Effect of Electron Mobility of the Electron Transport Layer on Fluorescent Organic Light-Emitting Diodes

Abstract

The charge balance mechanism in green fluorescent organic light-emitting diodes is investigated for different electron transport layers (ETLs) and electron mobilities. Carrier accumulation and an increase in the exciton recombination probability are shown to be critical for improving the current and power efficiencies by aligning the bands at the interface between the emitting layer (EML) and ETL. The peak in the electroluminescence (EL) spectra was found to shift slightly in response to changes in the width of the emission zone and reflected the electron mobility of the ETL. Higher electron mobility resulted in a wider recombination zone in the EML that was manifested by a blue-shift of the EL peak.Organic light-emitting diodes (OLEDs) 1-14 have the potential to be employed in applications such as flat panel displays, flexible electronics, and solid-state lighting. Considerable research has been devoted to uncovering exact OLED operation mechanisms and precisely determining their electrical and optical properties. The charge balance 1-14 is considered to be a key factor for controlling device performance, stability, and color coordinates; in a heterojunction device structure, holes in the hole transport layer (HTL) 1 are regarded as being transported too quickly, while electrons in the electron transport layer (ETL) 2 are transported too slowly, and as a result, holes simply pass through without generating excitons with the electrons in the emission layer (EML), which manifests as a low current efficiency.To overcome this discordance between the hole and electrons mobilities and to improve the carrier charge balance, many researchers have investigated novel structures such as a metal oxide buffer layer, 3,4 electron or hole blocking layers, and new materials for the ETL 5,6 that have a high electron mobility. In the early stages of OLED heterostructure development, tris(8-hydroxyquinolinolato)aluminum (Alq 3 ) was used for both the EML and ETL in a single device. 2 Other materials that have been introduced for the ETL include 3-phenyl-4(10-naphthyl)-5-phenyl-1,2,4-triazole (TAZ), 5,7 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BPhen), 1,5,6 and 2-[3,5-bis(1-phenylbenzimidazol-2-yl)phenyl]-1-phenylbenzimidazole (TPBi) 5,7,8 in an effort to enhance the exiton recombination and current efficiencies by producing a higher electron mobility and adequate band alignment. The electron mobility also has the potential to control the exciton recombinationzone width in the EML. 9 Tang and Chen et al. 9 have shown that a higher hole mobility through the EML resulted in a wider recombination zone, which was beneficial for a longer device lifetime, and that the electroluminescence (EL) peak shifts when the width of the recombination zone changes. Both the higher hole mobility in the EML and the larger emission zone generate an additional contribution to the EL peak from adjacent layers such as the HTL and ETL. 9 In other work, Meerholz et al. 12 adjusted the width of the emission zone in the EML by copolymerization of the HT...

Adjusting dopant concentrations in solution process to optimize the white phosphorescent organic light-emitting diodes

“…7. These phenomena are similar to the report of reference [27]. The potential barrier between the HOMO of the EML (CBP in this study) and that of TPBi is $0.6 eV, which induces carrier accumulation at the interface between EML and ETL, But the potential barrier of HOMO between CBP and Alq 3 is $0 eV.…”

“…The potential barrier between the HOMO of the EML (CBP in this study) and that of TPBi is $0.6 eV, which induces carrier accumulation at the interface between EML and ETL, But the potential barrier of HOMO between CBP and Alq 3 is $0 eV. In reference [27], they found that the improved exciton recombination originates from carrier accumulation in the EML/ETL interface, which reduces the chance that carriers do not generate excitons in the EML; this also improves the current efficiency. The current efficiency of the TPBi device is slightly better than that of the Alq 3 device because of the higher HOMO level.…”

“…6 and 7. Although Alq 3 has lower electron mobility of $8.03 Â 10 À6 cm 2 / Vs than that of TPBi $6.52 Â 10 À5 cm 2 /Vs [27], the current density of the device using Alq 3 as ETL is competitive with (higher than) that of the device using TPBi as ETL, as shown in the inset of Fig. 6, due to the lack of a potential barrier between the EML host and the ETL as shown in Fig.…”

“…6, due to the lack of a potential barrier between the EML host and the ETL as shown in Fig. 1 (almost no potential barrier between CBP and Alq 3 ) [27]. However, as seen in Fig.…”

Efficiency enhancement in a single emission layer yellow organic light emitting device: Contribution of CIS/ZnS quantum dot

“…In general holes are being transported very quickly, while the electron mobility's are smaller in polymeric semiconductors and this causes low exciton generation and current efficiency [40,41]. In order to study the effect of CIS/ZnS incorporation on charge injection into the system, electron-and hole-only devices based of bare ADS233YE and Fig.…”