Photogenerated carriers

In the presence of an electric field the drift length is the mobility-lifetime product times the electric field A.mfp = prE [576]. With typical values of pz and E the mean free path usually exceeds by far the thickness of the solar cell, and virtually all photogenerated carriers can be collected. However, under certain operating conditions, field-free regions in the / -layer may exist, and the collection efficiency is decreased because the diffusion lengths of the carriers are much smaller than the thickness of the solar cell [11, 577]. 

For capture and conversion of solar energy, semiconductors possess several key advantages, including relatively high absorption coefficients and the possibility to separate the photogenerated carriers at homo and heterojunctions. The energy... 

The overall process performance, as measured by photon efficiency (number of incident photon per molecule reacted, like the incident photon to current conversion efficiency, or IPCE, for PV cells), depends on the chain from the light absorption to acceptor/donor reduction/oxidation, and results from the relative kinetic of the recombination processes and interfacial electron transfer [23, 28]. Essentially, control over the rate of carrier crossing the interface, relative to the rates at which carriers recombine, is fundamental in obtaining the control over the efficiency of a photocatalyst. To suppress bulk- and surface-mediated recombination processes an efficient separation mechanism of the photogenerated carrier should be active. 

Illumination is a relevant parameter in the electrochemistry of silicon because photogenerated carriers may initiate or contribute to the charge exchange at the electrolyte-silicon interface. If an electrode is illuminated, photogenerated electron-hole pairs are generated corresponding to the number of absorbed photons. This number depends on spectral distribution, total illumination intensity and losses due to optical reflection and transmission. The number of electron-hole... 

Photogenerated carriers are needed for the formation of macropores in n-type electrodes, as discussed in Chapter 9. 

The SPAD detector is similar in design to other photodiodes except the electric field of its junction is better separated than in other photodiodes. This design better endures the flow of the avalanche current triggered by a photogenerated carrier and provides better performance than normal SSPD. SPAD have detection limits several orders of magnitude lower than conventional PMTs.(35) In addition, they have been shown to have higher quantum efficiency (QE) values than PMTs in the NIR region.(57)... 

Iron oxide (Fe Oj) and tungsten oxide (WO ) films have been studied and developed as candidate semiconductor materials for the PEC junction (photoanode). High-temperature synthesis methods, as reported for some high-performance metal oxides, have been found incompatible with multijunction device fabricatioa A low-temperature reactive sputtering process has been developed instead. In the parameter space investigated so far, the optoelectronic properties of WO3 films were superior to those of Fe Oj films, which showed high recombination of photogenerated carriers (Miller et al., 2004). 

The corrosion reactions cause another serious problem. They produce a number of surface reaction iniermediates such as dangling bonds and atomic vacancies, having electronic levels within the band gap. Namely, corrosion reactions produce a number of mid-gap surface states, which act as surface recombination centers for photogenerated carriers. The Voc is decreased largely by the production of such surface states.3,41... 

Beginning with composite electrodes the simplest scheme here is to coat the surface of a small gap semiconductor that is well matched to the solar spectrum but which is electro-chemically unstable with a thin film of a wide gap, electrochemically stable semiconductor. To demonstrate the feasibility of using such a composite electrode, the film must be thin enough - of the order of 50-100 A or less - so that at least some of the photogenerated carriers in the small bandgap material can tunnel through to the electrolyte. Furthermore the film must not have any cracks or pinholes since these would... 

On the other hand the measured current upon irradiation by the same source would be different if the solid has blocking contacts. Perfect blocking contacts are defined as those that permit the photogenerated carriers to drift out of the solid under the influence of the electric field but do not allow any charge to be injected into the solid. The current under these conditions is given by2. ... 

In most a-Si H solar cells, a built-in electric field (F) assists in the collection of photogenerated carriers, and efficient collection occurs as long as the drift length (/itF) is significantly larger than the film thickness. Crandall (1982) has shown that the transport in p-i-n cells can be charac-... 

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