Simulations indicate that 25-30% efficiency can be achieved with a four-terminal thin-film tandem structure. The bottom low band gap cell can be CuIn1-xGaxSe2, and CdSe is proposed as the top cell, as it has an ideal band gap of 1.7 eV. In addition to the efficiency requirements, the top cell must also be transparent to effectively transmit sub band gap light to the bottom cell. We have developed CdSe devices that meet many of the requirements of this tandem structure. High electronic quality CdSe has been deposited on SnO2 and ZnO, which serve as the transparent n-type contact. The p-type transparent contact is Znse/Cu. Voc ' s of 475 mV have been achieved and can be further improved with better contacts. However, record Jsc ' s in excess of 17 mA/cm(2) have been achieved. This is close to the target 18 mA/ cm(2) to meet the efficiency objectives. Transmission of 80% of the sub band gap radiation has been demonstrated for 2-mu m-thick absorber layers. This is also close to the 85% target to achieve the overall tandem efficiency objectives. Improvement of the contact layers to achieve the Voc target is the final challenge. (c) 2004 Elsevier B.V. All rights reserved.