Stable Organic Radicals as Hole Injection Dopants for Efficient

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Stable Organic Radicals as Hole-Injection Dopants for Efficient Optoelectronics Zhengyang Bin, Haoqing Guo, Ziyang Liu, Feng Li, and Lian Duan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b17385 • Publication Date (Web): 18 Jan 2018 Downloaded from http://pubs.acs.org on January 18, 2018

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ACS Applied Materials & Interfaces

Stable Organic Radicals as Hole-Injection Dopants for Efficient Optoelectronics Zhengyang Bin,† Haoqing Guo,‡ Ziyang Liu,† Feng Li,*‡ and Lian Duan*† †

Key Laboratory of Organic Optoelectronics & Molecular Engineering, Department of

Chemistry, Tsinghua University, Beijing, 100084, People’s Republic of China †

Center for Flexible Electonics Technology, Tsinghua University, Beijing, 100084, People’s

Republic of China ‡

State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin

University, Changchun, 130012, People’s Republic of China E-mail: [email protected], [email protected]

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ABSTRACT: Precursors of reactive organic radicals have been widely used as n-dopants in electron-transporting materials to improve electron-conductivity and enhance electroninjection. However, the utilization of organic radicals in hole-counterparts has been ignored. In this work, stable organic radicals have been proved for the first time to be efficient dopants to enhance hole injection. From the absorbance spectra and the ultraviolet photoelectron spectra, we could observe an efficient electron transfer between the organic radical (4-Ncarbazolyl-2,6-dichlorophenyl)bis(2,4,6-trichlorophenyl)methyl, TTM-1Cz) and the widely used hole injection material (1,4,5,8,9,11-hexaazatriphenylene hexacarbonitrile, HAT-CN). When the unpaired electron of TTM-1Cz is transferred to HTA-CN, it would be oxidized to TTM-1Cz cation with a newly formed lowest unoccupied molecular orbital (LUMO) which is quite close to the highest occupied molecular orbit (HOMO) of hole-transporting material (HTM). In this way, the TTM-1Cz cation would promote the electron extraction from the HOMO of HTM and improve hole injection. Using TTM-1Cz doped HAT-CN as the hole injection layer, efficient organic light-emitting diodes with extremely low voltages can be attained.

KEYWORDS: Sable Oganic Radical, Hole injection, Dopant, Charge Transfer, Organic Light-Emitting Diodes


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1. INTRODUCTION Organic semiconductors have been widely used in optoelectronics, such as organic lightemitting diodes (OLEDs), organic solar cells (OSCs), and organic thin-film transistors (OTFTs).1-7 However, compared with inorganic counterparts, organic semiconductors suffer from weak molecular interaction and low intrinsic carrier-density, which leads to relatively low film conductivity and severely influences the device performance.8-9 To overcome these limitations and thus to approach the performance of inorganic devices, global research efforts are currently directed towards applying strong dopants to increase carrier-density and improve film-conductivity.10-12 In 2004, Leo’s group firstly presented an approach of using the precursor of organic radical as an n-dopant in electron-transporting material to improve the electron conductivity.13 And over the past decade, a series of precursors of organic radicals have been reported and used as efficient dopants in optoelectronics, such as pyronine B14-16, crystal violet17-18, and o-MeO-DMBI19-21. But Up to now, organic radicals are primarily used to improve electron-conductivity of electron-transporting materials and enhance electroninjection, their performance in hole-counterparts has been ignored. Herein, we firstly use stable organic radicals as a new type dopants to promote hole injection and improve device performance. From the absorbance spectra and ultraviolet photoelectron spectroscopy, we could observe an efficient electron transfer between a normally used hole injection material (1,4,5,8,9,11-hexaazatriphenylene hexacarbonitrile, HAT-CN)22-24 and a stable organic radical ((4-N-carbazolyl-2,6-dichlorophenyl)bis(2,4,6trichlorophenyl)methyl, TTM-1Cz)25. When the unpaired electron of TTM-1Cz is hopped from its singly occupied molecular orbital (SOMO) to the lowest unoccupied molecular orbital (LUMO) of HAT-CN due to a favorable energy difference, TTM-1Cz would be oxidized to TTM-1Cz cation with a newly formed LUMO which is quite close to the highest occupied molecular orbit (HOMO) of hole-transporting material (HTM). In this way, TTM3 ACS Paragon Plus Environment


1Cz cation would promote the electron extraction from the HOMO of HTM and improve hole injection, as shown in Figure 1.

