Designing a Stable Cathode with Multiple Layers to Improve the Operational Lifetime of Polymer Light‐Emitting Diodes

Abstract

The short device lifetime of blue polymer light‐emitting diodes (PLEDs) is still a bottleneck for commercialization of self‐emissive full‐color displays. Since the cathode in the device has a dominant influence on the device lifetime, a systematic design of the cathode structure is necessary. The operational lifetime of blue PLEDs can be greatly improved by introducing a three‐layer (BaF2/Ca/Al) cathode compared with conventional two‐layer cathodes (BaF2/Al and Ba/Al). Therefore, the roles of the BaF2 and Ca layers in terms of electron injection, luminous efficiency, and device lifetime are here investigated. For efficient electron injection, the BaF2 layer should be deposited to the thickness of at least one monolayer (∼3 nm). However, it is found that the device lifetime does not show a strong relation with the electron injection or luminous efficiency. In order to prolong the device lifetime, sufficient reaction between BaF2 and the overlying Ca layer should take place during the deposition where the thickness of each layer is around that of a monolayer.

Role of Ultrathin Metal Fluoride Layer in Organic Photovoltaic Cells: Mechanism of Efficiency and Lifetime Enhancement

“…As the BaF 2 thickness increased up to < 3 nm, the series resistance (R s ) tended to decrease, the shunt resistance (R sh ) tended to increase, and thus the FF tended to increase (Table 1). [14] In OLEDs that operate at voltages higher than the V bi of a typical OPV, the device luminous efficiencies did not degrade meaningfully even with much thicker metal fluoride layers (e.g., 8 nm). A BaF 2 layer thicker than a single-coverage layer is an insulator and forms a barrier to electron tunneling.…”

“…[14] In OLEDs that operate at voltages higher than the V bi of a typical OPV, the device luminous efficiencies did not degrade meaningfully even with much thicker metal fluoride layers (e.g., 8 nm). [14,16] The grain size of BaF 2 tended to increase as the deposited nominal thickness increased as monitored by using a quartz crystal microbalance sensor. Although the V oc was still high in the device with a BaF 2 layer thicker than a single-coverage layer, the insulating nature of BaF 2 and electron accumulation at the interface because of the blockage of extraction increased the R s from 11.7 W cm 2 at 3 nm to 30.9 W cm 2 for a 5 nm BaF 2 interfacial layer and thus reduced the FF, which caused the S-shaped J-V behavior in the device with a 5 nm thickness (Figure 1 a).…”

“…[14] The effective WF of the BaF 2 / Al cathode estimated by a photovoltaic measurement [17] was 2.9 eV (with thicker BaF 2 than a single-coverage layer), whereas the Ba WF is 2.7 eV. [14] The effective WF of the BaF 2 / Al cathode estimated by a photovoltaic measurement [17] was 2.9 eV (with thicker BaF 2 than a single-coverage layer), whereas the Ba WF is 2.7 eV.…”

Computational Study on Carrier Injection in Ca/Poly(9,9'-dioctylfluorene) Interface by Using Quantum Chemistry and Monte Carlo Methods

“…[7][8][9][10] Many researchers have studied the electron injection from cathodes to organic layers. [11][12][13][14] Most of the light-emitting polymers, including a poly(9,9 0 -dioctylfluorene) (PDOF) [15][16][17][18][19] that is known as a good blue lightemitting polymer, have barriers of electron injection because of their low work functions. Therefore, a low work function metal such as calcium has been used as a cathode material to reduce the barriers.…”

“…Therefore, a low work function metal such as calcium has been used as a cathode material to reduce the barriers. 11,13,18) Another way to enhance the electron injection at the cathode/polymer interface is modification of the interfacial structures. [20][21][22] It is suggested that interfacial structures such as a polymer orientation, cluster formation of metals, and interface adhesion strongly affect the device properties.…”