High-pressure sputtering deposition and in situ plasma oxidation of TiOx thin films as electron selective contact for photovoltaic applications

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In this article, we show the structural, optical, and electrical characterization of TiO x deposited by the unconventional technique of High-Pressure Sputtering (HPS). This technique has the potential to reduce the plasma-induced damage of the samples. To fabricate the TiO x, a 2-step process was used. Firstly, a thin Ti film was deposited in an Ar atmosphere. Secondly, O 2 was introduced into the HPS chamber to create an Ar/O 2 plasma that, along with low temperatures (150 °C or 200 °C), induces the oxidation of the deposited Ti film. With this approach, the Ti film is expected to behave as a capping layer that will reduce the oxidation of the Si substrate. This study aims to obtain a TiO x layer with low specific contact resistivity ( ρ c ) and high minority carrier lifetime. These are crucial characteristics for obtaining high-quality selective contact. It was found that the 2-step process can oxidize the Ti layer. These HPS TiO x layers show a resistivity in the order of 0.3–10 Ωcm and a ratio Ti/O of ∼1.9. Moreover, the SiO x regrowth is minimal since this is comparable to the native oxide. This was confirmed by transmission electron microscopy (TEM) and Fourier transform infrared spectroscopy (FTIR). The samples fabricated with a Ti layer (∼4 nm) plus an oxidation temperature of 200 °C (duration of less than 2 h) show a low ρ c of 0.02 Ωcm 2 , an excellent transmittance (>87 %) in the visible region and an optical bandgap of 2.8 eV. These TiO x layers are amorphous, although some anatase phase crystalline clusters appear for the 200 °C processes. However, the minority carrier lifetime results of Si passivated by TiO x were inadequate for fabricating efficient solar cells. We also found that using the RCA oxide improved lifetime. This indicates that introducing alternative low-temperature passivating layers can solve this issue. • TiO x films as electron selective contacts for Si solar cells deposited by the uncommon technique of high-pressure sputtering. • Ar/O 2 plasma plus low temperatures were used to oxidize the previously sputtered Ti thin film. • TiO x films show good transparency in the visible region and a bandgap of 2.8 eV. • Sputtering TiO x at high pressure shows low specific contact resistivity of 20 ± 29 mΩ cm 2 . • The use of chemically growth SiO x (RCA) between c-Si and TiO X shows and improvement in lifetime.

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