73HIGHLIGHTS 2020

CAPTURING THE EXCITED STATE OF Cu-BASED OLED MATERIALS

Bright, efficient and inexpensive organic light-emitting diodes (OLEDs) are needed to reduce energy consumption. Cu-based materials with delayed fluorescence can fulfill these requirements. Pump-probe X-ray scattering performed at beamline ID09 gives information about structural changes in the excited state, providing insight to control non-radiative losses.

OLED materials appear in our everyday life in applications such as the screens of smartphones. Large-area flexible screens, which can be rolled out on the wall, or devices for large- area lighting based on OLED technology, are also very attractive. Nevertheless, such devices produced using existing technologies are either very expensive or not very bright. This is due to the use of expensive 5d elements to increase the efficiency of light emission. The challenge, therefore, is to develop OLED materials that simultaneously have high efficiency and do not contain 5d metals.

The light emission process in OLEDs is illustrated in Figure 59a. When electricity is switched on, electron-hole recombination leads to the formation of two types of excited states with singlet and triplet spin multiplicity. Materials containing 5d elements emit light from the triplet state due to strong spin-orbit coupling. If the triplet and singlet energies are similar, materials based on 3d elements, in particular Cu, can emit light from the singlet state with high quantum efficiency due to temperature-activated delayed fluorescence. This effect is used in the Cu-based OLED material CuPCP (Figure 59b) that has been explored at beamline ID09.

Non-radiative processes, in which energy is released as heat, always compete with light emission. Large changes of the excited-state structure in comparison to the initial ground state typically increase the probability of non- radiative transitions. Photoexcitation changes the charge localised at the Cu atom. Copper prefers different coordinations depending on the oxidation states. To minimise structural rearrangements in the excited state, the Cu atoms have to be constrained with ligands (see Figure 59b).

The structural changes occurring in the triplet excited state of this Cu-based OLED compound were investigated at ID09 using pump-probe X-ray scattering (Figure 59c). The same excited states that are formed during electroluminescence can be generated by laser excitation. The advantage of such an approach is precise control of the time between excitation and the actual observation of the system by means of a 100-ps duration X-ray pulse. In Figure 59c, data are presented for 100 ps, 1 ns, and 2 µs. Structural changes of CuPCP induce multiple oscillations in the pump-probe X-ray scattering signal (red line of Figure 59c) and the shape of these oscillations is sensitive to the

Fig. 59: a) Scheme of electronic transitions leading to light emission by OLED materials. b) Structure of CuPCP. c) Pump-probe X-ray scattering signal for CuPCP. Black lines are experimental data corresponding to different delays after photoexcitation, red line corresponds to the contribution of structural changes of CuPCP to the scattering signal,

blue line additionally takes into account the response of the solvent.