Triangular-shaped zinc oxide nanoparticles enhance the device performances of inverted OLEDs

https://doi.org/10.1016/j.nanoso.2015.01.001Get rights and content

Highlights

  • The device characteristics of the inverted bottom-emission organic light emitting diodes (IBOLEDs) containing ZnO nanoparticles (ZnO NPs) used as an electron injecting layer were studied.

  • ZnO particles were synthesized in different conditions resulting in different shapes.

  • It is found that triangular-shaped ZnO NPs enhanced the device performances of the IBOLEDs.

  • Three-fold increase in the efficiencies of IBOLEDs is achieved by using these ZnO NPs.

Abstract

Three-fold increase in the efficiencies of inverted bottom-emission organic light emitting diodes (IBOLEDs) is achieved with triangular-shaped zinc oxide nanoparticles (ZnO NPs) synthesized in methanol. ZnO nanoparticles with a diameter ranging from 5 to 10 nm are deposited by a solution process and used as an electron injecting layer (EIL). Influences of the process conditions on device performances such as concentration, annealing temperature and solvents are investigated. IBOLEDs are fabricated with ITO/ZnO NPs/polymer/V2O5/Al layout where poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene vinylene] (MEH-PPV) is used as an active layer. The highest brightness and luminous efficiency reaching 1340 cd m−2 and 0.65 cd A−1 are achieved with 2.5% wt triangular-shaped ZnO NPs synthesized in methanol. On the other hand, performances of the devices fabricated with ZnO NPs synthesized in isopropanol (IPA) and ethanol were 723 cd m−2/0.22 cd A−1 and 67 cd m−2/0.011 cd A−1, respectively. Through this work, it is found that for the performances of the devices, shape effect is more important than the size.

Introduction

Organic light emitting diodes (OLEDs) have aroused interest due to their applications in full-color displays and solid-state lighting sources  [1], [2]. Active-matrix organic light emitting diodes (AMOLEDs) are feasible to become one of the major flat panel displays of the near future because of their excellent performances compared to passive-matrix OLEDs. However, backplane technology is one of the critical problems for improving large-size AMOLEDs. Low-temperature polycrystalline silicon (LTPS) and amorphous silicon (a-Si) thin film transistor (TFT) backplane technologies have already been used in AMOLEDs. The a-Si-TFT backplane has more advantages for large panels because of its good uniformity and low manufacturing cost when compared to LTPS–TFT backplane. Therefore, there is an increasing demand in developing the a-Si-TFT process to adapt to AMOLEDs backplane [3], [4]. Since only n-channel TFT is used in a-Si backplanes, conventional OLEDs (Fig. 1(a)) are connected at the source end of the driving a-Si-TFT, which would affect the stability of the source voltage  [5], [6]. Transparent indium tin oxide (ITO) has been sputtered on organic layers to fabricate inverted top-emission OLEDs (ITOLEDs)  [7], [8], [9], [10], [11], but the sputter deposition of ITO is known to damage the organic layers  [12]. Inverted bottom-emission OLEDs (IBOLEDs) (Fig. 1(b)) integrated to n-channel TFTs are ideal recipes to deal with all of these problems. IBOLEDs have a bottom cathode that can be connected directly to a TFT, which is dislocated at the drain end of a-Si-TFT. Both the gate and the source voltages of the driving a-Si-TFT are directed through the gate which can decrease the driving voltage.

Efficient electron injection is a crucial issue for IBOLEDs, but there are only a few cathode materials, which are appropriate for device structures. Generally, ITO is employed as cathode in IBOLEDs because of its transparency. However, ITO has a high work function and it can limit the injection of electrons. In order to improve the electron injection, various methods have been reported such as insertion of thin metallic layers with low work functions  [13] and use of electron transporting layers (ETLs) doped with Li  [14] or Cs compounds  [15], [16], [17]. However, such methods may have operation instability caused by oxidation of metal or diffusion of metal dopants  [18], [19]. In this regard, metal oxides (such as CuO, NiO, ZnO, TiO2) are promising candidates because of their good charge injection feature, high transparency, easy fabrication, air stability and low resistance compared to organic materials. Metal oxides like TiO2   [20] and ZnO  [21], [22] were applied as an electron injection layer (EIL) and a cathode  [23], [24] in OLEDs and organic photovoltaics. Their applications range from ultra-thin layers (on the anode side) to nano-structured layers (on the cathode side) of devices. The use of a hole-blocking metal oxide NP on the anode side modifies the device efficiency by adjusting the charge balance in the device  [25], [26], [27]. Since NPs have high surface to volume ratio (S/V) which is directly related to the particle size and morphology, they show superior chemical and physical properties indeed. Especially when the size of NPs is below 5 nm, S/V becomes dramatically high. For 5 nm-sized particles, as used in this study, 50% of atoms are on the NPs’ surface and the surface energy increases with decreasing the amount of the surface atoms coordination. As a result, NPs are more chemically reactive than bulk materials  [28]. The influence of NPs’ size, especially quantum dots’ size on the electronic structure of OLEDs is represented by the band gap increasing with decreasing of the particle sizes, which is attributed to the quantum confinement effect. ZnO demonstrates this effect especially for the particle size smaller than 8 nm  [29], [30], [31]. Since ZnO NPs have a wide band gap (Eg3.5eV), good electron-transporting properties,  [32], [33] solution-based processibility at room temperature and low work function,  [34], [35] they can be used as an efficient and transparent EIL for OLEDs.

