Enhanced intrinsic photovoltaic effect in tungsten disulfide nanotubes.

Abstract

The photovoltaic effect in traditional p-n junctions-where a p-type material (with an excess of holes) abuts an n-type material (with an excess of electrons)-involves the light-induced creation of electron-hole pairs and their subsequent separation, generating a current. This photovoltaic effect is particularly important for environmentally benign energy harvesting, and its efficiency has been increased dramatically, almost reaching the theoretical limit¹. Further progress is anticipated by making use of the bulk photovoltaic effect (BPVE)², which does not require a junction and occurs only in crystals with broken inversion symmetry³. However, the practical implementation of the BPVE is hampered by its low efficiency in existing materials^4-10. Semiconductors with reduced dimensionality² or a smaller bandgap^4,5 have been suggested to be more efficient. Transition-metal dichalcogenides (TMDs) are exemplary small-bandgap, two-dimensional semiconductors^11,12 in which various effects have been observed by breaking the inversion symmetry inherent in their bulk crystals^13-15, but the BPVE has not been investigated. Here we report the discovery of the BPVE in devices based on tungsten disulfide, a member of the TMD family. We find that systematically reducing the crystal symmetry beyond mere broken inversion symmetry-moving from a two-dimensional monolayer to a nanotube with polar properties-greatly enhances the BPVE. The photocurrent density thus generated is orders of magnitude larger than that of other BPVE materials. Our findings highlight not only the potential of TMD-based nanomaterials, but also more generally the importance of crystal symmetry reduction in enhancing the efficiency of converting solar to electric power.