Excitonic materials

The size of the exciton is approximately 50 A in a material like silicon, whereas for an insulator the size would be much smaller for example, using our numbers above for silicon dioxide, one would obtain a radius of only 3 A or less. For excitons of this size, it becomes problematic to incorporate a static dielectric constant based on macroscopic crystalline values. 

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... 

PM spectra and their decays in DOO-PPV films and dilute solutions, we conclude that the primary excitations in DOO-PPV films are also singlet excitons [26]. The long excitonic lifetime and a corresponding high PL quantum efficiency [27] indicates that DOO-PPV is a high quality polymer material, which is very suitable for electrooptics and laser action applications [28],... 

In the previous section we have shown that for an excitation density above the PL line-narrowing threshold, the shape of the SE spectrum remain unchanged. This demonstrates that the spontaneous emission spike is not due to a new transition existing in the phoioexcited material, as would be the case for excitonic mol-... 

In low-dimensional systems, such as quantum-confined. semiconductors and conjugated polymers, the first step of optical absorption is the creation of bound electron-hole pairs, known as excitons [34). Charge photogcncration (CPG) occurs when excitons break into positive and negative carriers. This process is of essential importance both for the understanding of the fundamental physics of these materials and for applications in photovoltaic devices and photodctcctors. Since exciton dissociation can be affected by an external electric field, field-induced spectroscopy is a powerful tool for studying CPG. 

It has been demonstrated that the whole photoexcitation dynamics in m-LPPP can be described considering the role of ASE in the population depletion process [33], Due to the collective stimulated emission associated with the propagation of spontaneous PL through the excited material, the exciton population decays faster than the natural lifetime, while the electronic structure of the photoexcited material remains unchanged. Based on the observation that time-integrated PL indicates the presence of ASE while SE decay corresponds to population dynamics, a numerical simulation was used to obtain a correlation of SE and PL at different excitation densities and to support the ASE model [33]. The excited state population N(R.i) at position R and time / within the photoexcited material is worked out based on the following equation ... 

In electroluminescence devices (LEDs) ionized traps form space charges, which govern the charge carrier injection from metal electrodes into the active material [21]. The same states that trap charge carriers may also act as a recombination center for the non-radiative decay of excitons. Therefore, the luminescence efficiency as well as charge earner transport in LEDs are influenced by traps. Both factors determine the quantum efficiency of LEDs. 

Trilayer structures offer the additional possibility of selecting the emissive material, independent of its transport properties. In the case of small molecules, the emitter is typically added as a dopant in either the HTL or the ETL, near the interface between them, and preferably on the side where recombination occurs (see Fig. 13-1 c). The dopant is selected to have an cxciton energy less than that of its host, and a high luminescent yield. Its concentration is optimized to ensure exciton capture, while minimizing concentration quenching. As before, the details of recombination and emission depend on the energetics of all the materials. The dopant may act as an electron or hole trap, or both, in its host. Titus, for example, an electron trap in the ETL will capture and hold an election until a hole is injected nearby from the HTL. In this case, the dopant is the recombination mmo.-... 

The use of interpenetrating donor-acceptor heterojunctions, such as PPVs/C60 composites, polymer/CdS composites, and interpenetrating polymer networks, substantially improves photoconductivity, and thus the quantum efficiency, of polymer-based photo-voltaics. In these devices, an exciton is photogenerated in the active material, diffuses toward the donor-acceptor interface, and dissociates via charge transfer across the interface. The internal electric field set up by the difference between the electrode energy levels, along with the donor-acceptor morphology, controls the quantum efficiency of the PV cell (Fig. 51). 

---