Great effort has been placed into improving the performance of single-junction solar cells in recent years. Cells featuring the three most notable materials—Si, GaAs and CdTe—have sat at efficiencies of 25.6%, 28.8%, and 22.1% respectively for several years, sitting well below the Schockley-Queisser limit. Further improvements in efficiency are constrained by high manufacturing costs and/or poor material quality. Tandem cells are an excellent way to circumvent the challenges of improving single-junction device efficiencies. The proposed design of a II-VI (1.7 eV)/Si (1.1 eV) tandem solar cell featuring a MgxCd1-xTe top cell (x~13% Mg mol fraction) have the potential to reach an efficiency greater than 30% at low manufacturing cost.
This talk will summarize the latest study of structural and optical properties of CdTe/MgxCd1-xTe and Mg0.13Cd0.87Te/MgyCd1-yTe (y > 0.24) double-heterostructures, which are grown by molecular beam epitaxy. The MgCdTe barrier layers provide excellent passivation to the CdTe absorber, resulting in a longest carrier lifetime of 3.6 µs in undoped CdTe/MgxCd1-xTe double heterostructures and the lowest interface recommendation velocity 1.2 cm/s. Also the Mg0.13Cd0.87Te/MgyCd1-yTe double heterostructures revealed a long carrier lifetime of 560 ns, indicating excellent material quality.
Solar cells made of these double-heterostructures with 1- to 1.5-µm-thick n-type CdTe absorbers, and passivated hole-selective p-type a-SiCy:H contacts have achieved a highest Voc of 1.11 V and a maximum total-area efficiency of 18.5% and active-area efficiency of 20.3% measured under AM1.5G illumination. Using the similar device structure designs and measurement conditions, Mg0.13Cd0.87Te/MgyCd1-yTe double-heterostructure solar cells have shown a device with a Voc of 1.12 V and an efficiency greater of 15.2%.
X.-H. Zhao, S. Liu, C. M. Campbell, Y. Zhao, M. B. Lassise, and Y.-H. Zhang, Ultralow interface recombination velocity (~1 cm/s) at CdTe/MgxCd1-xTe heterointerface, IEEE Journal of Photovoltaics 7, 913 – 918 (2017).
Y. Zhao, X.-H. Zhao, Y.-H. Zhang, Radiative recombination dominated monocrystalline CdTe/MgCdTe double-heterostructures, IEEE J. of Photovoltaics 7, 690-694 (2017).
J. J. Becker, M. Boccard, C. M. Campbell, Y. Zhao, M. Lassise, Z. C. Holman, and Y.-H. Zhang, Loss analysis of monocrystalline CdTe solar cells with 20% active-area efficiency, IEEE J. of Photovoltaics 7, 900 – 905 (2017).