Electronic band gap

Another large band-gap electron transport host is 3-phenyl-4-(l -naphthyl)-5-phenyl-1,2,4-triazole (TAZ), which has a HOMO (-6.6 eV) and LUMO (-2.6 eV). Using TAZ1 (109) as the host, a maximum EQE (ext) of 15.5% and a luminous power efficiency of 40 lm/W can be achieved in a phosphorescent OLED the value of phosphorescent decay lifetime of 7% Ir(ppy)3 in the TAZ (t-650 ns) is longer than that in CBP (t-380 ns) and the phosphorescence efficiency is approximately proportional to the excited state lifetime [174]. 

Conductors (e.g., metals) have partially filled conduction bands or overlapping conduction and valence bands. Because of the easily accessible energy levels (i.e., no band gap), electrons can readily move to higher energy levels and conduct current. 

Apart from the wider band gaps, electrons and holes in ionic solids have mobilities several orders lower than those in the covalent semiconductors. This is due to the variation in potential that a carrier experiences in an ionic lattice. 

It indicates that the absorption coefficient for an indirect transition to energy E from an initial energy (see Eq. (1)) is proportional to the product of the initial density of states and the final density of states. When S3 absorbs a light having photon energy over the band gap, electrons in N 2p in the valence band are excited to Hf 5d related to Hf-N bonds in the conduction band. It is speculated that the increase of the absorption coefficient is low because the pDOS of Hf 5d and N 2p related to Hf-N bonds is small. When a photon with energy over 3.8 eV is absorbed, it is expected that the absorption coefficient increases abruptly because the pDOS of Hf 5d and N 2p related to Hf-N bonds is large. 

Since the carbon dioxide reduction is a complex process, the efficiency depends upon a number of factors such as electron migration through band gap, electron transfer reactions, adsorption of substrate on the semiconductor surface, surface reactions, nature of solvent medium, illumination source, reactor geometry and a number of other... 

Photodoping When transfor example, is exposed to radiation of energy greater than its band gap, electrons are promoted across the gap and the polymer undergoes photodoping. Under appropriate experimental conditions, spectroscopic signatures characteristic of, for example, solitons can be observed [22]. 

The Fermi level in the electrolyte has been left undefined since it depends on the initial relative concentrations of H2 and O2 in solution. Figure 3b shows the situation at equilibrium in the dark once the semiconductor and the metal are brought into contact with the electrolyte and a depletion layer is formed near the semiconductor surface. Fermi levels of the three phases equilibrate, giving rise to a band bending in the semiconductor. When the semiconductor is irradiated with photons of energy corresponding to the band gap, electron-hole pairs are created and the Fermi level in the semiconductor is raised towards the flat band potential Vfb by an amount Fph which is the photopotential generated. The maximum value the... 

The results indicate that donor units of carbazole influence the band gaps, electronic energy levels, and photovoltaic levels. The hole mobility was measured at 3.07 x 10 " cm A s. 

The excitation of light from materials stroked by electrons, known from fluorescent screens of TV tubes for example, is called cathodolumines-cence (CL). In semiconductors with a direct band gap, electrons that are excited from the valence to the conduction band can recombine with the holes by emission of radiation, whereas semiconductors with an indirect band gap have a reduced probability of radiative recombination. In semiconductors and most inorganic materials, CL depends strongly on the concentration of dopants, which can either enhance CL by forming luminescence centers for radiative transitions or quench CL by forming centers of nonradiative transitions. 

The first step is absorption of photons to form electron-hole pairs. Semiconductors have the band structure in which the conduction band is separated from the valence band by a band gap with a suitable width. When the energy of incident light is larger than that of a band gap, electrons and holes are generated in the conduction and valence bands, respectively. 

Fig. 3 Schematic of a photocatalytic process. When illuminated with light of energy higher than the band gap, electron-hole pairs are created in a semiconductor, thus allowing chemical reactions on its surface...

Non-ideal semiconductors. For non-ideal semiconductors (amorphous, partially crystalline, or other defects that lead to localized states in the band gap) illumination can lead to many different types of electron transitions as illustrated in Fig. 5 from the valence band to the conduction band (ideal case), and transfer involving localized states. I.e., electron transition can take place at photon energies smaller than the band gap. Electron transition leading to trapped electrons in the localized states can only contribute to the photocurrent if they can reach the conduction band, or alternatively the underlying metal or the electrolyte. For this, trapped electrons have different possibilities ... 

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