Organic Radicals Outperform LiF as Efficient Electron-Injection

Sep 18, 2017 - However, these n-dopants suffer from the strong tendency of metal diffusion and exciton quenching, which severely influence device perf...
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Organic Radicals Outperform LiF as Efficient Electron-Injection Materials for Organic Light-Emitting Diodes Zhengyang Bin, Ziyang Liu, and Lian Duan* Key Laboratory of Organic Optoelectronics & Molecular Engineering, Department of Chemistry, Tsinghua University, Beijing 100084, People’s Republic of China S Supporting Information *

ABSTRACT: One of the key issues for organic light-emitting diodes (OLEDs) is to achieve high electroluminescence efficiency and high power efficiency, which requires extremely efficient electron injection and thus low driving voltage. Here, we design a series of precursors for reactive organic radicals according to theoretical calculations and achieve efficient electron injection by using a highly reducing radical on the surface of the electron injection layer to reduce the electron injection barrier through an interface charge-transfer process. In contrast to bulk charge transfer in electrontransporting material, interface charge transfer allows us to make efficient electron injection at contact without introducing any structural and electronic disorder to electron-transporting material. 2-(2,4,6-Trimethoxyphenyl)-1,3dimethyl-1H-benzoimidazol-3-ium (R3), with the strongest electron-donating ability, could largely reduce the electron injection barrier and outperform the previously reported organic radical (2-(2-methoxyphenyl)-1,3-dimethyl-1H-benzoimidazol-3-ium, o-MeO−DMBI or R1) and the widely used electron injection material (LiF) to boost device performance.

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lower than that of traditional LiF, could be further improved with more efficient organic radicals. Herein, a series of precursors for reactive organic radicals are designed according to theoretical calculations and synthesized to study the mechanism for electron injection and improve electron injection efficiency. From thermal gravity analysis (TGA) and the quartz crystal microbalance (QCM) method, we demonstrate that these precursors are stable in ambient conditions and tend to lose iodine and form reactive organic radicals when heated in vacuum. Then, depositing a thin layer of these reactive organic radicals on the surface of ETMs could greatly reduce electron injection barriers and boost device current density due to the strong interface charge-transfer process between the organic radical and ETM inferred from Xray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS) measurements. Compared with bulk charge transfer in ETMs, interface charge transfer at its surface could minimize structural and electronic disorder by dopants in ETMs. Moreover, atomic force microscopy (AFM) shows that such a thin layer of organic radical could not fully cover the surface and leaves a lot of pores on the surface of the ETM; thus, the cathode could be partially deposited into these pores accompanied by efficient conductive channels for electrons with minimized influence of dopants. In OLEDs, 2(2,4,6-trimethoxyphenyl)-1,3-dimethyl-1H-benzoimidazol-3-

rganic light-emitting diodes (OLEDs) are regarded as the most promising candidates for next-generation displays and lightings, and these devices have evolved from lab curiosity to global industry.1−5 To fabricate OLEDs with high performance, efficient electron injection remains a crucial requirement to reduce the operation voltage and improve the device efficiency.6−8 N-doping in electron-transporting materials (ETMs) is an effective way to improve electron injection even in the case of large electronic barriers due to the formation of thin space charge layers at contacts.9,10 Unfortunately, it has been proven that n-doping is intrinsically rather difficult for the requirement of extremely strong electron-donating ability, which means that it is highly reactive and easily oxidized in ambient conditions.7 The most widely used n-dopants are inorganic materials, such as active metals (Li,11 Mg12) and metal salts (Cs2CO3,13,14 Li3N,15 LiH16). However, these ndopants suffer from the strong tendency of metal diffusion and exciton quenching, which severely influence device performance.17 The high evaporation temperature is another shortcoming for metal-based n-dopants. Thus, the low-temperature evaporable precursors of organic radicals have been reported as n-dopants in ETMs to increase film conductivity and decrease the electron injection barrier, thus largely improving device efficiency and enhancing device operation stability due to the eliminated exciton quenching by diffused metals.18,19 In the previous work, we found that moving an organic radical from the bulk of the electron-transporting layer to its surface could further decrease the electron injection barrier and increase the device current density.20 However, the mechanism has not been clarified and the electron injection efficiency, which is slightly © XXXX American Chemical Society

Received: August 11, 2017 Accepted: September 18, 2017 Published: September 18, 2017 4769

DOI: 10.1021/acs.jpclett.7b02125 J. Phys. Chem. Lett. 2017, 8, 4769−4773

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The Journal of Physical Chemistry Letters Scheme 1. Synthetic Routes and Molecular Structures of Complexes R1-I, R2-I, and R3-I

Figure 1. (a) TGA curves and (b) frequency shift versus mass loss characteristics of R1-I, R2-I, and R3-I.

