Exciton confinement

The picture presented above for confinement of the excitons within the device is for the EM layer sandwiched between the HTL and ETL. The EM need not be a discrete layer in the OLED, however, for exciton confinement to occur. Alternatively, the EM can consist of a luminescent molecule doped (- 1%) into a polymeric or molecular host material (40,41,54,55). So long as the energy gap (or band gap) of the host is higher than that of the EM dopant, excitons will be effectively trapped or confined on the dopant molecules leading to improved EL efficiency. An example of such a dopant-based device... 

The above approaches used the idea of conjugation length control in PTs by distorting the polymer backbone with bulky substituents as side groups. Hadziioannou and coworkers [509,510] demonstrated PL and EL tuning via exciton confinement with block copolymers... 

G.G. Malliaras, J.K. Herrema, J. Wildeman, R.H. Wieringa, R.E. Gill, S.S. Lampoura, and G. Hadziioannou, Tuning of the photo- and electroluminescence in multi-block copolymers of poly[(silanylene)-thiophene]s via exciton confinement, Adv. Mater., 5 721-723, 1993. 

A. Charas, J. Morgado, J.M.G. Martinho, A. Fedorov, L. Alcacer, and F. Cacialli, Excitation energy transfer and spatial exciton confinement in polyfluorene blends for application in light-emitting diodes, J. Mater. Chem., 12 3523-3527, 2002. 

F. Chen, G. He, Y. Yang, Triplet exciton confinement in phosphorescent polymer light-emitting diodes, Appl. Phys. Lett., 82 1006-1008 (2003). 

FC Chen, SC Chang, G He, S Pyo, Y Yang, M Kurotaki, and J Kido, Energy transfer and triplet exciton confinement in polymeric electrophosphorescence devices, J. Polym. Sci. B Polym. Phys., 41 2681-2690, 2003. 

In other white light devices, blue, green and red emitters are combined. Kido et al. [169, 170] designed multilayer systems using 6 (TPD) for blue, metal-chelate complexes for green and red emission, respectively. Similar devices have been developed by other groups, using Forster transfer or exciton confinement for the creation of the three primary colors [171, 172]. Exciplex emission was... 

In this section we analyze the surface investigation of molecular crystals by the technique of UV spectroscopy, in the linear-response limit of Section I, which allows a selective and sharp definition of the surface excited states as 2D excitons confined in the first monolayer of intrinsic surfaces (surface and subsurfaces) of a molecular crystal of layered structure. The (001) face of the anthracene crystal is the typical sample investigated in this chapter. 

For a coherent interpretation of the reported experimental data, we need a model of surface excitons, the structures I, II, and III being attributed to excitons confined, respectively, in the first, the second, and the third surface monolayer (see Fig. 3.5). The rapid decay of the van der Waals forces along the c axis explains the very fast transition, in a few molecular layers, from surface to bulk spectroscopy (the other two faces of the anthracene crystal do not show surface-confined excitons). 

As you may recall from Chapter 4, when an electron is promoted from the valence to conduction bands, an electron-hole pair known as an exciton is created in the bulk lattice. The physical separation between the electron and hole is referred to as the exciton Bohr radius (re) that varies depending on the semiconductor composition. In a bulk semiconductor crystal, re is significantly smaller than the overall size of the crystal hence, the exciton is free to migrate throughout the lattice. However, in a quantum dot, re is of the same order of magnitude as the diameter (D) of the nanocrystal, giving rise to quantum confinement of the exciton. Empirically, this translates to the strongest exciton confinement when D < 2r. ... 

Here, we mention the structure design of ETL. We have to form double ETLs (ETL-E/ETL-C), which are very similar to the double HTLs, for low electron injection from a cathode and the confinement of molecular excitons and hole carriers (Fig. 2.8b. For the efficient electron injection from a cathode into the ETL, the electron affinity (Ea) of ETL should be close to the level of the cathode s workfunction. The large Ea, however, mostly results in a small bandgap and insufficient exciton confinement. A typical example of the efficient combination of double ETLs is the OXD/Alq, in which OXD works well to confine excitons and the hole effect while the Alq layer injects and transports electrons from a cathode to an OXD layer. 

Rothe C, Brunner K, Bach I, Heun S, Monkman AP (2005) Effects of triplet exciton confinement induced by reduced conjugation length in polyspirobifluorene copolymers. J Chem Phys 122(8) 084706... 

A few reports in the last two decades have described exciton confinement in small TiOi particles. Anpo and co-workers (1987) prepared particles of titania with sizes (diameters) ranging from 55 A to 2000 A for rutile, and from 38 A to 530 A for the anatase form of TiOi, and noted that these crystallites display size quantisation in optical properties and in the photocatalytic hydrogenation of methylacetylene, even for such large sizes. For a 120 A rutile particle the bandgap increased by 67 meV relative to the bulk rutile bandgap Eg 3.03 eV) for anatase Eg increased by 156 meV (from the bulk Eg of -3.18 eV). Moreover, quantum yields of photocatalytic activity of TiOi appeared to increase with the magnitude of the blue shift of the effective bandgap. Kormann et at. (1988) indicated that small particles of TiOi, prepared by the arrested hydrolysis of either TiCU or Ti(i-PrO)4, showed size... 

Cascade hole-injection and effective electron-blocking/exciton confinement for light-emitting diodes (LEDs) have been reported using the blue phosphorescent emitter iridium bis(4/,6/-difluorophe-nylpyridinato) (pzT p).197... 

The experimental results are explained by a strong coupling between excitons confined in Si nanocrystals and Er3+ ions in surrounding S1O2. 

For nc-Si/SiO2 structures of type 1 the PL band maximum shifts from 1.3 to 1.7 eV when d decreases from 4.5 to 1.5 nm the intrinsic PL of nc-Si is commonly explained by the radiative recombination of excitons confined in nc-Si, while the size dependent spectral shift is attributed to the quantum confinement effect [21]. A considerable width of the PL band can be explained by nc-Si size distribution [21] as well as by phonon-assisted electron-hole recombination [22]. The external quantum yield of the exciton PL was found to reach -1 % for the samples with d = 3 - 4 nm at room temperature [18]. The lower quantum yield of the nc-Si/SiO2 structure in comparison with that observed for single Si quantum dots [22] and for III-V and II-VI compounds [22] can be explained by lower probability of the optical transitions, which are still indirect in nc-Si [21], as well as by the exciton energy migration in the assembly of closely packed nc-Si [18]. 

Goushi, K., Kwong, R., Brown, J. J. et al. 2004. Triplet exciton confinement and unconfinement by adjacent hole-transport layers. /. Aypl. Phys. 95 7798. 

Intensity of photo-luminescence, PL, of the RO-PPV film increased with the alkoxy chain length, which is attributable to the decrease of intermolecular interaction and exciton-confinement in the film. 

After fundamental stupes on soluble RO-PPV derivatives(9), we have focused on the synthesis of highly luminous polymers and fabrication of highly efficient devices to commercialize P-LED. Taking exciton confinement and charge transporting into account, we have copolymerized various kinds of arylene vinylene units to introduce structural and energetic irregularity into poly(arylene vinylene). 

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