Electric fields photoexcitation

The chapter is organized as follows in Section 8.2 a brief overview of ultrafast optical dynamics in polymers is given in Section 8.3 we present m-LPPP and give a summary of optical properties in Section 8.4 the laser source and the measuring techniques are described in Section 8.5 we discuss the fundamental photoexcitations of m-LPPP Section 8.6 is dedicated to radiative recombination under several excitation conditions and describes in some detail amplified spontaneous emission (ASE) Section 8.7 discusses the charge generation process and the photoexcitation dynamics in the presence of an external electric field conclusions are reported in the last section. 

From the framework depicted, it emerges that photocatalytic activity seems strictly related to the dipole moment generated by a distorted crystal structure, namely electron-hole separation upon photoexcitation is promoted by a local electric field due to a dipole moment and, in turn, this promotes vectorial movement of electron and holes. 

As shown in Fig. 10-9, the photoexcited reaction current occurs only when an appreciable electric field exists in the space chai ge layer. No photocurrent occurs at the flat band potential because no electric field that is required to separate the photoexcited electron-hole pairs is present. The photocurrent occurs at any potentials different from the flat band potential hence, the flat band potential may be regarded as the potential for the onset of the photocurrent. It follows, then, that photoexcited electrode reactions may occur at potentials at which the same electrode reactions are thermodynamically impossible in the dark. 

Figure 3.38. Principle of the photorefractive effect By photoexcitation, charges are generated that have different mobilities, (a) The holographic irradiation intensity proHle. Due to the different diffusion and migration velocity of negative and positive charge carriers, a space-charge modulation is formed, (b) The charge density proHle. The space-charge modulation creates an electric Held that is phase shifted by 7t/2. (c) The electric field profile. The refractive index modulation follows the electric field by electrooptic response, (d) The refractive index profile.

The study of the dispersion of photoinjected charge-carrier packets in conventional TOP measurements can provide important information about the electronic and ionic charge transport mechanism in disordered semiconductors [5]. In several materials—among which polysilicon, a-Si H, and amorphous Se films are typical examples—it has been observed that following photoexcitation, the TOP photocurrent reaches the plateau region, within which the photocurrent is constant, and then exhibits considerable spread around the transit time. Because the photocurrent remains constant at times shorter than the transit time and, further, because the drift mobility determined from tt does not depend on the applied electric field, the sample thickness carrier thermalization effects cannot be responsible for the transit time dispersion observed in these experiments. 

Such a photoinduced charge separation can proceed effectively provided an electric field (potential gradient) has been established at the position where the primary photoexcitation takes place. In general, a potential gradient can be produced at the interface between two different substances (or different phases). For example, a very thin (ca. 50 A) lipid membrane separating two aqueous solutions inside the chloroplasts of green plants is believed to play the essential role in the process of photosynthesis, which is the cheapest and perhaps the most successful solar conversion system available. 

The generation of photoexcited species at a particular position in the film structure has been shown in (6.19) and (6.20) to be proportional to the product of the modulus squared of the electric field, the refractive index, and the absorption coefficient. The optical electric field is strongly influenced by the mirror electrode. In order to illustrate the difference between single (ITO/polymer/Al) and bilayer (ITO/polymer/Ceo/Al) devices, hypothetical distributions of the optical field inside the device are indicated by the gray dashed line in Fig. 6.1. Simulation of a bilayer diode (Fig. 6.1b) clearly demonstrates that geometries may now be chosen to optimize the device, by moving the dissociation region from the node at the metal contact to the heterojunction. Since the exciton dissociation in bilayer devices occurs near the interface of the photoactive materials with distinct electroaffinity values, the boundary condition imposed by the mirror electrode can be used to maximize the optical electric field E 2 at this interface [17]. 

Undoped polymers, which remain in the semiconducting state. In this case charge injection is operated by photoexcitation (simultaneous creation of an electron in the conduction band and a hole in the valence band), or by action of an electric field at a junction. 

T. Ito, 1. Yamazaki, N. Ohta, External Electric Field Effect on Interlayer Vectorial Electron Transfer from Photoexcited Oxacarbocyanine to Viologen in Langmuir-Blodgett Films , Chem. Phys. Lett., 277, 125 (1997)... 

More recently, the parent system has been investigated under radio frequency plasma conditions. The plasma generated by an electric discharge provides an unusual medium for reactions in the gas phase. The free electrons that are present in the plasma are responsible for the chemical reactions that take place. The electrons are accelerated by an applied electric field and collide with the molecule, thereby activating it for the reaction. Most procedures employ radio frequency (RF) discharges so as to keep the pressure, temperature and energy of the system relatively low. The substrate is usually volatilized into the plasma zone with residence times of the order of a second. Because there is no need of chromo-phores, the technique has been used to activate those cases that are inert to the normal photoexcitation in solution, e.g. the parent 1,4-pentadiene. ... 

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