Efficient Electron Injection by Size- and Shape ... - ACS Publications

Subscriber access provided by GAZI UNIV

Article

Efficient Electron Injection by Size- and Shape-Controlled Zinc Oxide Nanoparticles in Organic Light Emitting Devices Yong-Jin Pu, Norito Morishita, Takayuki Chiba, Satoru Ohisa, Masahiro Igarashi, Akito Masuhara, and Junji Kido ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b07742 • Publication Date (Web): 29 Oct 2015 Downloaded from http://pubs.acs.org on October 30, 2015

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Applied Materials & Interfaces is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 12

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

Efficient Electron Injection by Size- and Shape-Controlled Zinc Oxide Nanoparticles in Organic Light Emitting Devices Yong-Jin Pu*1, Norito Morishita1, Takayuki Chiba1, Satoru Ohisa1, Masahiro Igarashi1, Akito Masuhara2, Junji Kido*1 1

Department of Organic Device Engineering, 2Department of Chemistry and Chemical Engineering,

Yamagata University, 4-3-16 Jonan, Yonezawa 992-8510, Japan [email protected]; [email protected]

Abstract Three different sized zinc oxide (ZnO) nanoparticles were synthesized as spherical ZnO (S-ZnO), rod-like ZnO (R-ZnO), and intermediate shape and size ZnO (I-ZnO) by controlling the reaction time. The average sizes of the ZnO nanoparticles were 4.2 nm × 3.4 nm for S-ZnO, 9.8 nm × 4.5 nm for I-ZnO, and 20.6 nm × 6.2 nm for R-ZnO. Organic light-emitting devices (OLEDs) with these ZnO nanoparticles as the electron injection layer (EIL) were fabricated. The device with I-ZnO showed lower driving voltage and higher power efficiency than those with S-ZnO and R-ZnO. The superiority of I-ZnO makes it very effective as an EIL for various types of OLEDs regardless of the deposition order or method of fabricating the organic layer, the ZnO layer, and the electrode.

Keyword: zinc oxide, nanoparticles, solution-process, electron injection, organic light emitting device

1. Introduction Zinc oxides (ZnO) have been widely used as n-type semiconducting materials in organic electronic devices.1-6 There are mainly three ways to form the ZnO layer: sputtering,1, 6 the sol–gel process,3,

4

and nanoparticle deposition.5,

7

The sol–gel process and nanoparticle deposition are

solution-based processes and can reduce production costs. The sol–gel process often requires high temperatures to convert a precursor to ZnO, whereas nanoparticle deposition takes place at low temperatures as long as the process solvent can be dried. There are many reports on the application of ZnO nanoparticles as an electron collection/extraction layer in organic photovoltaic (OPV) devices. On the other hand, there are fewer reports on the use of ZnO nanoparticles as an electron injection layer (EILs) in OLEDs.8 This is presumably because the ZnO nanoparticle layer tends to have a rough morphology, causing partial short circuits and/or poor electrical contact in the thin flat device, and the damage by ZnO ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

nanoparticle to OLEDs is more serious than that to OPVs. In OPVs, the short circuits and the poor contact results in small shunt resistance and large series resistance, respectively, but the device itself can still work and those defects in the devices are invisible. However, short circuits in OLEDs break the device and poor contacts cause dark spots, and those defects are visible. These types of damage are fatal to the lighting devices. In addition, the device thickness of OLEDs ideally has to be thin to reduce voltage and power consumption, whereas that of the OPV ideally has to be thick to absorb more light. Among reports on use of a ZnO layer in OLEDs, most device structures are limited to inverted structures in which the ZnO is formed on an ITO substrate because the ZnO on the ITO can be thermally annealed at high temperature to improve surface morphology and conductivity. On the other hand, there have been few reports on ZnO layers formed on organic layers in regular-structure OLEDs.9-13 Recently, Wilken et al. reported that more crystallized rod-like ZnO nanoparticles have fewer electron trap sites and are less sensitive to oxygen in OPVs than less crystallized spherical nanoparticles.14 Here, we report efficient electron injection by ZnO nanoparticles deposited on an organic layer in regular-structure OLEDs without the use of polyelectrolytes or alkali metal salts. We synthetically controlled the size and shape of the ZnO nanoparticles. ZnO nanoparticles whose shape and size were intermediate between spherical ZnO and rod-like ZnO showed higher electron injection and efficiency as an EIL than the spherical and rod-like ZnOs. This intermediate shape and size of the growth-controlled ZnO is universally effective, making the ZnO superior as an EIL not only on the organic layer in regular OLEDs but also on ITO in inverted OLEDs.

