OLED

In the last decades, several studies aimed at assessing some specified aspects of visual comfort characterizing the relationship between the human needs and the light environment, such as an available amount of light, light uniformity, light quality in rendering colors, and predicting the risk of glare for space occupants. For each of them, a (still growing) number of indices and metrics have been proposed in literature and standards. In the present work, they are described, categorized according to common features, and finally discussed. As in the case of long-term thermal comfort indices, such visual comfort indices and their summation over a specified calculation period might be used for driving optimization processes to support a more conscious integrated design of buildings. To that purpose, the choice among the available visual comfort indices needs to be informed by an analysis of their features and implications. We present recommendations for use, and suggest areas where improvement is needed for their use in optimization processes to support buildings' design.

use a low refractive index electron transport layer (ETL). To design a high-efficiency OLED, it is important to have a multilayer structure to control the radiative recombination such that excitons are confined to the emis-sive layer (EML) using high triplet charge blocking layers. In addition to exciton confinement , the electron and hole transport layers should be chosen in such a way to maintain the charge balance to avoid triplet-polaron quenching. In a multilayer OLED, the ETL also plays a key role in determining the light extraction efficiency since it is the intermediate layer between the EML and the metallic cathode, and a large portion of the dipole radiation from the EML is lost to the evanescent region where the in-plane wave-vector is larger than the total wave-vector, resulting in radiation that is coupled to the surface plasmon polariton (SPP) and the " lossy surface waves " on the metallic cathode. [4] The magnitude of the loss to the SPP mode is determined by the dielectric constants of the metallic cathode and the ETL, therefore the refractive index of the ETL can significantly impact the out-coupling efficiency of an OLED. There have been some reports demonstrating the effect of the ETL's refractive index on light extraction efficiency in OLEDs by simulation. [5,6] However, experimental study of the effect of ETL refractive index on device performance is rather limited. [7] In this work, we demonstrate the effect of ETL's refractive index on device efficiency using a solution processed OLED with a copper-based thermally activated delayed fluorescent (TADF) emitter, [(2-(Diphenylphosphino)-4-isobutylpyridine) (PPh3)2Cu2I2] (Cu(I)-iBuPyrPHOS). The emitter has a photolu-minescence quantum yield (PLQY) of 70%. [8] Based on the PLQY and assuming an out-coupling efficiency of 20%, a maximum external quantum efficiency (EQE) of 14% is expected. Using 2,9-di(naphthalen-2-yl)-4,7-diphenyl-1,10-phenanthroline (NBPhen) and 2,4,6-tris(biphenyl-3-yl)-1,3,5-triazine (T2T) as ETLs, we showed that an optimized device has a maximum EQE of 12% which is close to the estimate of the maximum EQE of 14%. Surprisingly, using tris-[3-(3-pyridyl)mesityl] borane (3TPYMB) as the ETL we achieved a maximum EQE of 21%, showing nearly a 76% enhancement just by changing the ETL alone. Upon investigation of the origin of the efficiency enhancement, we found that the refractive index of 3TPYMB is 1.65, which is the lowest among all commonly used ETLs, resulting in a significant enhancement in light extraction efficiency. Our device data are also confirmed by the optical simulation results. A low refractive index electron transport layer (ETL) can be very effective in enhancing the out-coupling efficiency of an organic light-emitting diode (OLED). However, most organic films show a refractive index close to 1.8. In this work, it has been discovered that tris-[3-(3-pyridyl)mesityl]borane (3TPYMB) has a low refractive index of 1.65 (at 550 nm), which is the lowest refractive index ETL among the commonly used ETLs up to date. Using 3TPYMB as an ETL, a solution processed OLED is demonstrated with nearly a 76% enhancement in external quantum efficiency (EQE). Optical simulation results of this study show that 59% of the enhancement comes from the low refractive index 3TPYMB, and the remaining 17% from the change in charge balance due to the 3TPYMB ETL in the OLED devices.

General expressions are deduced for the stresses developed in the individual thin layers of a multi-layer structure as a result of bending to a specified radius. These are appropriate for analysing flexible organic light emitting diode (FOLED) devices on flexible substrates. Residual stress (caused internally by temperature change and differential thermal expansion) after material deposition and return to ambient temperatures is not considered. The reduced elastic modulus of the typical small molecule OLED materials: N,N 0 -bis(naphthalen-1-yl)-N,N 0 -bis(phenyl)benzidine (NPD) and tris-(8-hydroxyquinoline)aluminum (Alq 3 ) are measured as thin-films using nano-indentation techniques. A conventional device: polyethylene terephthalate (PET)/Buffer layer (BL)/ITO/OLED/Al is considered from a modeling standpoint, as a preliminary to actual fabrication and subsequent comparative testing of OLED performance on rigid and flexible supports.

