Acceptor Blends Film to Achieve High

peaks of the D/A blends solution rarely showed any obvious red-shift or blue shift, ..... With the increase of the electric field, the slope of log(I)...
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C: Energy Conversion and Storage; Energy and Charge Transport

Tuning Microstructure of Donor/Acceptor Blends Film to Achieve High Performance of Ternary Data-Storage Device Fengjuan Zhu, Qi-Jian Zhang, Jiahui Zhou, Hua Li, and Jian-Mei Lu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b02704 • Publication Date (Web): 24 Apr 2019 Downloaded from http://pubs.acs.org on April 24, 2019

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Tuning Microstructure of Donor/Acceptor Blends Film to Achieve High Performance of Ternary Data-Storage Device Fengjuan Zhu, Qijian Zhang, Jiahui Zhou, Hua Li*, Jianmei Lu* College of Chemistry, Chemical Engineering and Materials Science, Collaborative Innovation Center of Suzhou Nano Science and Technology, Soochow University, Suzhou 215123 (P. R. China), E-mail: [email protected], [email protected]

ABSTRACT: Most of the high performance electronic devices are based on donoracceptor (D-A) structured molecules which are usually require complex reaction conditions and long-time synthesis periodicity, limiting the large-scale potential application in future. Encouraged by our previous research about blending the pristine electron-donor and electron-acceptor together as the active-layer in memory device, the tailor-made D/A blends with large π-conjugated planarity were designed for effective

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molecular interactions in thin films. The electron density of states distribution of individual D/A units indicated the non-negligible interactive coupling between the donor and acceptor, which were further confirmed by the UV-vis spectroscopy and cyclic voltammetry of the D/A blended films. Through mixing the blends with different D/A proportion, various microstructures were achieved to influence the corresponding memory behaviors, where the device with D/A proportion of 2/1 exhibited the typical ternary memory performance while other devices showed the binary memory properties. This work present here provides a novel mind in molecular designing and active-layer making to achieve excellent high-density memory devices in future.

1. Introduction

The ever-growing information explosion has spurred the exploration of multilevel materials for high-density data-storage (HDDS) devices.1 Till now, series of donoracceptor (D-A) structured systems concerning small-molecules,2 conjugated polymers3-5 and pendent polymers6 have been rationally designed for HDDS devices, and the charge transfer (CT) conducting mechanism3,

6-7

was widely applied to interpret the

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corresponding memory behavior. In the progress of molecular foundry, these D-A functional materials were always found to refer to expensive catalysts,8 nitrogen protection conditions,9 multistep complex reactions and time-consuming periodicity,10 significantly delaying the large-scale industrial application in next generation storage systems. To overcome these challenges, our previous work reported a brief synthetic route of carbazole derivative (TCz) donor and perylene imide (PDI) acceptor which were straightforwardly blended as the active layer to achieve fine-tunable memory performance (Figure 1).11 The large-scale blend functional film is easily accessible by the solution process due to the introducing of large alkyl chains in the molecule. However, this previous designed TCz donor was with a twist molecular configuration due to the singlebond linked functional carbazole units, thus, the microstructure of the pristine TCz film and the as-fabricated D/A blending films was difficult to clarify, which limited the better understanding of the mechanism of corresponding memory devices. Therefore, this simple but powerful strategy is set aside. To obtain an excellent molecular stacking for the D/A blending films, a tailor-made electron-donor with larger conjugated planarity is highly desirable. In our previous work,

