E-ISSN:2583-9152

Review Article

Anode Buffer

Journal of Condensed Matter

2023 Volume 1 Number 2 Jul-Dec
Publisherwww.thecmrs.in

A Mini Review on The Anode Buffer Layers Used in Organic Light Emitting Diodes

Pandey C1*, Bajpai M2, Malik R3
DOI:10.61343/jcm.v1i02.35

1* C K Pandey, Department Of Physics, S.N.S College Motihari 845401 Brabu, Muzafferpur, Bihar, India.

2 Manisha Bajpai, Department of Physics, Siddhartha University, Kapilvastu Siddharth Nagar 272202, UP, India.

3 Rakhee Malik, Department of Physics, Government Degree College, Nadabhood Badaun 243723, UP, India.

Research on organic light emitting diodes (OLEDs) are recently increasing due their unique advantages over inorganic devices. To explore up to the technology, there is a need to identify it with the fundamental device physics of OLED. The fundamental physics of the device consists of basically three steps i.e. device structure, device mechanism and device characteristics & their parameters. Device structure of OLED typically consist of three basic layers but there are several intermediate layers are used to enhance device efficiency. Anode buffer layers are used to are used to reduce the interface barriers present at anode/HTL interface. Here, in in this paper different used anode buffer layers are reviewed.

Keywords: Organic light emitting diodes, Buffer Layers, Polymers

Corresponding Author How to Cite this Article To Browse
C K Pandey, , Department Of Physics, S.N.S College Motihari 845401 Brabu, Muzafferpur, Bihar, India.
Email: 		Pandey C, Bajpai M, Malik R, A Mini Review on The Anode Buffer Layers Used in Organic Light Emitting Diodes. J.Con.Ma. 2023;1(2):75-78.
Available From
https://jcm.thecmrs.in/index.php/j/article/view/35

Manuscript Received Review Round 1 Review Round 2 Review Round 3 Accepted
2023-11-08 2023-11-13 2023-11-18 2023-11-24 2023-12-01
Conflict of Interest Funding Ethical Approval Plagiarism X-checker Note
None Nil Yes 22.87%

© 2023by Pandey C, Bajpai M, Malik Rand Published by Condensed Matter Research Society. This is an Open Access article licensed under a Creative Commons Attribution 4.0 International License https://creativecommons.org/licenses/by/4.0/ unported [CC BY 4.0].

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C K Pandey et al.: Review on the anode buffer layers used in organic light emitting diodes

Introduction

In the 20th century, OLEDs evolved and became the focus of attention due to their wide application not only in contemporary displays but also in solid-state lighting. OLEDs are fabricated using organic semiconductors (OSCs) [1-2]. The real interest in polymer-based OLEDs started in the early 1990’s, when electroluminescence (EL) was observed in thin films of poly (phenylene Vinylene) (PPV) [3]. Polymer based OLEDs namely termed as polymer LEDs (PLEDs) possess several advantages over small molecular OLEDs such as easy and cheap fabrication procedure. Both small molecular and polymer OLEDs suffers from several losses during operation and their efficiencies are still low [4]. Mainly, the losses in OLEDs occur at electrode/organic interfaces which cause a high energy mismatch between electrode, Fermi level and HOMO or LUMO of organic materials. To reduce these losses, several organic layers with matching HOMO or LUMO level have been synthesized and different techniques were used to improve the efficiency of OLEDs [5-6]. Introduction of hole or electron injection layer has by far remained the preferred choice for reducing these losses. Several inorganic and organic injection layers have been used for this purpose and improved the efficiencies of devices [7-8].

Need of Anode Buffer Layers

Anode buffer layers (ABLs) are used to reduce the interface barriers present at anode/HTL interface. The Fermi level of the anode ITO used is in the range of 4.8–5.1 eV, and most HTLs have HOMO values > 5.5 eV. Therefore, this interface shows evidence of a high injection barrier above 0.5 eV, accompanied by high barrier values at this interface. To reduce this barrier, several injection layers were been used and the list include transition metal oxides [9], doped transport layers [10], organic acceptors [11] etc. Table 1 lists some of the most commonly used buffer layers introduced at the anode/HTL interface. From the table, it is reflected that multiple buffer layers were used to reduce the barrier at this interface, which significantly improved the OLED efficiency. Metal oxides such as MoOx, WO3 etc. [12,13] have been found very effective to improve this interface, however, their deposition is very difficult and the parameters depend on the deposition conditions

such as pressure inside the chamber. It was found that the introduction of MoOx as the buffer layer in OLEDs not only reduced operational voltage but also has a high brightness compared to other organic injection layers [14].

