Transport Layers for Efficient Solution

Subscriber access provided by Otterbein University

Article

Nickel Oxide Hole Injection/Transport Layers for Efficient Solution-processed Organic Light-Emitting Diodes Shuyi Liu, Rui Liu, Ying Chen, Szuheng Ho, Jong Hyun Kim, and Franky So Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/cm501898y • Publication Date (Web): 15 Jul 2014 Downloaded from http://pubs.acs.org on July 16, 2014

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.

Chemistry of Materials 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 17

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

Chemistry of Materials

Nickel Oxide Hole Injection/Transport Layers for Efficient Solution-processed Organic Light-Emitting Diodes Shuyi Liu†, Rui Liu†, Ying Chen†, Szuheng Ho†, Jong H. Kim†, and Franky So†,* †Department of Materials Science and Engineering, University of Florida, Gainesville, Florida 32611, United States ABSTRACT: Solution-processed nickel oxides (s-NiOx) are used as hole injection and transport layers in solution-processed organic light-emitting diodes (OLEDs). By increasing the annealing temperature, the nickel acetate precursor fully decomposes and the s-NiOx film shows larger crystalline grain sizes, which lead to better hole injection and transport properties. UV-ozone treatment on the s-NiOx surface is carried out to further modify its surface chemistry, improving the hole injection efficiency. The introduction of more dipolar species of nickel oxy-hydroxide (NiO(OH)) is evidenced after the treatment. Dark injection-space charge limited (DI-SCL) transient measurement was carried out to compare the hole injection efficiency of s-NiOx and poly(3,4ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT: PSS) hole injection layers (HIL). The UV-ozone treated s-NiOx shows significantly better hole injection, with a high injection efficiency of 0.8. With a p-type thin film transistor (TFT) configuration, the high-temperature annealed s-NiOx film shows a hole mobility of 0.141 cm2 V-1 s-1, which is significantly higher compared to conventional organic hole transport layers (HTLs). Due to its improved hole injection and transport properties, the solution-processed phosphorescent green OLEDs with NiOx HIL/HTL show a maximum power efficiency of 75.5 + 1.8 lm W-1, which is 74.6 + 2.1 % higher than the device with PEDOT:PSS HIL. The device with NiOx HIL/HTL also shows a better shelf stability than the device with PEDOT:PSS HIL. The NiOx HIL/HTL is further compared with PEDOT:PSS HIL/N,N’-Di(1-naphthyl)-N,N’-diphenyl-(1,1’biphenyl)-4,4’-diamine (NPB) HTL in the thermal-evaporated OLEDs. The device with NiOx HIL/HTL shows a comparable efficiency at high electro-luminescence (EL) intensities.

INTRODUCTION Since their invention in 1987,1 organic light-emitting diodes (OLEDs) have received much attention due to the potential for flat displays and solid-state lighting applications.2-5 With the development of highly efficient phosphorescent emitters and out-coupling techniques, the current efficiency of OLEDs can reach as high as 200 Cd/A,6 meeting the requirements for flat panel display and lighting applications. However, most OLEDs with high efficiencies are fabricated by thermal evaporation, which is suffering from high manufacturing costs and difficulties to realize large area devices. Solution-processed OLEDs, on the other hand, offer an opportunity to significantly lower the fabrication costs and also enable the application on large area devices and flexible substrates, thanks to its compatibility to roll-to-roll processing. However, relatively low efficiency is often a challenge for solution processed OLEDs. To achieve high efficiencies, charge blocking layers are usually incorporated to maintain charge balance and to confine the excitons in the emitting layer (EML). The complicated device structures are usually achievable by thermal evaporation processing. In contrast, solution-based processing limits the fabrication of multilayer device architectures because the deposited layers can be dissolved or damaged by the solvent of the subsequent layer. Although several research groups have demonstrated solution-processed OLEDs using small molecules emitting materials,7-9 the EMLs are directly spin-coated onto the poly(3,4ethylenedioxythiophene):poly(sterene sulfonate) (PEDOT:PSS) hole injection layer (HIL), which lacks the ability to block electrons and excitons from EML. Additionally, the acidity of PEDOT:PSS could lead to issues of device stability.10,11

Therefore, a hole transporting layer (HTL) is usually required in a multilayer OLED to efficiently block electrons and excitons. Despite the development of numerous small molecule organic hole transport materials, most of them are not compatible with solution processing because they can be dissolved or physically washed away by the solvents of the subsequent functional layer. Therefore, either a robust orthogonal solvent system or cross-linkable organic materials which can withstand the subsequent wet processing are required. While cross-linkable materials have been used to serve as the HTL in solution-processed multilayer OLEDs,12-16 the hole mobility of the resulting HTLs after cross-linking is often reduced due to the more porous morphology compared to the thermally evaporated films, leading to lower efficiencies. On the other hand, the orthogonal solvent system is difficult to be realized in phosphorescent emitter systems because most of the materials developed for HTLs and EMLs are only soluble in organic solvents. Even with an orthogonal solvent system, the underlying HTL layer would be partially washed away or mixed the top layer. Therefore, there are only a few reports on high efficiency solution-processed OLEDs with a solution-processed HTL, and the development of alternative high mobility HTL materials compatible with solution processing is of utmost importance. In this work, instead of using organic materials, we used a solution processed inorganic p-type semiconductor nickel oxides (NiOx) as both the HIL and HTL in OLEDs. It is known that non-stoichiometric NiOx is a wide band gap insulator with a room temperature conductivity of 10−13 S cm− 1.17 On the other hand, non-stoichiometric NiOx with a large density of Ni2+ vacancies accompanied by compensation of holes or Ni3+,