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the carbazole (Cz) moiety has been recognized as a well-known electron-donor arising from its electron-rich secondary amine system. Thus, a rational modification of the previous TCz donor needs to made to improve the molecular configuration.12 Different from the single-bond linked functional carbazole units, the (oligo)-carbazole (eg. indolo[3,2-b]carbazole, ICz, Figure 1) possesses a much larger π-conjugated plane,13 making it even prefect for molecular self-assembly through the intermolecular π-π interactions.14 Meanwhile, the perylene diimide derivative (PDI) was also serving as the electron-acceptor due to the large π-conjugated planarity as well as the high electron mobility and thermal/chemical stability.15-17 According to the theoretical calculation shown in Figure S1, the electron density of states distribution of individual D/A units indicated the non-negligible interactive coupling between the donor and acceptor. Thus, in this paper, indolo[3,2-b]carbazole (ICz) donor and perylene diimide (PDI) acceptor were blended together as the active-layer for high-density storage devices. Through tuning the mixed ratios of this D/A blend system, different intermolecular stacking modes were obtained as confirmed by X-ray diffraction (XRD) tests, and the corresponding datastorage devices exhibited significant memory behaviors from the traditional binary

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property to the typical ternary performance, providing a novel strategy for functional molecular designing and HDDS active-layer fabrication.

Figure 1. Cross Section of the digital device and the PDI and ICz molecules used in the blend film.

2. Results and Discussion

The synthesis of target D/A molecules

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Scheme 1. Synthesis of ICz (D) and PDI (A) Molecules.

The synthetic routes of ICz donor and PDI acceptor were depicted in Scheme 1, and the detailed synthetic procedures of the intermediate compounds and the target molecules were presented in Supporting Information part. As is shown, the compound 5,11dihydroindolo[3,2-b]carbazole (3) was achieved through the one-pot approach at room temperature,18 then through the simple alkyl-chain substitution at the amine position to form the target molecule ICz. The electron-acceptor PDI was obtained according to the previous literatures, including the reduction of 7-tridecanone (4) to give the compound tridecan-7-amine (5) with a high yield of 99%. Then 3,4:9,10-Perylenetetracarboxylic anhydride reacted with compound 5 to produce the symmetric bisimide PDI molecule as

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a dark-red solid.19 After multiple water washing, the pure PDI was achieved. The NMR and MALDI-TOF-MS spectra of ICz donor and PDI acceptor were shown in Figure S7S12.

Thermal properties of D/A molecules

Figure 2. TGA and DSC plots of ICz and PDI molecules.

To investigate the long-term stability of the data-storage device material, the thermal stability of ICz donor and PDI acceptor were measured through thermo-gravimetric analysis (TGA) and differential scanning calorimeter (DSC) tests. As shown in Figure 2b, ICz donor and PDI acceptor both possessed a high melt point (Tm) of 120 oC and 155 oC, respectively, indicating an excellent heat endurance ability of the designed molecules,

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which can guarantee a stable D/A blend film for memory applications. Though ICz donor and PDI acceptor were achieved via simple synthetic procedures, the thermal decomposition temperatures (Td, with 5% weight loss) were estimated to be 267 oC and 349 oC, respectively, as calculated from the weight loss curves of TGA measurements (Figure 2a), further suggesting the high thermal stability of the D/A mixed system for a desired data-storage performance.

Photophysical properties

Figure 3. UV-vis spectra of CH2Cl2 solution with different electron-donor (ICz) and electron-acceptor (PDI) mixing ratios.

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The optical properties of the D/A mixed systems with various blending ratios in solution state and as thin films were measured through UV-vis spectroscopy, as depicted in Figure 3 and S2b. In dilute CH2Cl2 solution (CICz ≈ 10-6 M), the D/A blending systems showed similar spectral features, and the absorption peaks at 525 nm, 486 nm and 456 nm were attributed to the 0→0, 0→1 and 0→2 π−π* electronic transitions of the PDI acceptors,1920

while the high-energy band from 350 nm to 300 nm was assigned to the ICz donors.21

Furthermore, the absorbance intensity of the low-energy bands was improved with the increasing of the blend proportion of PDI acceptor. Compared with the solution absorbance of pure ICz donor and PDI acceptor in CH2Cl2 (Figure S2a), the absorbance peaks of the D/A blends solution rarely showed any obvious red-shift or blue shift, which was mainly caused by the extreme low concentration that prevented the effective interaction between the ICz donor and PDI acceptor. However, the D/A blending films exhibited different absorption spectral features when compared with the absorbance feature of corresponding systems in solution state, as shown in Figure S2b. The large red-shifting of the 0→0, 0→1 and 0→2 absorption peaks and the obvious change of the curve structure indicated the ordered J-aggregations22 in solid states and the non-

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negligible interactive coupling23 between ICz donor and PDI acceptor. The photoluminescence (PL) spectra of the D/A blended films was shown in Figure S3, and the results indicated that the charge transfer occurred between acceptor PDI and donor ICz in the blended films.

