Does Sb2Se3 Admit Nonstoichiometric Conditions? How Modifying the Overall Se Content Affects the Structural, Optical, and Optoelectronic Properties of Sb2Se3 Thin Films

Abstract

Sb₂Se₃ is a quasi-one-dimensional (1D) semiconductor, which has shown great promise in photovoltaics. However, its performance is currently limited by a high V_oc deficit. Therefore, it is necessary to explore new strategies to minimize the formation of intrinsic defects and thus unlock the absorber's whole potential. It has been reported that tuning the Se/Sb relative content could enable a selective control of the defects. Furthermore, recent experimental evidence has shown that moderate Se excess enhances the photovoltaic performance; however, it is not yet clear whether this excess has been incorporated into the structure. In this work, a series of Sb₂Se₃ thin films have been prepared imposing different nominal compositions (from Sb-rich to Se-rich) and then have been thoroughly characterized using compositional, structural, and optical analysis techniques. Hence, it is shown that Sb₂Se₃ does not allow an extended range of nonstoichiometric conditions. Instead, any Sb or Se excesses are compensated in the form of secondary phases. Also, a correlation has been found between operating under Se-rich conditions and an improvement in the crystalline orientation, which is likely related to the formation of a MoSe₂ phase in the back interface. Finally, this study shows new utilities of Raman, X-ray diffraction, and photothermal deflection spectroscopy combination techniques to examine the structural properties of Sb₂Se₃, especially how well-oriented the material is.

Keywords: MoSe2; Sb2Se3; chalcogenides; emerging materials; material characterization; photovoltaics; quasi-1D semiconductors.

Figure 1

XPS in-depth profiles of samples with a nominal composition of 2[Se]/(2[Se] + 3[Sb]): (a) 0.36, (b) 0.48, (c) 0.53, and (d) 0.54, (e, f) distribution of Se out of the stoichiometry for the 0.53 and 0.54 samples, respectively.

Figure 2

(a) SEM cross-sectional 30× images of samples with 2[Se]/(2[Se] + 3[Sb]): (I) 0.54, (II) 0.53, (III) 0.48, and (IV) 0.36. (b) Crystalline domain as a function of 2[Se]/(2[Se] + 3[Sb]), obtained from the XRD–Rietveld analysis.

Figure 3

(a) X-ray diffractograms of a set of samples with different 2[Se]/(2[Se] + 3[Sb]) relative amounts. ICDD Powder Diffraction File pattern numbers are Sb₂O₃: 01-072-1334 and Sb₂Se₃: 04-003-0715. (b) Texture coefficients of diffraction peaks of Sb₂Se₃ thin films with different Se amounts.

Figure 4

(a) Normalized Raman spectra of the surface of Sb₂Se₃ thin films with 2[Se]/(2[Se] + 3[Sb]): 0.36, 0.48, 0.50, 0.53, and 0.54. (b) Area of Raman peaks—expressed as A(peak of interest)/A(190 cm^–1) as a function of 2[Se]/(2[Se] + 3[Sb]). (c) Absolute Raman spectra of lift-off remain on the Mo substrate of Sb₂Se₃ samples with 2[Se]/(2[Se] + 3[Sb]): 0.50, 0.52, 0.53, and 0.54. (d) Absolute Raman spectra of the previous peeled-off Sb₂Se₃ films. (e) Texture coefficient (002) as a function of 2[Se]/(2[Se] + 3[Sb]).

Figure 5

Absorption coefficient (α) as a function of photon energy (hν) of glass/Sb₂Se₃ films prepared during the same synthesis process as the SLG/Mo/Sb₂Se₃ corresponding samples with 2[Se]/(2[Se] + 3[Sb]): 0.54, 0.53, 0.50, 0.48, and 0.36. The absorption coefficient obtained from the PDS spectra.

Figure 6

(a) Phase concentration extracted from XRD as a function of 2[Se]/(2[Se] + 3[Sb]). (b) Phases identified by Raman spectroscopy analysis as a function of 2[Se]/(2[Se] + 3[Sb]). (c) Texture coefficient (002) as a function of 2[Se]/(2[Se] + 3[Sb]). (d–g) Evolution of the optoelectronic parameters as a function of 2[Se]/(2[Se] + 3[Sb]).