A series of novel highly conjugated Donor-p-Acceptor (D-p-A) chromophores 3(a-g) are designed and synthesized through the Wittig Reaction. These molecules contain an aryl/heteroaryl groups as an electron donor (D) and electron transporting oxadiazole as acceptor (A). The structural integrity of the compounds was characterized by 1 H NMR, 13 C NMR and GC-MS analysis. The photophysical properties have been studied in detail using UV-Vis absorption, fluorescence spectroscopy and time-resolved fluorescence measurements. These synthesized compounds emit intense blue-green-yellow fluorescence with reasonably good quantum yields. In addition, steady-state absorption and fluorescence measurements were carried out in solvents of varying polarities to understand the intramolecular charge-transfer (ICT) characters of these compounds. The solvent dependency of the large Stokes shifts and high dipole moment of excited state also supported the charge-transfer character of the excited state. The combined experimental Cyclic Voltammetry (CV) and theoretical methods (DFT) were performed to investigate structure-property relationship and ICT property of these compounds. The computed values were found to be in good agreement with the experimental results. The results demonstrated that the novel D-p-A chromophores could play important role in organic optoelectronics.

In this research the effect of ZnO nanorods is simulated and investigated on the performance of poly (2-methoxy-5-(2-ethylhexyloxy)-1, 4-phenylene vinylene (MEH-PPV) based organic light-emitting diode (OLED) located between two electrodes (ITO as an anode and Calcium as a cathode) for emitting in near ultraviolet (UV) region. To study the irradiation in these regions, three structures (ITO/MEH-PPV/Ca, ITO/ZnO/MEH-PPV/Ca and ITO/ZnO/ZnO NR/MEH-PPV/Ca) were considered. Results showed that the OLED simulated with ZnO nanorods had a better performance in comparison with two other structures with higher irradiation intensity at the wave length of 380 nm.

The current work demonstrates e fficient utilization of 2D-MoO
3
nano-flakes as a hole injection layer (HIL) in organic light emitting diodes (OLEDs). Nano-flakes are synthesized using an organic solvent-assisted grinding and sonication method of liquid exfoliation for MoO
, and 8– 16 nm thick flakes are obtained. The effect of solar illumination on the hole injection properties of these nano-flakes is then studied by exposing the nano-flakes for 0, 15, 30, 45, 60 and 120 min and using them as HIL in green OLED. The device results are then compared with the OLED having bulk MoO
3
as HIL. OLEDs with nano-flakes as the HIL have shown better performance than the OLED with bulk MoO
3
as the HIL due to the better semiconducting properties in the nano-flake phase. The luminous intensity is increased by increasing the duration of irradiation and was found to be optimum in case of nano-flakes irradiated for 30 or 45 min and then started to decrease with the increase of duration of irradiation. The current density in the OLEDs with nano-flakes as the HIL shows a switching from high resistance to low resistance; however, the sequential pattern of switching voltage was missing with the duration o f irradiation. The current density also decreased for nano-flakes with 60 and 120 min of irradiation. Transition from the semiconducting to metal nature of nano-flakes by solar irradiation i s suggested t o b e the reason behind this decrease in current density and luminous intensity with a longer duration of illumination.

The authors report a quantum-dot (QD) organic light-emitting diode (OLED) structure formed on Si substrate. Here, the junction area is defined by a lithographically patterned oxide layer on Si substrate and is designed to allow a wide range of scalability of lateral dimension down to a nanometer
range. The device structure comprises: (from the top) indium-tin-oxide (70-nm thickness) as a hole-injecting transparent anode, Poly(3,4 ethylenedioxythiophene)-poly(styrenesulfonate) (30 nm) as a hole-transport layer, Poly(N-vinylcarbazole) (20 nm) as a hole-transport/electron-blocking
layer, CdSe/ZnS core–shell quantum-dots as an emissive layer, and n-Si substrate as an electroninjecting cathode. The authors observed a carrier injection mechanism originating from the twodimensional electron gas (2DEG) system available at the SiO2/Si interface. By varying the junction
dimensions and geometry, the electron injection process is found to occur predominantly at the junction’s periphery, not area, resulting in a low turn-on voltage (1–2 V). This 2DEG injection produces one-dimensional emission of light along junction edges. This edge injection/emission QD-OLED structure, when scaled down to a sub-10 nm range, offers an interesting approach to developing single quantum-dot light sources for quantum information processing.

Transient electroluminescence of an organic alloy-based light-emitting device (ITO/TPD/alloy/Alq/Al) was studied and the results
compared with a standard device without the alloy layer. Under forward bias condition, the electroluminesence (EL) of the alloy device
consisted of two components, a fast (10–20
s) and a slow (200–300
s) component. The slow component in the EL matched with a
corresponding matching current transient. These transients are identified with the leakage of electrons into the alloy region across the
alloy/Alq interface and subsequent exciton formation in the alloy layer. A large overshoot in EL intensity was also observed in the turn-off
transient of the standard and alloy devices. The variations in the EL overshoot and decay at different forward bias voltages, repetition rates
and temperatures were used to understand the role of traps in the device structures.