Transparent contact development for CdSe top cells in high efficiency tandem structures

Simulations indicate that efficiencies of 25 - 30% can be achieved with CdSe/CIGS thin-film tandem devices. The key to achieving this objective is development of a 16% transparent CdSe device. To address this problem we have been developing transparent contact/CdSe/transparent contact structures deposited on glass/SnO₂:F. With these transparent structures we have demonstrated record Jsc´s of 17.4 mA/cm². SnO₂ serves as the n contact, and the p contact is ZnSe:Cu. These structures have also exhibited sub band gap transmission of 80%. We have evaluated ZnO and CdS as alternative n contacts and ZnTe as an alternative p contact. Voc´s of only 300 mV are being attained with ZnSe:Cu because its Fermi level seems to be near the middle of the band gap of CdSe in spite of favorable valence band alignment. However, using ZnTe:Cu as the p contact we have achieved Voc´s up to 575 mV, a new high.