Recombination semiconductor/electrolyte junction

In the absence of surface recombination, all minority carriers that are collected by diffusion and migration in the semiconductor/electrolyte junction will eventually either transfer to redox species in the solution or react with the semiconductor itself leading to anodic or cathodic photodecomposition. Slow interfacial kinetics will result in the build up of photogenerated carriers at the interface, but unless photocurrent multiplication occurs, the saturation photocurrent will simply be determined by the light intensity, and the quantum efficiency will be unity. This means that the photocurrent contains no information about interfacial kinetics. In reality, most semiconductor/electrolyte interfaces are non-ideal, and a substantial fraction of the photogenerated electrons or holes do not take part in interfacial redox reactions because they recombine via surface states (see section 2.3.3). It is this competition between interfacial electron transfer and surface recombination that opens the way to obtain information about the rates of interfacial processes. 

Figure 23. Comparison of calculated current-voltage characteristics of n-type semiconductor-electrolyte junction device under illumination of 1 mA cm-2 (equivalent) photon flux.169 Parameters used are LD = 0.5 x 10-4 cm, 7V = 1016cm3, a = 3 x 104cm 1, nt = 107cm-3, e = 12, AV C = 0.7 V, r = 10-9 sec, and the electron and hole exchange current parameters are 1O 10 mA cm-2 and 10-5 mA cm-2, respectively. (a) Gartner s (Butler s) model [Eq. (103)]. (b) Reichman s model considering both electron and hole currents but neglecting the space charge recombination, (c) Reichman s model with the space charge recombination, (d) As (b) but in dark.

The Gaertner model, used for solid state devices, can be used to determine minority carrier diffusion lengths and the flatband potential at semiconductor-electrolyte junctions [53]. With the advent of photoelectrochemical energy conversion in the 1970s, models have been developed that were specifically addressed to the semiconductor-electrolyte boundary [54-59], taking into account the specific situation at the reactive boundary by introducing the charge transfer rate and the surface recombination velocity as parameters. 

When the semiconductor-electrolyte junction is illuminated with light, photons having energies greater than the semiconductor band gap are absorbed and create electron-hole pairs in the semiconductor. Photons absorbed in the depletion layer produce electron-hole pairs that separate under the influence of the electric field present in the space charge region. Electron-hole pairs produced by absorption of photons beyond the depletion layer will separate if the minority carriers can diffuse to the depletion layer before recombination with the majority carriers occurs. 

The overall charge separation process across a semiconductor liquid junction involves various contributions to the mechanism, details of which are still lacking. These include the light induced electron-hole formation, their various recombination mechanisms and their transport across the interface to react with the electrolyte. Time resolved techniques, that can now reach the femtoseconds time scale, should be a powerful tool to elucidate many mechanistic and kinetic aspects of these processes and provide the best interface with theory. Care should be exercised in... 

The liquid-junction photovoltaic cell has the advantages that the junction between electrolytic solution and semiconductor is formed easily and that polycrystalline semiconductors can be used. The principal disadvantage is that the semiconductor electrode tends to corrode under illumination. The electrochemical nature of the cell allows both production of electricity and generation of chemical products which can be separated, stored, and recombined to recover the stored energy. Liquid-junction cells also have the advantages that are attributed to other photovoltaic devices. Photovoltaic power plants can provide local generation of power on a small scale. The efficiency and cost of solar cells is independent of scale, and overall efficiency is improved by locating the power plant next to the load.72... 

The absence of solvents in such solid-polymer-electrolyte photovoltaic cells presents the possibility of fabricating corrosion-free systems. The thin-film solid-state cells also allow fabrication of multispectral cells composed of more than one semiconductor in optical and electrical series. A solid-state photovoltaic cell, n-Si/Pt/PP/PEO(K.I/ l2)/Pt/ITO, was studied. The surface modifications of n-Si with PP can dramatically reduce the large activation energy barrier against efficient charge transfer between semiconductor and polymer-solid electrolyte. The efficiency of this cell is limited by a high surface recombination velocity associated with surface states of the n-Si. The cell had V = 225 mV and 11 niA cm at 100 mW cm illumination with junction ideality factor of 1.5. This implies the existence of deleterious surface states acting as recombination centres. 

In the type of dye-sensitized solar cell, the dye and the semiconductor perform the processes of light absorption and charge separation, respectively, unlike the case of p-n junction type solar cells where the semiconductor performs both processes. This sensitization process minimizes losses due to recombination at the semicon-ductor/electrolyte interface (reaction 4). Since the electron transport in photoelectrochemical (PEC) cells is purely due to kinetic effects surfaces with structural defects do not therefore cause recombination and can be used in contrast to conventional solar cells [3]. That is the electron injection occurs anywhere in the cell and the time for electron recombination has been studied by flash photolysis and has been found to be very much longer than for electron injection [6]. 

A variant of photoiuminescence that in some cases requires the presence of a liquid junction is electroluminescence (EL) (28). In this case, the luminescence is not induced by optical excitation but by charge-transfer processes. The most widely used electrolyte for this purpose is alkaline peroxydisulfate (33) in which the SjOg ions are reduced by the conduction band electrons of an n-type semiconductor to yield the highly oxidizing sulfate radical anions that can inject a hole into the valence band. Recombination of an electron with the injected hole yields the luminescence. This process is much more localized to the surface as compared with photoiuminescence. Comparison between the two can... 

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