RESUMO
Sprayed transparent conductive oxides (TCOs) are an interesting alternative to sputtered TCOs for many applications due to the possible high throughput and a simple, atmospheric pressure process of spray deposition. In this work, the growth mechanism of sprayed ZnO:In was analyzed by transmission Kikuchi diffraction (TKD) analysis of the thin film's crystal orientation, which shows a preferred orientation of the growing grains and thus proves that the deposition occurs from the gas phase. It was observed that with increasing thickness of the layer, the average grain size increases and the measured resistivity significantly reduces to ≈5-6 × 10-3 Ω cm for layers of >500 nm thickness. Since many applications also require good electrical contact formation, the contact resistivity and the interface between sprayed IZO and n-type poly-Si and p-type GaAs, two materials that are commonly used in III-V/silicon tandem solar cells, were investigated by electrical measurements and high-resolution transmission electron microscopy (TEM) analyses. The interlayers observed in TEM were investigated by energy-dispersive X-ray spectroscopy (EDS) line scans. The results suggest that oxidic interlayers at the substrate/IZO interface are responsible for the observed higher contact resistivity compared to the contact resistivity of sputtered indium tin oxide (ITO) references. The results presented in this work lead to a better understanding of the deposition process occurring in spray pyrolysis and thus allow a more targeted optimization of process parameters depending on the future requirements of the application.
RESUMO
The production of crystalline silicon thin-film solar cells on cost effective ceramic substrates depends on a highly reliable diffusion barrier to separate the light absorbing layers from the substrate. Ideally this intermediate layer should be deposited with cost effective techniques, be conductive and should feature optical confinement. Furthermore the intermediate layer should withstand high temperatures and harsh chemical environments like they occur during solar cell processing. Especially stability against oxidizing solvents like HNO3 or inactivity during e.g., oxide removing steps with HF is required. Crystalline silicon carbide (c-SiC) deposited by atmospheric pressure chemical vapour deposition (APCVD) can match all those requirements and additionally fits the thermal properties of crystalline silicon. The c-SiC intermediate layer is deposited from methyltrichlorosilane (MTS) and H2 at 1100 degrees C. Under these conditions, growth of solely cubic 3C-SiC could be observed by X-ray diffraction measurements. Use of such intermediate layers during high temperature steps prevents diffusion of transition metals, originating from the substrates, into active silicon layers. Doping of these 3C-SiC layers with nitrogen results in specific resistivity of less than 100 ohms cm. The different potentially cost-effective substrates are made from graphite, crystalline silicon, sintered silicon carbide and sintered zircon (ZrSiO4). Surface properties of the coated substrates were investigated, explaining changes in surface roughness and influences on the solar cell processing.