Excitons are the bound state of electrons and holes created by the photoexcitation of semiconductors. Since excitons are electrically neutral, they do not yield photocurrent in conventional photovoltaic processes unless dissociated into free charge carriers. It has recently been theoretically predicted that excitons can be directly converted to electric current by employing the shift current mechanism, a quantum-geometric photocurrent generation induced in noncentrosymmetric compounds. However, there has been no experimental observation of this photocurrent from excitons as a direct current. In this study, an international research group has successfully demonstrated the photocurrent generation from exciton creation in epitaxial thin films of a noncentrosymmetric wide-gap semiconductor cuprous iodide (CuI). By irradiating a linearly polarized light to the film, a photocurrent was observed without external bias, which was identified as the shift current from the polarization dependence. The photocurrent was strongly enhanced when the photon energy of the light was tuned to the exciton transition energies, indicating the direct generation of photocurrent from exciton states. The photon-to-current conversion efficiency from exciton creation was much larger than that from the excitation of the continuum regime. They also performed first-principles calculations to reveal that the sign and magnitude of the exciton shift current strongly depend on the strain in the thin films. These findings provide a promising route for the development of next-generation optoelectronic devices that effectively use excitons.
Papers
Journal: Nature Communications
Title: Strongly enhanced shift current at exciton resonances in a noncentrosymmetric wide-gap semiconductor
Authors: Masao Nakamura, Yang-Hao Chan, Takahiro Yasunami, Yi-Shiuan Huang, Guang-Yu Guo, Yajian Hu, Naoki Ogawa, Yiling Chiew, Xiuzhen Yu, Takahiro Morimoto, Naoto Nagaosa, Yoshinori Tokura, Masashi Kawasaki
Photovoltaic effect by soft phonon excitation
