Ramchandra Pode

Low roll-off efficiency green phosphorescent organic light-emitting devices with simple double emissive layer structure

Using an Ir(ppy)3 metal complex doped in 4,4′4″-tris(N-carbazolyl)-triphenylamine hole and bis[2-(2-hydroxyphenyl)-pyridine]beryllium electron transport host materials, simple three layer green phosphorescent organic light-emitting devices comprising double emissive layers have been fabricated. A low driving voltage value of 3.3 V to reach a luminance of 1000 cd/m2 and maximum current- and power-efficiency values of 58.7 cd/A and 65.1 lm/W, and maximum external quantum efficiency (EQE) value of 18.6% are reported in this device. EQE exceeding 15% over a wide range of current density from 0.03 to 25 mA/cm2 is noticed. We demonstrate a low roll-off current efficiency value of 12% at a luminance of 10 000 cd/m2 in a highly efficient simple double emissive layer device, imperative to high brightness applications.

Highly efficient bilayer green phosphorescent organic light emitting devices

We present a highly efficient green phosphorescent device comprising only of two organic layers. A host material bis[2-(2-hydroxyphenyl)-pyridine]beryllium having a good electron transporting and energy transfer characteristics, and a wide band gap hole transport material N,N′-di(4-(N,N′-diphenyl-amino)phenyl)-N,N′-diphenylbenzidine lead to the fabrication of a simplified high efficiency device. The driving voltage value of 3.3V to reach a luminance of 1000cd∕m2 is reported. The maximum current- and power-efficiency values of 38.30cd∕A and 46.60lm∕W are demonstrated in this device. Results reveal a practical way to fabricate highly efficient truly bilayer organic devices for trouble-free manufacturing processes.

Low voltage efficient simple p-i-n type electrophosphorescent green organic light-emitting devices

We present simple p-i-n structures with double-emitting and mixed-emitting layers for highly efficient phosphorescent green devices. Using a wide band-gap hole transporting material of 4,4′4″-tris(N-carbazolyl)-triphenylamine and a wide band-gap electron transporting material of bis[2-(2-hydroxyphenyl)-pyridine]beryllium, the bilayered p-i-n structure with no heterointerface barriers has been realized. A very low onset voltage value of 2.4 V corresponding to the energy of 2.4 eV of green electroluminescence, which is close to the photon energy of dopant emitting molecules (2.3–2.4 eV), is achieved in this simple p-i-n device configuration. Maximum current- and power-efficiency values of 53.3 cd/A and 61.4 lm/W and low rolloff of current efficiency (6%) are demonstrated in the simple p-i-n green phosphorescent devices, promising for the practical and economical high brightness applications.

A highly efficient transition metal oxide layer for hole extraction and transport in inverted polymer bulk heterojunction solar cells

This paper demonstrates a comparative study of transition metal oxide materials, tungsten trioxide (WO3) and molybdenum trioxide (MoO3), as an effective hole extraction and transport layer in inverted polymer bulk heterojunction solar cells. Their device performances as an anode buffer layer are investigated based on thieno[3,4-b]thiophene/benzodithiophene (PTB7) and [6,6]-phenyl C70-butyric acid methyl ester(PC70BM) photoactive layers. The fabricated device with WO3 compared with MoO3 shows improved power conversion efficiency as high as 6.67% under a simulated AM1.5G illumination of 100 mW cm−2. The excellent performance of WO3 in inverted bulk heterojunction solar cells is attributed to an efficient hole extraction, excellent electron blocking capability, smooth morphology as well as better ohmic contact between the active layer and the metal electrode.

Efficient multiple triplet quantum well structures in organic light-emitting devices

We demonstrate the multiple quantum well (MQW) structures with the charge control layers (CCLs) to produce highly efficient red phosphorescent organic light-emitting devices (OLEDs). Various triplet quantum well devices from a single to five quantum wells are realized using wide band-gap hole and electron transporting layers, narrow band-gap host and dopant materials, and CCLs. Triplet energies in such MQW devices are confined at the emitting layers. The maximum external quantum efficiency of 14.8% with a two quantum well device structure is obtained. The described MQW device concept has been proposed to be very useful to future OLED display and lighting applications.

