White polymer light emitting diodes based on PVK: the effect of the electron injection barrier on transport properties, electroluminescence and controlling the electroplex formation

Abstract

The effects of the electron injection barrier on the charge transport, brightness and the electroluminescence (EL) properties of polymer light emitting diodes (PLEDs) with poly(9-vinylcarbazole) (PVK) as an emissive layer have been studied. By using Al and LiF/Al as the cathode in single layer PLEDs and diverse electron transporting layers (ETLs) such as 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP), 4,7-diphenyl-1,10-phenanthroline (BPhen) and 2,2',2''-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi) in the case of multilayer PLEDs, the charge transport, brightness, color tuning and the EL properties of the devices were drastically modified. The energy barrier for electrons affects the electron current flowing through the device, thereby affecting the operating voltage and the brightness of the PLEDs. The PLEDs with TPBi as the ETL possess the lowest injection barrier and give the maximum brightness of 426.24 cd m. The electron injection barrier is also found to play a major role in defining the EL spectra of the PLEDs. A larger injection barrier gives rise to electroplex formation in the EML-ETL interface of the PLEDs and an additional peak at ∼605 nm was observed in the EL spectrum. As a result, a near white emission with CIE coordinates of (0.30, 0.30) and (0.25, 0.23) at 20 V was obtained from devices with BCP and BPhen as ETLs. Furthermore, PVK doped with 2-phenyl-5-(4-biphenylyl)-1,3,4-oxadiazole (PBD) at 10, 20 and 30 wt% ratios modified the electron transport nature of PVK and had a remarkable influence on the aforesaid properties, especially on the electroplex formation.

First Example of White Organic Electroluminescence Utilizing Perylene Ester Imides

“…From these results, it has been observed that the devices with TPBi as ETL exhibited improved brightness and lowered turn on voltage as compared to the devices with BPhen as ETL. This is due to the lower electron injection barrier of TPBi‐PVK interface that facilitates more electron injection into the emissive layer resulting in an increased recombination at a comparatively lower voltage …”

Perylene Based Solution Processed Single Layer WOLED with Adjustable CCT and CRI

“…These results may be attributed to the deeper HOMO and LUMO energy levels of OXD-7, which may act as trap center for the injected holes and may cause unnecessarily excess electron injection in to the emission layer, respectively. Hole mobility values of PVK:PBD and PVK:OXD-7 hosts are calculated to be 2 × 10 −6 and 8 × 10 −7 cm 2 V −1 s −1 , whereas the electron mobility values are 8.7 × 10 −7 and 1.1 × 10 −5 cm 2 V −1 s −1 , respectively [25], OXD-7 [34], PBD [35], are taken from literature and PTE and PDI are calculated by using the first reduction potentials and optical band gaps (Figure S2)] and cross sectional view of fabricated device.…”

“…Figure 1. Energy band diagram of the devices, chemical structure of PTE and PDI [energy levels of PVK[25], OXD-7[34], PBD[35], are taken from literature and PTE and PDI are calculated by using the first reduction potentials and optical band gaps (FigureS2)] and cross sectional view of fabricated device.…”

White light emitting diode based on purely organic fluorescent to modern thermally activated delayed fluorescence (TADF) and perovskite materials

“…As shown in Fig. 9, the devices with ETLs BCP and/or BPhen gave close to white light emission with (0.30, 0.30) and (0.25, 0.23) as CIE values at 20 V [27].

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“…9 a The EL spectra and b the CIE co-ordinates diagram of the undoped PVK based PLEDs with or without different ETLs (DEV B-without ETL, DEV B-BCP, DEV B-Bphen and DEV B-TPBi)(Reproduced with permission from Ref. [27] Copyright 2016, ©Physical Chemistry Chemical Physics)…”