Photoelectrolysis electron-hole recombination

In photoelectrolysis cells, the concentrations of electrons and holes are changed from their dark equilibrium values due to creation of electron-hole pairs by absorption of photons. Since thermal equilibration with lattice vibrations is much faster than the electron-hole recombination of electrons, electrons and holes can be considered to be in thermal equilibrium with the lattice, even though they are metastable states. Therefore, Fermi-Dirac statistics can still be used by defining quasi-Fermi levels ( Ep and pEp) for electrons and holes in terms of their steady-state concentrations under illumination, n and p. ... 

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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