Excitons generation

Nozik AJ (2008) Multiple exciton generation in semiconductor quantum dots. Chem Phys Lett 457 3-11... 

Figure 17.7 Schematic illustration of the decay routes of an exciton generated in CdSe-ZnS quantum dots Reprinted with permission from reference [31] copyright [2003], American Chemical Society.

Figure 3 shows different forms of chemisorption for a C02 molecule. In the weak form of chemisorption the C02 molecule is bound to the surface by two valency bonds, as shown in Fig. 3a. This is an example of adsorption on a Mott exciton which is a pair of free valencies of opposite sign (i.e., an electron-hole pair). This may be either a free exciton wandering about the crystal or a virtual exciton generated in the very act of adsorption. As seen from Fig. 3a, in the case of the C02 molecule the weak form of chemisorption is a valency-saturated and electrically neutral form. As a result of electron capture, this form is transformed into a strong acceptor form shown in Fig. 3b, while as a result of hole capture it becomes a strong donor form shown in Fig. 3c. Both these forms are ion-radical ones. It should, however, be noted that the ion-radicals formed in these two cases are quite different and, having entered into a reaction, may cause it to proceed in different directions.

New Solar Cells Quantum Dot (QD) Structures and Multiple Exciton Generation (MEG)... 

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. 

Luque, A. Marti, A. Nozik, A. J. 2007. Quantum dots Multiple exciton generation and intermediate bands. MRS Bull. 32 236-241. 

The second type of solar cell is based on a /m-heterojunction in analogy to semiconductor devices [274]. Excitons generated by light, diffuse and dissociate at the interface between a hole and an electron-conducting material. The optimum layer thickness was calculated to be 1.5 times the exciton diffusion length [275]. 

Experiments using a two-layer heterostructure in which the photocurrent action spectra are observed both for front and rear (symbatic and antibatic) illumination of the interfaee between a photosensitizer and a hole transport layer have shown that the surface enhaneement of bound pair generation is due to a layer typieally 300-500 nm thiek [13]. Within this distance of the interface, excitons generated by the optieal absorption may diffuse toward the interface and initiate bound pair generation. The importanee of these excitons for a specific photoreceptor can be iden-... 

At present the IV-VI series of semiconducting materials comprises a number of the most promising materials for IR applications [1-4]. An interest in these materials is primarily because they are narrow band gap semiconductors and therefore have the potential to be employed in devices as optically active components in the near-infrared (NIR) and infrared (IR) spectral region and are hence beneficial to applications for solar cells, detectors, telecommunications relays, etc. The interest in the IV-VI materials has also grown in recent years because of the observation that they are thought to demonstrate efficient multiple exciton generation (MEG) [3,5-7]. This has implications for the efficiencies of solar cells and other applications based on these materials, especially as it provides a means by which the Shockley/Queisser efficiency limit may be overcome. 

The foregoing section attempted to provide an introduction to the dynamics of singlet excitons, generated either by photoexcitation or by polaron recombination, and the effects of polarons and TEs on the SE dynamics. We now turn to the basic structure and dynamics of OLEDs, which obviously reflect the basic processes described above. 

A simple example of this is the case of a molecule (modeled as an oscillating dipole) close to a perfect mirror. If the dipole is parallel to the mirror, destructive interference between directly emitted light and reflected light causes a reduction in the radiative rate. In the presence of competing nonradiative decay processes, this leads to a reduction in the efficiency of emission. The variation of radiative rate with position and orientation for a molecule within an arbitrary planar dielectric structure has been modeled by Crawford.81 This model has been applied to polymer LEDs by Burns et al.,82 and Becker et al.,83 who predict significant variations in the efficiency of radiative decay in polymer LEDs depending on the distribution of exciton generation within the device. 

Ford TA, Avilov I, Beljonne D, Greenham NC (2005) Enhanced triplet exciton generation in polyfluorene blends. Phys Rev B 71(12)... 

High conversion efficiency via multiple exciton generation in quantum dots... 

Figure 3.14 Multiple election-hole pair (exciton) generation (MEG) in quantum dots. Source Nozik (2002).

Ellingson R. J., Beard M. C., Johnson J. C., Yu P., Micic 0.1., Nozik A. J., Shabaev A. and Efros A. L. (2005), Highly efficient multiple exciton generation in colloidal PbSe and PbS quantum dots , Nano Lett. 5, 865-871. 

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