2. Results and Discussion ZnO nanoparticles were synthesized from zinc acetate with potassium hydroxide in methanol, according to the procedure in the literature;2, 15 however, no surfactants were added in our study. At the beginning of the reaction, spherical nanoparticles formed; these spherical nanoparticles gradually developed a rod-like shape with increasing reaction time. Three types of nanoparticles of different sizes were prepared: spherical ZnO (S-ZnO), rod-like ZnO (R-ZnO), and intermediate shape and size ZnO (I-ZnO). The average sizes of the ZnO nanoparticles were 4.2 nm × 3.4 nm for S-ZnO, 9.8 nm × 4.5 nm for I-ZnO, and 20.6 nm × 6.2 nm for R-ZnO, as determined from their transmission electron microscope (TEM) images (Figure 1). The crystalline growth of the ZnO particles was confirmed by X-ray diffraction (XRD), UV-vis absorption (UV), and photoluminescence (PL) measurements (Figure 2). The peaks in the XRD patterns are attributed to a wurtzite-type structure. The (002) reflection peak at 36.2° of I-ZnO and R-ZnO, which is sharper than that of S-ZnO, shows rod formation along the c-axis, which is ACS Paragon Plus Environment

Page 2 of 12

Page 3 of 12

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


consistent with previous detailed analyses.15, 16 The optical energy gaps (Eg) of R-ZnO and I-ZnO, estimated from the absorption edge, were 3.31 eV and 3.34 eV, respectively, values that are almost the same as that of bulk ZnO.17 On the other hand, the UV-vis spectrum of S-ZnO is blue-shifted from those of I-ZnO and R-ZnO, indicating a less crystalline structure. The PL spectra also indicate a difference in the crystallinity of the ZnO nanoparticles. There are two peaks in the PL spectra. The narrow peak at approximately 370 nm is attributed to the band transition of the ZnO crystal and the broad peak at approximately 540 nm is attributed to the trap-state emission.18 The band emission of S-ZnO and I-ZnO is blue-shifted from that of R-ZnO, due to quantum confinement effect of the smaller ZnO nanoparticles. The intensity of the trap-state emission is strongly dependent on the size of ZnO nanoparticles. From the intensity of these two peaks of the band emission and the trap-state emission, it is apparent that R-ZnO has a more crystallized structure and fewer trap-state emissions, S-ZnO has a less crystallized structure and more trap-state emissions, and I-ZnO has a structure intermediate between S-ZnO and R-ZnO. The uniformity of the ZnO nanoparticle layer is important for obtaining the best quality and production yield of the devices. We investigated the influence of an underlayer on the uniformity of the

ZnO

layer

coated

from

2-ethoxyethanol

solution

prepared

to

be

10

mg/mL.

Poly(9,9-dioctylfluorene-alt-benzothiadiazole) (F8BT) was used as a hydrophobic underlayer with a water contact angle of 101°, and indium tin oxide (ITO) was used as a hydrophilic underlayer with a water contact angle of 19°. AFM images of the ZnO layers are shown in Figure 3. The roughness (Ra) increased in the order of S-ZnO, I-ZnO, and R-ZnO, and each type of ZnO shows similar roughness regardless of the underlayer. The water contact angles of the ZnO layers, summarized in Table 1, also increased in the order of S-ZnO, I-ZnO, and R-ZnO, which also demonstrates the rougher surface of the rod-like ZnO layers. The contact angle of S-ZnO on the F8BT underlayer was close to that on the ITO underlayer. However, the contact angle of R-ZnO on the F8BT underlayer was much larger than that on the ITO underlayer. These results suggest that, in the case of the rough R-ZnO layer, the water contact angle is affected by the hydrophobicity of the F8BT underlayer, which was not fully covered with R-ZnO. On the other hand, the F8BT underlayer was almost fully covered with S-ZnO, and thus the water contact angle of S-ZnO was less affected by the hydrophobic underlayer. We fabricated three types of OLEDs to investigate the electron injection property of the ZnO nanoparticles. The device configurations and deposition method of each layer are shown in Figure 4. The type I devices had a regular structure in which the ZnO was formed on the emitting layer by a solution process. The type II devices had an inverted structure in which the emitting layer was formed on the ZnO by a solution process. The type III devices also had an inverted structure but the emitting layer was formed on the ZnO by an evaporation process. In the type I devices, R-ZnO showed a lower driving voltage than S-ZnO because the fewer trap ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

