Abstract
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/Cu2O subcell on top of a silicon-based subcell were studied by optical modelling. Cu2O 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 Cu2O thin films were prepared on quartz substrates by magnetron sputter deposition. Thermal annealing of the Cu2O 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 cm2/V×s and the resistivity decreases from 560 to 200 Ω×cm after annealing. A Cu2O absorber layer of 2 μm thickness will generate about 10 mA/cm2 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/Cu2O 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/Cu2O subcell are obtained for a thickness of approximately 80 nm for both the top and bottom AZO layers. The SiNx 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 SiNx layer from 80 nm to 120 nm, the total reflectance for the STHSC device is reduced from 12.7% to 9.7%.
Highlights
Rapid progress of thin film solar cells has resulted in the development of tandem solar cells based on crystalline silicon with conversion efficiencies beyond the c-Si single-junction limit [1] [2] [3]
The objective of this work is to evaluate the optical performance of a ZnO/Cu2O subcell in a Si-based tandem heterojunction solar cell (STHSC) configuration
The optical analysis presented in this work focused on calculating the spectral curves for reflectance, absorptance, and transmittance for different thicknesses of the thin film layers constituting the ZnO/Cu2O subcell of a silicon-based tandem heterojunction solar cell
Summary
Rapid progress of thin film solar cells has resulted in the development of tandem solar cells based on crystalline silicon with conversion efficiencies beyond the c-Si single-junction limit [1] [2] [3]. The manufacturing of silicon-based tandem heterojunction solar cells (STHSC) with a high efficiency at low cost has still not been accomplished This situation might soon be changed due to the swift development of perovskite thin films as photovoltaic absorbents, showing a remarkable progress in the last 5 years [7] [8]. Another promising material for implementation in STHSC is cuprous oxide (Cu2O). The highest conversion efficiency achieved experimentally is 8.1% for a ZnO/Cu2O solar cell based on thermally oxidized copper sheets [12], suggesting that further investigation of Cu2O-based solar cells is still needed in order to realize their full potential in photovoltaic applications
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