Shift current is a direct current generated from nonlinear light–matter interaction in a noncentrosymmetric crystal and is considered a promising candidate for next-generation photovoltaic devices. The mechanism for shift currents in real materials is, however, still not well understood, especially if electron–hole interactions are included. Here, we employ a first-principles interacting Green’s-function approach on the Keldysh contour with real-time propagation to study photocurrents generated by nonlinear optical processes under continuous wave illumination in real materials. We demonstrate a strong direct current shift current at subbandgap excitation frequencies in monolayer GeS due to strongly bound excitons, as well as a giant excitonic enhancement in the shift current coefficients at above bandgap photon frequencies. Our results suggest that atomically thin two-dimensional materials may be promising building blocks for next-generation shift current devices.When continuous wave light is shone on a noncentrosymmetric crystal, a direct current (DC) can arise due to a second-order optical response of the crystal. The origin of this current is interpreted to be related to the “shift” (
1–
4) of the intracell coordinates of the excited electron. This so-called shift current is proposed as an alternative to the photocurrent generated by traditional semiconductor p–n junctions (i.e., a junction between hole-doped [p-type] and electron-doped [n-type] semiconductors) for photovoltaic applications (
5,
6). Unlike conventional photovoltaic devices, shift current is a bulk phenomenon, which does not require a p–n junction to separate the optically generated electron–hole pair for a DC. Moreover, recent studies reveal that the photocarriers in shift current can have long travel distances, which is distinct from the usual drift transport mechanism in traditional solar cells (
7,
8) and makes shift current a promising candidate for efficient energy conversion.Despite many investigations over the past decade, a basic understanding of shift currents is far from complete. Most theoretical studies to date rely on the assumption of having noninteracting particles (
3–
6,
9–
11). Given that it is well known that light-induced electron–hole pairs can form bound or resonant excitons (correlated electron–hole states), which dominate and qualitatively change the absorption features of semiconductors, electron–hole interactions or excitons are expected to play a large role in shift currents, especially for reduced dimensional systems. However, it is not straight forward to generalize existing ab initio methods [such as the ab initio GW plus Bethe-Salpeter equation (GW-BSE) approach (
12)], used to understand and compute excitonic effects in linear optical absorption, to study nonlinear optical responses. Different model approaches to investigate the effects of many-electron interactions on nonlinear optical responses of materials have been proposed. For instance, a Floquet-based model Hamiltonian formalism showed that excitonic effects enhance nonlinear response (
13). In the specific case of second harmonic generation, first-principles approaches have been developed and applied to real materials, for instance, by making an approximation to the full many-body perturbation theory treatment (
14,
15) or to the time-dependent density function theory, in which electron interaction effects are taken into account via simplified kernels (
16). A real-time formulation based on propagating the time-dependent Schrodinger equation has also been developed (
17) and applied to second harmonic generation (
18). For shift currents in real materials, only one recent study considered the effects of excitons on the linear optical coefficient that might influence shift currents, but these authors included only the effects of excitons on the electromagnetic field profile in a bulk sample, and the crucial process of shift current generation itself is still treated within an independent-particle picture (
11). Thus, there is still no first-principles calculation and understanding of the role of many-electron interactions, particularly those due to excitons, on shift currents.Here, we show from first principles that 1) bound exciton states in the band gap can generate substantial shift currents, and 2) excitonic effects in the electron–hole continuum part of the spectrum can also greatly enhance shift currents due to the enhancement of the optical matrix elements from the coherence of the electron–hole pairs and to interexciton couplings that arose in the nonlinear responses.
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