# Understanding the Role of Transition Metal Oxides as Hole-Selective Contacts for Enhanced Efficiency in Selenium Solar Cells

## TL;DR

This paper explores how transition metal oxides improve the efficiency of selenium solar cells, especially under indoor lighting.

## Contribution

The study introduces optimized inorganic transition metal oxides as hole-selective contacts in selenium solar cells.

## Key findings

- Optimized TMO layers improve fill factor and power conversion efficiencies in selenium solar cells.
- A 20 nm MoOx HTL achieves a 5.5% outdoor power conversion efficiency.
- A 10 nm V2Ox HTL enables over 10% indoor efficiency across various light conditions.

## Abstract

Selenium solar cells (SeSCs) are gaining renewed interest
as wide
band gap photovoltaic absorbers suitable for indoor energy harvesting
and tandem applications. While significant progress has been made
through extensive optimization of electron transport layers (ETLs),
the role of hole transport layers (HTLs) has been comparatively less
explored. In this work, we investigate the integration of inorganic
transition metal oxides (TMOs), namely molybdenum oxide (MoO
x
), tungsten oxide (WO
x
), and vanadium oxide (V2O
x
), as hole-selective contacts in SeSCs. We systematically optimize
the TMO thicknesses and assess their effect on device performance
under both standard AM1.5G and indoor illumination conditions. Our
results demonstrate that incorporating optimized TMO layers substantially
improves the fill factor (FF) and parasitic resistances of the device,
leading to enhanced power conversion efficiencies (PCEs). The best
outdoor performance is achieved with a 20 nm MoO
x
HTL, delivering a champion PCE of 5.5%. For indoor conditions,
a 10 nm V2O
x
HTL enables PCE
values exceeding 10% across a wide range of light intensities and
spectra. Ultraviolet photoelectron spectroscopy and transmission electron
microscopy-energy dispersive X-ray spectroscopy analyses reveal strong
interfacial interactions between Se and the TMOs, including evidence
of spontaneous MoSe2 formation at room temperature, which
likely contributes to enhanced hole selectivity and suppressed recombination.
Additionally, preliminary indications suggest the possible formation
of VSe2 under similar conditions. These findings underscore
the crucial role of inorganic HTLs in unlocking the full potential
of SeSCs and highlight their suitability for emerging applications
such as indoor photovoltaics and monolithic tandem architectures.