Phosphorescent OLED Emitters
In organic light-emitting diodes (OLEDs) electric current is efficiently converted into visible light by means of luminescent emitters. In addition to being an energy-saving technology, thin and flexible OLED displays can be produced. A limiting factor is the operational stability of OLEDs – in particular, blue OLED emitters are prone to bleach quickly. For this reason, research on OLED emitters has focused lately on developing efficient blue-light emitters. Theoretical and computational chemistry can support this research by identifying the factors which control the internal quantum efficiencies. In an OLED, an electronically excited state is generated by charge recombination of a hole and an electron, preferably at or close to the emitter. Spin statistics leads to 75% of the molecules generated in the triplet state and only 25% as singlets. To harvest both, singlet and triplet molecules, several strategies have been pursued: Doping of the emissive layer with chromophores or chromophore pairs showing
(1) efficient intersystem crossing (ISC) and phosphorscence
(2) thermally actictivated delayed fluorescence (TADF)
In phosphorescent OLEDs (PhOLEDs), metal organic complexes containing precious heavy elements such as Ir or Pt are used as dopants. The combination of ISC and efficient phosphorescence in principle allows to harvest 100% of the excitons. The challenge for a computational treatment is to reliably predict the emission wavelength and phosphorescence lifetime and to give a realistic estimate of the triplet quantum yield. We have only recently started to study complexes of this type theoretically, but our first results are very promising (Kleinschmidt et al.). We use density functional theory for obtaining geometry parameters and the DFT/MRCI method in combination with the spin-orbit coupling kit SPOCK and VIBES for determining their spectroscopic properties.
TADF emitters invoke reverse intersystem crossing (rISC) to repopulate the excited singlet state and emit delayed fluorescence instead of phosphorescence. In principle, an internal quantum efficiency of 100% can be reached in this way, too. Two types of compounds are being used as TADF emitters: metal-free donor-acceptor compounds and complexes of earth-abundant d10 metals such as Cu (I). Funded by a DFG project, we have been working on both types of compounds with the goal of understanding the underlying principles that steer the (r)ISC and emission rate constants. In addition to a small singlet-triplet energy gap, environment effects and spin-vibronic interactions are seen to be very important in these charge transfer compounds (For a recent review see here). We closely collaborate with other theoreticians and experimentalists in this field. In a recent project (SPP 2102), we investigate the ability of Zn (II) to form TADF emitting complexes.
One possibility to shorten the time the TADF emitter resides in the excited state – and hence to avoid side reactions that lead to the degradation of the emitter – is to transfer the excitation energy to a nearby fluorescent emitter. This is the principle of hyperfluorescence. Hyperfluorescence has experimentally been proven to occur. The theoretical treatment of hyperfluorescence is still in its very early stages (Lyskov-2018). Currently, we are setting up quantum chemical methods to investigate the excitation energy transfer between the TADF auxiliary dopant and the fluorescence emitter.