Donor-<i>Spiro</i>-Acceptor Molecular Design: A Key toward High-Efficiency Simplified Single-Layer Phosphorescent Organic Light-Emitting Diodes

Abstract

Conspectus In organic electronic technologies, Single-Layer Phosphorescent Organic Light-Emitting Diodes (SL-PhOLEDs), made of only the electrodes and the Emissive Layer (EML), are a new generation of simplified PhOLEDs. In the field of the OLED, such a type of device has been studied for a long time as they represent “ideal” devices. Indeed, simplifying the classical multilayer structure of PhOLEDs (called ML-PhOLEDs) is important to reduce the amount of commodities, the manufacture complexity, and the production and recycling costs. However, removing the functional organic blocking/transporting layers of an ML-PhOLED stack leads to an intense decrease in the PhOLED performance. This is the main problem; the researchers have faced in this field. To keep a high performance without the different organic layers, the effective injection, transport, and recombination of charges in the device should be performed by the EML and more particularly by the host materials. This host material should then display a set of electronic and physical properties, essential to reach a high-efficiency SL-PhOLED. However, reaching high-performance SL-PhOLED is far more difficult than for ML-PhOLED and each molecular parameter of the host can dramatically decrease the PhOLED performance. The molecular design of the host is then crucial and a specific design has been particularly studied in the last years, which consists of linking an electron-rich unit to an electron-poor unit via a spiro linkage. This design called Donor- Spiro -Acceptor (D- Spiro- A) allows one to gather all the required properties in a single host and has allowed, in the past few years, important advances in the field. Nowadays, the most efficient SL-PhOLEDs use this molecular design and our group has particularly contributed to this research field. Thus, as the D- Spiro -A molecular design nowadays provides SL-PhOLEDs with the highest performance, a good understanding of its impact in the device efficiencies appears particularly important. This is the story we want to tell herein. In this Account, we discuss through a structure/properties/device performance relationship study (triplet state energy level, HOMO/LUMO energy levels, charge carriers mobilities, thermal and morphological properties, and device performances), the recent advances made by this design in the field of SL-PhOLEDs. This Account will mainly focus on the association of phenylacridine-like fragments (namely, phenylacridine, indoloacridine, quinolinophenothiazine, and quinolinoacridine) as the electron-rich unit and fluorene/phosphine oxide as the electron-poor unit, which is, in the light of the literature, the combination that has allowed the most important advances in the last years. Trying to unravel why this design has led to high performance appears to be important for the future of SL-PhOLED technology and may lead to new directions in terms of molecular design.

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