Flexible Multifunctional Aromatic Polyimide Film: Highly Efficient

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A Flexible Multifunctional Aromatic Polyimide Film: Highly-Efficient Photoluminescence, Resistive Switching Characteristic and Electroluminescence Lunjun Qu, Lishuang Tang, Runxin Bei, Juan Zhao, Zhenguo Chi, Siwei Liu, Xudong Chen, Matthew Phillip Aldred, Yi Zhang, and Jiarui Xu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b02712 • Publication Date (Web): 30 Mar 2018 Downloaded from http://pubs.acs.org on March 31, 2018

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ACS Applied Materials & Interfaces

A Flexible Multifunctional Aromatic Polyimide Film: Highly-Efficient Photoluminescence, Resistive Switching Characteristic and Electroluminescence Lunjun Qu, Lishuang Tang, Runxin Bei, Juan Zhao, Zhenguo Chi, Siwei Liu, Xudong Chen, Matthew P. Aldred, Yi Zhang,* and Jiarui Xu PCFM Lab, GD HPPC Lab, Guangdong Engineering Technology Research Centre for High-performance Organic and Polymer Photoelectric Functional Films, State Key Laboratory of Optoelectronic Materials and Technologies, School of Chemistry, Sun Yat-sen University, Guangzhou 510275, China KEYWORDS: aromatic polyimide, photoluminescence, resistive switching behavior, memory device, PLED

ABSTRACT: We report a flexible multifunctional aromatic polyimide (BTDBPI) that shows yellow-green fluorescence with high photoluminescence quantum yield (PLQY) of 30 % in the film-state. The nonvolatile “write once-read many” (WORM) characteristic in a memory device with the configuration of ITO/BTDBPI/Au, indicates that BTDBPI possesses organic semi-conductor behavior. Moreover, polymer light-emitting diodes (PLED) with the structure of ITO/PEDOT:PSS/BTDBPI/TPBI/Mg-Ag, exhibits an interesting dual-emission phenomenon that 1

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originates from the electroluminescence (EL) of the BTDBPI nanometer film (yellow-green, 525 nm) and TPBI (deep blue, 380 nm), demonstrating that BTDBPI shows both the charge-transporting and EL properties.

In recent years, with the development of the electronic and optoelectronic technology fields, organic and polymer light-emitting diodes (OLED and PLED) have rapidly developed for applications in smartphones, emerging large-area displays (i.e., televisions) and other wearable devices.1-2 Currently, π-conjugated polymers based on poly(p-phenylene),3-4 poly(p-phenylene vinylene)5-6 and polyfluorene,7-8 are the most investigated electroactive polymers and have mostly been applied to PLED as emissive layers, hole and electron transport materials, mainly due to their facile thin-film formation by inkjet printing or spin-coating. Aromatic polyimides (Ar-PIs) are high-performance polymeric materials with unique properties, such as excellent thermal stability, chemical resistance, mechanical strength and flexibility, and have been widely applied to microelectronics and aerospace industries as insulating materials from the 1960s.9-10 In the past decades, Ar-PIs have evolved gradually from traditional insulating materials to functional materials owing to the chemical structure modifications of the dianhydride and diamine intermediate materials, and have demonstrated PL,11-12 electrochromism,13-14 and resistive memory properties.15-18 However, only a few studies19-20 have investigated them as emissive material layers (EMLs) in EL devices, because traditional Ar-PIs (such as Kapton, 2


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Upilex, etc) are usually non-luminescent or exhibit extremely low PLQY values in the solid-state and are insulating.

In order to improve the solid-state fluorescence efficiency of Ar-PIs, some investigations have been carried out, such as utilizing alicyclic dianhydrides or diamines,21 embedding fluorescent

aggregation-induced

emission

(AIE)-based

moieties

in

the

diamines

or

dianhydrides,22-23 and controlling the electron push-pull effects between the diamine and dianhydride moieties.24 All these methods are found to be effective in preventing charge transfer (CT) complex formation in Ar-PIs and thus improving the PLQY in the solid-state of the Ar-PIs. However, the PLQY values of these Ar-PIs in the solid-state are relatively poor with values rarely exceeding 30%, due to the strong intramolecular and intermolecular CT interactions in Ar-PIs.25 Therefore, it is still challenging to design and synthesize Ar-PIs that exhibit high PLQY in the solid-state.