sites of R-ZnO improved electron injection from Al to the organic layer. However, surprisingly I-ZnO showed a driving voltage that was much lower even than that of R-ZnO, contrary to our initial expectation. We presume that the rough morphology of R-ZnO resulted in poor electrical contact, resulting in the deterioration of the device. Figure S1 shows optical microscope images of the emitting devices. Among the type I devices, only the device with R-ZnO showed uneven emitting image owing to the rough ZnO surface. We also should note that the efficiencies of the device with I-ZnO were quite high, 10.8 cd/A and 13.8 lm/W at 100 cd/m2, although only ZnO was used as EIL between F8BT and Al cathode.8 Generally, the conduction band of ZnO is not enough shallow for the electron injection to LUMO of most organic compounds. Therefore, in most OLEDs with ZnO, Cs2CO3 or aliphatic polyamine such as polyethyleneimine (PEI) and polyethyleneimine ethoxylated (PEIE) have been used to improve the electron injection. In the type II devices, we tried to improve the electrical contact between the ZnO and the organic layer by depositing the organic layer onto the ZnO layer from a solution, in reverse order of type I. However, the order of the driving voltage was the same as that of the type I devices, and I-ZnO showed the highest performance. Only R-ZnO showed many dark spots in the emitting image, as well as the type I devices. This result suggests that the poor contact of R-ZnO was not only with the organic layer but also with the electrode. Finally, we deposited an organic layer onto the ZnO layer by an evaporation process in the type III devices. In this case, R-ZnO showed the highest driving voltage, in contrast to the other type devices. The performance of the device with I-ZnO was also the highest among the devices, and only the emitting image with R-ZnO showed many dark spots, as well as the other type devices. This is probably because of the poor contact not only between ZnO and the electrode but also between ZnO and the evaporated organic layer. The device with R-ZnO and I-ZnO showed larger leakage current than that of S-ZnO with the type III strucuture (Figure S2). This result also supports the poor electrical contact of the rodlike ZnOs in the interface. The large leakage current and unbalanced charge injection caused wide-ranging charge recombination, and HTL emission appeared in the type III devices (Figure S3). We consider that there are two effects of the ZnO nanoparticle size and shape on the performance of the devices. One is that the rod-like ZnO nanoparticles reduced the electron trap state in their crystals, improving the electron injection. Another effect is that the rougher morphology of the rod-like ZnO nanoparticles caused poor electrical contact with the adjacent layers. The intermediate-size ZnO nanoparticles, I-ZnO, showed characteristics intermediate between those of the spherical and rod-like nanoparticles, thus exhibiting the best performance in the devices. This superiority of I-ZnO is universal in any type of OLED device.

ACS Paragon Plus Environment

Page 4 of 12

Page 5 of 12

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


3. Conclusion Using gradually increasing reaction times, we synthesized three types of size- and shape-controlled ZnO nanoparticles: spherical ZnO, rod-like ZnO, and ZnO intermediate between the spherical ZnO and the rod-like ZnO. TEM, XRD, UV, and PL measurements clearly showed the sizeand shape-controlled features of each type of nanoparticle. OLEDs with these ZnO nanoparticles as the EIL were fabricated. The device with the intermediate ZnO showed a lower driving voltage and higher power efficiency than those with spherical ZnO and rod-like ZnO. The intermediate ZnO is thus a superior EIL for various types of OLEDs regardless of the deposition order or method of fabricating the organic layer, the ZnO layer, and the electrode.

