pure emitter type II
Figure 6: Escape rate ke as a function of the external electric field. Data points were obtained from equation (1) in the text. Stars indicate data for the pure emitter and dots for the type II structure.
Conclusions
In conclusion, we have investigated the electric field induced quenching in the PL intensity of the deep-blue emitter Ir(cnbic)3 in combination with several host materials building type I and type II molecular heterostructures. We report a strong field dependent quenching of the phosphorescence for thin films of the pure emitter and the type II heterostructure.
These results highlight the importance of energy level alignment in the choice of suitable matrix materials for hosting deep-blue phosphors, in addition to the well-known guideline of a higher triplet level. By time resolving the PL in the presence of the field we provide useful insights in the quenching mechanism. In particular, we demonstrated that dissociation occurs preferentially for Coulomb stabilized excitons, when the external field overcomes the binding energy and accepting states are available for charge transfer.
Additionally, we report a long lasting formation of polaron pairs, if one of the two charge carriers forming the exciton remains trapped on the emitter. The phosphorescence
quenching has immediate consequences for the optimization of deep-blue emitting OLEDs.
While several aspects contribute to the efficiency of electroluminescence, such as charge injection, carrier balance, triplet-polaron quenching[20, 22] and exciton annihilation,[11, 20]
electric field induced quenching becomes critical when large local field are building up at interfaces[23] and for particular HOMO-LUMO alignment between emitter and host, as demonstrated in this study. For deep-blue emitters the design of interfaces becomes critical as a consequence of high band-gap materials. While triplet-polaron quenching has been recognized as less critical in explaining the roll-off of the electroluminescence efficiency with respect to TTA, this might not hold true for deep-blue emitters where charge accumulation at organic heterojunctions or high driving voltages could results in high field We conclude that type I structures can reduce substantially the quenching for fields up to 2 MV/cm. On the contrary an effect can be observed at relatively low driving voltages (6.25V = 0.5 MV/cm) in the case of type II configurations. These experiments should provide valuable insights for the design of blue electrophosphorescent OLEDs and more generally other organic electroluminescent devices, where large driving electric fields are involved and high electroluminescence efficiencies are critical.[39, 40]
Experimental
Details about synthesis and purification of the materials can be found in the supporting information. The HOMO and LUMO of the materials used were determined via density functional calculations. For the ionization potential (HOMO) and the electron affinity (LUMO) first the geometry of the neutral as well as the charged states were optimized using the BP86-functional[41, 42], in combination with a split-valence basis set (SV(P)) including polarization functions on all heavy atoms.[43] For iridium an effective core potential was employed.[44] For the energetics single point calculations at the optimized geometries using the same functional in combination with a valence triple zeta basis set (TZVP) were conducted. To account for dielectric solid state effects a ultraviolet photoelectron spectroscopy/inverse electron photoemission spectroscopy UPS/IEPS-calibrated version of the conductor like screening model (COSMO)[45] was used in conjunction with the single point calculations. All calculations were carried out with the
turbomole program package.[46] These data were compared with cyclic voltammetry curves and UPS showing a reasonable agreement, with a variance smaller than 0.3 eV.
Sample preparation was carried on in a nitrogen glove-box equipped with a vacuum chamber for the evaporation of the SiOx and Al layers. As mentioned above SiOx blocking layers were used to minimize charge carrier injection upon the application of voltage and to avoid quenching by the surface plasmons at the metallic interfaces. Similar results in the PL intensity modulation were obtained using 10 nm of LiF instead of SiOx (not shown) clarifying the intrinsic nature of the effects in contrast to interfacial phenomena.[47] For the solubilization of the materials before spin-coating only spectroscopic grade solvents were used. The average thickness of the organic films (100 ± 20 nm) was determined by a scanning profilometer. PL excitation was performed at 337 nm with a nitrogen pulsed laser (500 ps, 50 Hz rep. rate) exciting both the emitter and the matrix molecules. Light excitation and detection was performed on the ITO side of the structures. Such an excitation wavelength, well above the emitter first excited state, mimics the electroluminescence processes taking place in real devices, where hot excitons are formed on the emitter, after electron and hole capture from the host material. All the measurements were performed at low excitation power and in absence of bi-excitonic quenching processes, as proved by the monoexponential decays of phosphorescence (see inset of Fig. 3(a)). Therefore, all the experiments were performed in the absence of triplet-triplet annihilation and exciton-polaron quenching, in order to provide a genuine estimation of electric field induced effects.
For the all the PL measurements samples were kept under a dynamic vacuum of 10-6 mbar at room temperature, only the spectrum in the lower panel of Fig. 1(a) was measured at T = 5K. PL spectra were recorded using an intensified CCD camera or a single sweep streak camera for the time resolved decays. PL quantum yield was measured with an integrating sphere saturated with nitrogen at room temperature. The application of an electric field was accomplished by Keithley source or a pulse generator in the case of the data reported in Fig.5.
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