Figure 1. Electron extraction and charge transfer process for hole injection. Moreover, the transferred electrons in HAT-CN could fill the trap states in HATCN film, which is also favorable for electron extraction and further improve hole injection. Combined with our previous study for using the precursor of highly reactive organic radical as an efficient electron-injection layer21,26, here we are able to use the two different types of organic radicals to improve electron- and hole injection, separately, and fabricate efficient organic light-emitting diodes with extremely low voltages. 2. EXPERIMENTAL METHODS Materials and Device Fabrication. TTM-1Cz and TTM (tri(2,4,6-trichlorophenyl)methyl) were synthesized as reported previously25. Materials for device fabrication were purchased from Jilin Optical and Electronic Materials Company and used as received. We prepared the films under a high vacuum chamber at 10-4 Pa and the devices were deposited onto ITO substrates with sheet resistances about 15 Ω/ square after ultraviolet-ozone treatments. Measurements. Ultraviolet-vis spectrophotometer (Jobin Yvon, FluoroMax-3) was used to measure the absorption spectra. Keithley 4200 semiconductor characterization system was used to measure the current density-luminance-voltage curves in devices. Brucker Icon was used for conductive atomic force microscopy (c-AFM) measurements. Mono hromatized HeI


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radiation with the energy of 21.2 eV was used for ultraviolet photoelectron spectroscopy (UPS) characterization. 3. RESULTS AND DISCUSSIONS Figure 2(a) shows the molecular structures of hole injection material (HAT-CN) and two stable organic radicals (TTM-1Cz and TTM)25. Unlike traditionally used n-dopants in electron-transporting materials which have strong electron-donating ability, the relatively low SOMO levels makes TTM-1Cz and TTM stable in air and able to withstand oxygen and light for a long time. The SOMO energies of TTM and TTM-1Cz were measured by ultraviolet photoelectron spectroscopy (UPS). As calculated by the UPS spectra shown in Figure 2(b), TTM-1Cz, with an electron-donating group of carbazole at benzene ring, has a relative higher SOMO energy of -5.6 eV than TTM (-6.2 eV). Thus it is easy for the unpaired electron of TTM-1Cz to hop from its SOMO level to the LUMO of HAT-CN (-5.7 eV)22-24 due to a favorable energy difference, while the similar charge transfer between TTM and HAT-CN is negligible because of the unmatched energy levels, as shown in Figure 2(c).



Figure 2. (a) Molecular structures of HAT-CN, TTM and TTM-1Cz. (b) UPS spectra of TTM and TTM-1Cz. (c) Energy levels and charge transfer between HAT-CN, TTM and TTM-1Cz. To investigate the charge transfer process between HAT-CN and organic radical, absorbance spectra and UPS spectra of the pristine HAT-CN and radical doped HAT-CN films were measured, as shown in Figure 3. As shown in Figure 3(a), there exists a big difference for absorption spectra when TTM-1Cz or TTM is doped into HAT-CN. The pristine TTM-1Cz film shows a significant absorption with a maximum absorption at 380 nm. However, when TTM-1Cz is doped into HAT-CN, the absorption peak at 380 nm completely vanished, which is a strong signal for an efficient electron transfer between TTM-1Cz and HAT-CN. Moreover, UPS spectra shown in Figure 5(b) also demonstrate this charge transfer process. In UPS spectra, the pristine HAT-CN film has an obvious peak at the binding energy of 2-3 eV, which means some trapped states around Fermi level in HAT-CN film. But when TTM-1Cz is co-evaporated, the transferred electrons would fill the trapped states and increase its Fermi level due to the efficient electron transfer process. For comparison, when TTM is doped into HAT-CN, the absorption peak and binding energy in the UPS spectra remained unchanged and stay at the same position, indicating a rather weak electron transfer between TTM and HAT-CN.