Electrochemical Properties

To further consider the intermolecular charge transfer (CT) effect between ICz donor and PDI acceptor in thin films, the highest-occupied molecular orbital (HOMO) and lowestunoccupied molecular orbital (LUMO) energy levels were investigated through cyclic voltammetry (CV) measurement.24 As shown in Figure 4a, ICz donor and PDI acceptor illustrated significant curves with the first oxidation onset at 0.71 V and 1.65 V, and the corresponding HOMO/LUMO energy levels were calculated to be -5.08 eV/-2.28 eV and -6.02 eV/-3.97 eV, respectively. Thus, the intermolecular bandgap between the HOMO of ICz donor and the LUMO of PDI acceptor was much lower than the bandgap of the individual ICz donor or PDI acceptor, as Figure 4b illustrates the partial charge transfer process of the D/A blending films in ground state. Subsequently, the CV measurements

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of the blending films with different D/A mixing proportions were also investigated. Interestingly, besides the original oxidation peaks associated with the ICz donor and PDI acceptor, an additional oxidation peak was observed for all the blending films at around 1.3 V, confirming the intermolecular electronic interaction between ICz donor and PDI acceptor in thin films for the ground state.25 Meanwhile, the HOMO and LUMO energy levels of the blending films were summarized in Table 1, and the energy barrier between ITO electrode and HOMO energy level (Φ1) was much lower than that between top Al electrode and LUMO energy level, indicating the mixture materials was predominately hole-transporting material.26

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Figure 4. Cyclic voltammetry curves of the blending films with different D/A mixing proportions (a,c,d,e), and energy levels diagram of ICz donor and PDI acceptor (b).

Table 1. Optical and Electrochemical property of the blend film with different D/A ratios.

D/A blend

λonse Egopt

proportions

t

Eoxonse HOMO

LUMO

Φ1

Φ2

(eV)

(eV)

(eV)

(eV)

t

(eV)

(nm)

(eV)

2/1

605

2.05

0.69

-5.06

-3.01

0.26

1.29

1/1

606

2.05

0.69

-5.06

-3.01

0.26

1.29

1/2

610

2.03

0.70

-5.07

-3.05

0.27

1.25

a

The data were calculated by the following equation: bandgap = 1240/λonset of the

three synthesized molecular films. b Vs Ag/AgCl. c Calculated from cyclic voltammetry. d Calculated from the HOMO energy level and optical bandgap.

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Molecular stacking in thin films

Figure 5. X-ray diffraction patterns of different D/A ratios.

To investigate the microstructures in blending films, XRD patterns were measured to characterize the molecular stacking properties, as depicted in Figure 5. The pristine ICz film showed a strong (100) diffraction peak at 6.80o with the distance calculated to be 13.04 Å, and the second order diffraction peak was appeared at 13.62o with the corresponding dsapceing of 6.50 Å. Additionally, the weak diffraction peaks observed at 9.15o and 20.50o could be assigned to the (001) and (010) reflections with the d-value estimated to be 9.67 Å and 4.32 Å, respectively.27 While for PDI acceptor in solid state, the XRD pattern only showed a moderate weak diffraction peak at 4.99o with the d-space

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calculated to be 17.69 Å, which was mainly induced by the large branched alkyl chains that inhibit the ordered crystal formation in thin films. To better understand the microstructure in solid states, the optimized crystalline structures of pristine ICz and PDI were simulated through material studio, and the detailed packing results were depicted in Figure S4a and S4b.