ACS Paragon Plus Environment


(CBP) host and fac-tris[2-(p-tolyl)pyridine]iridium(III) (Ir(mppy)3) phosphorescent dopant in chlorobenzene solution was used for EML, with the weight ratio of 0.92 : 0.08. After spin-coated onto PEDOT:PSS or s-NiOx, the EMLs were dried at 60 °C for 30 min. Afterwards, the samples were transferred into an evaporator. Tris-(2,4,6-triMethyl-3-(pyridine-3yl)phenyl)borane (3TPYBM), lithium fluoride (LiF) and aluminum (Al) were vacuum deposited as electron transport layer (ETL) and cathode, respectively. To compare the performance with an OLED with an organic HTL, a 30 nm thick N,N’Di(1-naphthyl)-N,N’-diphenyl-(1,1’-biphenyl)-4,4’-diamine (NPB) layer was deposited onto the PEDOT:PSS HIL by thermal-evaporation. However, due to the difficulty of making a solution-processed OLED with organic HTLs, we thermalevaporated the emitting layer of CBP doped with 8 wt% of fac-tris(2-phenylpyridine)iridium(III) (Ir(ppy)3) onto both sNiOx HIL/HTL and PEDOT:PSS HIL/NPB HTL. 1,3,5-tris(Nphenylbenzimiazole-2-yl)benzene (TPBi) was deposited as a thin exciton/hole blocking layer and Alq3 was deposited as the ETL. 100

EXPERIMENTAL SECTION

90

The nickel oxide precursor was synthesized according to our previous publication.11 To prepare the nickel ink precursor, nickel acetate tetrahydrate (Ni(CH3COO)2·4H2O) and ethanolamine (NH2CH2CH2OH) were dissolved in ethanol with a molar ratio of 1:1. The film thickness was controlled by the concentration of the precursor ink. The solution was stirred in a sealed vial for 2 hours, and a homogeneous and clear dark green solution was formed. A 40 nm thick s-NiOx film was formed via spin-coating the precursor solution onto the precleaned substrates, followed by annealing at 275 °C for 1 hour. From the X-ray Diffraction (XRD) data, the resulting films were polycrystalline.11 To use the s-NiOx films for HILs/HTLs in OLEDs, we further modified the annealing temperature to 500 °C for better crystallization of NiOx films. The annealing process was performed in ambient air. Figure 1 shows the transmittance and absorbance of the 500 °C annealed s-NiOx film on a quartz substrate. A high transmittance of above 90% over the entire visible region was obtained. From the absorption onset wavelength of 355 nm, we determine the band gap (Eg) to be 3.5 eV. After cooling down to room temperature, this s-NiOx film was loaded into a UV-ozone chamber for further surface treatment. The UV-ozone exposure time was around 5-10 min. There was no significant change of the film transmittance after UV-ozone treatment. To investigate the annealing temperatures and UV-ozone treatment effects on the surface properties of the s-NiOx films , atomic force microscopy (AFM) was used to examine the morphology, and X-ray photoemission spectroscopy (XPS) measurements were carried out to study the surface chemistry. The hole injection efficiencies of s-NiOx and PEDOT:PSS were characterized by dark-injection space charge limited (DISCL) transient measurements. The field-effect mobility of sNiOx was measured with a p-channel thin film transistor (TFT). To fabricate solution-processed OLEDs, the s-NiOx films were deposited onto the indium tin oxide (ITO) substrates. The substrates were then transferred into a nitrogen glove box. A mixture of 4,4'-Bis(N-carbazolyl)-1,1'-biphenyl

80

Transmittance Absorbance (a.u.)

70 60 50 40 30 20

Absorbance (a.u.)

is one of the few p-type metal oxide semiconductors with a good hole selectivity.18-25 Recently, solution-processed NiOx (s-NiOx) has been extensively studied as HTLs in organic photovoltaics (OPVs).11, 26-28 However, there are a very few reports on s-NiOx as a functional layer in OLEDs. Specifically, only sputtered NiOx has been attempted to be used as an HIL in tris(8-hydroxyquinolinato)aluminum (Alq3) based fluorescent OLEDs. However, these devices showed very low efficiencies.29-31 Here, we demonstrate high efficiency solutionprocessed phosphorescent OLEDs using s-NiOx as HIL/HTL. By varying the annealing temperatures and treating the s-NiOx surface with UV-ozone, we were able to modify the morphology and chemical stoichiometry of the s-NiOx surface, resulting in improvement in injection and transport properties. The resulting solution-processed phosphorescent green OLEDs with s-NiOx showed a maximum current efficiency of 70.0 + 1.6 Cd A-1 and a power efficiency of 75.5 + 1.8 lm W-1, which was significantly higher than the device with a PEDOT:PSS HIL. To the best of our knowledge, this is one of the highest efficiency in OLEDs with solution-processed metal oxide functional layers.

Transmittance (%)

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 2 of 17

10 0 300

400

500

600

700

800

Wavelength (nm) Figure 1. Transmittance (solid line) and absorbance (dashed line) spectra of a 40 nm thick s-NiOx film prepared on quartz substrates.

