Charge carrier transport metal-semiconductor interface

It should be emphasized that the previous models only describe the current due to minority carriers. Under near-flatband conditions, the majority carriers start to contribute to the overall current. This can be recognized by an increase in the dark current, and under those conditions (2.64) and (2.65) are no longer useful. Another implicit assumption is the absence of mass transport hmitations in the electrolyte, but this is rarely an issue in photoelectrochemistry due to the high electrolyte concentrations and modest efficiencies reported thus far. Finally, it should be realized that the models described above cannot accormt for recombination at the semiconductor/electrolyte interface. Wilson pubhshed an extension of G tner s original model that included interfacial recombination effects [56]. Although this somewhat complicated model is not often used because it does not account for the dark current and recombination in the space charge, it is of value for metal oxides that show extensive surface recombination. 

As new metal oxide semiconductors are identified and explored for water photoelectrolysis, a persistent problem that arises is the low drift mobility of electrons and holes and the short lifetimes of photogenerated carriers. Many materials that seem promising on the basis of bandgap and stability are found inadequate for photoelectrolysis because the low mobility of minority carriers prevents rapid charge transport to the aqueous interface. Metal oxides are also prone to nonstoichiometry, which can result in traps that promote recombination and shorten carrier lifetimes. When short lifetimes and low mobility arise in the same material, device efficiency can drop rapidly and photocurrents are far below those that might be otherwise expected. 

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