Nanoporous ZnMnO Organic–Inorganic Hybrid Light-Emitting Diode

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Multicolor Emission from Poly(p‑Phenylene)/Nanoporous ZnMnO Organic−Inorganic Hybrid Light-Emitting Diode Sejoon Lee,*,†,‡ Youngmin Lee,‡ Deuk Young Kim,†,‡ and Gennady N. Panin‡,§ †

Department of Semiconductor Science and ‡Quantum-Functional Semiconductor Research Center, Dongguk University-Seoul, Seoul 04623, Korea § Institute of Microelectronics Technology and High-Purity Materials, Russian Academy of Sciences, 142432 Chernogolovka, Russia ABSTRACT: The voltage-tunable multicolor emission was realized in a poly(p-phenylene)/nanoporous ZnMnO organic−inorganic hybrid light-emitting diode. Red, green, and blue (RGB) colors sequentially appeared with increasing magnitude of the bias voltage (i.e., R → RG → RGB with V↑). At a higher voltage (>2.4 V), eventually, the device emitted the visible light with a mixture of colors including RGB. These unique features may move us a step closer to the application of organic−inorganic hybrid solid-state lighting devices for the full-color display and/or the electrical-to-optical data converter for multivalue electronic signal processes. In-depth analyses on electrical and optical properties are presented, and voltage-controllable multicolor-emission mechanisms are discussed. KEYWORDS: poly(p-phenylene), ZnMnO, organic−inorganic hybrid light-emitting diode, electroluminescence, voltage-controlled multiple color emission

1. INTRODUCTION Solid-state lighting devices (SSLDs) are of huge interest for the various display applications; for example, portable display units, cellular phones, and personal palmtop smart organizers. For the demonstration of the full-color display, three primary colors (i.e., red, green, and blue (RGB)) are mandatory. In recent years, plenty of SSLDs showing either of RGB has been demonstrated on various types of light-emitting devices. With respect to the material issues, for instance, there are three main categories. First, the light-emitting diodes (LEDs) composed of various inorganic semiconductor materials are the typical example of SSLDs.1−6 The second group of SSLDs can be categorized by the organic material-based LEDs,7−13 which are widely used as elementary devices for the full-color display and also are feasible for transparent-flexible display systems. Finally, the organic−inorganic hybrid (OIH) material systems have been intensively exploited for better performances of SSLDs (e.g., improvements of color purity, color stability, color saturation, etc).14−21 Particularly, the OIH systems would enable us to demonstrate multicolor emission in a single device, because the wide spectrum of the material choice and its mixture system could allow color-tuning15−18 and/or colorswitching operations.22,23 Furthermore, the OIH material systems can provide low-cost and drawback techniques for the full-color display system. All of the above have garnered ample attention to the color-tunable OIH-SSLDs for the demonstration of multiple- and/or variable-color emission in SSLDs. In light of this, we devised the poly(p-phenylene) (PPP)/ nanoporous ZnMnO (ZMO) OIH-LED by using simple fabrication steps, including spin-coating and shadow-masking © 2016 American Chemical Society

techniques. The device shows the voltage-tunable multicolor emission, which might enable us to demonstrate the full-color emissions and/or to convert the multivalued electronic signals to the identical optical signals. In this article, we examine the electrical and optical properties of the fabricated PPP/ZMO OIH-LED and discuss the color-tuning mechanism in our PPP/ ZMO OIH-LED.

2. RESULTS AND DISCUSSION Figure 1 illustrates the device scheme of the p-PPP/n-ZMO OIH-LED. First, ZMO multilayers (i.e., Zn0.93Mn0.07O (∼8 nm)/Zn0.65Mn0.35O (∼8 nm) × 20 periods) were sequentially deposited on an (0001) Al2O3 substrate by r.f. magnetron sputtering (Figure 1a). The scanning electron microcopy (SEM) image of the top layer exhibits a smooth surface with no visible pinholes or hillocks (Figure 1e). After the deposition of the ZMO multilayers, high-temperature annealing was performed at 1100 °C so as to form a ZMO porous superlattice structure (Figure 1b). As shown in Figure 1f, the sample clearly displays a lot of hexagonal pores with the average diameter of ∼500 nm. The formation of porous ZMO is thought to result from thermal grooving and agglomeration in multilayered ZMO nanosheets.24,25 Because of the formation of porous superlattices, ZMO becomes an n-type semiconductor, because the oxygen vacancies (VO) at the wall of ZMO pores act as the donors (Figure 1g). Onto the ZMO porous superlattice layer, the hole injection layerPPPwas spin-coated by using PPPReceived: September 11, 2016 Accepted: December 6, 2016 Published: December 6, 2016 35435

