Photovoltaic and Photothermoelectric Effect in a Double-Gated
WSe<sub>2</sub> Device
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Abstract
Tungsten diselenide (WSe<sub>2</sub>), 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 WSe<sub>2</sub> 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 WSe<sub>2</sub> 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 2 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 WSe<sub>2</sub> where a region of strong optical absorption, caused by, for example,
an asymmetry in flake thickness or optical absorption of the electrodes,
generates a sizable thermal gradient upon illumination