Enabling High-Efficiency Organic Light-Emitting Diode with

C , 2018, 122 (33), pp 18836–18840 ... as the host, bis[2-(2-pyridinyl-N)phenyl-C](acetylacetonato)iridium(III) [(ppy)2Ir(acac)] as the green emitte...
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Enabling High-Efficiency Organic Light-Emitting Diode with Trifunctional Solution-Processable Copper(I) Thiocyanate Sudam D. Chavhan,† Tsu Hao Ou,† Ming-Ruei Jiang,† Ching-Wu Wang,‡ and Jwo-Huei Jou*,† †

Departments of Materials Science and Engineering, National Tsing Hua University, Hsin-Chu, Taiwan 30013, Republic of China Institute of Optoelectronic & Electrical Engineering, National Chung Cheng University, Chiayi, Taiwan 62102, Republic of China

‡

J. Phys. Chem. C 2018.122:18836-18840. Downloaded from pubs.acs.org by UNIVERSITE DE SHERBROOKE on 01/15/19. For personal use only.

S Supporting Information *

ABSTRACT: We report on a low-temperature solution processed trifunctional inorganic p-type semiconductor, copper(I) thiocyanate (CuSCN), as a hole injection/transporting and electron-blocking layer for high-efficiency organic light-emitting diodes (OLEDs). The electroluminescence (EL) characteristics of CuSCN and poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) based devices were studied with the structure of 4,4′-bis(Ncarbazolyl)-1,1′-biphenyl as the host, bis[2-(2-pyridinyl-N)phenyl-C](acetylacetonato)iridium(III) [(ppy)2Ir(acac)] as the green emitter, 2,2′,2″-(1,3,5-benzinetriyl)-tris(1-phenyl1H-benzimidazole) as the electron transporting layer, and lithium fluoride/aluminum as the cathode electrode. The power efficacies for the CuSCN based devices are found to be 51.7 and 40.3 lm/W at 100 and 1000 cd/m2, respectively, which are 13 and 60% higher than the PEDOT:PSS based counterparts. These are the highest power efficacies ever reported for this particular device architecture. The superior EL characteristics may be explained by its unique electronic properties. We believe that the high lowest unoccupied molecular orbital (−1.8 eV) and deep highest occupied molecular orbital (−5.5 eV) of CuSCN assist to confine the electron injected into the emission layer and facilitate the injection of hole, likewise enhancing recombination. The present study will serve to enable highly efficient white OLEDs for general lighting purposes.

1. INTRODUCTION Solution-processed organic light-emitting diode (OLED) is one of the promising alternatives to conventional solid-state lighting and display technologies because of their low-cost, flexibility and roll-to-roll fabrication process.1−5 Therefore, it is vitally important to pay more attention to enhance the overall performance of the solution-processed OLEDs to substitute expensive vacuum-based fabrication techniques. So far, poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) has been widely used as a hole injection/transport layer (HIL/HTL) for solution-processed OLEDs. However, its low work function and problematic chemical properties, such as being hygroscopic and corrosive, compromises the power efficiency and device lifetime.6−9 Moreover, it lacks the ability to effectively block electrons and excitons crossing from an emissive layer (EML). Efforts to use customized organic small molecules as a HIL/HTL appears to be immaterialized because the deposited thin layer of small molecules can either be dissolved or physically washed away by the solvents of the subsequent EML deposition.10,11 Hence, to get rid of the difficulty, it is crucial to search a new class of HIL/HTL materials, which can be stable in an ambient atmosphere as well as possess superior electronic properties as those of organic materials. To keep an eye out for new HIL/HTL materials, a prerequisite condition for selecting the HIL/HTL material is © 2018 American Chemical Society

to possess excellent electronic properties, which can effectively block the electrons and excitons, to exhibit chemical compatibility with the EML, and be environmentally robust to avoid the degradation at the HTL/EML interface. One promising approach is to use inorganic wide band gap semiconductor materials that are easy to process over largearea substrates because of their excellent optoelectronic properties such as optical transparency and high conduction band value and deep valence band level.12 Recently, much research has been devoted to utilize metal oxide hole transporting materials (HTMs) such as nickel oxide (NiO),13−15 vanadium oxide (V2O5),16−18 molybdenum oxide (MoO3),16,19 and tungsten oxide (WO3) for solutionprocessed OLEDs.20,21 However, the employment of these materials requires the additional extra step of thermal decomposition to remove unwarranted hydroxide species steamed from the precursor solution to act as an HIL/HTL efficiently. As such, wide band gap metal oxide materials harbor high resistivity that limits their singular role as an HIL rather than a multifunctional HIL/HTL layer and requires an additional layer of HTL for high power efficiency, and hence the device fabrication process becomes time-consuming and an Received: May 25, 2018 Revised: July 23, 2018 Published: July 27, 2018 18836