Efficiency Control in Iridium Complex-Based Phosphorescent Light-Emitting Diodes

Key factors to control the efficiency in iridium doped red and green phosphorescent light emitting diodes (PhOLEDs) are discussed in this review: exciton confinement, charge trapping, dopant concentration and dopant molecular structure. They are not independent from each other but we attempt to present each of them in a situation where its specific effects are predominant. A good efficiency in PhOLEDs requires the triplet energy of host molecules to be sufficiently high to confine the triplet excitons within the emitting layer (EML). Furthermore, triplet excitons must be retained within the EML and should not drift into the nonradiative levels of the electron or hole transport layer (resp., ETL or HTL); this is achieved by carefully choosing the EML’s adjacent layers. We prove how reducing charge trapping results in higher efficiency in PhOLEDs. We show that there is an ideal concentration for a maximum efficiency of PhOLEDs. Finally, we present the effects of molecular structure on the efficiency of PhOLEDs using red iridium complex dopant with different modifications on the ligand to tune its highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energies.

Luminescence properties of Sn1-xFexO2 nanoparticles

Nanoparticles of Sn1−xFexO2 (x = 0, 0.02, 0.05, 0.1) have been prepared by co-precipitation method using SnCl2 and FeSO4 precursors and subsequent heat-treatment at 600°C. X-ray Diffraction (XRD) study shows that these nanoparticles crystallise in tetragonal system. The unit cell volume increases slightly with Fe2+ doping indicating substitution of Sn4+ sites by Fe2+ ions. The unit cell volume increases from 72.24 A3 to 72.90 A3 as x varies from 0 to 0.1. With increasing Fe2+ concentration, the peaks in X-ray diffraction pattern become broad because of strain effect produced from substitution of smaller ionic radius Sn4+ (0.69 A) by large one Fe2+ (0.77 A). The average crystallite size was found to decrease from 21 nm to 11 nm as x changed from 0 to 0.1. In Transmission Electron Microscopy (TEM) study of pure SnO2, the particles are spherical in shape and particle size is found to be 25 nm, which is close to 21 nm from XRD study. Its Selected Area Electron Diffraction (SAED) confirms the tetragonal system. In infrared study, a broad peak centred around 650 cm1− was observed due to Sn-O/Fe-O vibration. From steady state luminescence study, it is found that pure SnO2 and Fe2+ doped SnO2 show the band-edge emission around 400 nm and the emission intensity decreases with increasing Fe2+ concentration. The band-edge absorption occurs in 300-350 nm.

Solar Photovoltaic Electricity

The importance of green energies is continuously growing in our societies not only due to environmental concerns but also to solve the problem of access to electricity in a number of rural areas in developing countries. Photovoltaic energy is one of the most promising environment friendly sources of electricity. It does not involve any moving mechanical part and is one of the most reliable sources of renewable energies. It is a real opportunity for developed countries to lower their carbon print and for developing countries to grant access of electricity to their populations in remote rural areas.

Effectiveness of a polyvinylpyrrolidone interlayer on a zinc oxide film for interfacial modification in inverted polymer solar cells

This paper investigate the effectiveness of non-conjugated polymer polyvinylpyrrolidone (PVP) at the interface of an n-type metal oxide buffer layer and the photoactive layer in inverted bulk heterojunction solar cells. A 15% enhancement in power conversion efficiency (PCE) is realized after the incorporation of a thin PVP layer between zinc oxide (ZnO) and polythieno[3,4-b]-thiophene-co-benzodithiophene (PTB7):[6,6]-phenyl C71-butyric acid methyl ester (PC70BM) based photoactive layer in inverted polymer solar cells. The fabricated devices with the PVP layer show enhanced PCE as high as 7.30% under simulated AM 1.5 G (100 mW cm−2) illumination. The ZnO/PVP improves the electron extraction property of the ITO electrode, effectively blocks holes from the highest occupied molecular orbital of the donor, suppresses charge recombination at the interface of ZnO and the photoactive layer, and decreases the interfacial contact resistance.

Low-Voltage, Simple-Structure, High-Efficiency p–i–n-Type Electrophosphorescent Blue Organic Light-Emitting Diodes

We present simple p–i–n structures with single-emitting and mixed emitting layers for highly efficient phosphorescent blue organic devices. Using a high-triplet-energy-hole-transporting material of 1,1-bis(4-methylphenyl)-aminophenyl-cyclohexane (TAPC) and a high-triplet-energy-electron-transporting material of 1,3,5-tris(m-pyrid-3-ylphenyl)benzene (Tm3PyPB), the p–i–n structure has been realized by doping with MoO3 as a p-dopant and Cs2CO3 as an n-dopant. A very low onset voltage of 3.0 V and a driving voltage of 4.0 V to obtain a brightness of 1000 cd/m2 are achieved in this p–i–n device configuration. A maximum external quantum efficiency of 23.9% and a power efficiency of 36.7 lm/W are reported.