Table 1: Recent advancement in Anode buffer layer used

Buffer layerPerformance improvementReference
MoO3Formation of efficient ohmic contact, reduction in turn-on voltage to 2.4 V and increase in efficiency by more than 50 %9
WO3No barrier at interface and increase in efficiency by two times10
p-doped HTLs (F4-TCNQ as dopant)Reduction in operating voltage to almost half and 30-70 % increase in efficiency11
PANI-PSSOperating voltage reduction by 15 – 30 % and luminance improvement by about 30-50 %12
F4-TCNQTurn on voltage reduction to 2.5 V and efficiency improvement more than 2 times13
Organic acceptors such as F2-HCNQ etc.Turn on voltage reduction to 2.5 V and efficiency improvement by more than 50 %14

Han You et al. [15] have compared MoO3 and CuPc anode buffer layers, have observed that the turn-on voltage defined as the voltage required for the luminance of 1 cd/m2 of OLEDs with the MoO3 buffer layer is reduced to 2.4 V from 6.9 V for OLEDs with the CuPc buffer layer. The important thing about MoO3-based devices is that it maintains a low turn-on-voltage across a wide thickness range of 5 to 20 nm. This is completely different from the case of the CuPc buffer layer where the turn-on-voltage increases with the CuPc thickness. Devices with MoO3 buffer layers have lower operating voltages and are therefore more energy efficient. When the MoO3 thickness is 5–10 nm, an energy efficiency of at least 10 lm/W is achieved, and when the MoO3 thickness is 15 nm, the maximum power efficiency reaches 15 lm/W. This increases by a factor of 7.5 comparison with devices using 15 nm CuPc. For devices with MoO3 buffer layer, the HTL/ITO offset energy is reduced, making hole injection layer (H-I-L) from ITO to the hole transport layer more effective.

Zhou et al. [16] have used a p-doped HTL (2 mol % F4-TCNQ doped TDATA) as ABL to improve the efficiency of OLED. They designed an efficient OLED with a p-doped H-I-L. Doping results in lower operating voltage and higher EL efficiency as compared to the undoped devices. These OLEDs have very low operating voltages of about 3.4 V at 100 cd/m2 and near turn-on voltages of about 2.5 V at 1 cd/m2. These device properties are suitable for the required level of integrated drive electronics.


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This impressively demonstrates the promising potential of controlled doping if it can be extended to devices with optimized structures. However, the introduction of these layers increases the bulk resistance, since the thickness of these layers ranges from 10 to 30 nm.

Organic acceptor layers such as F4-TCNQ, F2-HCNQ have been found effective and also can be easily deposited [17-18]. Among these materials, F4-TCNQ is a popular material. Bao Xiu Mi [19] used F2-HCNQ as ABL to enhance the efficiency of OLEDs. They examined OLED devices that were doped with the p-type dopant F2-HCNQ. Doping not only NPB but also the host material 2-TNNA can improve the current density and power efficiency. Because of its strong electron acceptance capability, F2-HCNQ offers more host material options than the state-of-the-art p-type dopant F4-TCNQ. In addition, F2-HCNQ is thermally stable, which makes the doped layer more stable and enables better control of co-deposition through thermal evaporation.

Conclusions

Anode buffer layers (ABLs) are widely used for the application of organic devices. Here we have reviewed various organic/inorganic ABLs used in organic light emitting diodes to improve LED parameters such as interface barrier, turn on voltage, luminance efficiency and hence power efficiency.

Acknowledgement

The author (MB) would like to acknowledge department of higher education, Uttar Pradesh for providing financial assistance (F.No. :89/2022/1585/70-4-2022/001-4-32-2022).

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