RESULTS AND DISCUSSION AFM and XPS of the s-NiOx Films. The nickel ink precursor was spin-coated onto ITO substrates and annealed at either 275 °C or 500 °C. The topography and phase images of the sNiOx films are shown in Figure 2. The s-NiOx film annealed at 275 °C has an average crystalline grain size of 10 nm, with a root mean square (RMS) roughness of 0.87 nm. The s-NiOx film annealed at 500 °C shows larger crystalline grain size of 30 nm and a larger RMS roughness of 1.12 nm. The crystalline size of our s-NiOx films under different annealing temperatures is in good agreement with that in the literature.32 Interestingly, we found that after a short UV-ozone treatment, the phase image of the 500 °C annealed s-NiOx film significantly changed with a lot of pinhole-like patterns. This morphological change corresponded to the increased film RMS roughness: from 1.12 nm to 2.66 nm. These results suggest that the UV-ozone treatment might lead to changes in surface composition of the s-NiOx film.


Page 3 of 17

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)

(c)

(e)

(b)

(d)

(f)

Figure 2. AFM phase images of (a) 275 °C annealed and (b) 500 °C annealed s-NiOx films with a scan size of 200 nm × 200 nm; AFM topography images of 500 °C annealed s-NiOx (c) before and (d) after UV-ozone treatment; AFM phase images of 500 °C annealed sNiOx (e) before and (f) after UV-ozone treatment. The scan size for (c)-(f) is 1 µm × 1 µm. To verify the surface chemistry of s-NiOx films, we analyzed the surface compositions of the s-NiOx films using XPS with the adventitious C 1s peak referenced to 284.8 eV. The results suggested that the nickel acetate precursor did not fully decompose in the s-NiOx film annealed at 275 °C, as revealed by the residual acetate peak in its C 1s signals (given in Supporting Information). Figure 3 shows the peaks of Ni 2p3/2 orbital and O 1s orbital in sNiOx films with and without UV-ozone treatment. The measurement was performed at a take-off angle of 20 °. The main peaks in the Ni 2p3/2 and O 1s signals correspond to the stoichiometric NiO. To further investigate the species, the shoulder peak in the O 1s signals was fitted with signals from nickel hydroxide (Ni(OH)2), nickel oxy-hydroxide (NiO(OH)) and water (H2O). Similarly, the shoulder peak in the Ni 2p3/2 signals was fitted with three peaks, for Ni(OH)2, NiO(OH) and the nickel intersite respectively.33,34 As shown in Table 1, the UV-ozone treatment introduced more dipolar NiO(OH) species onto the film surface, which could result in a change in the surface dipole and hence a vacuum level shift at the inorganic-organic interface, facilitating hole injection from s-NiOx to EML.33 The optimum surface treatment time is 5 minutes and further treatments do not make significant changes. These findings are in good agreement with other reports on O2-plasma treated NiOx films.27, 33 It should be noted that the H2O composition is doubled after the UV-ozone treatment. Indeed, another possibility is that the UV-ozone treatment improves the wetting of s-NiOx and thus the adhesion to the adjacent EML, resulting in enhanced hole injection.

Injection and Transport Properties of the s-NiOx Films. DI-SCL transient measurements were conducted to compare the hole injection efficiency of s-NiOx HIL/HTL and the con-

ventional PEDOT:PSS HIL. The hole only devices consist of HIL followed by a 2.0 µm thick of NPB HTL. UV-ozone treated ITO and gold (Au) served as the counter electrodes. The transient current density (J-t) curves for devices with sNiOx HIL annealed at 275 °C and 500 °C before and after UVozone treatment, along with the conventional PEDOT:PSS (P VP Al4083 from CleviosTM) HIL are presented in Figures 4ad, respectively. The measurement configuration and calculation details are given in Supporting Information. The ratio of the peak current density (JDI) to steady-state current density (Jss) is kept around 1.21 for both PEDOT:PSS and UV-ozone treated s-NiOx samples under different applied voltages, indicating an Ohmic contact for holes.35,36 The s-NiOx annealed at 500 °C followed by the UV-ozone treatment shows the best hole injection with the highest Jss. Figure 4e shows the hole mobility and Figure 4f shows the injection efficiency of NPB. The injection efficiency of the UV-ozone treated s-NiOx is about 0.8, which is significantly higher than that of PEDOT:PSS. However, without UV-ozone treatment, the sNiOx film shows a lower hole injection injection. The 275 °C annealed NiOx has the lowest hole injection among all samples, which could be attributed to the low work function of the incompletely decomposed nickel acetate precursor components. And since the lower temperature annealed s-NiOx has a stronger chemical adsorption property,37 the carbonaceous and hydroxyl species adsorbed on its surface could also result in a reduction of work function and thus hole injection efficiency.38-40


Chemistry of Materials Original Ni 2p3/2 peak

Ni 2p3/2

Fitted Ni 2p3/2 peak Ni

2+

(NiO) peak

Ni

2+

(Ni(OH)2) peak

Ni

3+

(NiO(OH)) peak

Original O 1s peak Fitted O 1s peak 2O (NiO) peak 2-

Original Ni 2p3/2 peak 867 864 861

2-

O (NiO(OH)) peak

858

855

Fitted Ni 2p3/2 peak

Ni 2p3/2 852

Binding Energy (eV)

(d)

2-

O (H 2O) peak

Original O 1s peak 534 532

Fitted O 1s peak 2Binding O (NiO) peak

Ni

2+

Ni

2+

(Ni(OH)2) peak

O (Ni(OH)2) peak

Ni

3+

(NiO(OH)) peak

O (NiO(OH)) peak

(NiO) peak

530

528

O526 1s

Energy (eV)

2-

Intensity

2-

Intersite Shake-up peak

867

O 1s

O (Ni(OH)2) peak


(c)

(b)

Intensity

Intensity

(a)

Intensity

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 4 of 17

864

861

858

855

852

2-

O (H2O) peak

534

Binding Energy (eV)

532

530

528

526

Binding Energy (eV)

Figure 3. High resolution (a) Ni 2p3/2 and (b) O 1s XPS acquisition for as-prepared s-NiOx; High resolution (c) Ni 2p3/2 and (d) O 1s XPS acquisition for UV-ozone treated s-NiOx.

