Direct observation of exciton shift current in a noncentrosymmetric widegap semiconductor

Photocurrent generation via nonlinear light-matter interaction has recently attracted great interest. One example is the shift current, a photocurrent emerging in noncentrosymmetric compounds, driven by the change in the geometric phase upon the optical transition, not by the drift and diffusion of photoexcited free carriers. Since shift current does not require photoexcited free carriers, it can be generated by creating charge-neutral excitons that do not carry electrical current in general. Although the exciton shift current has been predicted theoretically, there have been no direct experimental observations. In this presentation, we show a successful demonstration of the exciton shift current in epitaxial thin films of CuI, a widegap semiconductor with a noncentrosymmetric zinc-blende structure. A clear shift current is observed at exciton resonances well below the bandgap, of opposite sign and much larger in magnitude than that in the electron-hole continuum. First-principles calculations elucidate that tiny strain applied in the thin films plays a crucial role in the sign reversal. The present results reveal that excitons have a potential for enhancing photovoltaic performance below the bandgap spectral region through the nonlinear shift current process.