Research on silicon-based tandem heterojunction solar cells (STHSC) incorporating metal oxides is one of the main directions for development of high-efficiency solar cells. In this work, the optical characteristics of a STHSC consisting of a ZnO/Cu 2 O subcell on top of a silicon-based subcell were studied by optical modelling. Cu 2 O is a direct-gap p-type semiconductor which is attractive for application in solar cells due to its high absorptance of ultra-violet and visible light, nontoxicity, and low-cost producibility. Highly Al-doped ZnO and undoped Cu 2 O thin films were prepared on quartz substrates by magnetron sputter deposition. Thermal annealing of the Cu 2 O layer at 900 ° C enhances the electrical properties and reduces optical absorption, presumably as a result of increased grain size. Hall effect measurements show that the majority carrier (hole) mobility increases from 10 to 50 cm 2 /V × s and the resistivity decreases from 560 to 200 Ω × cm after annealing. A Cu 2 O absorber layer of 2 μm thickness will generate about 10 mA/cm 2 of photocurrent under AM1.5G illumination. The optical analysis of the STHSC involved calculating the spectral curves for absorptance, transmittance, and reflectance for different thicknesses of the thin film layers constituting the ZnO/Cu 2 O subcell. The complex refractive indices of the thin films were derived from spectroscopic ellipsometry measurements and implemented in the simulation model. The lowest reflectance and highest transmittance for the ZnO/Cu 2 O subcell are obtained for a thickness of approximately 80 nm for both the top and bottom AZO layers. The SiN x anti-reflection coating for the c-Si bottom subcell must be optimized to accommodate the shift of the photon spectrum towards longer wavelengths. By increasing the thickness of the SiN x layer from 80 nm to 120 nm, the total reflectance for the STHSC device is reduced from 12.7% to 9.7%.
KeywordsTandem Solar CellsHeterojunctionCuprous OxideMagnetron SputteringOptical PropertiesOptical Modelling
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