Table 1. Binding Energies and Component Ratios of Ni 2p3/2 and O 1s Species from Figure 3 Related Components

Ni 2p3/2 Binding Energy (eV)

O 1s Binding Energy (eV)

Component ratio (from Ni 2p3/2)

Component ratio (from O 1s)

Stoichiometric Nickel Oxide [NiO]

854.1a // 854.1b

529.5 // 529.5

63.3% // 58.0%

66.0% // 54.6%

Nickel Hydroxide [Ni(OH)2]

855.6 //855.5

531.0 // 530.9

21.4%// 17.9%

16.9% // 14.9%

Nickel Oxy-hydoxide [NiO(OH)]

856.1 // 856.1

531.9 // 531.8

15.3% // 24.2%c

14.7% // 23.1%

Absorbents [H2O, O2]

-

533.1 // 533.0

-

2.4% / 7.4%

a

The numbers before double-slash (“//”) is acquired from the as-prepared s-NiOx; b The numbers after double-slash is acquired from the UV-ozone treated s-NiOx; c The bold numbers show the ratio of dipolar NiO(OH) component increased after UV-ozone treatment.

In addition to the hole injection property, the hole transport property was studied. The field-effect mobility of the s-NiOx film was measured using a p-channel thin film transistor (TFT). The nickel ink precursor was spin-coated onto the Si/SiO2 substrates with a 300 nm thick of SiO2 (capacitance: 10 nF cm-2), followed by a 60 nm thick gold evaporated as the drain and source contacts. The TFT has a channel length of 40 µm and width of 1 mm. Figure 5 shows the device structure and the TFT output current data. The calculation details are given in the Supporting Information. The field effect mobility for 500 °C-annealed s-NiOx is 0.141 cm2 V-1 s-1, which is significantly higher than the value of organic HTLs. This mobility is also comparable to the reported values of 0.43 cm2 V-1 s-1 for the sputtered nickel metal films followed by a similar oxi-

dization process (500 °C annealing in air.)41 However, for the NiOx film annealed at 275 °C, the TFT source-drain current level was low and no saturation region was found (Supporting Information). This is possibly due to the smaller crystalline grains of the s-NiOx annealed at low temperatures, as shown in the AFM images. The grain boundaries in polycrystalline films scatter carriers and hinder the carrier transport. Therefore, it is expected that 500 °C-annealed s-NiOx exhibits a higher hole injection efficiency and hole mobility than that for the film annealed at 275°C. It should also be noted that the bulk transport properties of the s-NiOx films are not affected by the UV-ozone treatment, which is proved by the similar current density-voltage (J-V) curves of the s-NiOx hole-only



0.4

0.3

-2

0.5

0.3

0.2

0.2 0.1

0.1 0.0

15

10

2

25 30 Time (µs)

35

40

0.0

10

15

20

25

Time (µs)

30

35

-3

10

-4

-5

With PEDOT:PSS HIL With UV-ozone treated s-NiOx HIL 10

6 5 4 3 2 1 0

40

-6

7

5

10

15

20

25 Time (µs)

30

35

UV-ozone

8

40

7

10V 12V 14V 16V 18V 20V 22V 24V

(d)

6 5 4 3 2 1 0

5

10

15

20

25

Time (µs)

30

35

40

1.0

(e)

10

10

20

10V 12V 14V 16V 18V 20V 22V 24V

(c)

7

-2

-1

-1

NPB Hole Mobility (cm V s )

10

since

8

Current Density (mA cm )

0.4

(b)

-2

(a)

expectation 10V 12V 14V 16V 18V 20V

Hole Injection Efficiency

0.5

0.6

10V 12V 14V 16V 18V 20V


-2


0.6

-2

devices (given in Supporting Information). This is within our

8

9

1/2

Sqrt E (kV

10

0.8 0.6 0.4 0.2 0.0

11

-1/2

(f)

PEDOT:PSS HIL UV-ozone treated s-NiOx HIL 50

60

70

cm )

80

90

100

-1

110

120

E (kV cm )

Figure 4. The transient current density of (a) 275 °C annealed s-NiOx, (b) 500 °C annealed s-NiOx, (c) 500 °C annealed s-NiOx followed by UV-ozone treatment, and (d) PEDOT:PSS; (e) The NPB hole mobility extracted from the devices with PEDOT:PSS (black square) and UV-ozone treated s-NiOx (red circle); (f) The hole injection efficiency curves of PEDOT:PSS (black square) and UV-ozone treated s-NiOx (red circle).

-25 -20 -15 -10 -5 0

(c)

0.007 1/2

-30

0.008

(b) 1/2

VG= -30 V VG=-25 V VG= -20 V VG= -15 V VG= -10 V VG=-5 V V G= 0 V

IDS (saturated) (A )

-35

(a) IDS (µ A)

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 5 of 17

0.006 0.005 0.004 0.003 0.002

0

-5

-10

-15

-20

-25

-30

-35

-40

-45

0.001

0

-5

-10

-15

-20

-25

-30

-35

-40

VGS (V)

VDS (V)

Figure 5. (a) The structure of s-NiOx thin film transistor, (b) the IDS-VDS output curves, and (c) the IDS(saturated)1/2-VG transfer curve at VDS = -40 V. The linear fit of the curve yields a slope value of 1.33 × 10-4, corresponding to a hole mobility of 0.141 cm2 V-1 s-1. treatment only reaches to a few nanometers within the s-NiOx top surface, while the transport properties are related to the film bulk properties. We could therefore conclude that the UV-ozone treatment plays a dominant role of altering the s-NiOx film surface species, which significantly improves its hole injection efficiencies while has little effect on its hole transport properties.