DOI: 10.1021/acs.jpcc.8b05029 J. Phys. Chem. C 2018, 122, 18836−18840

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The Journal of Physical Chemistry C

Figure 1. Schematic diagrams of the (a) molecular structures of the organic materials and the CuSCN compound and (b) green OLED device architecture; energy levels of the studied green OLED devices containing (c) PEDOT:PSS as the HIL and (d) CuSCN as an electron-blocking and hole-injecting and -transporting layer.

expensive one.21 To overcome the problems associated with metal oxide materials, we have opted to use copper(I) thiocyanate (CuSCN) inorganic p-type semiconductor material because of its excellent transparent properties across the visible range of the electromagnetic spectrum. Besides, it is very stable in an ambient atmosphere, easily deposited by the solution process, earthly abundant, and thereof low cost, which makes it a very attractive material to utilize it in optoelectronic device applications. CuSCN has been widely used in dye-sensitive, organic− inorganic hybrid perovskite, and polymer solar cells as an HTL because of its excellent optoelectronic properties.22 Perumal et al. successfully demonstrated the role of CuSCN as an HTL into OLED devices.23 Ding et al. fabricated quantum dot-based light-emitting diodes by employing CuSCN as an HIL.24 However, the power efficiency of these devices is low at high luminance. In the present study, we have fabricated high-efficiency solution processed green OLEDs by employing CuSCN as an HIL/HTL and compared it with PEDOT:PSS based counterparts. It is revealed that the CuSCN based device shows 51.7 and 40.3 lm/W power efficacies at 100 and 1000 cd/m2, respectively, which are 13 and 60% higher than the PEDOT:PSS based counterparts. These are the highest power efficacies ever reported for this particular device architecture.

were measured by an ultraviolet−visible (UV−vis) spectrophotometer. 2.2. Device Fabrication and Characterization. Indium tin oxide (ITO)-coated 3 × 3 cm2 glass substrates were cleaned with acetone and isopropyl alcohol via water bath sonication for 30 min. Ultraviolet (UV)−ozone treatment was carried out for 15 min to remove the residue of organic moieties. PEDOT:PSS as a HIL/HTL was deposited on a cleaned ITO substrate via spin coating at 4000 rpm for 20 s and dried at 120 °C for 20 min. The as-prepared CuSCN solution was utilized to deposit the CuSCN layer via spin coating at 3000 rpm for 60 s. The EML solution consists of CBP, and Ir(ppy)2(acac) was prepared by dissolving the molecules in tetrahydrofuran at 40 °C and was subjected to sonication for 90 min. Prior to EML deposition, the solution was filtered by a 0.45 μm PET filter and spin-coated at 2500 rpm for 20 s in a nitrogen-filled glovebox. The deposition of TPBi, LiF, and Al was carried out under a high-vacuum thermal evaporator. The characterization of the final device was done in an ambient atmosphere without encapsulation. The current−voltage−luminance characterization was done by a computer interfaced Keithley 2400 electrometer and a Minolta CS-100A luminance-meter. Emission spectra were recorded by a PR-655 spectroradiometer. The emission area of the OLED device was 0.09 cm2, calculated from the overlapping of ITO and Al electrodes.

3. RESULTS AND DISCUSSION Figure 1a shows the molecular structures of organic materials and the CuSCN compound. The studied device architecture along with the energy band diagram consisting of PEDOT:PSS and CuSCN as hole-transporting materials are shown in Figure 1b−d. Energy level diagrams reveal that the PEDOT:PSS comprised device will have poor electron blocking properties to CuSCN counterparts because of its relatively low lowest unoccupied molecular orbital (LUMO) level. Absorption and transmittance of the PEDOT:PSS and CuSCN thin films deposited on quartz substrates were carried out using an UV−vis spectrophotometer, and it is shown in Figure 2. The absorption spectra reveal that the CuSCN thin

2. EXPERIMENTAL DETAILS 2.1. Materials and Thin-Film Characterization. The host material 4,4′-bis(carbazol-9-yl)biphenyl (CBP), guest material bis[2-(2-pyridinly-N)phenyl-C] (acetylacetone)iridium(III) [Ir(ppy)2(acac)], and electron-transport material 2,2′,2″-(1,3,5-benzinetriyl)-tris(1-phenyl-1H-benzimidazole) (TPBi) were purchased from Luminescence Technology Corporation Ltd., Taiwan. Hole injection/transport material PEDOT:PSS was obtained from Heraeus and CuSCN was brought from Sigma-Aldrich. Absorption and transmittance spectra of quartz/PEDOT:PSS and quartz/CuSCN thin films 18837

