Quantitative relationship between polarization differences and the zone-averaged shift photocurrent

Abstract

A relationship is derived between differences in electric polarization between bands and the "shift vector" that controls part of a material's bulk photocurrent, then demonstrated in several models. Electric polarization has a quantized gauge ambiguity and is normally observed at surfaces via the surface charge density, while shift current is a bulk property and is described by shift vector gaugeinvariant at each point in momentum space. They are connected because the same optical transitions that are described in shift currents pick out a relative gauge between valence and conduction bands. We also discuss subtleties arising when there are points at the Brillouin zone where optical transitions are absent. We conclude that two dimensional materials with significant interband polarization differences should have high bulk photocurrent, meaning that the modern theory of polarization can be used as a straightforward way to search for bulk photovoltaic material candidates.

Spatio-temporal dynamics of shift current quantum pumping by femtosecond light pulse

“…where σ curr >σ light is the duration of transient photocurrent, and plot it as a function of Ω in figure 3(a) for gapless and in figure 3(c) for gapped device. In gapped device nonzero I t 0 á ñ ¹ a ( ) appears initially at subgap frequency ÿΩ ; Δ/2, which is at first sight surprising since in the 'standard model' analyses [14,17] of an infinite Rice-Mele TB chain DC photocurrent is zero in the gap. However, it is compatible with an electron from the valence band in figure 2(a) absorbing two-photons at the same time to transition to the conduction band.…”

“…whose parameters have been tuned to capture electronic structure of realistic materials like polyacetylene, BaTiO 3 and monochalcogenides [16,17]. Here c n † (c n ) creates (annihilates) electron in s-wave orbital nñ | centered at site n; B 2 g g = -+ + and γ − =−γ−B/2 are the alternating hoppings between the nearest neighbor sites with B parameterizing the dimerization of the chain; and ±D/2 is the staggered on-site potential.…”

“…However, photocurrent generated by BPVE in a closed circuit remains small compared to pn-junctions. This has ignited renewed experimental [4,[6][7][8][9][10][11] and theoretical [12][13][14][15][16][17][18] interest in BPVE with the ultimate goal [15] to optimize its power conversion efficiency.…”

“…The widely used [12][13][14][15][16][17] 'standard model' [25,26] for computing the magnitude of shift current is based on a perturbative expansion which yields its DC component…”

“…Thus, equation (1) can only describe photocurrent linear in intensity, as typically encountered also in conventional photovoltaics [27,28]. The third-rank tensor σ pqr (ω) can be expressed as a product of two terms carrying intuitive meaning [12,15,17]-diagonal imaginary part of the dielectric function, which is proportional to the density of states (DOS); and the so-called shift vector (of magnitude [2] 10-100 nm) as the average distance traveled by coherent photoexcited charge carriers during their lifetime. The shift vector depends on the Berry connection [19,20] of the Bloch energy bands of an infinite periodic crystal and it is, therefore, an intrinsic property of the material insensitive to elastic and inelastic scattering processes [25].…”