Up to now, Ar-PIs have been reported to exhibit resistive switching characteristics in memory devices, including the nonvolatile WORM memory,26-27 flash memory,28-29 volatile dynamic random access memory (DRAM)30-31 and static random access memory (SRAM).32-33 The Ar-PI-based resistive memory devices record data based on the low and high conductivity response under an applied voltage, in which the conductive mechanism of Ar-PIs is related to the field induced CT formation.34 However, to our knowledge, most of these reported Ar-PIs are usually non-luminescent or exhibit low PLQY, and ignore the relationship between the EL 3



behavior and resistive switching behavior. In other words, these Ar-PIs that exhibit resistive switching characteristics may have potential applications in the fields of wearable electronics and PLED.

Figure 1. (a) and (b) Chemical structures of BTDA and BTDBPI, (c) and (d) BTDA powder under optical microscope and fluorescence microscopy, (e) BTDBPI film under sunlight, (f) and (g) flexible BTDBPI film (30 µm) under 365 nm UV light, (h) BTDBPI thin film coated on the quartz plate (100 nm) under 365 nm UV-light, (i) BTDBPI in NMP solution (2×10-2 mg· mL-1) under 365 nm UV light, (j) UV-vis absorption and PL spectra of the BTDA powder (excited at 365 nm), (k) PL spectra of the BTDBPI thick film (30 µm), thin film (100 nm), in NMP solution (2×10-2 mg·mL-1), (l) PL lifetime of BTDBPI in film-state (30 µm) and NMP solution (2×10-2 mg·mL-1).

In this work, we report a flexible multifunctional Ar-PI BTDBPI that exhibits PL, resistive switching characteristic and EL properties. BTDBPI is synthesized from its monomer BTDA (Figure 1a). The experimental section, characterization, infrared spectra and thermal stability of BTDA are all described and shown in Scheme S1 and Figure S1-S5. BTDA is a 4


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benzothiadiazole-based diamine and shows red fluorescence under 365 nm UV-light (Figure 1c-1d). In the UV-vis absorption spectra of BTDA (Figure 1j and Figure S6), the maximum absorption peak at 313 nm originates from the π-π* transition of the aromatic benzothiadiazole-dibenzene unit. The absorption peak in the visible region (470 nm), which is also observed in NMP solution (1×10-5 M), originates from intramolecular CT effect. When excited at 480 nm, BTDA shows a strong red emission peak at 635 nm and a short lifetime of 10.8 ns with PLQY of 13.2%, as shown in Figure S7-S8. However, The PL emission of BTDA in NMP solution (1×10-5 M) shows a red-shift to 658 nm and a lower PLQY of 1.5% with a short lifetime of 12.2 ns because of the strong polarity effect of the NMP solvent. In addition, BTDA exhibits a strong orange emission peak at 555 nm in CCl4 solution, and has a large red-shift to 655 nm in DMSO solution by increasing the solvent polarity (Fig. S9-S10). This Stoke’s shift phenomenon is related to the large conjugated and coplanar structure of BTDA, which forms a typical D-π-A-π-D structure with a strong intermolecular charge transfer (ICT) nature.34