Figure 2. (a) Molecular structures of BCP and BPBiPA. (b) Current density−voltage characteristics of electron-only devices using different organic radicals to deposit on the surface of the ETM with various film thicknesses. (c) SOMO energies (left axis) and optimized layer thicknesses for R1, R2, and R3 (right axis).

methoxy groups, named R1 (2-(2-methoxyphenyl)-1,3-dimethyl-1H-benzoimidazol-3-ium iode, also named o-MeO−DMBI), R2 (2-(2,6-dimethoxyphenyl)-1,3-dimethyl-1H-benzoimidazol3-ium), and R3 (2-(2,4,6-trimethoxyphenyl)-1,3-dimethyl-1Hbenzoimidazol-3-ium), are chosen, and their iodine-salt precursors (R1-I, R2-I, and R3-I) are synthesized for further study, as shown in Scheme 1. The TGA curves shown in Figure 1a indicate that these precursors are all very stable in air. However, when heated in vacuum, they tend to decompose or evaporate at rather low temperatures of below 300 °C, which is favorable for device fabrication. The QCM method is widely used to study the thermal decomposition of inorganic salts, such as sodium acetate and cesium carbonate.22,23 Here, we use this method to verify the thermal decomposition process of these precursors. 1,10-Phenanthroline (BPhen), a thermally stable ETM, is selected for comparison, Figure 1b. In order to reduce possible experimental errors, we compare the slope of the linear relationship frequency shift versus mass loss of these organic precursors with the slopes of thermal stable BPhen molecule.

ium (R3), with the strongest electron-donating ability, could largely reduce the electron injection barrier and outperform the previously reported organic radical (2-(2-methoxyphenyl)-1,3dimethyl-1H-benzoimidazol-3-ium, o-MeO−DMBI or R1) and the widely used electron injection material (LiF) to boost device performance. 1,3-Dimethyl-1H-benzoimidazol-3-ium iodide (DMBI-I) is a category of organic precursor that could decompose to a reactive DMBI radical during vacuum heating; then, the freshly formed DMBI radical could undergo electron transfer with the ETM and improve its conductivity. Thus, the electron-donating ability of the organic radical is a crucial factor that severely influences electron-transfer efficiency. As reported in our previous work,21 the singly occupied molecular orbital (SOMO) of the DMBI radical is mainly located at the ortho and para positions of the benzene ring compared with its meta position. Thus, introducing some electron-donating groups like methoxy- in the ortho and para positions could largely increase its SOMO energy and enhance its electron-donating ability. In this work, three organic radicals with different numbers of 4770

DOI: 10.1021/acs.jpclett.7b02125 J. Phys. Chem. Lett. 2017, 8, 4769−4773

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The Journal of Physical Chemistry Letters

Figure 3. (a) XPS spectra of C 1s and N 1s core levels of BPBiPA deposited on the pure ITO substrate or on a thin layer of R2 (2.5 nm). (b) UPS spectra of BPBiPA (3 nm) on Al and BPBiPA (3 nm)/R2 (2.5 nm) on Al. (c) AFM image of 2.5 nm R2 on the surface of 100 nm BPBiPA. (d) Comparison between bulk charge-transfer and interface charge-transfer processes. (e) Current density−voltage characteristics of an electron-only device utilizing a bulk charge-transfer film or an interface charge-transfer film as the electron injection layers. The device structures is ITO/BCP (10 nm)/BPBiPA (100 nm)/electron injection layer/Al. For bulk charge transfer, the electron injection layer is 10 wt % for R2 in 10 nm BPBiPA, while for interface charge transfer, the electron injection layer is 2.5 nm R2 on the surface of 10 nm BPBiPA.

