Hole transporting layer plays a crucial role to realize high efficiency and long lifespan by balancing the charge carrier into the desired recombination zone.

Organic light emitting diodes (OLEDs) have been well known for their potential usage in the lighting and display industry. The device efficiency and lifetime have improved considerably in the last three decades. However, for commercial applications, operational lifetime still lies as one of the looming challenges. In this review paper, an in-depth description of the various factors which affect OLED lifetime, and the related solutions is attempted to be consolidated. Notably, all the known intrinsic and extrinsic degradation phenomena and failure mechanisms, which include the presence of dark spot, high heat during device operation, substrate fracture, downgrading luminance, moisture attack, oxidation, corrosion, electron induced migrations, photochemical degradation, electrochemical degradation, electric breakdown, thermomechanical failures, thermal breakdown/degradation, and presence of impurities within the materials and evaporator chamber are reviewed. Light is also shed on the materials and device structures which are developed in order to obtain along with developed materials and device structures to obtain stable devices. It is believed that the theme of this report, summarizing the knowledge of mechanisms allied with OLED degradation, would be contributory in developing better-quality OLED materials and, accordingly, longer lifespan devices.

Carbazoles decorated with both donor and acceptor fragments offer a classical way to optimize bipolar functional properties. In this work, a series of carbazoles featuring triphenylamine donors and cyano acceptors are synthesized and their structure-property relationship is studied. The effects of connectivity and the chromophore number density on photophysical and electroluminescence properties are investigated. The position of the triphenylamine donor on the 3,6-dicyanocarbazole nucleus significantly affected the photophysical and electroluminescence properties. The dye possessing triphenylamine on C2 and C7 displayed a red shift in absorption when compared with the structural analogue with triphenylamine tethered to C1 and C8. The emission wavelength of the dyes are tunable from blue to green, by altering the position of triphenylamine and cyano substituents. All of the dyes exhibited positive solvatochromism in emission, attributable to the photoinduced intramolecular charge transfer from the triphenylamine donor to the cyano acceptor. However, the extent of charge transfer and hybridization of local and charge-transfer-excited states is highly dependent on the position of triphenylamine and cyano groups on the carbazole nucleus. Dyes containing cyano substituents at C2 and C7 showed a prolonged excited state lifetime, broad emission, and large Stokes shifts, indicating the presence of a higher charge transfer component in the excited state. The dyes displayed exceptional thermal stability with the onset decomposition temperature (10% weight loss) > 350 °C. Electrochemical measurements revealed low oxidation potential for dyes containing triphenylamine at C3 and/or C6. Addition of a cyano acceptor on carbazole led to the stabilization of lowest unoccupied molecular orbital. Furthermore, the materials were tested as emitting dopants in solution-processable multilayer organic light emitting diodes and found to display deep-blue/sky-blue electroluminescence with external quantum efficiency as high as 6.5% for a deep-blue emitter (CIE y ∼ 0.06).

A deep-blue emitter was developed by modifying carbazole nuclear positions C2 & C7 with a triphenylamine donor and C3 & C6 with a cyano acceptor. The molecular design features cross-conjugated localized and charge transfer chromophores which results in a hybridized local charge transfer (HLCT) excited state. An organic light emitting diode (OLED) using this material exhibited high external quantum efficiency (6.5%) with excellent color saturation (CIEy ∼ 0.06) and small full-width at half maximum (48 nm).

Abundant molecules enable countless combinations of device architecture that might achieve the desirable high efficiency from organic light-emitting diodes (OLEDs). Due to the relatively high cost of OLED materials and facilities, simulation approaches have become a must in further advancing the field faster and saver. We have demonstrated here the use of state-of-art simulation approaches to investigate the effect of molecular orbital energy levels on the recombination of excitons in OLED devices. The devices studied are composed of 1,1-bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC) as hole transporting material (HTM), 4,4′-Bis(9-carbazolyl)-1,1′-biphenyl (CBP) as host, 2,2',2”-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi) or bathophenanthroline (Bphen) as electron transporting materials. The outcomes reveal that exciton recombination highly sensitive to the energy-level alignment, injection barriers, and charge mobilities. A low energy-barrier (<0.4 eV) between the layers is the key to yield high recombination. The lowest unoccupied molecular orbital (LUMO) levels of the organic layers have played a more pivotal role in governing the recombination dynamics than the highest occupied molecular orbital (HOMO) level do. Furthermore, the Bphen based device shows high exciton recombination across the emissive layer, which is >106 times greater than that in the TPBi based device. The high carrier mobility of Bphen whose electron mobility is 5.2 × 10−4 cm2 V−1 s−1 may lead to low charge accumulation and hence high exciton dynamics. The current study has successfully projected an in-depth analysis on the suitable energy-level alignments, which would further help to streamline future endeavours in developing efficient organic compounds and designing devices with superior performance.