Thermoelectric and Photovoltaic Properties of Mn-Doped Kesterite Cu2Zn1–xMnxSnSe4

Abstract

The semiconductor Cu2ZnSnSe4 (CZTSe) is a promising candidate for both thermoelectric and photovoltaic energy harvesting applications due to a combination of features such as direct band gap, high absorption coefficient, and low thermal conductivity. We report the solid-state synthesis and characterization of Mn-doped Cu2Zn1–xMnxSnSe4 (x = 0, 0.05, 0.10, and 0.15) in an attempt to explore the effect of isovalent substitution at the Zn site. X-ray diffraction and Raman spectroscopy of all specimens confirmed the formation of a single-phase tetragonal kesterite structure (space group I4̅). The band gap obtained by UV–visible diffuse reflectance measurements was 1.42 eV for all compositions. Thermoelectric properties were measured in the range 300–785 K. Electrical resistivity was metallic and reduced on Mn doping, while the Seebeck coefficient exhibited a p-type semiconducting behavior that enhanced on Mn doping, with associated enhancement of the power factor. Lattice thermal conductivity showed a 1/T behavior, falling from about 1.9–2.7 W m–1 K–1 at 300 K to 0.51–0.9 W m–1 K–1 above 750 K. The combined effect of enhanced power factor and reduced lattice thermal conductivity resulted in a figure of merit ZT in the range of 0.19–0.42 above 750 K. Thin-film photovoltaic devices with a CZTSe absorber and an SnSe electron transport layer gave 3.2% efficiency.