Photocurrents direct transition

Efficiency of the photocurrent Tijg of a CU/CU2O electrode in alkaline solution with a redox system within the electrolyte (AsOf"/AsO and [Co(NH3)5Clp+) as a function of the energy hv of the incident light beam with direct and indirect band gap, (a) (n, hvf - hv plot for the determination of band gap of direct transition, (b) (ti, fev) /2 ijy pjQt for the determination of band gap of indirect transition. (From Collisi, U. and Strehblow, H.-H., /. Electroanal. Chem., 219,213,1986.)... 

The direct proof that H is present in certain centers in Ge came from the substitution of D for H, resulting in an isotopic energy shift in the optical transition lines. The main technique for unraveling the nature of these defects, which are so few in number, is high-resolution photothermal ionization spectroscopy, where IR photons from an FTIR spectrometer excite carriers from the ls-like ground state to bound excited states. Phonons are used to complete the transitions from the excited states to the nearest band edge. The transitions are then detected as a photocurrent. 

In the case of a semiconductor electrode, the existence of the energy gap makes a qualitatively different location of energy levels quite probable (Figs. 23b, 23c). One of them, either the ground or excited, is just in front of the energy gap, so that the direct electron transition with this level involved appears to be impossible. This gives rise to an irreversible photoelectro-chemical reaction and, as a consequence, to photocurrent iph. The photoexcited particle injects an electron into the semiconductor conduction band... 

For positive lit electrodes one can register the drift of holes, and for negative ones- the drift of the electrons. The photosensitizer (for example Se) may be used for carrier photoinjection in the polymer materials if the polymer has poor photosensitivity itself. The analysis of the electrical pulse shape permits direct measurement of the effective drift mobility and photogeneration efficiency. The transit time is defined when the carriers reach the opposite electrode and the photocurrent becomes zero. The condition RC < tlr and tr > t,r should be obeyed for correct transit time measurement. Here R - the load resistance, Tr -dielectric relaxation time. Usually ttras 0, 1-100 ms, RC < 0.1 ms and rr > 1 s. Effective drift mobility may be calculated from Eq. (4). The quantum yield (photogenerated charge carriers per absorbed photon) may be obtained from the photocurrent pulse shape analysis. 

In I/E curves the onset of photocurrent is expected from classical theories to occur near the Hatband potential as measured in the dark (Efb (d)), i.e. where the majority carrier current starts too. However, a large shift of the onset potential is seen especially if no additional redox couple is present in the aqueous electrolyte, in cathodic direction for p-, in anodic direction for n-type materials (Fig. 1). This shift depends on the light intensity but saturates already at relatively low intensities (Memming, 1987). If minority carrier acceptors (oxidants for p- and reductants for n-type semiconductors) are added to the solution, the onset can be shifted back to Efb (d) if they have the appropiate redox potential. In principal two types of redox couples can be found those which lead to a shift of the photocurrent onset potential and those which don t. The transition between the two classes occurs at a specific redox potential. 

In the case of porphyrin systems, there is an agreement that electron injection occurs from the lowest singlet excited state (438, 439). In the tetraruthenated porphyrins, the mechanisms involved in ET are more complex, since both components are responsible for photocurrent generation. As already discussed, from the HOMO and LUMO compositions, the peripheral ruthenium complexes can effectively transfer electronic charge to the porphyrin center via Ru( Jtt) porphyrin MLCT transitions. In addition, the direct interaction between the ZnTPyP core and Ti02 plays an important role in the photoresponse efficiency. 

Let us calculate the correction to the proton polaron direct current density conditioned by light-induced transitions between the sites. This photocurrent calculation is analogous to that of the correction to the drift activation current carried out in the previous subsection [the analogy lies in the fact that operator (276) is similar to the correction to the Hamiltonian if the electric field is taken into account, with the correction being nondiagonal on the operator of the coordinate see expression (236)]. 

The Efb is a property of the semiconductor interface that depends on the electrolyte in which the measurement is made. The onset of photocurrent does not necessarily define the potential because other interfacial effects may delay the onset to a point beyond the transition from accumulation to depletion. The error from such interfacial effects could be on the order of a few millivolts to over a volt. One such interfacial effect might be the kinetic overpotential required to drive the reaction. This overpotential shifts the photocurrent onset in the cathodic direction for p-type samples and in the anodic direction for n-type samples. Therefore, catalysts are often deposited onto the electrode surface to minimize the overpotentials (see section Catalyst Surface Treatments ). However, the modification of electrode surfaces with catalysts may influence the semiconductor/electrolyte junction and surface states and additionally shift the Efb in unexpected ways. Ideally, the catalyst treatment will improve the accuracy of the measured by this technique, although effects such as Fermi level pinning may introduce a change in the band structure at the surface which may negate the improvement from a reduced kinetic overpotential. 

The better linearity of these plots is used to distinguish direct and indirect transitions and to determine the band gap energy from photocurrent measurements. 

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