TITLE:
Optical Analysis of a ZnO/Cu2O Subcell in a Silicon-Based Tandem Heterojunction Solar Cell
AUTHORS:
Ørnulf Nordseth, Raj Kumar, Kristin Bergum, Laurențiu Fara, Sean Erik Foss, Halvard Haug, Florin Drăgan, Dan Crăciunescu, Paul Sterian, Irinela Chilibon, Cristina Vasiliu, Laurentiu Baschir, Dan Savastru, Edouard Monakhov, Bengt Gunnar Svensson
KEYWORDS:
Tandem Solar Cells, Heterojunction, Cuprous Oxide, Magnetron Sputtering, Optical Properties, Optical Modelling
JOURNAL NAME:
Green and Sustainable Chemistry,
Vol.7 No.1,
February
27,
2017
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%.