Experimental Section Synthesis. S-ZnO: Zinc acetate (1.678 g, 9.15 mmol) was dissolved in methanol (84 mL) under nitrogen, and then, deionized water (250 µL) was added into the solution. After heating the solution to 60 °C, a methanol solution (46 mL) of potassium hydroxide (0.98 g, 17.5 mmol) was slowly added to the zinc acetate solution over 15 min. The mixture was stirred at 60 °C for 150 min and then left for 90 min at room temperature. The supernatant was slowly removed, and methanol (100 mL) was added to the mixture. In total, the supernatant was removed and methanol was added three times each. After leaving the mixture overnight, the supernatant was slowly removed again until the volume of the ZnO mixture reached 3–5 mL. A small amount of the ZnO mixture (100 µL) was removed and dried, and the weight of the ZnO solid was measured. The average concentration of the ZnO was 80–120 mg/mL. Finally, 2 mL of the ZnO mixture was placed in a vial, and chloroform and 2-ethoxyethanol were added sequentially until reaching a concentration of 9.0 mg/mL and the volume ratio of chloroform and 2-ethoxyethanol was 1:4. I-ZnO and R-ZnO: The initial reaction of zinc acetate with potassium hydroxide in methanol was same as for S-ZnO. After the reaction, the mixture was concentrated to be 20 mL by evaporation of methanol in vacuum. The concentrated mixture was refluxed for 6 h for I-ZnO and 10 h for R-ZnO. The washing and solution preparation procedures were same as that of S-ZnO. Characterization. TEM images were obtained by JEOL JEM-2100 TEM. XRD was measured with a Rigaku SmartLab diffractometer equipped with a rotating anode (Cu Kα radiation, λ = 1.5418 Å). UV-vis absorption spectra were recorded on a Shimadzu UV-3150 spectrometer. Ionization potentials were determined by ultraviolet photoelectron yield spectroscopy under a vacuum (ca. 10-3 Pa) using a Sumitomo Heavy Industries PYS-202-Y. PL spectra were measured using a Jobin Yvon Fluoromax-4 fluorometer. AFM images were obtained by Bruker Dimention Icon AFM with a Bruker Si cantilever. OLEDs were characterized after encapsulation using epoxy glue and a glass cover. The active area of the device was 2 mm2. EL spectra were recorded using Hamamatsu PMA− ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

11 photonic multichannel analyzer. The current density–voltage and luminance–voltage characteristics were measured using a Keithley Source Measure Unit 2400 and a Minolta CS200 luminance meter, respectively. Device fabrication. Indium tin oxide (ITO) substrates were cleaned in an ultrasonic bath containing deionized water and 2-propanol, followed by UV – ozone treatment for 20 min. PEDOT:PSS solution in water (CLEVIOS CH8000) was passed through a filter (0.45 µm) to remove any large particles, then spin-coated and annealed at 200 °C for 10 min in atmospheric condition. All other spin coating process were performed in a dried nitrogen-filled glove box. TFB (American Dye Source) was spin coated from p-xylene solution (7 mg/mL) and annealed at 180 °C for 60 min. F8BT (Sumitomo Chemical) was spin coated from p-xylene solution (12 mg/mL) and annealed at 130 °C for 10 min. The ZnO nanoparticle dispersion in 2-ethoxyethanol and chloroform (9 mg/mL) was spin coated and annealed at 100 °C for 5 min. Other organic compunds and Al cathode were deposited by thermal evaporation under high vacuum (~10-5 Pa). The active area of the device was 2 × 2 mm2.

Acknowledgments The authors would like to thank the “Strategic Promotion of Innovative R&D Program” and “Japan Regional Innovation Strategy Program by Excellence” of Japan Science and Technology Agency (JST) for financial support, Sumitomo Chemical for material support, and Mr. Y. Hayakawa for assistance in TEM measurement. Y.-J. Pu thanks the PRESTO (Sakigake), JST for support.