Polyamic acid (PAA) BTDBPAA is synthesized by the polymerization of BTDA acting as the donor and 4,4'-bisphenol A dianhydride (BPADA) acting as the acceptor (Figure S11). Then BTDBPI (Figure 1b) is prepared successfully by a thermal imidization process for cyclization of BTDBPAA (Scheme S1). The infrared band absorption at 3440 cm-1 that is peaked in BTDBPAA disappears in BTDBPI that are characteristics for stretching vibration of oxhydryl and amino groups. Stretching vibration of the carbonyl group in the carboxyl and amide units at about 1710 5



and 1660 cm-1 in BTDBPAA and appearance of new bands at 1775 and 1715 cm-1 for the imide ring in BTDBPI confirm the fully imidization of BTDBPAA (Figure S12). BTDBPI is insoluble in DMF, DMAc and DMSO, but soluble on heating in NMP solvent. Its molecular weight can be measured and replaced by that of BTDBPAA (Mn=2.6×10-4, Mw=2.5×10-4, DPI=Mn/Mw=1.04) (Figure S13). BTDBPI shows excellent thermal stability with a Td5% at 550℃ and a high glass transition temperature (Tg) at 212 ℃ , as shown in Figure S14-S15. Compared with the high thermal stability of BTDBPI film, the thermal imidization condition of BTDBPAA to BTDBPI begins and ends within the range 180℃ to 240℃, as shown in Figure S15. BTDBPI can form flexible yellow films (30 µm thickness, Figure 1e) and exhibits yellow-green fluorescence (550 nm) in the film-state with high PLQY of 27.1% (Figure 1f-1g). When spin-coated on the quartz plate (100 nm thickness), it shows a blue-shift to 525 nm with absolute PLQY of 30.0% and a short lifetime of 2.5 ns (Figure 1h-1k). Compared with the thin film (100 nm), BTDBPI exhibits a red-shift to 531 nm with PLQY of 45.0% and a short lifetime of 2.9 ns in NMP solution (2×10-2 mg·mL-1) (Figure 1i-1l). In addition, the absorption edge wavelength (λedge) of the BTDBPI film

is around 475 nm and thus the optical energy band gap (Eg) is estimated to be 2.68 eV (Figure S16), according to the Planck equation.35 The PL emission of a thick BTDBPI film (30 µm) shows a 25 nm red-shift compared with BTDBPI thin-film on quartz plate due to the strong intermolecular CT interactions with an increase in film thickness (Figure S17). According to the previous literature, this result is the highest PLQY for Ar-PIs up to now.36-38

6


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Table 1. Optical properties of BTDA and BTDBPI in NMP solution and solid-state. a

Materials

a

bs λamax (nm)

b

λemaxx (nm)

c

λemaxm (nm)

d

φPL (%)

e

τ (ns)

BTDA powder

313 (470)

480 (320)

635

13.2

10.8

BTDA in NMP (1×10-5 M)

316 (478)

490 (331)

658

1.5

12.2

BTDBPI film (100 nm)

295 (405)

478 (360)

525

30.0

2.5

BTDBPI in NMP (2×10-2 mg·mL-1)

285 (390)

395 (312)

531

45.0

2.9

maximum absorption wavelegth,

b

maximum excitation wavelength,

c

maximum emission wavelength,

d

PLQY determined using a calibrated integrating sphere and BaSO4 as a reference, and e PL lifetime.

Optical properties of BTDA and BTDBPI in NMP solution and solid-state are listed in Table 1. When comparing the optical behavior of BTDA and BTDBPI in NMP solution, the obvious difference of the PLQY and emission wavelength between the starting diamine and the related polyimide is quite interesting. To gain a better insight into the photoluminescence phenomenon, theoretical calculations of the frontier molecular orbitals, oscillator strengths and the assignment of S0 to Si transitions for the BTDA and the model basic unit (MBU) of BTDBPI are carried out, and the results are shown in Figure S18 and Table S1. The result shows that the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of the benzothiadiazole have higher overlapping integral than that of the aminos by a value of oscillator strength of 0.26 in BTDA. The energy gap of MBU from HOMO to LUMO is 3.19 eV, which is larger than that of BTDA (2.83 eV). This difference can lead the PL emission of BTDBPI to a blue-shift compared to BTDA. Different to the most Ar-PIs,39-40 the HOMO and LUMO of the MBU are predominantly distributed on the diamine moiety. The total overlapping of these two frontier molecular orbitals would lead to the first excited state (S1) via the electron 7



from HOMO to LUMO transition, which is characterized by a strong value of oscillator strength of 0.41. These calculated results indicate that the strong electron-withdrawing effect of diamine with the benzothiadiazole moiety in BTDBPI tends to exhibit high PLQY.

