Organic light-emitting diodes (OLEDs) have become a mainstream display technology in consumer electronics. Self-emitting ability, transparency, true dark tone, and capability of being made flexible, are some of the features of OLED displays, leading to a superior performance compared with liquid crystal displays. In addition to displays, OLEDs are also a strong candidate for lighting applications. Despite great advances in improving the internal quantum efficiency of an OLED to nearly 100%, the external quantum efficiency is still lacking behind due to optical losses. This review reports the latest advances in the optical design of OLEDs that address the external coupling efficiency of OLEDs. Discussed at first are the fundamentals of OLED optics and how the refractive indices of different layers in an OLED stack affect the extraction efficiency. Then, this paper reviews how microlens arrays, scattering layers, and corrugated structures can be used to recover the optical losses and improve the external efficiency, and the general optical designs for different optical structures for light extraction are presented.

A trans-N-heterocyclic carbene (NHC) platinum(II) acetylide complex bearing phenyl acetylene ligands (NPtPE1) has been synthesized via the Hagihara reaction in 64% yield. The complex features spectrally narrow deep blue emission with a phosphorescence quantum yield (0.30) and lifetime (∼10 μs) in the solid state. The modest quantum yield and lifetime make NPtPE1 a candidate for incorporation into an organic light emitting diode (OLED). Prototype devices exhibited a maximum EQE of 8% with CIE (0.20,0.20). To the best of our knowledge, this is the first example of a platinum(II) acetylide bearing NHC ligands to be incorporated into an OLED.

In a conventional organic light-emitting diode (OLED), only a fraction of light can escape to the glass substrate and air. Most radiation is lost to two major channels: waveguide modes and surface plasmon polaritons. It is known that reducing the refractive indices of the constituent layers in an OLED can enhance light extraction. Among all of the layers, the refractive index of the electron transport layer (ETL) has the largest impact on light extraction because it is the layer adjacent to the metallic cathode. Oblique angle deposition (OAD) provides a way to manipulate the refractive index of a thin film by creating an ordered columnar void structure. In this work, using OAD, the refractive index of tris(8-hydroxyquinoline)aluminum (Alq3) can be tuned from 1.75 to 1.45. With this low-index ETL deposited by OAD, the resulting phosphorescent OLED shows nearly 30% increase in light extraction efficiency.

It is commonly accepted that a full bandgap voltage is required to achieving efficient electroluminescence (EL) in organic light-emitting diodes. In this work, we demonstrated organic molecules with a large singlet-triplet splitting can achieve efficient EL at voltages below the bandgap voltage. The EL originates from delayed fluorescence due to triplet fusion. Finally, in spite of a lower quantum efficiency, a blue fluorescent organic light-emitting diode having a power efficiency higher than some of the best thermally activated delayed fluorescent and phosphorescent blue organic light-emitting diodes is demonstrated. The current findings suggest that leveraging triplet fusion from purely organic molecules in organic light-emitting diode materials offers an alternative route to achieve stable and high efficiency blue organic light-emitting diodes.

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 emissive 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 photoluminescence 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. 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 (...

To better understand the correlation of the dielectric properties with the photovoltaic response in conjugated polymer:fullerene bulk heterojunction materials, the concept of introducing minimal structural change is employed to increase the polymer dielectric constant via polar cyano groups added to the end of butyl or octyl side chains in the poly(dithienosilole-thienopyrrolodione) system. Density functional theory calculations confirm that the polar groups do not affect the polymer electronic structure but can lead to an increase in overall dipole moment depending on the polymer chain conformation. Despite the increased dielectric constant (from 2.7 to 4.3 for cyano-octyl side chains and from 2.7 to 3.2 for the cyano-butyl analogues), the device characteristics employing the cyano-containing polymers are inferior to those of the devices made with unfunctionalized alkyl chains. It is found that the hole mobilities for the cyano-containing polymers are two orders of magnitude lower compared to those for the parent polymers and suggest this is due to an increase in energetic disorder caused by the strong local permanent dipoles associated with the cyano groups. The study highlights the complexity in the relationship between the dielectric constant of organic materials, the morphologies that are induced, and their photovoltaic performance.

ZnO is a potential replacement for TiO 2 as an electron transport layer (ETL) used in low-temperature processed hybrid perovskite solar cells. However, poor thermal stability of perovskites on ZnO and interfacial traps introduced during processing are obstacles to achieve a good device performance. Here, we demonstrate perovskite solar cells using aluminum doped zinc oxide (AZO) nanoparticles for the ETL having a better thermal stability compared with ZnO. However, the device shows a lower short circuit current density and a large photocurrent hysteresis, which are attributed to the poor interfacial properties between the ETL and the perovskite layer. To address this issue, a thin interfacial modification layer of phenyl-C61-butyric acid methyl ester (PCBM) was employed. The resulting device shows the efficiency is improved from 13 to 17% along with a significant reduction in hysteresis. Results from our thermal admittance spectroscopy show that the interface defect states are significantly reduced with the PCBM passivation layer.

Interfaces between donor and acceptor in a polymer solar cell play a crucial role in exciton dissociation and charge photogeneration. While the importance of charge transfer (CT) excitons for free carrier generation is intensively studied, the effect of blending on the nature of the polymer excitons in relation to the blend nanomorphology remains largely unexplored. In this work, electroabsorption (EA) spectroscopy is used to study the excited-state polarizability of polymer excitons in several polymer:fullerene blend systems, and it is found that excited-state polarizability of polymer excitons in the blends is a strong function of blend nanomorphology. The increase in excited-state polarizability with decreased domain size indicates that intermixing of states at the interface between the donor polymers and fullerene increases the exciton delocalization, resulting in an increase in exciton dissociation efficiency. This conclusion is further supported by transient absorption spectroscopy and time-resolved photoluminescence measurements, along with the results from time-dependent density functional theory calculations. These findings indicate that polymer excited-state polarizability is a key parameter for efficient free carrier generation and should be considered in the design and development of high-performance polymer solar cells.