In the last twenty years, organic light-emitting diodes (OLEDs) have been developed and have reached a stage of commercialization, finding utilities in a large range of applications such as full-colour displays for smartphones and TVs. Unlike traditional inorganic diodes, they also have a great potential for large, light, and flexible flat-panel displays.[1]

However, OLEDs still present several issues to deal with to improve their performance. One of the key challenges is to boost the external quantum efficiency (EQE). The first emitter generation based on fluorescent molecules shows limited IQE, which cannot exceed 25% based on spin statistics. In order to obtain more efficient devices, 100% of the excitons should be harvested.[2] Phosphorescent emitters (PhOLED) have been developed as a second generation but despite an IQE approaching 100%, these materials, containing noble metals, are costly, toxic and critical in terms of available resources.[3,4]

As a response to these drawbacks, the use of Thermally Activated Delayed Fluorescence (TADF) appears to be a promising approach to reach the efficiency of PhOLED with purely organic materials. TADF is observed from materials with a significantly small singlet-triplet energy gap (∆E_ST), enabling an up-conversion of triplets to singlets via reverse intersystem crossing (RISC).[5] As a result, the harvesting of 100% of singlet excitons leads to a remarkably high IQE.

In this work, we investigate new donor-acceptor small organic molecules with a spiro connection as potential deep-blue TADF emitters; the spiro connection is introduced to limit intermolecular interactions and get rid of the host matrix typically used. First, a theoretical screening approach based on Density Functional Theory (DFT) and Time-Dependent DFT (TD-DFT) is carried out to evaluate the ∆E_ST and the degree of charge-transfer character in the singlet and triplet excited states of potential candidates recently synthesized in Rennes. This led to the selection of SPAF-(POPh₂)₂ as the most promising candidate; this compound yields an ∆E_ST of 0.17 eV due to the small spatial overlap between the HOMO and LUMO and 2 triplet states (one with a locally excited and the other with a charge-transfer character) below the lowest singlet state. Moreover, the experimental emission wavelength of the single molecule is located at 360 nm near UV range.

In a second part, SPAF-(POPh₂)₂ has been integrated in OLED devices to assess their actual performance.

Interestingly, a device built with a pure layer of SPAF-(POPh₂)₂, without a host matrix, generates a white light, with a luminance of 260 Cd/m² at 7V and an EQE higher than 0.4% for a non-optimized device. We tentatively attribute the significant red shift of the emission in the thin film and the broad tail at long wavelengths to the formation of exciplex-like emission between the conjugated core of one molecule and the phosphine moieties of adjacent units.