The origin of the photo-emf in ferroelectric and non-ferroelectric materials

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Abstract
A photo-emf arises from an optical excitation that is either asymmetric in k-space or occurs at asymmetries in real space, as is shown by a discussion of the Boltzmann transport equation. Excitation asymmetric in k-space leads to the photogalvanic effect. A photo-current is impressed with unpolarized light in the bulk of ferro- and pyroelectric crystals, and with polarized light even in non-pyroelectric piezoelectric crystals. Photon drag and optical rectification are outlined in connection with the photogalvanic effects. Excitation at asymmetries in real space leads to a barrier photo-emf, if both electrons and holes are mobile and if the electric and chemical potentials are not spatially constant in the unilluminated semiconductor. The Dember effect as being based on one mobile charged species only, does not represent a photo-emf. Anomalous photovoltages of a magnitude larger than the band gap, in particular in polytype zinc sulfide, that have been accounted for by a battery of barrier photo cells, are reviewed with respect to the possibility of the contribution of the photogalvanic effect.