Approaches for Long Lifetime Organic Light Emitting Diodes

Abstract

Organic light emitting diodes (OLEDs) have been well known for their potential usage in the lighting and display industry. The device efficiency and lifetime have improved considerably in the last three decades. However, for commercial applications, operational lifetime still lies as one of the looming challenges. In this review paper, an in-depth description of the various factors which affect OLED lifetime, and the related solutions is attempted to be consolidated. Notably, all the known intrinsic and extrinsic degradation phenomena and failure mechanisms, which include the presence of dark spot, high heat during device operation, substrate fracture, downgrading luminance, moisture attack, oxidation, corrosion, electron induced migrations, photochemical degradation, electrochemical degradation, electric breakdown, thermomechanical failures, thermal breakdown/degradation, and presence of impurities within the materials and evaporator chamber are reviewed. Light is also shed on the materials and device structures which are developed in order to obtain along with developed materials and device structures to obtain stable devices. It is believed that the theme of this report, summarizing the knowledge of mechanisms allied with OLED degradation, would be contributory in developing better-quality OLED materials and, accordingly, longer lifespan devices.

Keywords: OLED; degradation; device architecture; lifetime; materials.

Figure 1

Overview of OLED global market revenue by a) display and b) lighting application. c) Contribution of OLED technology by application in next‐generation electronic devices.

Figure 2

A brief history of OLED technology evolution.

Figure 3

Number of research articles on WOLED per year (Web of Science).

Figure 4

Highest power efficiency WOLED per year (Web of Science).

Figure 5

Number of research articles on blue OLED per year (Web of Science).

Figure 6

Highest power efficiency of blue OLED per year (Web of Science).

Figure 7

Number of research articles on red OLED per year (Web of Science).

Figure 8

Highest power efficiency of red OLED per year (Web of Science).

Figure 9

Number of papers of lifetime per year (Web of Science).

Figure 10

Illustrates the number of patents on WOLEDs filed per year (Web of Science).

Figure 11

Electrolysis of water molecules and formation of dark spots inside an OLED device.

Figure 12

The reaction of Alq₃with moisture and oxygen^{[ 110 ]}

Figure 13

Pinholes in metal layer causing device failure.

Figure 14

Molecular migration causing OLED device failure.

Figure 15

Atomic migration causing OLED device failure.

Figure 16

General schematic illustration of the influence of glass transition temperature (T _g) in the thermal degradation of OLED materials.

Figure 17

High injection barrier generating high joule heat.

Figure 18

External dust particles causing formation of dark spots.

Figure 19

Molecular structures of heavy metal complexes based phosphorescent emitters.

Figure 20

Molecular structures of host materials using in both fluorescent and phosphorescent OLEDs.

Figure 21

Molecular structures of ETL materials using in OLEDs.

Figure 22

Molecular structures of TADF emitters using in OLEDs.

Figure 23

Molecular structures of TADF host materials using in OLEDs.

Figure 24

Molecular structures of HTL materials using in OLEDs.

Figure 25

Device architectures of step‐wise energy transfer in OLEDs.

Figure 26

Schematic representation of a two stack tandem OLED device.

Figure 27

Device architectures of conventional and p–i–n OLEDs.

Figure 28

General device structure of a) conventional and (b) inverted OLED device.

Figure 29

(Left) Schematic representation of excitons in conventional and exciplex OLEDs. (Right) Essential parameters for donor and acceptor to obtain TADF mechanism enabling exciplex forming cohost systems.