  Understanding the carrier excitation and transport processes at the
single-charge level plays a key role in quantum-dot-based solar cells and
photodetectors. Here, we report on Coulomb-induced giant photocurrent
enhancement of positive charged trions (\emph{X$^+$}) in a single
self-assembled InAs/GaAs quantum dot embedded in an \emph{n-i-}Schottky device
by high-resolution photocurrent (PC) spectroscopy. The Coulomb repulsion
between the two holes in the \emph{X$^+$} increases the tunneling rate of the
hole, and the remaining hole can be reused as the initial state to regenerate
\emph{X$^+$} again. This process brings the PC amplitude of \emph{X$^+$} up to
30 times larger than that of the neutral exciton. The analysis of the hole
tunneling time gives the equivalent change of hole tunnel barriers caused by
Coulomb interaction between two holes with a value of 8.05 meV during the
tunneling process. Our work brings a fundamental understanding of energy
conversion for solar cells in nanoscale to improve internal quantum efficiency
for energy harvesting.
