Characterization of reactively sputtered molybdenum oxide films for solar cell application

Abstract

Molybdenum oxide (MoO3) thin films were prepared via Radio Frequency (RF) sputtering at different ambient composition and post-deposition annealing. The effects on the structural, optical, and surface properties of the deposited films were investigated. The ambient oxygen concentration O2/(O2 + Ar) was varied from 10% to 100% at 10 mTorr. Post deposition anneals were performed in Ar at 300–500 °C. The films were analyzed using glancing incidence x-ray diffraction (GIXRD), UV/Vis/NIR spectrophotometry, and x-ray photoelectron spectroscopy (XPS). As-deposited films have amorphous structures, independent of the oxygen partial pressure. Annealing at 300 °C in air resulted in crystallization of the molybdenum oxide films to the monoclinic β-MoO3 phase. Samples annealed at 400 and 500 °C were identified as pure orthorhombic α-MoO3 phase with (020) preferred orientation. High resolution XPS studies showed the presence of Mo6+ (MoO3) and Mo5+ (Mo4O11) oxidation states at the surface of as deposited and low temperature (300 °C) annealed films, and the Mo6+ to Mo5+ did not change much with deposition oxygen partial pressure. Annealing at 400–500 °C suppressed the oxygen deficiency at the surface, resulting in films with composition close to stoichiometric phases. UV/Vis/NIR spectrophotometry revealed that all films have a high optical transmittance (>80%) in the visible range, followed by a steep drop at λ ≈ 400 nm indicating a strong absorption due to band-to-band transition. Increasing the oxygen partial pressure had no significant effect on optical transmittance of the films, and the bandgaps in the range of 2.6 eV to 2.9 eV were obtained. Annealing at 300 °C had a negligible effect on the optical properties of the MoO3 films, but samples annealed at 400 °C and 500 °C exhibited wider bandgaps within the range of 3.1–3.4 eV.

A van der Waals Density Functional Study of MoO3 and Its Oxygen Vacancies

“…Vacancy induced band gap states in MoO 3 have been utilized for sub-band gap transitions and enhancement of solar cell device performance. 10,19,21 Critical for operation, the stability of oxygen defects was found to be dependent on the method of reduction, due to differences in bonding dependent on position in the structure. 9 The bonding types were examined in the context of surface reconstruction in a theoretical study by Coquet et al on slab models.…”

Thermal stability and photoluminescence property of hexagonal MoO3·0.55H2O microrods

“…For the MoO 3 ¢0.55H 2 O, the relatively sharp peaks at 974 and 905 cm ¡1 are likely due to the stretching of MoDO double bond. The peaks [26,27] revealing that band gap value of the obtained MoO 3 is reasonable and credible. The optical band gap energy has minimal changes when the treatment temperature is below 350 C, displaying a weak temperature dependence.…”

Thickness dependence of dispersion parameters of the MoOx thin films prepared using the vacuum evaporation technique

“…Many studies have been reported in which the transition metal oxides are employed in organic-based optoelectronic devices such as organic solar cells [18e20], organic light emitting diodes [21e24] and organic thin film transistors [25,26]. Molybdenum oxide can be regarded as a promising candidate for the transparent back contact component at tandem and superstrate devices due to its high transparency higher than 80 percent in the visible and near infrared spectral region and also its relatively wide band gap energies of 2.6e2.9 eV [27,28]. It is also an appropriate material for the primary back contact in CIGS thin film solar cells owing to its high work function (~5.7 eV) [29].…”