Photovoltaic and Photothermoelectric Effect in a Double-Gated WSe2 Device

• Published in: Nano letters Vol. 14; no. 10; pp. 5846 - 5852
• Main Authors: Groenendijk, Dirk J, Buscema, Michele, Steele, Gary A, Michaelis de Vasconcellos, Steffen, Bratschitsch, Rudolf, van der Zant, Herre S. J, Castellanos-Gomez, Andres
• Format: Journal Article
• Language: English
• Published: Washington, DC American Chemical Society 08-10-2014
• Subjects: Condensed matter: electronic structure, electrical, magnetic, and optical properties; Exact sciences and technology; Fullerenes and related materials; Optical properties and condensed-matter spectroscopy and other interactions of matter with particles and radiation; Physics; Visible and ultraviolet spectra; electrostatic control; Tungsten diselenide; PN junction; photovoltaic; photothermoelectric; Photovoltaic effects; Doses; Display devices; Optoelectronic devices; Photocurrents; Energy gap; Asymmetry; Transition elements; Tungsten; Illumination; Absorption spectra; Photoconductivity; Active material; Optical absorption; Gates
• ISSN: 1530-6984; 1530-6992
• DOI: 10.1021/nl502741k

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 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 WSe2 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.