Oxygenated cadmium sulfide (CdS:O) is commonly used as the n-type window layer in high-performance CdTe heterojunction solar cells. This layer is deposited by reactive sputtering, but the optimal amount of oxygen in the sputtering ambient is highly dependent on the specific system and process employed. In this work, the intrinsic properties of CdS:O were measured as a function of the oxygen content (0%–10%) in the sputtering ambient and correlated to device performance with the goal of better defining optimal CdS:O properties for CdTe solar cells. Optimal performance was found using CdS:O films that contained ∼40 at. % oxygen as measured by Rutherford backscattering spectrometry. X-ray photoelectron spectroscopy confirmed these results and showed that oxygen is incorporated primarily as oxygenated sulfur compounds (SOx). Device efficiency improved from 10.5% using CdS to >14% with CdS:O due largely to increases in short-circuit current density as well as a modest improvement in open-circuit voltage. The transparency of the CdS:O films was well correlated with observed improvements in blue quantum efficiency with increasing oxygen content. The optical bandgap of as-deposited CdS:O was identified as a simple metric for process optimization and transfer, with 2.8 eV being ideal for the device architecture employed.
In this work, we report on the development of a reactive sputtering process for CdS:O for high efficiency CdTe solar cells. X-ray diffraction, UV-Vis-NIR spectrophotometry, and Rutherford backscattering spectrometry were used to characterize the crystal structure, composition, and optical properties, respectively. All films were slightly Cd-rich, while the bulk oxygen content increased up to 45 at. % in direct proportion to the O 2 partial pressure. Optical absorption in cells was reduced by increasing the oxygen fraction in the sputtering ambient. Optimal performance was obtained from cells with CdS sputtered in a 6% O 2 /Ar ambient, yielding efficiency >14% and V OC >840 mV.Index Terms -cadmium telluride, cadmium sulfide, thin-film photovoltaic cells, sputtering, optical band gap.
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