Duan et al. demonstrated that ZnO nanostructures can be achieved via indium (In) doping. The In-doped ZnO nanowires were grown by one-step vapor transport deposition. The effect of In doping content on the morphology, S/V increment, structure, and optical properties of the nanowire has been investigated  [36]. Jian-Min Li et al. presented shape-selected tetragonal ZnO μnano-crystals grown with controlling morphological process during the crystal growth and synthesis of large but thin transparent regularly-shaped single-crystalline ZnO (0001) hexagonal nanodisks (or nanohexagons) by an indium assisted vapor-phase transport (VPT) for nanoscale solar cells, and photonics  [37], [38]. It was also reported that the synthetic strategy of Au nanoseed assisted open static atmospheric pressure vapor transport method established a greater structural control towards designing and creating 3D elaborate highly UV luminescent ultranarrow ZnO nanoarchitectures and an understanding of the complicated nucleation and growth processes of hybrids  [39]. Tian et al. reported a low-temperature, environmental, solution-based approach for the preparation of complex and oriented ZnO nanostructures, and the systematic modification of their crystal morphology for sensing, catalysis, optical emission, piezoelectric transduction, and actuations  [40].

In this contribution, we report an effective bottom cathode structure using ZnO NPs as an EIL for IBOLEDs. In literature, effect of different thickness of electron injection layers containing ZnO NPs  [22], effect of doping ZnO with carbon nanotubes  [41] and the effect of different sizes of ZnO NPs in OLEDs  [42] have been investigated. However, effects of ZnO NPs’ shape on the device performances are discussed for the first time by this study. ZnO NPs synthesized in different solvents (methanol, ethanol, and isopropanol) gave nanostructures with different morphologies. By coating these nanostructures on top of ITO layer, device performances of IBOLEDs were dramatically changed. ZnO NPs’ concentration, annealing time and temperature were optimized for the best device performances. We believe that the IBOLEDs developed by optimization of electron injection layer (EIL) in this work can be integrated with a-Si-TFTs that would notably speed up the placing of large size AMOLEDs on the market in a close future.

Section snippets

Synthesis of ZnO nanoparticles

Zinc oxide nanoparticles (ZnO NPs) were prepared in three different solvents with some minor modifications of the procedure previously described in the literature  [43], [44], [45], [46], [47]. The alcohols used in this study were methanol, ethanol, and isopropanol, respectively. In a typical synthesis procedure, zinc acetate dihydrate (Zn(CH3COO)2 2H2O) (1.70 g, 7.75 mmol) was dissolved in methanol (70 mL) under vigorous stirring for 30 min at 60 °C (at 70 °C for ethanol and 80 °C for

Results and discussions

In this study, inverted devices with the following structures were fabricated;

ITO/ZnO NPs (50 nm)/MEH-PPV (120 nm)/V2O5(10 nm)/Al (130 nm).

MEH-PPV (Fig. 2(a)) was chosen for the emissive layer because of both its widespread usage for the polymer devices and the particular sensitivity of its photo-physics to the solid-state morphology. Similar to most PPV derivatives  [48], the essentially non-polar MEH-PPV chains are readily soluble in aromatic solvents such as toluene, chlorobenzene and xylene

Conclusion

Here, a class of hybrid and inverted light-emitting diodes (IBOLEDs) have been fabricated. The metal oxide ZnO NPs were introduced as EIL between the emissive layer and the ITO cathode in the investigated devices. MEH-PPV and V2O5 were used as an emissive layer and hole injection layer, respectively. The optimum preparation condition of EIL which enhance the device characteristics was searched. Therefore, the concentration, annealing time and annealing temperature of the ZnO NPs were varied and

Acknowledgment

The authors would like to thank Dr. Özgür Duygulu for obtaining TEM images at TÜBİTAK Marmara Research Center, Materials Institute.

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