Ballistic and shift currents in the bulk photovoltaic effect theory

The bulk photovoltaic effect (BPVE) - the generation of electric currents by light in noncentrosymmetric materials in the absence of electric fields and gradients - was intensively investigated at the end of the last century. The outcomes, including all main aspects of this phenomenon, were summarized in reviews and books. A new upsurge of interest in the BPVE occurred recently, resulting in a flood of misleading theoretical and experimental publications centered around the so-called shift current. Numerous top-rated recent publications ignore the basic principles of charge-transport phenomena and the previous results of joint experimental-theoretical studies. Specifically, leading (or substantial) contributions to currents caused by asymmetry of the momentum distributions of electrons and holes are missed. The wide-spread basic relation for the shift current ignores the kinetic processes of relaxation and recombination of photo-excited electrons and leads to nonvanishing shift currents in thermal equilibrium. The goals of this methodological note is to specify and substantiate the benchmarks of the BPVE theory and return studies to the right track in the interest of developing photovoltaic devices.