Figure 2. (a) Schematic diagram and photograph of the BTDBPI-based memory device, (b) current–voltage (I–V) characteristics of the ITO/BTDBPI/Au sandwich device under an applied voltage, (c) effects of operation time in the OFF and ON states of the memory device under a constant 1 V bias voltage, and (d) I–V characteristics of the ITO (200 nm), PEDOT:PSS (60 nm), BTDBPI (60 nm) and Kapton (60 nm) thin films.

Cyclic voltammetry41 was conducted to determine the oxidation potential of the BTDBPI thin film in CH3CN solution, as shown in Figure S19. For BTDBPI, the onset potential was measured to be 1.68 V versus Ag/AgCl. Therefore the HOMO and LUMO level of BTDBPI are calculated to be -6.40 eV and -3.72 eV, respectively. The surface resistivity (Rs) of the flexible 8


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BTDBPI film (30 µm) is about 5.21×1011 Ω, and the volume resistivity (Rv) is about 4.48×1013 Ω·m, which indicates that the flexible BTDBPI film is insulated. However, BTDBPI exhibits resistive switching behavior in a memory device with the configuration of ITO/BTDBPI (60 nm)/Au (Figure 2a). The current-voltage (I-V) performance of the memory device fabricated based on BTDBPI is depicted in Figure 2b. In the first sweep (sweep-1) from 0 to 6 V, an abrupt increase in current can be observed at a threshold voltage of 2.3 V, indicating that the BTDBPI thin-film undergoes a switching behavior from a low-conductivity (OFF) state to a high-conductivity (ON) state. The ON state can be maintained during the subsequent sweep (sweep-2) from 6 V to 0. The next dual sweeps are performed after turning the power off for less than 1 minute, the subsequent negative scan (sweep-3 and -4) from 0 V to -6 V and postive scan (sweep-5 and -6) from 0 V to 6 V can not turn the device from the ON state to the OFF state, indicating that the device is non-erasable. This switching behavior in conductivity can be repeated in other cells (Figure S20), and represents WORM memory nature of the device. The two states with a distinct current ratio (ON/OFF) of 104 indicates that the device possesses a bistable memory performance (Figure 2c). Moreover, compared with the conductive materials ITO, poly(3,4-ethylenedioxythiophene): poly(styrenesulfonic acid) (PEDOT:PSS, spin-coated on the ITO) and the traditional Ar-PI material Kapton (spin-coated on the ITO) with similar nanometer thickness (Figure 2d), the resistive switching performance of BTDBPI indicates that it can behave as a semi-conductor material under a suitable applied voltage, and exhibits charge-transporting properties. 9



Figure 3. (a) EL curves of BTDBPI-based PLED device P0 and P1, PL curves of BTDBPI thin film (100 nm) and TPBI powder, (b) voltage−luminance (V-L) characteristics of device P0 and P1, (c) CIE coordinates of the device P0 and P1, and (d) image of the device configuration of P0 and P1, with HOMO and LUMO values.

In addition to the resistive switching behavior and PL properties of BTDBPI, EL properties are also investigated by fabricating PLED devices with the configuration of ITO/PEDOT:PSS (40 nm)/BTDBPI/1,3,5-tri(1-phenyl-1H-benzimidazol-2-yl)phenyl (TPBI, 40 nm)/Mg-Ag alloy, where PEDOT:PSS and TPBI served as the hole-injection layer (HIL) and electron transporting layer (ETL),42-43 respectively (Figure S21). The device P0 exhibits single emission at around 525 nm with CIE coordinates of (0.34, 0.50) (Figure 3a and 3c). We reduced the thickness of BTDBPI as the emissive layer from 80 nm (P0) to 20 nm (P1) and optimized the annealing time of PEDOT:PSS from 10 minutes to 5 minutes. The device P1 shows interesting dual-emission, 10


