K. Thangaraju

Study on photoluminescence from tris-(8-hydroxyquinoline)aluminum thin films and influence of light

Tris-(8-hydroxyquinoline)aluminum (Alq3), which is the most widely used material in organic electroluminescent devices, has been synthesized. Alq3 thin films have been deposited on glass and silicon substrates. The influence of light exposure on the optical properties of Alq3 thin films has been studied. It is confirmed that the photoluminescence (PL) of Alq3 thin film originates from its two geometrical isomers, namely, facial and meridional, which result from PL decay analysis (biexponential fit). It is also confirmed that the PL from both the isomers decreases for increasing light exposure time leading to the creation of luminescent quencher in Alq3 thin film.

A new electron transporting material for effective hole-blocking and improved charge balance in highly efficient phosphorescent organic light emitting diodes

A highly efficient new benzoimidazole phosphine oxide based electron transporting material, bis(1-phenyl-1H-benzo[d]imidazole)phenylphosphine oxide (BIPO), was designed, synthesized by condensation, nucleophilic substitution and oxidation reactions, and confirmed using various spectroscopic studies. It shows a thermal stability (ΔT5%) of 451 °C with a glass transition temperature of 129 °C from the thermogravimetric analysis and differential scanning calorimetry studies. BIPO used as an efficient electron transport layer (ETL) in a green emitting phosphorescent organic light emitting diode, ITO/4,4′-bis[N-(1-naphthyl)-N-phenyl-amino]biphenyl (30 nm)/4,4′,4′′-tris(carbazole-9-yl)triphenylamine 10 nm)/(4,4′-N,N′-dicarbazole)biphenyl (CBP) host doped with Ir(ppy)3 dopant (5%) (30 nm)/ETL (40 nm)/LiF (1 nm)/Al (100 nm), serves as an effective hole-blocking material and improves the charge balance in the device, resulting in higher device efficiencies of 22.19%, 68.3 cd A−1 and 24.4 lm W−1 with a maximum luminance of 72 080 cd m−2 compared to those (17.03%, 52.0 cd A−1 and 20.9 lm W−1 with a maximum luminance of 33 490 cd m−2) of a device using a widely used ETL, 1,3,5-tris(m-pyrid-3-yl-phenyl)benzene. These results show that the new BIPO ETL could be very useful in efficient organic light emitting diodes.

Highly efficient green phosphorescent organic light emitting diodes with improved efficiency roll-off

A 10-nm thick 4,4′,4″-tris(carbazole-9-yl)tri-phenylamine (TcTa) interlayer effectively confines triplet excitons within the emissive layer (EML) of phosphorescent organic light emitting diodes (PHOLEDs) based on green-emitting Ir(ppy)3 dopant and improves the charge balance in the EML of the device, resulting the higher device efficiencies of 61.7 cd/A, 19.7 %, and 43.2 lm/W with the maximum luminance of 75,310 cd/m2 and highly improved efficiency roll-off (22.2% at 20 mA/cm2) when compared to those (61.1 cd/A, 19.6 %, and 47.2 lm/W with a maximum luminance of 38,350 cd/m2) of the standard device with efficiency roll-off of 62.3 % at 20 mA/cm2.

Photoluminescence of Tris-(8-hydroxyquinoline)aluminum Thin Films and Influence of Swift Heavy Ion Irradiation

The effect of swift heavy ion (SHI) irradiation on the optical properties of Tris-(8-hydraxyquinoline) aluminum (Alq3) thin films has been studied. It is confirmed that PL and lifetime of SHI irradiated Alq3 thin films irradiated both at room temperature and low temperature decrease for increasing ion fluences indicating the transfer of excitons energy to unstable cationic Alq3 species. It is also confirmed that PL and lifetime of Alq3 thin films irradiated at low temperature decrease largely compared to that of at the room temperature indicating that a large number of unstable cationic Alq3 species are generated by SHI irradiation.

Swift heavy ion irradiation-induced modifications of tris-(8-hydroxyquinoline)aluminum thin films

Tris-(8-hydroxyquinoline)aluminum (Alq3), one of the most widely used light emitting and electron transport materials in organic luminescent devices, has been synthesized. Alq3 thin films have been deposited by a thermal evaporation process on glass substrates. The effect of swift heavy ion (SHI) irradiation using 40 MeV Li3+ on the Alq3 thin films has been studied by UV-visible, infrared, photoluminescence (PL) and time-resolved photoluminescence (TRPL) spectroscopy. From TRPL studies, it is found that the PL of Alq3 thin films arises from two species corresponding to its two geometrical isomers, namely facial and meridional having two different life times. It is also confirmed that the PL and lifetimes of excitons decrease with the increase of ion fluences of SHI of 40 MeV Li3+, indicating a transfer of exciton energy to unstable cationic Alq3 species generated by SHI irradiation.

