  Van der Waals (vdW) heterojunctions, based on two-dimensional (2D) materials,
show great potential for the development of eco-friendly and high-efficiency
nano-devices. Considerable research has been performed and has reported
valuable applications of photovoltaic cells, photodetectors, etc. However,
simultaneous energy conversion and storage in a single device has not been
achieved. Here, we demonstrate a simple strategy to construct a vdW p-n
junction between a WSe2 layer and quasi-2D electron gas. After once optical
illumination, the device stores the light-generated electrons and holes for up
to seven days, and then releases a very large photocurrent of 2.9 mA with bias
voltage applied in darkness; this is referred to as chargeable
photoconductivity (CPC), which completely differs from any previously observed
photoelectric phenomenon. In normal photoconductivity, the recombination of
electron-hole pairs takes place at the end of their lifetime, causing a release
of heat; in contrast, infinite-lifetime photocarriers can be generated in CPC
devices without a thermal loss. The photoelectric conversion and storage are
completely self-excited during the charging process. The ratio between currents
in full- and empty-energy states below the critical temperature reaches as high
as 109, with an external quantum efficiency of 4410000% during optical
charging. A theoretical model developed to explain the mechanism of this effect
is in good agreement with the experimental data. This work paves a path towards
storage-type photoconductors and high-efficiency entropy-decreasing devices.
