Development of II-VI high band gap devices for high efficiency tandem solar cells

Simulations indicate that efficiencies of 25-30% are attainable in 4-terminal, compound semiconductor thin film devices. CIGS can serve as the bottom cell, but a high band gap, transparent top cell with greater than 17% efficiency is needed. CdSe with an Eg of 1.7 eV and Cd_xZn_1-xTe (CZT) that can also be tuned to this range show high potential for this role. Using MIS structures we have demonstrated the attainment of high electronic quality CdSe. External Jsc´s of 14.7 mA/cm² have been attained without correction for substantial losses in the Cu contact. Similar progress has been made with CZT. High quality, single phase material has been demonstrated.