Supporting Infomation Summary of the performance, optical microscope images, current density – voltage plots, and EL spectra of the OLEDs are available in supporting information.

References 1. Shirakawa, T.; Umeda, T.; Hashimoto, Y.; Fujii, A.; Yoshino, K. Effect of ZnO Layer on Characteristics of Conducting Polymer/C60 Photovoltaic Cell. J. Phys. D: Appl. Phys. 2004, 37, 847-850. 2.

Sun, B.; Sirringhaus, H. Solution-Processed Zinc Oxide Field-Effect Transistors Based on Self-Assembly of Colloidal NanorodsSolution-Processed Zinc Oxide Field-Effect Transistors Based on Self-Assembly of Colloidal Nanorods. Nano Lett. 2005, 5, 2408-2413.

3.

White, M. S.; Olson, D. C.; Shaheen, S. E.; Kopidakis, N.; Ginley, D. S. Inverted Bulk-Heterojunction Organic Photovoltaic Device Using a Solution-Derived ZnO Underlayer. Appl. Phys. Lett. 2006, 89, 143517. ACS Paragon Plus Environment

Page 6 of 12

Page 7 of 12

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


4.

Bolink, H. J.; Coronado, E.; Repetto, D.; Sessolo, M. Air Stable Hybrid Organic-Inorganic Light Emitting Diodes Using ZnO as the Cathode. Appl. Phys. Lett. 2007, 91, 223501.

5.

Lee, H.; Park, I.; Kwak, J.; Yoon, D. Y.; Lee, C. Improvement of Electron Injection in Inverted Bottom-Emission Blue Phosphorescent Organic Light Emitting Diodes Using Zinc Oxide Nanoparticles. Appl. Phys. Lett. 2010, 96, 153306.

6.

Kim, Y. H.; Han, T. H.; Cho, H.; Min, S. Y.; Lee, C. L.; Lee, T. W. Polyethylene Imine as an Ideal Interlayer for Highly Efficient Inverted Polymer Light-Emitting Diodes. Adv. Funct. Mater. 2014, 24, 3808-3814.

7.

Sun, B.; Sirringhaus, H. Solution-Processed Zinc Oxide Field-Effect Transistors Based on Self-Assembly of Colloidal Nanorods. Nano Lett. 2005, 5, 2408-2413.

8.

Lee, T.-W.; Hwang, J.; Min, S.-Y. Highly Efficient Hybrid Inorganic–Organic Light-Emitting Diodes by Using Air-Stable Metal Oxides and a Thick Emitting Layer. ChemSusChem 2010, 3, 2021.

9.

Youn, H.; Yang, M. Solution Processed Polymer Light-Emitting Diodes Utilizing a ZnO/Organic Ionic Interlayer with Al Cathode. Appl. Phys. Lett. 2010, 97, 243302.

10. Qian, L.; Zheng, Y.; Choudhury, K. R.; Bera, D.; So, F.; Xue, J. G.; Holloway, P. H. Electroluminescence from Light-Emitting Polymer/ZnO Nanoparticle Heterojunctions at Sub-Bandgap Voltages. Nano Today 2010, 5, 384-389. 11. Chiba, T.; Pu, Y.-J.; Hirasawa, M.; Masuhara, A.; Sasabe, H.; Kido, J. Solution-Processed Inorganic-Organic Hybrid Electron Injection Layer for Polymer Light-Emitting Devices. ACS Appl. Mater. Interfaces 2012, 4, 6104-6108. 12. Hofle, S.; Schienle, A.; Bernhard, C.; Bruns, M.; Lemmer, U.; Colsmann, A. Solution Processed, White Emitting Tandem Organic Light-Emitting Diodes with Inverted Device Architecture. Adv. Mater. 2014, 26, 5155-5159. 13. Pu, Y.-J.; Chiba, T.; Ideta, K.; Takahashi, S.; Aizawa, N.; Hikichi, T.; Kido, J. Fabrication of Organic Light-Emitting Devices Comprising Stacked Light-Emitting Units by Solution-Based Processes. Adv. Mater. 2015, 27, 1327-1332. 14. Wilken, S.; Parisi, J.; Borchert, H. Role of Oxygen Adsorption in Nanocrystalline ZnO Interfacial Layers for Polymer–Fullerene Bulk Heterojunction Solar Cells. J. Phys. Chem. C 2014, 118, 19672-19682. 15. Pacholski, C.; Kornowski, A.; Weller, H. Self-assembly of ZnO: from Nanodots to Nanorods. Angew. Chem. Int. Ed. 2002, 41, 1188-1191. 16. Wilken, S.; Scheunemann, D.; Wilkens, V.; Parisi, J.; Borchert, H. Improvement of ITO-Free Inverted Polymer-Based Solar Cells by Using, Colloidal Zinc Oxide Nanocrystals as Electron-Selective Buffer Layer. Org. Electron. 2012, 13, 2386-2394. ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