Figure 6. The optimized structure of different D/A ratios.

To get an insight into the molecular stacking in the blending films, XRD patterns were also measured. For the film with the blending proportion of 2/1, the XRD pattern exhibited four significant diffraction peaks at 4.99o, 6.80o, 13.62o and 20.50o, illustrating an effective cooperation of ICz donor and PDI acceptor to form a desired co-crystal stacking28 in this

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mixture. The diffraction peak at 20.50o with the d-value of 4.32 Å could contributed to the quasi π–π stacking due to the large π-conjugated plane of ICz unit, as shown in Figure 6a. However, with the increasing of PDI fraction (1/1 and 1/2), the XRD tests only showed a diffraction peak that was consistent with the pristine PDI film, indicating a preferred longrange ordering of the PDI acceptor in blending films, which was mainly induced by the vast amount of branched alkyl chains with a large steric hindrance that hindered the ordered stacking of ICz, and the optimized intermolecular stacking modes were depicted in Figure 6b and 6c. Therefore, various molecular stacking modes could be achieved through mixing the ICz donor and PDI acceptor with different D/A blending proportions.

Electronic Properties

It is widely accepted that the molecular stacking in film state has an important influence on device performance. Thus, the current-voltage (I-V) measurements were applied to the D/A blending system based devices to investigate the effect of microstructures on memory behavior. As shown in Figure S5a, the sandwich-structured ITO/ICz/Al device showed a high-conductivity state with no memory performance owing to the electron-rich

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characteristic and ordered long-range packing that formed a conducting channel for the charge carriers to move through the ICz film, while ITO/PDI/Al device was stayed in the low-conductivity state due to the electron-deficient environment within the material that blocked the effective transporting of the charge carriers throughout the film (Figure S5b). Subsequently, the I-V characteristics of the device with the D/A blending ratio of 2/1 were measured in the same condition, as shown in Figure 7a. Two sharp increases in current were found at the threshold voltage of -1.6 V and -4.1 V, indicating the transitions from the OFF state to the intermediate conducting state (ON1 state) and further to the (high-conducting state) ON2 state for this device. Furthermore, the ON2 state could be maintained even the external voltage was shut down or reversed, indicating the sandwich-structured device ITO/ICz:PDI, (2:1)/Al exhibiting a typical ternary WORM memory behavior.29 Additionally, the current ratio of the ON2/OFF state was distinct enough for the low misreading. More importantly, the data-storage density could be potentially hundreds of millions times greater than that of the traditional binary memory systems because of the binary-to-ternary memory performance switch.4

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Figure 7b was the I-V curve of ITO/ICz:PDI, (1:1)/Al device, which was initially in the low-conductivity state (OFF state). During the external voltage sweep from 0 to -5 V on one cell of the device, an abrupt increase in current was observed at the threshold voltage of -2.8 V, illustrating the device undergoing the transformation from the OFF state to the high-conductivity state (ON state). Furthermore, the device could stay in the ON state during the following negative sweep from 0 to -5 V and the reverse positive sweep from 0 to 5V, indicating that the sandwich-structured device ITO/ICz:PDI, (1:1)/Al was with a traditional binary WORM (write-once write many times) memory behavior. With the further increase of PDI in the D/A blending proportion, a current switching at the threshold voltage around -3.5 V indicated the transition from the original OFF state to the final ON state for the memory devices ITO/ICz:PDI, (1:2)/Al, showing the binary data-storage properties (Figure 7c). Given the above, the observations further confirmed the conclusion that the molecular stacking in thin films had a crucial effect on the data-storage device performance.

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Figure 7. I-V characteristics and log (I)-log (V) curves of ITO/ICz:PDI/Al devices.

