Designing highly efficient Organic Semi-Conductors (OSCs) for Organic Electronics (OE) has led to the fantastic development of this technology. In this context of OE, Phosphorescent Organic Light-Emitting Diodes (PhOLEDs) are the 2nd generation of OLEDs (after fluorescent OLEDs) and have encountered a significant development for the last twenty years as they can in principle reach internal quantum efficiency of 100% by harvesting both singlet and triplet excitons. PhOLED technology is more mature than the recent 3rd generation of OLEDs based on Thermally Activated Delayed Fluorescence (TADF). A PhOLED uses a Host-Guest EMitting Layer (EML) which consists in a Triplet Emitter (Guest) disperses into an appropriate OSC (Host). For the last decade, the design of high triplet energy (ET) hosts (>2.7 eV), essential to be used with red, green and blue phosphors, has been an intense research field worldwide and has led to high-efficiency multi-layer PhOLEDs. This proposal finds its origin in an important fact of the literature: Almost all the high-efficiency PhOLEDs are multi-layer devices. These multi-layer PhOLEDs are constituted of a stack of organic layers in order to improve the injection, transport and recombination of charges within the EML. There are usually in a PhOLED stack, hole and electron transporting layers, hole and electron blocking layers and these layers are even often doubled. Despite the technology is mastered, it suffers from the complexity of the stack, a high-cost, and is time-consuming. In addition, interfacial phenomena can lead to parasite emissions and a low stability. Simplifying the multi-layers structure with the so-called Single-Layer PhOLEDs (SL-PhOLEDs), the simplest device only made of the electrodes and the EML is therefore one key step for the future. However, high-efficiency SL-PhOLEDs are very rarely reported in literature (especially blue emitting devices), due to the lack of host materials possessing all the required properties. In SPIRO-QUEST proposal, we aim to address this feature through the design of universal host materials for high performance red, green and blue SL-PhOLEDs. The goal of this project is to gather in a single molecule all the required properties to insure the energy transfers cascade within the PhOLED (particularly a high triplet state energy level), a high thermal/morphological stability for device lifetime, adequate HOMO/LUMO levels for charge injection and more importantly a good and well balanced mobility of electron and hole (ambipolar character). This ambipolarity is a key property for SL-PhOLED. To fulfil all the above mentioned criteria, this project uses a Donor-Spiro-Acceptor molecular design which implies the judicious connection of a Donor unit to an Acceptor unit. One of the novelty of this proposal is the nature of the donor unit which is a new electron rich fragment: the Quinolinophenothiazine. This fragment is highly promising as shown in preliminary works led by this consortium. Thus, molecular design of host materials is the heart of this proposal, which possess very solid foundations as demonstrated by preliminary results published in the last 30 months. As the development of new materials leading to high efficiency devices is one of the key direction of OE, SPIRO-QUEST targets striking results. More precisely, we aims to go beyond the state of the art in term of performance and stability of SL-PhOLEDs. This multidisciplinary project is led by three complementary research groups, internationally recognized in their fields. SPIRO-QUEST involves a group specialized in chemistry of pi-conjugated systems (C. Poriel/J. Rault-Berthelot, ISCR - Rennes), a group specialized in microelectronics and especially in charge transport (E. Jacques, IETR- Rennes) and a group specialized in physics of OLEDs (D. Tondelier, Ecole Polytechnique - LPCIM).