DOI: 10.1021/acsami.6b11539 ACS Appl. Mater. Interfaces 2016, 8, 35435−35439

Research Article

ACS Applied Materials & Interfaces

Figure 2. I−V characteristic curve in a semilogarithmic scale for the pPPP/n-ZMO hybrid diode. (upper inset) The I−V characteristic curves in a linear scale for the p-PPP/n-ZMO OIH-LED and the ohmic contacts to each layer. (lower inset) The schematic configuration of the bias setup.

Figure 3 displays the optical properties of the materials used in this experiment. To verify the intrinsic properties of each

Figure 1. Fabrication procedures for the p-PPP/n-ZMO OIH-LED. (a) Growth of the ZMO multilayers on the Al2O3 substrate. (b) Formation of nanoporous ZMO by thermal annealing. (c) Deposition of the p-PPP layer on nanoporous ZMO. (d) Fabrication of the pPPP/n-ZMO OIH-LED by the formation of Au and In electrodes. (e) Surface SEM image of the ZMO multilayers before thermal annealing. (f) Surface SEM image of nanoporous ZMO after thermal annealing. (inset) The enlarged view of the ZMO nanopores. (g) Schematic illustration of the expected lattice structures at the edge of the ZMO pores. (h) Surface SEM image of the PPP layer. (inset) The symbol of PPP. Figure 3. PL spectra of the nanoporous ZMO layer, the PPP layer, and the PPP/ZMO OIH structure. (inset) The green luminescence-related optical transitions in ZMO represented in the energy-band scheme.

blended chloroform solution (Figure 1c). Here, we note that PPP not only covers the ZMO porous superlattice layer but also fills the ZMO pores (Figure 1h). Hence, the effective area of p−n junction (i.e., n-ZMO/p-PPP) could be increased. Using the simple shadow-masking techniques, finally, In and Au electrodes are formed onto the n-ZMO and p-PPP, respectively (Figure 1d). After the device fabrication, we bonded Au wires onto the Au and In contact-pads and coated silica of glass (SOG) on the whole device area to prevent transmutation of the OIH junction properties (see the lower inset of Figure 2). Then, we examined the electrical characteristics of the n-ZMO/p-PPP OIH-LED. As shown in the upper inset of Figure 2, a good linearity in the current−voltage (I−V) relationship confirms the ohmic contacts to be well-formed for both n-ZMO (i.e., In−In) and p-PPP (i.e., Au−Au). In addition, the I−V characteristics between p-PPP and n-ZMO (i.e., Au−In) clearly reveal a rectifying behavior with the turn-on voltage (VT) of ∼1.2 V. The breakdown voltage (VB) is ca. −4 V, and the reverse saturation current (IS) is ∼3.5 × 10−6 A. These obviously depict that the p−n junction is formed between n-ZMO and p-PPP. From the devices fabricated by the identical procedures, more than 65% of the devices showed the similar characteristics.

layer, we analyzed the photoluminescence (PL) characteristics of PPP, ZMO, and PPP/ZMO. For porous ZMO, the green luminescence peak strongly appears with small peaks from the ultraviolet (UV) emission and the red emission. The strong green luminescence is attributed to both the VO-related radiative transition in ZMO (i.e., VO → EV)26,27 and the impact ionization-induced intraband transition in Mn2+ core− shells (i.e., 4T1 → 6A1)28,29 (see the inset of Figure 3). The UV peak originates from the near-band-edge emission in host material ZnO,30,31 and the red emission comes from the radiative recombination related to excess oxygen, possibly involving zinc vacancy complexes.30,31 For PPP, three emission peaks are clearly observable at the blue, green, and red regions. The blue emission is associated with the S1−S0 (0−0) transition and its dominant vibronic satellite.32,33 In addition, the emission at the longer wavelength region (i.e., green and red) is thought to arise from the lowenergy tails due to the aggregate states, because the aggregates give rise to the pronounced low-energy contribution to the PL properties in PPP.32,33 The broadening of spectral emissionband widths is responsible for the random distribution of PPP 35436

