**Author details**

reduction of OLEDs and performance enhancement. The potential inexpensive synthesis methods, OLEDs fabrication process, flexibility and lightweight make them one of the promising materials for energy devices. A large number of phosphorescent and fluorescent organic materials have been already synthesized for their use in OLEDs. Recently, TADF materials have been introduced in OLEDs fabrication to achieve 100% IQE and high EQE, which can be

120 Light-Emitting Diode - An Outlook On the Empirical Features and Its Recent Technological Advancements

The actual development of organic TADF emitters achieves a status that surpasses a simple curiosity. The new OLED framework based on highly efficient TADF materials opens a new field of applications for display and lightening. These materials, based on separated donor-acceptor moieties in one molecule, leads to a unique luminescent process of a triplet harvesting in the excited state with a further huge increase of internal efficiency up to the 100% limit. This simple way to tailored pure organic emitters is really one of the most recent advances in chemistry and OLEDs filed. Achieving very high external efficiencies without using an expensive and rare transition metals, puts the organic emitters towards new possibilities. Particularly important is the application in lighting from large area emitters, a field that is until now, a technological problem. The possibility to have a full range color emitters with high efficiency (as shown, and particularly in blue) is also an outstanding achievement towards a new device design and application, particularly in white emission. Nevertheless, improving the efficiency towards the theoretical efficiency limit, can only be achieved with a deep understand of the TADF process in these organic molecules. This is due to the fact that the interactions between energy levels of TADF emitter and the host can condition the emission due to the TADF process itself as referred before. Thus, the next steps must be focused on the physical models for this *rISC* pathway and the relationship with the molecular structure. In the OLEDs point of view, the design of reliable host materials that fulfills the requirements needs to allow an efficient luminesce emission form the

TADFs have been studied and used for OLEDs so far, and this field is relatively young but it has developed significantly during the past 5 years. By the incorporation of TADFs 40–50% EQE can be achieved. However, at the present stage despite of numerous characterization techniques to understand the TADF behavior, more rigorous efforts are required for their understanding and use for commercially production. The fabrication cost of the OLEDs based on TADF emitter should be less to make them major candidate in both display and lightening industry and the cost is critical. To reduce the cost, it is necessary to develop solution processed OLED fabrication methods and synthesis of polymeric or dendrimeric TADF emitters. These TADFs have already been found a significant role in next-generation displays and lightning materials and their use can only be realized as their synthesis characterization and

The authors would like to acknowledge the i3N support from UID/CTM/50025/2013 and the EXCILIGHT Project from the European Union's Horizon 2020 research and innovation pro-

gram under the Marie Sklodowska-Curie grant agreement No 674990.

achieved near 50%.

TADFs. A new era is opened.

device fabrication progresses.

**Acknowledgements**

Manish Kumar1,2, Miguel Ribeiro1 and Luiz Pereira<sup>2</sup> \*

\*Address all correspondence to: luiz@ua.pt

1 CeNTI – Centre for Nanotechnologies and Smart Materials, R. Fernando Mesquita, Portugal

2 Department of Physics and i3N – Institute for Nanostructures, Nanomodulation and Nanofabrication, University of Aveiro, Portugal