By using the QCM method, we could calculate that for all three precursors only a fraction (R1: 65.0%, R2: 67.8%, and R3: 68.0%) of the initial weight for source material is deposited on the quartz crystal, which agrees well with the experimental result for the iodine loss (R1: 66.9%, R2: 69.3%, and R3: 71.4%). It demonstrates that when heated in vacuum, these precursors tend to lose iodine and form reactive organic radicals. Current−voltage characteristics of electron-only devices are used to compare the electron injection efficiency of these organic radicals. The device structures are ITO/BCP (10 nm)/ BPBiPA (100 nm)/organic radicals (x nm)/Al. As shown in Figure 2a, BCP stands for 2,9-dimethyl-4,7-diphenyl-1,10phenanthroline and is used as a blocking layer, while BPBiPA stands for 9,10-bis(4-(2-phenyl-1H-benzo[d]imidazol-1-yl)phenyl)anthracene and is used as an ETM. Without any organic radical, the device current density is at a rather low level, indicating a huge electron injection barrier between the cathode and ETM. However, depositing a thin layer of organic radical on the surface of an ETM could largely decrease the electron injection barrier and boost the device current density. For example, Figure 2b shows that R1 could increase the device current density by nearly 3 orders of magnitude from 4.3 to 2118.3 A/m2 at a 12 V bias. Also, R2 and R3, with stronger electron-donating ability, could further improve electron injection, and the device current density is increased to 5855.8 A/m2 for R2 and 6928.1 A/m2 for R3. More importantly, as shown in Figure 2c, with a stronger electrondonating ability, less organic radical is enough to realize efficient electron transfer with an ETM; thus, the layer thickness of the organic radical could be greatly decreased. For example, the optimized layer thickness for R1 is 4.5 nm, but

when we use R2 and R3 as electron injection layers, only 2.5 and even 1.5 nm are enough to reduce the electron injection barrier and increase the device current density. To probe into the electron injection improvement by organic radicals, deep analysis of the interaction between the organic radical (R2) and stable ETM (BPBiPA) was carried out. We initially compare the binding energy of C 1s and N 1s for BPBiPA deposited on a pure ITO substrate or on a thin layer of R2 using XPS, as shown in Figure 3a. The core levels of C 1s and N 1s for BPBiPA both shift toward higher binding energy by 0.3 eV, which indicates efficient electron transfer from R2 to BPBiPA and thus a strong n-doping effect at the interface. Then, ultraviolet photoelctron spectroscopy (UPS) shown in Figure 3b was measured to further study the charge-transfer process bettween R2 and BPBiPA. Compared with the pristine BPBiPA film, typical electronic features are shifted by 0.4 eV uniformly toward higher binding energy when BPBiPA is deposited on 2.5 nm R2. Together with the XPS study discussed above, the energy shift here strongly indicates efficient charge transfer from the organic radical to BPBiPA at the interface. Moreover, the UPS spectra show that depositing a 3 nm BPBiPA film on 2.5 nm R2 gives rise to a Fermi level shift of 0.4 eV away from the highest occupied molecular orbital (HOMO) and thus toward the lowest unoccupied molecular orbital (LUMO) of BPBiPA, leading to a largely reduced electron injection barrier and an improved device current density due to efficient charge transfer between R2 and BPBiPA. The AFM in Figure 3c shows that such a thin layer (2.5 nm) of R2 could not fully cover the interface, and it leaves a lot of pores on the surface of BPBiPA; thus, the cathode could be partially deposited into these pores. Overall, on the surface of 4771

DOI: 10.1021/acs.jpclett.7b02125 J. Phys. Chem. Lett. 2017, 8, 4769−4773

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Figure 4. (a) Current density−voltage, (b) luminance−voltage, (c) current efficiency−luminance, and (d) power efficiency−luminance curves of OLED devices. The device structures are ITO/HAT-CN (5 nm)/NPB (20 nm)/TCTA (10 nm)/(Ir(mphmq)2(tmd): DIC-TRZ = 3.5% (30 nm)/ BPBiPA (40 nm)/R1 (4.5 nm), R2 (2.5 nm), R3 (1.5 nm), or LiF (0.5 nm)/Al, where HAT-CN stands for 1,4,5,8,9,11-hexaazatriphenylene hexacarbonitrile, NPB stands for N,N′-di(naphthalene-1-yl)-N,N′-diphenyl-benzidine, TCTA stands for tris(4-carbazoyl-9-ylphenyl)amine, (Ir(mphmq)2(tmd) stands for (bis(4-methyl-2-(3,5-dimethylphenyl) quinoline)) Ir(III) (tetramethylheptadionate), and DIC-TRZ stands for 2,4diphe-nyl-6-bis(12-phenylindolo)[2,3-a] carbazole-11-yl)-1,3,5-triazine.

