Capturing the excited state of Cu-based OLED materials

08-09-2020

Bright, efficient, and inexpensive organic light-emitting diodes (OLEDs) are needed to reduce energy consumption. Cu-based materials with delayed fluorescence can fulfil 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.

  • Share

OLED materials appear in our everyday life in applications such as the screens of smartphones. The ability to produce large area flexible screens, which one can roll out on the wall, or simpler devices for large area lighting, is very attractive.  Nevertheless, such devices produced using existing technologies will be either very expensive or not very bright. This is due to the use of expensive 5d elements, in particular Ir or Pt, to increase the efficiency of light emission. Limited availability of these 5d metals restricts large-scale production of large-area OLED devices. 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 1a. When one switches-on the electricity, electron-hole recombination leads to the formation of two types of excited states with singlet and triplet spin multiplicity. The triplet states forms with a three-times higher probability than the singlet state. Which of these states can be used for light emission depends on the spin-orbit coupling and this is the reason why 5d elements are often used. In the classical fully organic materials, the spin-orbit coupling is small and only the singlet state is emissive. Therefore, the maximal theoretically-possible efficiency of such devices is limited to 25%. If the spin-orbit coupling is high as in the case of materials that contain 5d metals, both singlet and triplet states are emissive. Moreover, the transition from singlet to triplet state (intersystem crossing) is probable. The triplet state has lower energy and therefore most of the light is emitted from this state. The quantum efficiency of such a light emission process can be up to 100%. What would happen if one were to add a 3d element, for example, Cu instead of a 5d element in the OLED? The spin-orbit coupling will not be strong enough to enable efficient light emission from the triplet, but the transition between states will be possible. If the triplet and singlet energies are similar, the temperature-activated transition from the triplet state to the light-emitting singlet excited state is possible and such material can emit light with high quantum efficiency. This effect is called temperature-activated delayed fluorescence and it is used in the Cu-based OLED material CuPCP (Figure 1b) that has been explored in an experiment at beamline ID09.

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.

Figure 1. 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.

Non-radiative processes, in which energy is released as heat, always compete with light emission. The probability of non-radiative transitions strongly depends on structural changes in the material. 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 a different type of coordination in different oxidation states: tetrahedral for Cu(I) and square planar for Cu(II). Therefore, to minimise structural rearrangements in the excited state, the Cu atoms have to be constrained with ligands (see Figure 1b).

The structural changes occurring in the triplet excited state of this Cu-based OLED compound has been investigated at ID09 using pump-probe X-ray scattering (Figure 1c). The same excited states that are formed during electroluminescence can be generated by exciting the system using a short laser pulse. The advantage of such an approach is the possibility to control precisely the time between the system excitation and the actual observation of the system by means of a 100 ps duration X-ray pulse.  In Figure 1c, data is 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 1c) and the shape of these oscillations is sensitive to the details of the occurring structural changes. For a given model of structural rearrangements, these signals can be calculated theoretically and then compared with the experimental ones, also taking into account other contributions (namely the associated solvent response) to the experimental signal. This allows the structural rearrangements of the material under investigation to be modelled, thus revealing how Cu atoms and electron-rich ligands move relative to each other. 

Complementary tools to X-ray scattering were also used including pump-probe X-ray absorption spectroscopy at Cu K-edge, measured at the SuperXAS beamline of the SLS synchrotron, and pump-probe X-ray emission spectroscopy at P Kα line, measured at Alvra beamline of SwissFEL (Swiss X-ray free electron laser) at PSI. This has allowed how charge redistributes between Cu and P atoms in the triplet state to be probed. The data from both synchrotron and XFEL based time-resolved X-ray techniques indicate that the structural rearrangements that accompany the transition from the ground to the triplet excited state are rather small: Cu atoms do not have a possibility to significantly rearrange their coordination environment. That is the property required to minimise non-radiative losses and it is the reason why CuPCP is a good candidate for applications in OLEDs. Moreover, the experimental information that was obtained using three pump-probe X-ray techniques has allowed theoretical methods based on density functional theory (DFT) to be validated and to compare the different approaches used for the prediction of the properties of the excited state. This is relevant to predict which new OLED materials will be efficient thus speeding up the discovery of new materials for cheap and efficient OLEDs.

 

Principal publication and authors
Taking a snapshot of the triplet excited state of an OLED organometallic luminophore using X-rays, G. Smolentsev (a), C.J. Milne (a), A. Guda (b), K. Haldrup (c), J. Szlachetko (d), N. Azzaroli (a), C. Cirelli (a), G. Knopp (a), R. Bohinc (a), S. Menzi (a), G. Pamfilidis (a), D. Gashi (a), M. Beck (a), A. Mozzanica (a), D. James (a), C. Bacellar (a,e), G.F. Mancini (a,e), A. Tereshchenko (b), V. Shapovalov (b), W.M. Kwiatek (d), J. Czapla-Masztafiak (d), A. Cannizzo (f), M. Gazzetto  (f), M. Sander (g), M. Levantino (g), V. Kabanova (g), E. Rychagova (h), S. Ketkov (h), M. Olaru (i), J. Beckmann  (i) and M. Vogt (i), Nature Communications 11, 2131 (2020); doi: 10.1038/s41467-020-15998-z.
(a) Paul Scherrer Institute, Villigen (Switzerland)
(b) Southern Federal University, Rostov-on-Don (Russia)
(c) Technical University of Denmark, Kongens Lyngby (Denmark)
(d) Institute of Nuclear Physics, Polish Academy of Sciences, Kraków (Poland)
(e) École Polytechnique Fédérale de Lausanne (Switzerland)
(f) University of Bern (Switzerland)
(g) ESRF
(h) G.A. Razuvaev Institute of Organometallic Chemistry, Russian Academy of Sciences, Nizhny Novgorod (Russia)
(i)  University of Bremen (Germany)