Excitons devices

As discussed in Sect. 7.3.1, fundamental thermodynamics limits conversion efficiency for both single- and multijunction semiconductor PEC devices have also been established in numerous studies. The thermodynamic tandem limit under ideal conditions, i.e., including thermalization loss, but with no overpotential loss, has been calculated at approximately 40% STH. This is consistent with the optical limits shown in Fig. 7.23. It is also substantially higher than the 30% STH limit established for the single-junction case. Moreover, employing advanced multi-exciton device schemes which eliminate part of the thermalization loss, the theoretical tandem limit is further increased to 46% STH [58]. 

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... 

However, not all excitons have sufficiently long lifetimes to reach the interface before recombining. To circumvent this problem and increase device efficiency, heterostmcture devices have been fabricated. In these devices, donors and acceptors are mixed together to create a network that provides many internal interfaces where charge separation can occur. Heterostmcture devices made from the donor polymer... 

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. 

The recombination rate Ptlx, which is defined as the number of generated exci-tons in A/ divided by the number of generated excitons in At plus the number of minority carriers passing through the device in At, for an unbalanced LED (where j(x) jp(x) 3> j (x) and pp pH) is given by 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. 

The efficient formation of singlet excitons from the positive and negative charge carriers, which are injected via the metallic contacts and transported as positive and negative polarons (P+ and P ) in the layer, and the efficient radiative recombination of these singlet excitons formed are crucial processes for the function of efficient electroluminescence devices. 

The processes of charge injection, transport, and recombination dictate the recombination efficiency h(/), which is the fraction of injected electrons that recombine to give an exciton. The recombination efficiency, which is a function of the device current, plays a primaty role in determining the amount of emitted light, therefore determining the OLED figurcs-of-meril. For example, the quantum efficiency /y(/) (fraction of injected electrons that results in the emission of a photon from the device) is, to a first approximation, given by ... 

An approach for improving the response of conjugated polymcr/fullerene bilayer devices, which is based on an additional excitonic middle layer inserted into the D-A interface, was suggested by Yoshino el al. [94. In the middle layer light absorption produces electron-hole pairs, which migrate towards the interlace and... 

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). 

PbS has attracted much attention due to its special direct band gap energy (0.4 eV) and a relatively large exciton Bohr radius (18 nm) and their nanoclusters have potential applications in electroluminescent devices such as light-emitting diodes. PbS nanocrystals with rod like structures with diameters of 20-60 nm and lengths of 1-2 pm have been obtained using the sonochemical method and by using PEG-6000 [66]. Addition of PEG and the time of sonication have been found to play a key role in the formation of these rods. 

Red electrophosphorescent devices employing exciton blocking have been demonstrated.222 These devices contained the luminescent dye Pt(OEP)], doped into a 4,4 -/V, N dicarbazolebi-... 

The QD s absorption can be used directly or it can be used to get multiple exciton generation. The latter has recently been shown in suitably chosen QDs, such as PbSe and Si.36,37 This discovery allows for the potential of a variety of devices employing both upconversion and downconversion. 

---