Figure 3. (a) UV-vis-near-IR absorbance spectra of pristine and radical-doped films (10 w.t.% 100 nm). (b) UPS spectra of pristine HAT-CN, TTM-doped and TTM-1Cz-doped HAT-CN films (10 w.t.%, 5 nm). 6 ACS Paragon Plus Environment

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To further study the influence of TTM-1Cz-doping on the performance of electrical conductivity in HAT-CN, we compare the conductivity of the films using conductive atomic force microscopy (c-AFM) measurement27, as described in Figure 4(a). The c-AFM images of pristine HAT-CN and TTM-1Cz-doped HAT-CN films clearly show a huge difference of current level and distributions between the two films, as can be seen in Figure 3(b) and 3(c). A significant increase of vertical current is observed when TTM-1Cz is added, indicating a greatly increased conductivity of the doped film. The current-voltage curves shown in Figure 3(d) also demonstrate a more than two orders of magnitude increase in film conductivity for average due to the efficient charge transfer process and filling of trapped state in HAT-CN by transferred electrons.

Figure 4. (a) Schematic diagram of c-AFM measurement, in which the film thickness is 5 nm and the doping ratio is 10 w.t.%. Current images of (b) pristine HAT-CN film and (c) TTM-1Cz-doped HAT-CN film at a bias of 1.0 V. (d) Current-voltage curves of pristine HAC-TN and TTM-1Cz-doped HAT-CN films measured by c-AFM.



The doped films are used in hole-only devices to study the hole injection properties upon radical doping. As shown in Figure 5(a), without HAT-CN, the device current density is relatively low, which means holes are hardly to be injected from anode (indium tin oxide, ITO) to hole-transporting layer (N,N’-di(naphthalene-1-yl)-N,N’-diphenyl-benzidine, NPB). A thin layer of HAT-CN between ITO and NPB could largely increase the current density from 1.3 A/m2 to 180.1 A/m2 at a 10 v bias due to the electron extraction process between NPB and HAT-CN.28-29 Here, depositing some TTM-1Cz in HAT-CN could further boost its current density to 611.1 A/m2 at an optimized doping concentration of 10% due to the efficient charge transfer process between TTM-1Cz and HAT-CN. For comparison, when TTM with a lower HOMO energy is doped into HAT-CN, the current density is even slightly decreased (Figure 3(b)). It is due to a weak electron-donating ability of TTM than TTM-1Cz and hence a negligible electron transfer between TTM and HAT-CN.

Figure 3. (a) Current density-voltage characteristics of hole-only devices with TTM-1Cz doped HAT-CN hole injection layer in different doping concentrations. (b) Current densityvoltage characteristics of hole-only devices with different dopants for comparison. The device structure is ITO/ pristine or radical-doped HAT-CN (5 nm)/ NPB (100 nm)/ Al. The optimized dopant concentration is about 10 w.t. %. With the above study, we provide a promising method for using stable organic radical to improve hole injection. And as reported in our previous work, a stable precursor of highly reactive organic radical (2-(2-Methoxyphenyl)-1,3-dimethyl-1H-benzoimidazol-3-ium, o8 ACS Paragon Plus Environment

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MeO-DMBI) could be used as an efficient electron-injection material to enhance electroninjection and improve the performance of OLEDs instead of using conventional LiF.21, 26 Here, we could use organic radical of TTM-1Cz to improve hole injection and o-MeO-DMBI to improve electron-injection in OLED devices. The device structure shown in Figure 6(a) is ITO/ Pristine or TTM-1Cz-doped HAT-CN (5 nm)/ NPB (40 nm)/ TCTA (10 nm)/ 5TCzBN : CzTrz (25 %, 35 nm)/ BPBiPA (20 nm)/ o-MeO-DMBI (4.5 nm)/ Al, where TCTA is N,N,Ntris(4-(9-carbazolyl)phenyl)amine,