Solution-Processed OLEDs with s-NiOx HIL/HTL. Encouraged by the enhanced hole injection and transport in the 500 °C-annealed s-NiOx film, OLEDs with s-NiOx HIL/HTL were fabricated. Figure 6a shows the device architecture and the band diagram of the solution-processed phosphorescent green OLEDs fabricated with PEDOT:PSS HIL or 500 °C annealed s-NiOx HIL/HTL before and after UV-ozone treatment.6,8 Figure 6b shows the device current density-voltageluminescence (J-V-L) curves and Figure 6c shows the device current efficiency (CE) and power efficiency (PE) curves. To

verify the reproducibility of the device performance, 12 devices of each group were fabricated and characterized, with the statistics of device performance summarized in Table 2. The electro-luminescence (EL) spectra of the devices are shown in Supporting Information. With the as-prepared s-NiOx HIL/HTL, the OLED devices show a larger turn-on voltage (Von) compared to the devices with PEDOT:PSS due to the inefficient hole injection. The devices also show a relatively low power and current efficiency. After UV-ozone treatment on the s-NiOx HIL/HTL, the device performance is significantly improved, due to the enhanced hole injection. The devices have the lowest Von with a maximum power efficiency of 75.5 + 1.8 lm W-1 and current efficiency of 70.0 + 1.6 Cd A-1, which are significantly higher than those of the devices with PEDOT:PSS. We also fabricated OLED devices with s-NiOx annealed at 275 °C. The resulting devices have a higher Von


Chemistry of Materials and a lower power efficiency (see Supporting Information). As the low-temperature processed s-NiOx surface is rich of Ni(OH)2 species,11 which is easily turned into NiO(OH) upon

10

4

10

3

1E-3

10

2

1E-4

10

1

1 0.1 0.01

1E-6

0

1

2

3

4

5

6

7

8

9 10 11

90 80 70 60 50

10

40 30

1

Voltage (V)

20

PEDOT:PSS As-prepared s-NiOx UV-O3 treated s-NiOx

1

10

100

10 1000

0

-2

Luminescence (Cd m )

Figure 6. (a) Device structure and energy levels with respect to vacuum level of the materials used in solution-processed OLEDs. b) J-V-L characteristics and c) current efficiency/power efficiency curves of the OLEDs with PEDOT:PSS (grey line) HIL, as-prepared (black square) and UV-ozone (UV-O3) treated (red circle) s-NiOx HIL/HTL.

Table 2. Device Characteristics of the Solution-Processed OLEDs Incorporating PEDOT:PSS HIL, As-Prepared and UVozone Treated s-NiOx (500 °C) HIL/HTL. Devices

Von (V)

V at 103 Cd m-2 (V)

Maximum CE (Cd A-1)

CE at 103 Cd m-2 (Cd A-1)

Maximum PE (lm W-1)

PE at 103 Cd m-2 (lm W-1)

PEDOT:PSS

3.23 + 0.05

6.04 + 0.03

61.2 + 1.3

47.1 + 1.4

43.2 + 1.0

24.5 + 0.7

As-prepared s-NiOx

3.80 + 0.02

7.40 + 0.07

55.3 + 2.3

24.7 + 1.4

42.1 + 2.2

10.5 + 0.7

UV-ozone treated s-NiOx

2.55 + 0.03

5.94 + 0.04

70.0 + 1.6

40.3 + 1.7

75.5 + 1.8

21.2 + 1.0

ozone treatment show reduced turn-on voltages due to the improved hole injection. However, the devices still show an overall poorer performance compared to the devices with 500 °C annealed NiOx HTLs (see Supporting Information). This could be attributed to a larger amount of defect states within the low-temperature processed NiOx films, and the interfacial gap states could also act as the quenching sites for the excitons formed in the EML. Therefore, the high annealing temperature and UV-ozone treatments are obviously the key to improve the device performance. To the best of our knowledge, the efficiency value reported here in the 500 °Cannealed NiOx followed by UV-ozone treatment is one of the highest value reported on OLEDs with solution processed metal oxide layers. To evaluate the device shelve life, solution processed OLEDs with a PEDOT:PSS HIL and UV-ozone treated s-NiOx HIL/HTL were both driven at a constant current of 1 mA/cm2. With both devices encapsulated; the devices were electrically driven in ambient condition. Photos of the active areas of these devices are presented in Supporting Information. Dark spots appeared in the PEDOT: PSS device within two weeks and grew with time. The short shelf life of the PEDOT:PSS device is due to the moisture uptake during storage, resulting in decreased hole injection and luminance degradation.11 On the other hand, the s-NiOx devices showed no dark spots after 6

