title: TiO2 Electron Transport Layers for Sb2Se3 Solar Cells
Creators: name: Don, Christopher
Publisher: name: University of Liverpool Repository
Year: 2024

description: In this thesis, TiO2 is evaluated as a potential electron transport layer (ETL) in Sb2Se3 photovoltaic (PV) solar cell devices. Various methods of fabricating planar TiO2 thin-films are investigated, including; spin-coating from precursor solution and magnetron sputtering from both a compound TiO2 target, and O2 reactive sputtering from a metallic Ti target.

Sb2Se3 is an emerging thin-film material gaining interest in the PV research community, reaching power conversion efficiencies (η) &gt;10% in less than 15 years of development. The highest performing Sb2Se3 device structures employ a CdS ETL, which has potential drawbacks due to the environmentally toxic cadmium inclusion, and parasitic absorption losses due to the relatively narrow 2.4 eV band gap. The functionality of non-toxic and wide band gap metal oxide ETL alternatives have been tested, with TiO2 the most numerous and highest performing.

The primary aim of this work was to fabricate high performance Sb2Se3 solar cells with industrially scalable TiO2 ETLs fabricated via magnetron sputtering. Non-ideal J-V characteristics, in the form of S-shape distortion, were observed in some devices fabricated with TiO2 ETLs fabricated from RF magnetron sputtering from a compound TiO2 target. Further analysis of the obtained TiO2 films revealed poor crystallinity and multi-phase nature, with both anatase and rutile signatures observed via bulk Raman spectroscopy and surface X-ray photoemission spectroscopy. The champion FTO/TiO2/Sb2Se3/P3HT/Au device fabricated with the defective TiO2 ETLs returned a 6.88% η cell.

Due to the irreproducibility of TiO2 films obtained via sputtering from a compound TiO2 target, an alternative approach was subsequently tested. The compound TiO2 target was replaced with a metallic Ti target, and titanium oxide films were obtained via introduction of a reactive O2 ambient during the sputter deposition process.

Sb2Se3 devices fabricated with reactively sputtered and post-deposition air annealed TiO2 devices delivered much improved reproducibility. The anatase-TiO2 obtained via this method demonstrated enhanced crystallinity compared to both the compound target TiO2 and spin-coated TiO2 ETLs. This was reflected in the cell performance, as an FTO/TiO2/Sb2Se3/P3HT/Au device produced an 8.12% η, which is the highest reported in literature for TiO2 ETL Sb2Se3 devices.

Device modelling simulations, along with band position measurements, suggest a spike-like conduction band offset exists between TiO2 and Sb2Se3. This large band offset was shown to induce J-V distortion for cases where the electron mobility and donor density were low. This may explain the significant Sb2Se3 device performance sensitivity to TiO2 processing.

Despite the progress made in this work, TiO2/Sb2Se3 devices are still inferior to CdS/Sb2Se3. However, with a model now established for the functionality of the TiO2/Sb2Se3 interface, and a reproducible method of fabricating high quality TiO2 material outlined, an eventual
transition to metal oxide ETL Sb2Se3 solar cells may yet be achievable.