To further understand the different memory behaviors of the D/A blending devices, the operational models between voltage and current at different stages were simulated, and the plotted log (I)-log (V) curves were shown in Figure 7. For device ITO/ICz:PDI, (2:1)/Al at the first stage (red line), the slope of log(I)-log(V) was fitted to be 0.9, indicating the operational model of I-V was in line with the Ohmic emission induced by the higher thermally generated charge carrier density than the injected carrier density. However, the Schottky barrier (Figure S6) between the electrode and active-layer blocked the transport of the charge carriers; thus, the device was in the OFF state under a low voltage sweep.

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With the increase of the electric field, the slope of log(I)-log(V) raised to 1.9 (green line) and 2.6 (blue line), namely, the conducting behavior was changed from the Ohmic conduction to the Child's square law (I ∝ V2) and further to the traps-filled-limit (TFL) transport phenomena, indicating the conducting mechanism was dominated by the space-charge-limited-current (SCLC) transport model. It is worthy of noting that the PDI acceptor could serve as the charge traps to block the mobility of the carriers. Upon the device switching to the ON1 state, the carriers also needed to overcome the additional energy barrier arising from the grain boundary30-31 of ICz donor and PDI acceptor formed co-crystal in film. Therefore, the charge carriers were trapped in the grain boundary to form a second depletion layer (grain boundary depletion), which would further increase the energy barrier for the carriers thus resulting a negative differential current (dI/dV < 0).32 Then the slope of log(I)-log(V) was fitted to be 1.8 (wether line), indicating that under the increased electric field the carriers have enough power to overcome the barrier33 of the second depletion layer to form a current switching rapidly (grain boundary limited current, GBLC).34 For the last region (yellow line), the slope of log(I)-log(V) also illustrated a straight line of the I-V curve, indicating the formation of an effective tunnel for the

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carriers to move across the grain boundaries (Ohmic model), as shown in Figure 8. For device ITO/ICz:PDI, (1:1) and (1:2)/Al, due to the lack of co-crystal structure in solid state, the conducting mechanism were both attributed to the SCLC transport model, as shown in Figure 7e and 7f.

Figure 8. The conducting mechanism of the memory device.

Conclusion

In summary, the tailor-made electron-donor ICz and electron-acceptor PDI were successfully designed with improved π-conjugated planarity. Besides, these two target molecules were synthesized through simpler procedures compared to that of the complex D-A structured molecules, which largely shortened the molecular synthesis periodicity

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and enhanced the accessibility of large-scale film for potential applications. ICz donor and PDI acceptor possessed prefect thermal stabilities that are comparable to the D-A functionalized materials. The films with different D/A blending proportions were confirmed to have partial charge transfer process between ICz donor and PDI acceptor. More importantly, different microstructures were achieved through the simple mixing of ICz donor and PDI acceptor in film state, and the corresponding memory devices were exhibited totally different data-storage behaviors that were proved to be induced by different conducting mechanisms owing to the various molecular stacking modes in film state. This work provided a simple but powerful strategy of molecular designing and device fabrication for high-density data-storage devices with superior thermal property and lowest cost.

Acknowledgement

This work was financially supported by the NSF of China (21878199, 21808149), National Excellent Doctoral Dissertation funds (201455), and NSF of the Jiangsu Higher Education Institutions of China (grant no.17KJA4300151).

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ASSOCIATED CONTENT

Supporting Information. The materials and synthesis procedure of the target molecules; the device fabrication experiment; the 1H NMR and13C NMR spectra of ICz and PDI; Proposed molecules stacking; I-V performance of memory devices with pure ICz or PDI functional

layer.

The following files are available free of charge.

AUTHOR INFORMATION

Corresponding Authors

[email protected]

[email protected]

Author Contributions F. Zhu synthesized the two target molecules, did the electronic measurement; Q. Zhang executed the XRD and CV measurement; J. Zhou did the simulation calculation. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

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Funding Sources NSF of China (21878199, 21808149),

National Excellent Doctoral Dissertation funds (201455),

NSF of the Jiangsu Higher Education Institutions of China (grant no.17KJA4300151).

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Diketopyrrolopyrroles:

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