weeks of storage in ambient. Thus, it is a clear demonstration that s-NiOx device outperforms conventional PEDOT:PSS one in terms of environmental stability. s-NiOx HIL/HTL compared with PEDOT:PSS HIL/NPB HTL. Since PEDOT:PSS only serves as a HIL in solutionprocessed OLEDs while s-NiOx serves as both HIL and HTL, a further comparison of s-NiOx HIL/HTL with conventional evaporated organic HTLs provides a more complete study. Here, we conducted a comparison between solution processed NiOx and thermal evaporated HTL. The devices studied have the following structures: ITO/30 nm PEDOT:PSS/30 nm NPB /40 nm CBP: 8 wt% Ir(ppy)3 /10 nm TPBi /30 nm Alq3/1 nm LiF/100 nm Al and ITO/40 nm UV-ozone treated s-NiOx /40 nm CBP: 8 wt% Ir(ppy)3/10 nm TPBi /30 nm Alq3/1 nm LiF/100 nm Al. The device performances are shown in Figures 7a-b. The EL spectra are given in Supporting Information. Compared to the reference device with a PEDOT:PSS HIL/NPB HTL, the device with s-NiOx HIL/HTL shows a similar J-V-L curve, with a power efficiency higher than the reference device at low EL intensities and a comparable efficiency as the reference device at high EL intensities. These results show that the performance of UV-ozone treated s-NiOx as HIL/HTL for OLEDs is comparable to conventional thermal evaporated HTLs.


-1

PEDOT:PSS As-prepared s-NiOx 0 UV-O3 treated s-NiOx 10

1E-5

(c)

-1

-2

5

Power Effciency (lm W )

10

-2


100

(b)

10

Current Efficiency (Cd A )

100

(a)

oxidation, the 275 °C annealed s-NiOx film turned much darker after UV-ozone treatment. The OLED devices with the 275 °C annealed s-NiOx films followed by UV-


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 6 of 17

Page 7 of 17

-2

4

10

1 3

10

0.1

2

0.01

10

1E-3

1

10

1E-4 0

1E-5

PEDOT:PSS HIL/NPB HTL UV-O3 treated s-NiOx HIL/HTL

0

1

2

3

4

5

6

7

10

-2


10

(a)

10

1E-6

highly efficient solution-processed multi-layer phosphorescent OLEDs.

5

100


-1

10 9 10 11

8

Voltage (V) 90

-1

(b)

80 70 60 50

10

40 30 20 PEDOT:PSS HIL/NPB HTL UV-O3 treated s-NiOx HIL/HTL

1

1

10

100

1000

10 0

-1


100

Power Efficiency (lm W )

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


-2

Luminescence (Cd m ) Figure 7. (a) J-V-L characteristics and (b) current efficiency/ power efficiency curves of the thermal-evaporated OLEDs incorporating PEDOT:PSS HIL/NPB HTL and UV-ozone treated sNiOx HIL/HTL.

CONCLUSIONS Solution-processed NiOx films were used as both HILs and HTLs to fabricate OLED devices. AFM and high-resolution XPS analysis were carried out to study the morphology and chemical composition of the s-NiOx films. The hole injection and transport properties of the s-NiOx films were characterized via DI-SCL and TFT measurements. Compared to the s-NiOx film annealed at 275 °C, the s-NiOx film annealed at 500 °C showed enhanced hole injection and transport properties due to the fully decomposition of nickel acetate precursor and larger crystalline grains. The hole injection efficiency of the sNiOx film was further improved after a short UV-ozone treatment. XPS analysis confirmed the development of more NiO(OH) species on s-NiOx surface after the UV-ozone treatment, which helps to improve the hole injection. The optimized s-NiOx film shows high hole injection efficiency of 0.8 and field-effect hole mobility of 0.141 cm2 V-1 s-1. The solution-processed phosphorescent green OLED device with UVozone treated s-NiOx HIL/HTL shows a maximum power efficiency of 75.5 + 1.8 lm W-1, which is significantly higher than the device with PEDOT:PSS HIL with improved stability. The UV-ozone treated s-NiOx HIL/HTL was also compared with OLEDs with thermally evaporated HTL and both devices show similar performance at high luminescence intensities. Our study of s-NiOx HIL/HTL demonstrate a pathway toward

ASSOCIATED CONTENT Supporting Information. The high-resolution XPS acquisition of Ni 2p3/2, O 1s and C 1s signals of s-NiOx annealed at 275 °C and 500 °C, theoretical summary of dark injection measurement, theoretical summary of hole mobility measurement using thin film transistor configuration, the source-drain current curves of the thin film transistor with 275 °C annealed s-NiOx, the J-V curves of the s-NiOx hole-only devices, the electro-luminescence spectrum of solution-processed and thermal-evaporated phosphorescent green OLEDs, the solution-processed phosphorescent green OLED device performance with 275 °C annealed s-NiOx before and after UV-ozone treatment, the electro-luminescence pictures taken for shelf-stability demonstration. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * Email: [email protected] (Franky So).

ACKNOWLEDGMENT The authors would like to acknowledge the support of Wintek Corporation, for their funding of this project. The authors would also like to thank Eric Lambers of the University of Florida Major Analytical Instrumentation Center for his help in XPS measurements.

