Stoichiometry and Morphology Analysis of Thermally Deposited V2O5-x Thin Films for Si/V2O5-x Heterojunction Solar Cell Applications

Abstract

In recent decades, dopant-free Si-based solar cells with a transition metal oxide layer have gained noticeable research interest as promising candidates for next-generation solar cells with both low manufacturing cost and high power conversion efficiency. Here, we report the effect of the substrate temperature for the deposition of vanadium oxide (V₂O_5-x, 0 ≤ X ≤ 5) thin films (TFs) for enhanced Si surface passivation. The effectiveness of SiO_x formation at the Si/V₂O_5-x interface for Si surface passivation was investigated by comparing the results of minority carrier lifetime measurements, X-ray photoelectron spectroscopy, and atomic force microscopy. We successfully demonstrated that the deposition temperature of V₂O_5-x has a decisive effect on the surface passivation performance. The results confirmed that the aspect ratio of the V₂O_5-x islands that are initially deposited is a crucial factor to facilitate the transport of oxygen atoms originating from the V₂O_5-x being deposited to the Si surface. In addition, the stoichiometry of V₂O_5-x TFs can be notably altered by substrate temperature during deposition. As a result, experimentation with the fabricated Si/V₂O_5-x heterojunction solar cells confirmed that the power conversion efficiency is the highest at a V₂O_5-x deposition temperature of 75 °C.

Keywords: heterojunction solar cell; passivation; transition metal oxide; vanadium oxide.

Figure 1

(a) Reported work functions (Φ) for TMOs (V₂O₅, MoO₃, WO₃, and TiO₂) and (b) energy band diagram with electron- and hole-selective contacts for c-Si/TMO HSCs.

Figure 2

Minority carrier lifetimes plotted against the minority carrier density. Carrier lifetimes were measured at different V₂O_5−x deposition temperatures for the cell structure in the inset. (Note: The vertical black dashed arrow indicates the carrier injection level of 1.5 × 10¹⁵ cm⁻³ at which the ${\tau }_{eff}$ values in the inset were derived).

Figure 3

Si 2p spectra of SiO_y formed at the V₂O_5−x (15 nm)/Si interfaces at each of the different temperatures.

Figure 4

Peak area ratios of Si states calculated by measured Si 2p doublet spectra.

Figure 5

AFM image of V₂O_5−x (5 nm) TF deposited at different substrate temperatures: (a) RT, (b) 50 °C, (c) 75 °C, (d) 100 °C, and (e) 125 °C. (f) Surface coverage and roughness (RMS) and (g) cross-sectional profile of each deposited V₂O_5−x TF.

Figure 6

AFM images measured in 3D, and height and width of V₂O_5−x islands at (a) RT, (b) 50 °C, (c) 75 °C, (d) 100 °C, and (e) 125 °C; (f) calculated aspect ratios (ARs). Schematic of the effect of the AR on the surface area of SiO_y for (g) low-AR and (h) high-AR V₂O_5−x islands.

Figure 7

Schematic illustration of the solid-state V₂O_5−x TF formation processes. (a) crucible heating (b) material evaporation (c) substrate wetting (d) surface transport and nucleation and (e) formation of V₂O_5−x layer.

Figure 8

V 2p_3/2 XPS profiles of V₂O_5−x (15 nm) TFs deposited on Si surfaces at various deposition temperatures: (a) RT, (b) 50 °C, (c) 75 °C, (d) 100 °C, and (e) 125 °C.

Figure 9

Integrated peak area ratios of vanadium oxidation states based on the measured V 2p XPS profiles.

Figure 10

Current density vs. voltage curves of Si/V₂O_5−x HSCs measured under (a) dark and (b) 100 mW/cm² illumination (AM1.5) conditions.