17. Srikant, V.; Clarke, D. R. On the Optical Band Gap of Zinc Oxide. J. Appl. Phys. 1998, 83, 5447-5451. 18. van Dijken, A.; Meulenkamp, E. A.; Vanmaekelbergh, D.; Meijerink, A. The Luminescence of Nanocrystalline ZnO Particles: the Mechanism of the Ultraviolet and Visible Emission. J. Lumin. 2000, 87-89, 454-456.


Page 8 of 12

Page 9 of 12

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 1. Characterization of the ZnO nanoparticles. Rac)

WC (°)d) F8BT ITO e) F8BT e) ITO e) S-ZnO 4.2 × 3.4 6.34 2.88 3.46 367 (3.38), 533 (2.33) 1.57 1.31 29 21 I-ZnO 9.8 × 4.5 6.30 2.96 3.34 366 (3.39), 553 (2.24) 3.55 3.23 41 29 R-ZnO 20.6 × 6.2 6.34 3.03 3.31 373 (3.32), 542 (2.29) 5.36 5.13 63 33 a) Ip: Ionization potential, b) Eg: Optically obtained energy gap, c) Ra: roughness obtained from AFM, d) WC: water contact angle, e) underlayer of the ZnO nanoparticles Size (nm)

a)

Ip (eV)a)

Ip-Eg (eV)

b)

Eg (eV)b)

PL [nm (eV)]

e)

c)

Figure 1. TEM images of (a) spherical, (b), intermediate, and (c) rod-like ZnO nanoparticles.

a)

b)

c)

Figure 2. (a) XRD, (b) UV-vis, and (c) PL spectra of S-ZnO, R-ZnO, and I-ZnO.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 12

a) on F8BT (nm) 25

(nm) 25

0

0

(nm)

0.2

(nm)

0.2

0.4

0.6

I-ZnO

0.4 0.8

0.8

0.4

0.6

S-ZnO

0 (nm)

0.2

0.8

0.8 0.4 0.6

(nm) 25

0.6

0.4 0.8

0.2

0.6

R-ZnO

0.2

0.6

0.4 0.8

(nm)

(nm)

0.2 (nm)

b) on ITO (nm) 25

(nm) 25

0

0

(nm)

0.2

(nm)

0.2

0.8 0.4 0.6 0.6

S-ZnO

0.4 0.8

0.2 (nm)

0.8 0.4

(nm) 25 0 (nm)

0.2 0.8 0.4 0.6

0.6

I-ZnO

0.6

0.6

0.4 0.8

0.2 (nm)

R-ZnO

0.4 0.8

0.2 (nm)

Figure 3. AFM images of the ZnO layers (a) on F8BT and (b) on ITO.


Page 11 of 12

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


a)

b)

c)

Figure 4. Device structures and performances of the OLEDs: a) type I, b) type II, and c) type III with S-ZnO (blue), I-ZnO (green), R-ZnO (red).



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 12

Graphical Abstract

Organic layer

ZnO nanoparticles Electrode Spherical ZnO

Intermediate ZnO

Rod-like ZnO

Spherical ZnO

Intermediate ZnO

Rod-like ZnO

Organic layer

ZnO nanoparticles Electrode


Efficient Electron Injection by Size- and Shape ... - ACS Publications

Recommend Documents