REFERENCES (1) Tang, C. W.; VanSlyke, S. A. Appl. Phys. Lett. 1987, 51, 913. (2) Pardo, D. A.; Jabbour, G. E.; Peyghambarian, N. Adv. Mater. 2000, 12, 1249. (3) Subramanian, V. presented at Solid-State Dev. Res. Conf. Edinburgh, UK, Septemper, 2008. (4) Han, T.-H.; Lee, Y.; Choi, M.-R.; Woo, S.-H.; Bae, S.-H.; Hong, B. H.; Ahn, J.-H.; Lee, T.-W. Nat. Photonics 2012, 6, 105-110. (5) Gevaux, D. Nat. Photonics 2007, 1, 567-568. (6) Xiang, C.; Koo, W.; So, F.; Sasabe, H.; Kido, J. Light: Sci. & Appl. 2013, 2, e74. (7) So, F.; Krummacher, B.; Mathai, M. K.; Poplavskyy, D.; Choulis, S. A. J. Appl. Phys. 2007, 102, 091101. (8) Cai, M.; Xiao, T.; Hellerich, E.; Chen, Y.; Shinar, R.; Shinar, J. Adv. Mater. 2011, 23, 3590-3596. (9) Jiang, W.; Duan, L.; Qiao, J.; Dong, G.; Zhang, D.; Wang, L.; Qiu, Y.; J. Mater. Chem. 2011, 21, 4918-4926. (10) Yan, H.; Scott, B. J.; Huang, Q.; Marks, T. J. Adv. Mater. 2004, 16, 1948-1953. (11) Manders, J. R.; Tsang, S.-W.; Hartel, M. J.; Lai, T.-H.; Chen, S.; Amb, C. M.; Reynolds, J. R.; So, F. Adv. Funct. Mater. 2013, 23, 2993-3001. (12) Liaptsis, G.; Meerholz, K. Adv. Funct. Mater. 2013, 23, 359365. (13) Ma, B.; Lauterwasser, F.; Deng, L.; Zonte, C. S.; Kim, B. J.; Frechet, J. M. J. Chem. Mater. 2007, 19, 4827-4832. (14) Li, J.; Marks, T. J. Chem. Mater. 2008, 20, 4873-4882. (15) Liu, M. S.; Niu, Y.-H.; Ka, J.-W.; Yip, H.-L.; Huang, F.; Luo, J.; Kim, T.-D.; Jen, A. K.-Y. Macromolecules, 2008, 41, 9570-9580. (16) Adhikari, R.; Postma, A.; Li, J.-H.; Hirai, T.; Bown, M.; Ueno, K. J. Soc. Inf. Display 2013, 21, 151-158. (17) Betancur, R.; Maymó, M.; Elias, X.; Vuong, L. T.; Martorell, J. Sol. Energy Mater. Sol. Cells 2011, 95, 735.



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

(18) Adler, D.; Feinleib, J. Phys. Rev. B 1970, 2, 3112. (19) Hüfner, S.; Steiner, P.; Sander, I.; Reinert, F.; Schmitt, H.; Neumann, M.; Witzel, S. Solid State Commun. 1991, 80, 869. (20) Hüfner, S.; Osterwalder, J.; Riesterer, T.; Hulliger, F. Solid State Commun. 1984, 52, 793. [21] Juybari, H. A.; Bagheri-Mohagheghi, M.-M.; ShokoohSaremi, M. J. Alloys Compd. 2011, 509, 2770. (22) Sawatzky,G. A.; Allen, J. W. Phys. Rev. Lett. 1984, 53, 2339. (23) Steirer, K. X.; Ndione, P. F.; Widjonarko, N. E.; Lloyd, M. T.; Meyer, J.; Ratcliff, E. L.; Kahn, A.; Armstrong, N. R.; Curtis, C. J.; Ginley, D. S.; Berry, J. J.; Olson, D. C. Adv. Energy Mater. 2011, 1, 813. (24) Kröger, F. A.; J. Phys. Chem. Solids 1968, 29, 1889. (25) Mrowec, S.; Grzesik, Z. J. Phys. Chem. Solids 2004, 65, 1651. (26) Ratcliff,E. L.; Meyer, J.; Steirer, K. X.; Armstrong, N. R.; Olson, D.; Kahn, A. Org. Electron. 2012, 13, 744. (27) Steirer, K. X.; Chesin, J. P.; Widjonarko, N. E.; Berry, J. J.; Miedaner, A. D.; Ginley, S.; Olson, D. C. Org. Electron. 2010, 11, 1414. (28) Jung, J.; Kim, D. L.; Oh, S. H.; Kim, H. J. Sol. Energy Mater. Sol. Cells 2012, 102, 103-108. (29) Chan, I.-M.; Hsu, T.-Y.; Hong, F. C. Appl. Phys. Lett. 2002, 81, 1899-1901. (30) Chan, I.-M.; Hong, F. C. Thin Solid Films 2004, 450, 304-311.

Page 8 of 17

(31) Im, H. C.; Choo, D. C.; Kim, T.W.; Kim, J. H.; Seo, J. H.; Kim, Y. K. Thin Solid Films 2007, 515, 5099-6102. (32) Park, Y. R.; Kim, K. J.; J. Crystal Growth. 2003, 258, 380384. (33) Ratcliff, E. L.; Meyer, J.; Steirer, K. X.; Garcia, A.; Berry, J. J.; Ginley, D. S.; Olson, D. C.; Kahn, A.; Armstrong, N. R. Chem. Mater. 2011, 23, 4988-5000. (34) Payne, B. P. Doctor of Philosophy Thesis, The University of Western Ontario, August, 2011. (35) Koo, Y.-M.; Choi, S.-J.; Chu, T.-Y.; Song, O.-K.; Shin, W.-J.; Lee, J.-Y.; Kim, J. C.; Yoon, T.-H.; J. Appl. Phys. 2008, 104, 123707. (36) Small, C. E.; Tsang, S.-W.; Kido, J.; So, S. K.; So, F. Adv. Funct. Mater. 2012, 22, 3261-3266. (37) Göpel, W. Surf. Sci. 1977, 62, 165. (38) Langell, M. A.; Berrie, C. L.; Nassir M. H.; Wulser, K. W. Surf. Sci. 1994, 320, 25–38. (39) Langell M. A.; Nassir, M. H. J. Phys.Chem. 1995, 99, 4162– 4169. (40) Roberts M. W.; Smart, R. S. C. Surf. Sci. 1980, 100, 590–604. (41) Jiang, J. Wang, X.; Zhang, Q.; Li, J.; Zhang, X. X. Phys. Chem. Chem. Phys. 2013, 15, 6875.


