It has been known for some time that materials lacking a center ofinversion can produce bulk photocurrents and photovoltages. In polarmaterials, this can even be in reponse to unpolarized light. Theprimary mechanism is a non-linear optical effect known as 'shiftcurrent.' Shift current is distinct from other photovoltaic effects:the photocurrent is carried by coherent excited carriers rather thanballistic carriers, and these coherent carriers are not separated byan electric field, but proceed in opposing directions as a consequenceof the asymmetry. The response tensor for shift current can be derivedusing time dependent perturbation theory to second order in thedensity matrix and yields an expression consisting of a transitionprobability multiplied by a 'shift' vector. This shift vectordescribes the averaged distance traveled by electrons in coherentstates during the lifetime of those states. This expression can bemodified for computation on a discrete grid, and used to calculate thebulk photovoltaic response from the wavefunctions provided by firstprinciples calculations. Using ferroelectrics BaTiO3 and PbTiO3, wehave analyzed in detail the effect of illumination frequency andpolarization and material properties, and determined guidelines forthe design and identification of ferroelectrics producing largephotovoltaic effects.  Furthermore, this mechanism allows fordifferences in response magnitude and direction depending on lightpolarization and frequency, including current flow parallel,antiparallel, and perpendicular to the material polarization. Thispotential for fine control over the response suggests application todevices including sensors and switches, in addition to solarphotovoltaics.

Optical processes leading to photovoltaic effect in single-phase polar materials

244th National Meeting (2012)

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