Electrical transport properties of spray deposited transparent conducting ortho-Zn2SnO4 thin films

Ortho Zinc Stannate (Zn2SnO4) exhibits excellent electrical and optical properties to serve as alternate transparent electrode in optoelectronic devices. Here we have optimized ortho-Zn2SnO4 thin film by spray pyrolysis method. Deposition was done onto a pre-heated glass substrate at a temperature of 400 °C. The XRD pattern indicated films to be polycrystalline with cubic structure. The surface of films had globular and twisted metal sheet like morphologies. Films were transparent in the visible region with band gap around 3.6 eV. Transport properties were studied by Hall measurements at 300 K. Activation energies were calculated from Arrhenius’s plot from temperature dependent electrical measurements and the conduction mechanism is discussed.Ortho Zinc Stannate (Zn2SnO4) exhibits excellent electrical and optical properties to serve as alternate transparent electrode in optoelectronic devices. Here we have optimized ortho-Zn2SnO4 thin film by spray pyrolysis method. Deposition was done onto a pre-heated glass substrate at a temperature of 400 °C. The XRD pattern indicated films to be polycrystalline with cubic structure. The surface of films had globular and twisted metal sheet like morphologies. Films were transparent in the visible region with band gap around 3.6 eV. Transport properties were studied by Hall measurements at 300 K. Activation energies were calculated from Arrhenius’s plot from temperature dependent electrical measurements and the conduction mechanism is discussed.

Improved electron injection in spin coated Alq3 incorporated ZnO thin film in the device for solution processed OLEDs

We deposit tris-(8-hydroxyquinoline)aluminum (Alq3) incorporated zinc oxide (ZnO) thin films by spin coating method under the normal ambient. It showed the higher transmittance (90% at 550 nm) when compared to that (80% at 550 nm) of spin coated pure ZnO film. SEM studies show that the Alq3 incorporation in ZnO film also enhances the formation of small sized particles arranged in the network of wrinkles on the surface. XRD reveals the improved crystalline properties upon Alq3 inclusion. We fabricate the electron-only devices (EODs) with the structure of ITO/spin coated ZnO:Alq3 as ETL/Alq3 interlayer/LiF/Al. The device showed the higher electron current density of 2.75 mA/cm2 at 12V when compared to that (0.82 mA/cm2 at 12V) of the device using pure ZnO ETL. The device results show that it will be useful to fabricate the low-cost solution processed OLEDs for future lighting and display applications.We deposit tris-(8-hydroxyquinoline)aluminum (Alq3) incorporated zinc oxide (ZnO) thin films by spin coating method under the normal ambient. It showed the higher transmittance (90% at 550 nm) when compared to that (80% at 550 nm) of spin coated pure ZnO film. SEM studies show that the Alq3 incorporation in ZnO film also enhances the formation of small sized particles arranged in the network of wrinkles on the surface. XRD reveals the improved crystalline properties upon Alq3 inclusion. We fabricate the electron-only devices (EODs) with the structure of ITO/spin coated ZnO:Alq3 as ETL/Alq3 interlayer/LiF/Al. The device showed the higher electron current density of 2.75 mA/cm2 at 12V when compared to that (0.82 mA/cm2 at 12V) of the device using pure ZnO ETL. The device results show that it will be useful to fabricate the low-cost solution processed OLEDs for future lighting and display applications.

Improved electroluminescence in organic light emitting diodes by thermal annealing of indium tin oxide anode

We demonstrate the improved electroluminescence (EL) in OLEDs by using annealed ITO anode at different temperatures under the normal ambient. SEM studies show the smooth surface morphology of ITO film upto 300°C and it turned wrinkle kind of structure at 400°C. The hole-only device (HOD) based on ITO annealed at 300°C exhibits the higher hole-current density when compared to pristine ITO based device and it is drastically decreased for 400°C. The higher EL emission intensity was observed for OLEDs using ITO annealed at 300°C and it is drastically decreased for 400°C annealed ITO based device. These results show that the annealing of ITO anode upto 300°C improves the hole-injection in OLEDs, balancing the charge carriers and improving the device performance, whereas the change in surface properties of ITO anode annealed at 400°C in turn alters the ITO/TPD interface leading to the charge imbalance, resulting in the decreased device performances.

(4,4′-N,N′-dicarbazole)biphenyl (CBP) as efficient host in cost-effective green phosphorescent OLEDs

(4,4′-N, N′-dicarbazole) biphenyl (CBP) serves as an efficient host material in the phosphorescent organic light emitting diodes (PHOLEDs) based on green-emitting Ir(ppy)3 dopant with relatively low-cost charge transporting layers, resulting the higher device efficiencies of 31.45 cd/A, 10 %, and 11.3 lm/W with the maximum luminance of 60,170 cd/m2 with highly improved efficiency roll-off when compared to those (24.08 cd/A, 7.98 %, and 14.16 lm/W with a maximum luminance of 19,090 cd/m2) of 9,9′-bis-(9,9-dimethyl-9H-fluoren-2-yl)-9H,9′H-[3,3′]bicarbazolyl (BDFC) host based device.

Tris-(8-hydroxyquinoline)aluminum thin film as ETL in efficient green phosphorescent OLEDs

Tris-(8-hydroxyquinoline)aluminum thin film as ETL in green phosphorescent OLEDs improves the device performances to a maximum of 34.2 cd/A, 11.3% with the maximum brightness of 63,150 cd/m2 and broadens the device emission in yellow-green region suitable in the white OLEDs for the lighting applications.