  Tungsten diselenide (WSe2), a semiconducting transition metal dichalcogenide
(TMDC), shows great potential as active material in optoelectronic devices due
to its ambipolarity and direct bandgap in its single-layer form. Recently,
different groups have exploited the ambipolarity of WSe2 to realize
electrically tunable PN junctions, demonstrating its potential for digital
electronics and solar cell applications. In this Letter, we focus on the
different photocurrent generation mechanisms in a double-gated WSe2 device by
measuring the photocurrent (and photovoltage) as the local gate voltages are
varied independently in combination with above- and below-bandgap illumination.
This enables us to distinguish between two main photocurrent generation
mechanisms: the photovoltaic and photothermoelectric effect. We find that the
dominant mechanism depends on the defined gate configuration. In the PN and NP
configurations, photocurrent is mainly generated by the photovoltaic effect and
the device displays a maximum responsivity of 0.70 mA/W at 532 nm illumination
and rise and fall times close to 10 ms. Photocurrent generated by the
photothermoelectric effect emerges in the PP configuration and is a factor of
two larger than the current generated by the photovoltaic effect (in PN and NP
configurations). This demonstrates that the photothermoelectric effect can play
a significant role in devices based on WSe2 where a region of strong optical
absorption, caused by e.g. an asymmetry in flake thickness or optical
absorption of the electrodes, generates a sizeable thermal gradient upon
illumination.
