Ultrafast Zero-Bias Photocurrent in GeS Nanosheets: Promise for Photovoltaics

Abstract

Ferroelectric semiconductors have been predicted to exhibit strong zero-bias shift current, spurring the search for ferroelectric semiconductors with band gaps in the visible range as candidates for socalled shift current photovoltaics with efficiencies not constrained by the Schockley−Queisser limit. Recent theoretical works have predicted that two-dimensional IV−VI monochalcogenides are multiferroic and capable of generating significant shift currents. Here we present experimental validation of this prediction, observing ultrafast shift currents by detecting terahertz electromagnetic pulses emitted by the photoexcited GeS nanosheets without external bias. We explore excitation fluence, orientation, and excitation polarization dependence of the terahertz emission to confirm that shift currents are indeed responsible for the observed emission. Experimental observation of zero-bias photocurrents puts GeS nanosheets forth as a promising candidate material for applications in third-generation photovoltaics based on shift current, or bulk photovoltaic effect.

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

“…There is numerical evidence that 2D ferroelectric single-layer IV-monochalcogenides have large shift current 22,23 . A recent experiment measuring shift current on thin films of GeS is consistent with our results 27 . We also expect large shift current in the recently discovered 2D ferroelectric SnTe 28 .…”

Terahertz Emission and Ultrafast Carrier Dynamics of Ar-Ion Implanted Cu(In,Ga)Se2 Thin Films

“…The τ 1 and τ 2 are considered to be determined by surface and bulk defect states, respectively, and depend on the quality of the CIGS thin film with respect to various defect densities [14,16,21]. The decay time constants of samples are similar to those of previous studies [22]. According to the fitting results, τ 1 and τ 2 increase as the density of Ar-ion increases.…”

Spectral dynamics of shift current in ferroelectric semiconductor SbSI

“…These quantum theories have clearly unveiled the topological nature of the shift current, and the concept of the topological shift current is now extended to the cases of excitonic process [9] and spin current [10]. Experimentally, the shift current has been demonstrated in various systems [11][12][13][14][15], and is proposed to be a major factor for realizing highly-efficient solar cells [16][17][18]. Since the shift current initiates upon an interband photoexcitation, its dynamics is inherently ultrafast, which is in fact observed recently [19].…”