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including not only the emission at 525 nm but also a new emission peak of TPBI at 380 nm (Figure 3a). The dual-emission of device P1 results in white light with CIE coordinates of (0.26, 0.34) (Figure 3c). To the best of our knowledge, this is the first report for a Ar-PI-based PLED device to achieve both the single-emission and high brightness white light dual-emission. The EL of P0 only shows good overlap with the PL emission of the BTDBPI, and the EL of P1 shows good overlap with the PL emission of the BTDBPI thin-film spin-coated on quartz plate (100 nm) and TPBI powder. Although TPBI is not suitable for the emissive layer, the emission of TPBI proves the charge-transporting ability of BTDBPI. Reducing the thickness of the emissive layer could improve the EL intensity and the current efficiency of the PLED device.44 For device P0, a maximal luminance (Lmax) of 52 cd m-2 with a turn-on voltage (Von) of 8 V is demonstrated (Figure 3b). However, device P1 shows improved brightness reaching 360 cd m-2 with a maximum external quantum efficiency (EQEmax) of 0.10%

and the Von is reduced to 5 V

(Figure 3b). Other EL data of BTDBPI-based decives are listed in Table 2 and described in Figure S22-23. In order to clarify the electroluminescence process of the BTDBPI based devices, the energy-band diagram of each layer is illustrated (Figure 3d). Therefore, the PLED device configuration successfully confines the holes and electrons and forms excitons in the BTDBPI and TPBI layers, which further indicates BTDBPI exhibits charge-transporting properties. Table 2. EL data of the BTDBPI-based PLED devices. Device

Von (V)

a

Lmax (cd m-2)

b

CE50 (cd A-1) c

PE50 (lm W-1) d

e

11


EQE (%)

CIE (x, y) f


a

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P0

8.0±0.2

52±2

0.05±0.01 0.010±0.001 0.020±0.001 (0.34±0.01, 0.50±0.01)

P1

5.0±0.1

360±5

0.10±0.01 0.210±0.002 0.100±0.001 (0.26±0.01, 0.34±0.01)

turn-on voltage,

b

around 50 cd m , -2

the maximum luminance, c current efficiency at around 50 cd m-2, e

d

power efficiency at

the maximum EQE, and CIE coordinates at 50 around cd m . The numbers are the f

-2

averages and the measurement error calculated from 3 samples respectively.

In summary, we have synthesized a flexible multifunctional Ar-PI BTDBPI material that exhibits PL and EL properties, and resistive switching behavior. This work shows the evolution of Ar-PI BTDBPI from the high performance and heat resistant insulating materials to the multifunctional semi-conductive material for applications in electronic and optoelectronic applications. More importantly, it demonstrates the possibility of non-conjugated Ar-PIs that may have potential applications in the fields of wearable electronics and multifunctional integrated devices in the future.

ASSOCIATED CONTENT

The experiment section, characterization of synthesis, infrared spectra, thermal stability and theoretical calculation of BTDA and BTDBPI; the fabrication and characterization of BTDBPI-based memory and PLED devices are described in the Electronic Supporting Information (ESI). The Electronic Supporting Information is available free of charge on the ACS Publications website.

AUTHOR INFORMATION

12


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Corresponding Author

E-mail: [email protected]

Notes

The authors declare no competing financial interests.

ACKNOWLEDGMENT

The financial support by the National 973 Program of China (No. 2014CB643605), the National Natural Science Foundation of China (Nos. 51373204, 51173214, 51233008), the National 863 Program of China (No. 2015AA033408), the Science and Technology Project of Guangdong Province (Nos. 2015B090915003 and 2015B090913003), the Leading Scientific, Technical and Innovation Talents of Guangdong Special Support Program (No. 2016TX03C295), and the Fundamental Research Funds for the Central Universities (No. 161gzd08) are gratefully acknowledged.

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Flexible Multifunctional Aromatic Polyimide Film: Highly Efficient

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