DOI: 10.1021/acsami.6b11539 ACS Appl. Mater. Interfaces 2016, 8, 35435−35439

Research Article

ACS Applied Materials & Interfaces with different lengths.34,35 In the PPP/ZMO OIH structure, the mixture colors (i.e., R, G, B) are visible, depicting that the PPP/ ZMO OIH junction has multiple light-emission sources. To investigate further insight into the light-emission sources at the PPP/ZMO junction, we examined the cathodoluminescence (CL) properties. To identify the luminescence centers, first, we compared the CL features for each layer with the PL results (Figure 4). Similar to PL, ZMO exhibits the

Figure 5. EL spectra at 300 K for the p-PPP/n-ZMO OIH-LED under various bias conditions (VA = 1.2−2.7 V). (inset) The photograph for the EL emission at VA = 2.7 V.

further feasibility (e.g., effective filling of PPP into ZMO pores for better interfacial properties). One thing that should be noticed here is the VA-dependent sequential appearance of the emitting color (i.e., R → RG → RGB with VA↑). The voltage gap for color-tuning from R to RG is ∼0.6 V, and that from RG to RGB is ∼0.2 V. These are smaller than the energy differences for R-G (∼0.65 eV) and GB (∼0.53 eV) bands. We attribute the above features to the broad electron-distribution function of ZMO, as discussed later. To help understand the operation scheme, we explain the voltage-tunable multicolor-emission mechanism for our device. Figure 6a illustrates the energy-band diagram at the equilibrium

Figure 4. CL spectra of the nanoporous ZMO layer, the PPP layer, and the depletion region of the p-PPP/n-ZMO OIH junction under Vacc = 2−5 keV.

strong green emission, and PPP reveals three emission bands at blue, green, and red regions. Next, to detect the CL signals from the PPP/ZMO junction, we arranged and focused on the expected depletion region (scanned area: 200 nm × 200 nm) by using the SEM machine equipped with the CL system. When applying the e-beam’s acceleration voltage (Vacc) of 2 kV, three emission peaks of R-G-B are observable near the depletion region. When increasing Vacc to 5 kV (i.e., when increasing the e-beam’s penetration depth), the CL intensity is much increased as much as the R-G-B peaks become more resolvable. The mixturization of R-G-B colors in the depletion region could allow the voltage-controllable multicolor emission in the n-ZMO/p-PPP OIH-LED, as discussed later. On the basis of all the above, we characterized the EL properties of the device in a dark vacuum chamber. Figure 5 shows the EL spectra of the n-ZMO/p-PPP OIH-LED with respect to the applied voltage (VA). During the EL measurements, the injection current at each VA was followed from the I−V characteristics. When VA of 1.2 V is applied, the device exhibits no significant EL emission. As VA increases up to 1.5 V, however, the n-ZMO/p-PPP OIH-LED begins to emit the red light at ∼700 nm. At VA = 2.1 V, the device displays the twocolor-emission (i.e., red and green), and the multiple colors of R-G-B are observable from VA = 2.1 V. As the magnitude of VA increases, the EL emission peaks become great. For example, three mixture colors of R-G-B are clearly visible when VA exceeds 2.4 V. As shown in the inset of Figure 5, at VA = 2.7 V, the device clearly displays a bright light emission. At VA = 2.7 V, the luminance value was ∼780 cd/m2, and the external quantum efficiency was ∼0.34%. Although the efficiency of our device is comparable to literature values,15,16 improving the external quantum efficiency should be a next step for the

Figure 6. Energy-band diagrams of the p-PPP/n-ZMO OIH-LED under various bias conditions: (a) VA = 0 V, (b) VA ≈ VT, (c) VA ≫ VT, and (d) VA ≫ VT.

state. Because of the energy difference in electron affinity of PPP (qχe(PPP) = 1.7 eV)36 and ZMO (qχe(ZMO) = 4.1 eV),37 the heterojunction is created between p-PPP and n-ZMO. According to PL and CL results, we here assume that the RG-B bands exist within the bandgap of PPP (Eg(PPP) = 3.5 eV)36 and that the G band is located underneath the conduction band of ZMO (Eg(ZMO) = 3.3 eV).37 When an appropriate positive voltage is applied to the device (e.g., VA > VT), the electrons in n-ZMO start to jump into the R 35437

DOI: 10.1021/acsami.6b11539 ACS Appl. Mater. Interfaces 2016, 8, 35435−35439

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ACS Applied Materials & Interfaces