DOI: 10.1021/acs.jpcc.8b05029 J. Phys. Chem. C 2018, 122, 18836−18840

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increments in ηp, ηc, and EQE were found to be 60.5, 33.9, and 32.4%, respectively. With the increase in the emitter dopant concentration from 15 to 17.5 wt %, the power efficacy (PE) of the device decreased by 46.8%. The deleterious effect of high dopant concentration on the EL characteristic of the device is imputed for low exciton generation originating from the adverse morphology of the EML caused by the self-aggregation of the emitter material after spin coating.25 Figure 4a shows the current density versus voltage (J−V) plot for 15 wt % dopant concentration green OLED device based on PEDOT:PSS and CuSCN HIL/HTL. J−V curve was obtained by connecting the positive terminal to the ITO electrode and negative to the Al electrode. It is noticed that by an increase in the positive bias voltage, the forward bias current of the device increases significantly, however, the reverse bias current remains low, which indicates that the device is excellently working as a rectifying diode. PEDOT:PSS based device shows a high forward bias current as compared to the CuSCN counterpart at a high bias voltage. Figure 4b shows the plot of luminance versus applied bias voltage of PEDOT:PSS and CuSCN devices. It reveals that the operating voltage of the CuSCN based device is lower than the PEDOT:PSS counterparts by ∼0.5 V. The low operating voltage for CuSCN is attributed to its high work function (5.5 eV) as compared to the PEDOT:PSS (5.0 eV) device that reduces the hole injection energy barrier between CuSCN and EML interface by 0.5 eV, as depicted in Figure 1c,d. In the case of PEDOT:PSS, the hole injection energy barrier height at HIL/ HTL and EML interface is around 1.0 eV that is beyond the permissible limit of ohmic contact and hence requires extra energy to effectively inject the holes into EML, resulting in the high operative voltage, and the experimental results corroborate the hypothesis. The power efficacy (PE) shown in Figure 4c exhibits that CuSCN HIL/HTL enhance the PE by 13, 60, and 47% at 100, 1000, and 10 000 cd/m2, as compared to the PEDOT:PSS counterpart, respectively. The superior performance of CuSCN based device at high luminance is attributed to its high conduction band value, which is located at 1.8 eV. The energy barrier height between the conduction band of CuSCN and the LUMO of EML is close to 1.1 eV. By contrast, the LUMO of PEDOT:PSS is 0.4 eV lower than the LUMO of CBP EML. This explains that CuSCN is capable of effectively blocking the electrons than PEDOT:PSS. More importantly, the high conduction band of CuSCN confines the excitons tightly within the EML at a high operating voltage and eventually help to give high PE at a high luminance. In both types of devices, the PE roll-off is slow at low luminance; however, it accelerated from 1000 cd/m2 and significantly declined beyond 10 000 cd/ m2. The current efficiency (CE) and EQE plots follow a similar

Figure 2. UV−vis absorption spectra of solution-processed PEDOT:PSS and CuSCN thin films on a quartz substrate.

film absorbs strongly near the UV region and weakly in the visible region. Conversely, PEDOT:PSS films absorb less light in the UV region and more light in the visible spectrum. The absorption of light in the visible region is attributed to the internal optical transitions involved in the intragap polaron levels of PEDOT:PSS.23 Transmittance of CuSCN thin films is higher in the visible region as compared to that of PEDTO:PSS films, as shown in Figure 2. Tapping mode atomic force microscopy was employed to study the surface morphology of ITO, ITO/PEDOT:PSS, and ITO/CuSCN thin films, and it is shown in Figure 3a−c. Surface morphology of bare ITO reveals the crystalline grain of ITO with surface roughness around 3.8 nm. PEDOT:PSS film exhibited 1.1 nm surface roughness indicating the very smooth layer deposition. The decreased surface roughness indicates that a uniform deposition of PEDOT:PSS fills the voids present in bare ITO substrates. The surface roughness of the CuSCN film is found to be 2.3 nm. The morphology of CuSCN-covered ITO substrate looks like bare ITO substrates except with some dark and light contrast. CuSCN layer thickness was optimized by changing the spin-coating speed from 3000 to 4000 rpm, and it corresponds to 15 and 9 nm, respectively (Figure S1). The effect of the CuSCN layer thickness on green OLED device performance is shown in the Supporting Information. The best green OLED performance is obtained by employing a 15 nm-thick layer of CuSCN. Table 1 shows the comparison of electroluminescence (EL) characteristics such as power efficiency (ηp), current efficiency (ηc), and external quantum efficiency (EQE) at 100, 1000, and 10 000 cd/m2 of three dopant concentrations of the green emitter with the CBP host OLED devices made up of PEDOT:PSS and trifunctional CuSCN. The best performance was noticed for CuSCN-based OLED devices at 15 wt % dopant concentration, revealing ηp of 40.3 lm/W, ηc of 53.7 cd/A, and EQE of 14.7% at 1000 cd/m2. By comparing the results of the PEDOT:PSS counterpart at 1000 cd/m2, the

