Unintentional Hydrogen Incorporation into the SnO₂ Electron Transport Layer by ALD and Its Effect on the Electronic Band Structure

This work reports the influence of atomic layer deposition (ALD) using its variants as thermal ALD, remote plasma ALD (RPALD), and direct plasma ALD on the physical parameters of the as-deposited SnOx films. The deposition process and chemical composition are related to their electronic band structure such as valence band maximum, conduction band minimum, band gap, and work function. Oxidant agents such as H2O, O2, and O3 were evaluated with deposition temperatures of 80 and 200 °C. Each of the SnOx films were integrated into a solar cell prototype as an electron transport layer, yielding the maximum photovoltaic efficiency of 15.15% for the cell using the SnOx film obtained with oxygen-assisted RPALD at 80 °C. Tin oxides deposited at 200 °C using ozone and oxygen, respectively, feature surface plasmon resonance (SPR) under near-infrared irradiation (0.5 eV), which was detected by reflection electron energy loss and X-ray photoelectron spectroscopies. The unintentionally incorporated substitutional hydrogen acting as a shallow donor is responsible for the SPR effect, defect states in the band gap, and considerably reducing photovoltaic efficiency of the solar cells studied. This doping can be detected by in situ quadrupole mass spectroscopy characterization in the absence of CH2 among reaction byproducts at the ALD oxidation step.