4.5. Photoluminescence Measurements. Photoluminescence (PL) measurements were performed at 300 K using an excitation source of a He−Cd laser (3250-Å line) and a 75 cm monochromator equipped with a GaAs photomultiplier tube. 4.6. Cathodoluminescence and Electroluminescence Measurements. CL measurements were performed by using a Gatan MONO CL2 system equipped with a scanning electron microscopy system (FE-SEM XL-30). During CL measurements, we applied the ebeam’s acceleration voltage of 2−5 kV, and the scanned area was 200 nm × 200 nm. Electroluminescence (EL) properties were characterized by using our home-built EL system, which is equipped in a dark chamber.

band of p-PPP (Figure 6b). Eventually, the electrons will recombine with holes from p-PPP, resulting in light emission in a red color region. When VA is increased far beyond VT (e.g., VA ≫ VT), the slope of the energy band will become more steep (Figure 6c). At this stage, because of the large potential gradient in tilted energy bands, the electrons could also reach the G band as well. At room temperature, since the electron distribution function of semiconductor ZMO should be broadened toward the higher energy region, the voltage required for color-tuning from R to RG could be smaller than the energy difference between R and G bands. Similarly, when the bias voltage is further increased (e.g., VA ≫ VT), the B band in PPP and the G band in ZMO can contribute to light emission in the present device (Figure 6d). For this bias state, due to both the large potential gradient and the broad electrondistribution function, one can easily tune the emission colors from RG to RGB by adding a small magnitude of VA. Consequently, multiple color emission can be demonstrated on our p-PPP/n-ZMO OIH-LED by controlling the magnitude of the bias voltage.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +82-2-2260-3946. Fax: +82-2-2260-3945. ORCID

Sejoon Lee: 0000-0002-4548-7436 Notes

The authors declare no competing financial interest.

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3. CONCLUSION The n-ZMO/p-PPP OIH-LED was fabricated through the simple process. The PPP-blended solution was spin-coated onto the nanoporous ZnMnO multilayer, which had been formed by thermal nucleation of the 20-period Zn0.93Mn0.07O/ Zn0.65Mn0.35O multilayers. And, the ohmic electrodes were formed through shadow-masking techniques. The fabricated device clearly exhibited a rectifying behavior with VT ≈ 1.2 V, VB ≈ −4 V, and IS ≈ 3.5 × 10−6 A. For EL measurements, the device exhibited the unique light-emitting characteristics (i.e., voltage-tunable multicolor emission). Namely, three colors of R-G-B sequentially appeared with increasing the bias voltages; eventually, all of the R-G-B colors were observed at the higher bias voltage. This feature arose from the voltage-controlled sequential contribution of R-G-B bands in the n-ZMO/p-PPP OIH-junction. The results suggest that the device holds promise for the applications in full-color displays and/or electrical-to-optical data converters representing the identical light signals upon the multivalued electronic signals.

ACKNOWLEDGMENTS This research was supported by Basic Science Research Program (2016R1A6A1A03012877) through the National Research Foundation of Korea (NRF) funded by the Ministry of Education.

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REFERENCES

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4. METHODS 4.1. Preparation of Poly(p-phenylene). PPP was obtained by the oxidative polymerization of benzene in an aluminum chloride and N-butylpyridinium chloride-based ionic liquid. For the chemical synthesis of PPP, we followed the procedures described in literature.38−40 4.2. Growth of Nanoporous ZnMnO. Nanoporous ZnMnO layers were prepared by the thermal nucleation of the sputter-grown ZnMnO multilayer. First, 20 periods of Zn0.93Mn0.07O (∼8 nm)/ Zn0.65Mn0.35O (∼8 nm) were subsequently deposited at 600 °C on (0001) Al2O3 substrates in Ar and O2 mixture gases (Ar: 15 sccm, O2: 15 sccm), where the working pressure was kept at 10 mtorr, and the r.f. power of 120 W was supplied. Next, high-temperature annealing was performed at 1100 °C in N2 to form nanoporous ZnMnO. 4.3. Fabrication of Poly(p-phenylene)/ZnMnO OIH-LED. First, PPP was blended with chloroform (30 mg/20 mL) and spin-coated onto the nanoporous ZnMnO layer. Then, the active areas were patterned by using conventional photolithography and shadow-mask techniques. Through the e-beam evaporation of In and Au, finally, ohmic electrodes were formed onto ZnMnO and PPP, respectively. 4.4. Electrical Measurements. The electrical properties were measured in an atmosphere ambience using a Keysight B1500A semiconductor device parameter analyzer. 35438

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DOI: 10.1021/acsami.6b11539 ACS Appl. Mater. Interfaces 2016, 8, 35435−35439