Photovoltaic and photothermoelectric effect in a double-gated WSe2 device

TL;DR

This study investigates photocurrent mechanisms in a double-gated WSe2 device, distinguishing between photovoltaic and photothermoelectric effects, and shows how gate configuration influences the dominant photocurrent generation process.

Contribution

It provides a detailed analysis of photocurrent generation mechanisms in WSe2 devices, highlighting the role of gate configuration in controlling photovoltaic and photothermoelectric effects.

Findings

Photocurrent is mainly photovoltaic in PN and NP configurations.

Photothermoelectric effect dominates in PP configuration.

Photothermoelectric current can be twice as large as photovoltaic current.

Abstract

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.