Correlation of energy band alignment and turn-on voltage in organic light emitting diodes

Choi

Applied Physics Letters

et al. 2012

We investigate the enhancement mechanism of the electroluminescence (EL) of alkali metal based n-doped organic light-emitting diodes (OLEDs). The dual role of the n-dopant (carrier transport and lowering of the injection barrier) induces a trade-off. When the electron transport layer (ETL) is optimally doped by the n-dopant for the highest conductivity, the amount of n-dopant at the ETL/cathode interface is insufficient to form enough chemical bonds with the cathode for efficient carrier injection. This insufficient amount of n-dopant limits the carrier injection properties. To solve this problem, we demonstrated that the addition of an electron injection layer (EIL) comprised of the n-dopant could increase its presence at the interface and, thereby, improve the carrier injection properties and, consequently, the EL efficiency. Moreover, simply using an alkali-metal alloy (rather than co-deposition) on the n-doped ETL as a cathode, instead of using the additional EIL, greatly improves the EL efficiency of the OLEDs. The alkali-metal alloy cathode increased the interfaced states at the ETL/cathode. The proposed model was confirmed by x-ray photoemission spectroscopy experiments on the alkali-metal n-dopant/electrode interface.

contrasting

confidence: 99%

Role of n-dopant based electron injection layer in n-doped organic light-emitting diodes and its simple alternative

Choi

Applied Physics Letters

et al. 2012

“…Though, 3d and 3e have similar LUMO energy levels the HOMO is showing small difference. Wu and co-workers 47 showed that the turn-on voltage is mainly dependent on the HOMO and LUMO energies of the constituent layers. However, while using the HOMO and LUMO values to rationalize the differences in electroluminescence performances care must be exercised.…”

Section: Electroluminescence Characteristicsmentioning

confidence: 99%

Deep-blue emitting pyrene–benzimidazole conjugates for solution processed organic light-emitting diodes

et al. 2015

Applied Physics Letters

“…The energy level alignment at organic and molecular semiconductor heterojunctions is generally recognized to be essential for the function and performance of organic electroluminescent 1,2 and photovoltaic devices. 3,4 Because of its direct relevance for understanding the underlying fundamental mechanisms, such as charge carrier recombination and generation, [5][6][7] the energy level alignment at these interfaces has been extensively studied by ultraviolet and X-ray photoelectron spectroscopy (UPS and XPS, respectively).…”

mentioning

confidence: 99%

Fermi level pinning induced electrostatic fields and band bending at organic heterojunctions

Akaike

Koch

Oehzelt

2014

The energy level alignment at interfaces between organic semiconductors is of direct relevance to understand charge carrier generation and recombination in organic electronic devices. Commonly, work function changes observed upon interface formation are interpreted as interface dipoles. In this study, using ultraviolet and X ray photoelectron spectroscopy, complemented by electrostatic calculations, we find a huge work function decrease of up to 1.4 amp; 8201;eV at the C60 bottom layer zinc phthalocyanine ZnPc, top layer interface prepared on a molybdenum trioxide MoO3 substrate. However, detailed measurements of the energy level shifts and electrostatic calculations reveal that no interface dipole occurs. Instead, upon ZnPc deposition, a linear electrostatic potential gradient is generated across the C60 layer due to Fermi level pinning of ZnPc on the high work function C60 MoO3 substrate, and associated band bending within the ZnPc layer. This finding is generally of importance for understanding organic heterojunctions when Fermi level pinning is involved, as induced electrostatic fields alter the energy level alignment significantl