the thin films were irradiated by UV or vis light.

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Fig. 5. The gradual transitions of the UV–vis absorption spectra of SP thin film spin-coated on a quartz substrate under UV (a) and vis (b) light irradiation. The UV irradiated film is irradiated by natural light for 30 min and then transferred to a photoreactor with a >400 nm vis light.

In summary, we demonstrated a facile approach to fabricate photoresponsive OFETs based on a tri-component active layer of an n-type semiconductor (NDI2OD-DTYM2), SP and PS (as the polymer matrix to improve the film morphology) with a weight ratio of 1:1:1. The device carrier mobility and current on and off ratio were responsive to UV and vis light. To clarify the mechanism of the switch process, we studied the transfer and output characteristics of the OFET devices and found that the off state drain/source current increased from 1 × 10−9 A to 3 × 10−7 A when the device was irradiated by UV light and then recovered to 1 × 10−9 A after by irradiating vis light. The photoisomerization of SP molecules modulated the conductivity of the active channel repeatedly with a 300 times of drain/source current switch was achieved. Although the results we got were limited, we initially realized photoresponsive n-channel OFETs by a tri-component strategy and made a step forward in the study of photoresponsive n-channel OFETs by using photochromic molecules, which is important for the development of multi-functional organic optoelectronic devices.

Acknowledgments

We thank the National Natural Science Foundation of China (Nos. 21302212 and 21522209) and the "Strategic Priority Research Program" (No. XDB12010100) for funding this work.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cclet.2017.05.014.

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