Defect Density Analysis of WO x and MoO x Thin Films Grown by Pulsed Laser Deposition for Heterojunction Solar Cell Applications

Abstract

This study presents a comprehensive analysis of the optical and electronic properties of thin films of molybdenum oxide and tungsten oxide to implement hole-selective contact for heterojunction solar cells. These contacts are currently viewed as an alternative for the fabrication of doping-free solar cells. However, the spreading of this technology is still limited due to the development of S-shaped J-V curves, which affect the electrical performance of the cells, and further optimization in the material deposition process is therefore crucial to overcome these challenges. To improve transition metal oxide-based heterojunction technology, this work investigates the impact of oxygen vacancies on electrical performance, particularly their role in S-shaped J-V curves. Defect density evaluation through nondestructive techniques like photothermal deflection spectroscopy together with a detailed experimental characterization is presented in this paper to highlight the structural and optical properties of the films. Prototypes of solar cells incorporating hole-selective contacts with tungsten and molybdenum oxide are prepared to show S-shaped J-V characteristics under AM 1.5 illumination. An equivalent circuit modeling was used for understanding the electrical characteristics of the prototypes. Furthermore, this approach offers insights into the optimization of the performances of devices.

Keywords: defect density; heterojunction solar cells; molybdenum oxide; photothermal deflection spectroscopy; pulsed laser deposition; transition metal oxides; tungsten oxide.

1

Graphical representation of the solar cell prototypes realized by implementing a hole selective contact made of MoO _x or WO _x . The structure of the cell is composed of a stack of c-Si passivated with aSi:H, the electron selective contact is made of a-Si:H­(n), and the hole-selective contact is made, alternatively, of MoO _x or WO _x .

2

XPS spectra for WO _x samples deposited at T _amb with PO₂ = 3 × 10^–2 mbar: (a) wide scan, (c) curve fitting of the W 4f spectrum, (e) curve fitting of the O 1s spectrum; and with PO₂ = 10 × 10^–2 mbar: (b) wide scan, (d) curve fitting of the W 4f spectrum, and (f) curve fitting of the O 1s spectrum.

3

XPS spectra for MoO _x samples deposited at T _amb with PO₂ = 3 × 10^–2 mbar: (a) wide scan, (c) curve fitting of the Mo 3d spectrum, (e) curve fitting of the O 1s spectrum; and with PO₂ = 10 × 10^–2 mbar: (b) wide scan, (d) curve fitting of the Mo 3d spectrum, and (f) curve fitting of the O 1s spectrum.

4

Raman spectra of the substoichiometric tungsten oxide samples deposited at (a) ambient temperature and 200 °C compared with (c) the tungsten trioxide deposited at 400 °C. Raman spectra of the substoichiometric molybdenum oxide samples deposited at (b) ambient temperature and 200 °C compared with (d) the molybdenum trioxide deposited at 400 °C.

5

Absorption coefficient of the substoichiometric (a) WO _x and (b) MoO _x samples grown under different deposition conditions.

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Resulting parameters for the samples under different deposition conditions: WO _x (a) A _D and (b) A _p; MoO _x (c) A _D and (d) A _p.

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Resulting DOS (black line) with Gaussian defect distributions, the valence, conduction bands, and tails for the samples deposited at ambient temperature and 6 × 10^–2 mbar: (a) WO _x and (b) MoO _x .

8

Experimental J–V curves of a TMO-based HJT solar cell showing an S-shape curve under AM 1.5. The black line with squares is referred to WO _X , while the red line with circles is referred to MoO _x .

9

Equivalent electrical model of an HJT solar cell including a SJ.

10

Dark saturation current vs oxygen pressure of the prototypes of solar cells.

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Simulation of the electrical characteristic of the solar cells prototype with hole contacts based on (a) WO _x deposited at an oxygen pressure of 6 × 10^–2 mbar and (b) MoO _x deposited at an oxygen pressure of 10 × 10^–2 mbar.