Figure 3. Surface morphologies of (a) pristine ITO and solution-processed films of (b) PEDOT:PSS and (c) CuSCN coated on ITO. 18838

DOI: 10.1021/acs.jpcc.8b05029 J. Phys. Chem. C 2018, 122, 18836−18840

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Table 1. Comparison of the Green OLED Device Employing Tri-functional CuSCN against That of PEDOT:PSS from the Perspectives of OV, PE, CE, EQE, CIE Coordinates, and Maximum Luminance OV (V)

HTL PEDOT:PSS

CuSCN

PE (lm/W)

CE (cd/A)

Ir(ppy)2(acac) con. (wt %) 12.5 15.0 17.5 12.5 15.0 17.5

EQE (%)

@100/1000/10 000 cd/m 4.4/5.3/9.0 4.1/5.0/6.7 3.6/4.3/ 3.2/4.1/ 3.5/4.2/6.4 3.1/3.7/

31.8/15.9/1.7 45.6/25.1/9.1 20.5/13.4/ 46.7/30.0/ 51.7/40.3/13.4 21.4/19.6/

44.8/26.7/4.8 59.8/40.1/19.5 23.2/18.3/ 47.4/38.7/ 56.8/53.7/26.9 20.9/22.8/

CIE (x,y) max. luminance (cd/m2)

2

11.8/7.0/1.3 15.6/10.5/5.1 6.0/4.7/ 12.3/10.0/ 14.7/13.9/7.0 5.4/5.9/

(0.33, (0.33, (0.32, (0.33, (0.33, (0.33,

0.63)/(0.33, 0.63)/(0.33, 0.64)/(0.32, 0.63)/(0.33, 0.63)/(0.33, 0.63)/(0.33,

0.63)/(0.33, 0.63) 0.63)/(0.33, 0.63) 0.64)/ 0.63)/ 0.63)/(0.34, 0.63) 0.63)/

10 390 18 200 7072 7569 15 180 8582

Figure 4. Comparison of the green OLED device employing the tri-functional CuSCN against that of PEDOT:PSS from the perspectives on the resulting (a) current density, (b) luminance, (c) PE, (d) CE, and (e) EQE.

is attributed to different optical properties of both HIL/HTLs rather than their light absorption property.

trend at high luminance, as shown in Figure 4d,e. The high CE and EQE demonstrated by PEDOT:PSS, as compared to the CuSCN counterpart at 100 cd/m2, is attributed to its high charge-carrier mobility due to oxidative doping, whereas CuSCN is an intrinsic wide band gap material and possesses low charge-carrier mobility. Figure 5 shows the normalized EL spectra of the devices containing 15 wt % green Ir(ppy)2(acac) dye in the CBP host corresponding to the PEDOT:PSS and CuSCN HIL/HTL devices. The peaks of the emission spectra exhibit the emission from both devices that are in the green region. The slight spectral difference observed between 540 and 560 nm regions

4. CONCLUSIONS We have successfully demonstrated the tri-functional role of CuSCN as an excellent HIL/HTL for highly efficient solutionprocessed green OLEDs. The high PE of the green OLED devices with CuSCN relative to PEDOT:PSS HIL/HTL counterparts is mainly attributed to its high conduction band level and deep valence band position. These excellent optoelectronic properties play an important role to reduce the operation voltage (OV) of the device as well as for effective blocking of the electrons and efficient injection of holes at the HIL/HTL and EML interface. The present results will be helpful to fabricate high efficacy white OLEDs for general lighting purposes.

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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.jpcc.8b05029. Thickness depth profiles of CuSCN thin films prepared Figure 5. Comparison of the green OLED device employing the trifunctional CuSCN against that of PEDOT:PSS from the perspectives of EL spectra at 1000 nits.

at 3000 and 4000 rpm by using spin coating and CuSCN thickness-dependent green OLED performance (PDF). 18839

DOI: 10.1021/acs.jpcc.8b05029 J. Phys. Chem. C 2018, 122, 18836−18840

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +886-(0)3-5742617. ORCID

Sudam D. Chavhan: 0000-0001-7134-4968 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to acknowledge the support from the Ministry of Science and Technology and Ministry of Economic Affairs, Taiwan. This work was financially supported in part by grants 106-2811-M-007-065 and 104-EC-17-A-07-S3-012.

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DOI: 10.1021/acs.jpcc.8b05029 J. Phys. Chem. C 2018, 122, 18836−18840