Conventional PV solar cells are reaching a limit in practical efficiency; the next step in commercial PV is a low-cost, high efficiency tandem. A mono-crystalline Cadmium Zinc Telluride (CZT) cell that can act as the top cell in a c-Si/CZT four terminal hybrid tandem with a conversion efficiency of 25%-31% is achievable. The CdZnTe mono-crystalline top cell, with thickness about 3 mm, can convert the AM 1.5 spectrum from approximately 300 nm-70 0 nm with efficiency >17% while passing the red and near IR to the silicon bottom cell. The incremental manufacturing cost to the original c-Si in volume production is more than offset by the value of increased efficiency, which lowers the cost per peak watt to levels equal to or better than obtained today in conventional silicon cells. The CZT top cell is designed to have a band gap of 1.73eV-1.80eV, which is well matched to the c-Si bandgap of 1.1eV, giving a theoretical maximum efficiency of approximately 40%. The CZT single crystal cell can be more than 90% transparent at wavelengths longer than 750 nm. By band-gap tuning compounds in the CdTe family, using zinc to optimize a hybrid tandem, the efficiency and economics of conventional c-Si (including multi crystalline), which constitutes 90% of the PV market, is greatly improved. In volume production a LCOE target of $0.02/kWh, consistent with the DOE Solar Target for utility PV, is achievable in locations receiving good solar irradiance. (c) 2020 Elsevier Ltd. All rights reserved.