5TCzBN

is

2,3,4,5,6-pentakis(3,6-di-tertbutyl-9H-

carbazol-9-yl)benzonitrile, CzTrz is 3′-(4,6-diphenyl-1,3,5-triazin-2-yl)-(1,1′-biphenyl)-3-yl)9-carbazole

and

BPBiPA

is

9,10-bis(4-(2-phenyl-1H-benzo[d]imidazol-1-yl)phenyl)

anthracene. In the devices, TTM-1Cz is an air-stable organic radical to improve hole injection, while o-MeO-DMBI-I is the precursor of reactive organic radical o-MeO-DMBI to enhance electron-injection. Both of the devices give the typical sky blue emission at 484 nm, indicating that the organic radical added in the devices have negligible influence for light-emission, as shown in Figure 6(b). But when TTM-1Cz is added in HAT-CN, it could improve hole injection, which increased device luminance and decrease device driving voltage (Figure 6(c)).

With

improved charge balance due to the enhanced hole injection, the current efficiency of device with TTM-1Cz is improved from 31.4 cd/A to 33.8 cd/A at 1000 cd/m2 (Figure 6(d)). And the power efficiency, which is highly concerned for real applications, is further enhanced from 30.2 lm/W to 34.9 lm/W at 1000 cd/m2, compared with the control device without TTM-1Cz. As far as we know, the voltage required to get 1000 cd/m2 (3.0 V) for TTM-1Cz-doped device is the lowest for sky blue OLEDs based on thermally activated delayed fluorescent emitters.30



Figure 6. (a) The structure of the OLED devices. (b) The normalized electroluminescence spectra of the devices (c) Current density-voltage, and luminance-voltage curves of the devices. (d) Current efficiency-luminance and power efficiency-luminance curves of the devices. 4. CONCLUSIONS To conclude, stable organic radicals are for the first time to be used as efficient dopants to enhance hole injection in optoelectronics. We demonstrate that the efficient charge transfer between organic radical and organic semiconductor is a crucial point for electron extraction and hole injection. And it is believed further improvement in hole injection efficiency can be achieved by tuning the energy levels of the organic radicals. It is a promising way to use organic radicals to improve hole injection, not only meaningful for organic light-emitting diodes, but potential for other advanced opotelectronics as well, such as organic solar cells, and organic thin-film transistors. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected], [email protected]


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Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS We would like to thank the National Science Fund of China (Grant Nos. 51525304 and U1601651), the National Key Basic Research and Development Program of China (Grant Nos. 2016YFB0400702, 2016YFB0401003), and the National Basic Research Program of China (Grant No. 2015CB655002) for ﬁnancial support. REFERENCES (1) Tang, C. W.; VanSlyke, S. A. Organic electroluminescent diodes. Appl. Phys. Lett. 1987, 51, 913-915. (2) Baldo, M. A.; Thompson, M. E.; Forrest, S. R. High-efficiency fluorescent organic lightemitting devices using a phosphorescent sensitizer. Nature 2000, 403, 750-753. (3) Uoyama, H.; Goushi, K.; Shizu, K.; Nomura, H.; Adachi, C. Highly efficient organic light-emitting diodes from delayed fluorescence. Nature 2012, 492, 234-238. (4) Shao, Y.; Yuan, Y.; Huang, J. Correlation of energy disorder and open-circuit voltage in hybrid perovskite solar cells. Nat. Energy 2016, 1, 15001. (5) Brenner, T. M.; Egger, D. A.; Kronik, L.; Hodes, G.; Cahen, D. Hybrid organic-inorganic perovskites: low-cost semiconductors with intriguing charge-transport properties. Nat. Rev. Mater. 2016, 1, 15007. (6) Naab, B. D.; Himmelberger, S.; Diao, Y.; Vandewal, K.; Wei, P.; Lussem, B.; Salleo, A.; Bao, Z. High Mobility n-type transistors based on solution-sheared doped 6,13bis(triisopropylsilylethynyl)pentacene thin films. Adv. Mater. 2013, 25, 4663-4667. (7) Li, H.; Tee, B. C. K.; Cha, J. J.; Cui, Y.; Chung, J. W.; Lee, S. Y.; Bao, Z. High-mobility field-effect transistors from large-area solution-grown aligned C60 single crystals. J. Am. Chem. Soc. 2012, 134, 2760-2765.



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GRAPHICAL ABSTRACT


Stable Organic Radicals as Hole Injection Dopants for Efficient

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