such as alkali metals, LiF has an outstanding property of airstability and is widely used in OLEDs, though at costs of high evaporation temperature and strong diffusion tendency of Li+, which behaves as an exciton quenching center to reduce device efficiency. In this work, using organic radicals as a new type of electron injection layer instead of LiF could greatly lower the evaporation temperature and avoid exciton queching by metals. Thus, compared with the LiF-based device, using BPBiPA/ R1(4.5 nm) as an electron injection layer could enhance device efficiency from 25.4 to 28.1 cd/A, and using the organic radical (R3) with the strongest electron-donating ability could further reduce the electron injection barrier and largely enhance device performance. Thus, the turn-on voltage (2.3 V) of the device using BPBiPA/R3 as an electron injection layer is extremly low and the power efficiency is further improved to 30.3 lm/W. To conclude, a series of precursors for highly reactive organic radicals have been reported to improve the electron injection efficiency and enhance device performance. We demonstrate that using interface charge transfer instead of traditionally used bulk charge transfer in an ETM could decrease the electron injection barrier and improve the device current density. This methode of using organic radicals to improve electron injection through interface charge transfer with an ETM could be applicable to a wide range of transporting systems, such as nanotubes, perovskites, and two-dimensional materials, and will eable a great range of applications, such as organic solar cells and organic thin-film transistors.

BPPiPA, part of the surface area is covered with R2, which could form an efficient interface charge transfer and strong ndoping effect, while the other part of the area is covered by a cathode with an efficient conductive channel for electrons.24 Thus, compared with charge transfer in the bulk of the ETM, interface charge transfer between the organic radical and ETM possesses a lot of advantages, as shown in Figure 3d. First, interface charge transfer means a separate layer of organic radical and ETM, which could avoid a complicated codepositing process and save materials. Second, doping in the bulk of the ETM often disrupts the molecular packing to incorporate a severe structural and electronic disorder, which is detrimental for carrier transport.25 Third, depositing organic radicals as a separate layer on the surface of electrontransporing materials means that all of the electron transfer occurred in the same direction, which is promoted by an applied device electrical field, thus largely improving the chargetransfer efficiency. Overall, interface charge transfer has a much higher efficiency than a bulk charge-transfer process, which is more prospective to improve electron injection and increase the current density of electron-only devices. As shown in Figure 3e, using an interface charge-transfer film as an electron injection layer could largely increase the device current density from 1.1 × 10−3 to 76.2 A/m2 at a 4 V bias, an almost 6 orders of magnitude improvement when compared with the bulk chargetransfer process. Finally, the interface charge-transfer films of BPBiPA/R1, BPBiPA/R2, and BPBiPA/R3 are used as electron injection layers in OLEDs, and the performance is shown in Figure 4. All of the devices give typical green emission at 608 nm from Ir(mphmq)2(tmd), indicating that the organic radicals added in the devices have negligible influence for light emission. Compared with other reactive elelctron injection materials,

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.7b02125. 4772

DOI: 10.1021/acs.jpclett.7b02125 J. Phys. Chem. Lett. 2017, 8, 4769−4773

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The Journal of Physical Chemistry Letters

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Detailed experimental methods, synthesis and characterization of the new designed precursors, and additional figures and data for devices, such as thickness optimization of R1-, R2-, and R3-based electron-only devices and the comparison of device performance for R2- and R3-based devices (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: (008610) 6277998. Fax: (008610) 62795137. ORCID

Lian Duan: 0000-0001-7095-2902 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

We would like to thank the National Key Basic Research Development Program of China (Grant No. 2015CB655002), the National Natural Science Foundation of China (Grant Nos. U1601651 and 51525304), and the Open Fund of the State Key Laboratory on Integrated Optoelectronics (Grant No. IOSKL2015KF13) for financial support.

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DOI: 10.1021/acs.jpclett.7b02125 J. Phys. Chem. Lett. 2017, 8, 4769−4773