Page 9 of 17

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

Shuyi Liu, Rui Liu, Ying Chen, Szuheng Ho, Jong H. Kim, and Franky So* Chem. Mater. 2014, XX, XXXX Solution-processed Nickel Oxide Hole Injection/Transport Layers for Efficient Solutionprocessed Organic Light-Emitting Diodes


The solution processed nickel oxide (s-NiOx) annealed at a high temperature of 500 °C shows larger crystalline sizes with a high hole mobility. A further UV-ozone treatment changes its surface species, which significantly facilitate the hole injection, leading to enhanced device performance compared to the device with PEDOT:PSS.



Figure 1.

100 Transmittance Absorbance (a.u.)

80 70 60 50 40 30 20

Absorbance (a.u.)

90

Transmittance (%)

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 17

10 0 300

400

500

600

Wavelength (nm)

700


800

Page 11 of 17

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


Figure 2



Figure 3 Original Ni 2p3/2 peak

Ni 2p3/2

Ni

2+

(NiO) peak

Original O 1s peak Fitted O 1s peak 2O (NiO) peak

Ni

2+

(Ni(OH)2) peak

O (Ni(OH)2) peak

Ni

3+

(NiO(OH)) peak

2-

Intensity

O (NiO(OH)) peak


Original864 Ni 2p3/2 peak 867 861 Fitted Ni 2p3/2 peak

858

855

Ni 2p3/2 852

2+

Ni

2+

(Ni(OH)2) peak

Ni

3+

(NiO(OH)) peak

Original O 1s peak 534 532

Fitted O 1s peak 2Binding O (NiO) peak

(NiO) peak

864

530

528

O526 1s

Energy (eV)

2-

O (Ni(OH)2) peak 2-

O (NiO(OH)) peak


867

2-

O (H2O) peak

Binding Energy (eV)

Ni

O 1s

2-

Intensity

Intensity

Fitted Ni 2p3/2 peak

Intensity

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 17

861

858

855

852

2-

O (H2O) peak

534

Binding Energy (eV)


532

530

528

Binding Energy (eV)

526

Page 13 of 17

Figure 4

0.3

0.2

0.2 0.1

0.1

10

20

25 30 Time (s)

35

40

0.0

10

15

20

-2

25

Time (s)

30

2

35

40

5 4 3 2 1 0

10

-3

10

-4

10

-5

10

With PEDOT:PSS HIL With UV-ozone treated s-NiOx HIL

-6

10

5

10

1.0

-1

-1

15

6

7

8

9

Sqrt E (kV

1/2

cm

10

-1/2

)

11

Hole Injection Efficiency

0.0

10V 12V 14V 16V 18V 20V 22V 24V

7

15

20

25

Time (s)

30

35

40

8 -2

-2

0.4

8 -2

10V 12V 14V 16V 18V 20V

0.5


0.3

0.6


0.4

NPB Hole Mobility (cm V s )

-2

10V 12V 14V 16V 18V 20V

0.5


0.6


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


10V 12V 14V 16V 18V 20V 22V 24V

7 6 5 4 3 2 1 0

5

10

15

20

25

Time (s)

30

0.8 0.6 0.4 0.2 0.0

PEDOT:PSS HIL UV-ozone treated s-NiOx HIL 50


60

70

80

90

-1

E (kV cm )

100

110

120

35

40


Figure 5 -35

0.008

VG= -30 V VG=-25 V

-30

0.007

IDS (saturated) (A )

VG= -20 V

-25

1/2

VG= -15 V VG= -10 V

-20

1/2

IDS (A)

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 14 of 17

VG=-5 V

-15

VG= 0 V

-10

0.005 0.004 0.003 0.002

-5 0

0.006

0

-5

-10 -15 -20 -25 -30 -35 -40 -45

VDS (V)


0.001

0

-5

-10

-15

-20

VGS (V)

-25

-30

-35

-40

Page 15 of 17

Figure 6 100

10

4

0.1

3

10

0.01

2

1E-3

10

1E-4

10

1

1E-6

0

1

2

3

4

5

6

7

Voltage (V)

8

9 10 11


80 70 60 50

10

40 30

1

20

PEDOT:PSS As-prepared s-NiOx UV-O3 treated s-NiOx

1

10

100

10 1000 -2


0

-1

PEDOT:PSS As-prepared s-NiOx 0 UV-O3 treated s-NiOx 10

1E-5

90

-1

1


10

Power Effciency (lm W )

-2

5

10

-2


100


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



Figure 7a 5

-2

10

10

4

10

1 3

10

0.1

2

0.01

10

1E-3

1

10

1E-4 0

1E-5 1E-6

PEDOT:PSS HIL/NPB HTL UV-O3 treated s-NiOx HIL/HTL

0

1

2

3

4

5

6

7

8

10

-2


100


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

-1

10 9 10 11

Voltage (V)


Page 16 of 17

Page 17 of 17

Figure 7b

90

-1

80 70 60 50

10

40 30 20 PEDOT:PSS HIL/NPB HTL UV-O3 treated s-NiOx HIL/HTL

1

1

10

100

1000

10 0

-1


100

Power Efficiency (lm W )

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


-2



Transport Layers for Efficient Solution

Recommend Documents