Wafer-scale pulsed laser deposition of ITO for solar cells: reduced damage vs. interfacial resistance

Abstract

Transparent conducting oxides (TCOs) used in solar cells must be optimized to achieve minimum parasitic absorption losses while providing sufficient lateral conductivity. Low contact resistance with the adjacent device layers and low damage to the substrate during deposition of the TCO are also important requirements to ensure high solar cell efficiencies. Pulsed laser deposition (PLD) has been proposed as an alternative low-damage TCO deposition technique on top of sensitive layers and interfaces in organic and perovskite solar cells but is yet to be studied for the more mature silicon technology. Focusing on the PLD deposition pressure as the key parameter to reduce damage, we developed tin-doped indium oxide (ITO) with a sheet resistance of 60 Ω □⁻¹ at different pressures and implemented it in silicon heterojunction (SHJ) solar cells. Buffer-free semi-transparent perovskite cells with the same PLD ITO electrodes were also fabricated for comparison. While in the perovskite cells increased ITO deposition pressure leads to an improved open circuit voltage and fill factor indicative of damage reduction, SHJ cells with PLD ITO at all conditions maintained a high passivation quality, but increased pressures lead to high series resistance. Transmission electron microscopy and time-of-flight secondary ion mass spectrometry confirmed the formation of a parasitic SiO_x at the ITO/a-Si:H interface of the SHJ cell causing a transport barrier. The optimized ITO films with the highest carrier density were able to obtain >21% SHJ efficiency with 75 nm-thick PLD ITO. Moreover, reducing the ITO thickness to ∼45 nm and using TiO_x for optical compensation enables fabrication of SHJ devices with reduced indium consumption and efficiencies of >22%.