II–VI compounds as the top absorbers in tandem solar cell structures

Efficiencies of 19% have been achieved with copper indium gallium diselenide (CIGS) solar cells with a band gap of about 1.1 eV. Simulations indicate that if these are combined with a transparent top cell of band gap in the range 1.6–2.0 eV and with comparable efficiency, tandem efficiencies in the 25–30% range can be attained. We are developing II–VI based top cells for this application on both glass as well as lightweight substrates for space applications. To achieve these objectives, short circuit currents (Jscs) in the range 18–19 mA/cm2 are needed. We have attained internal Jscs in this range for 1.7 eV CdSe absorbers, and external Jscs are rapidly approaching these values. Single-phase Cd1−XZnXTe (CZT; EG = 1.6–1.8 eV) films have been deposited by co-deposition on glass and flexible polyimide film substrates from the binary compounds using two deposition technologies. CZT absorber performance is presently limited by poor transport properties and influence from the contact layers. To develop open circuit voltages (Vocs) in the range of 1 V, a nearly degenerate, transparent p-type contact is needed. We have focused our efforts on ZnSe and ZnTe for this contact and thus far have attained Vocs approaching 500 mV for ZnSe/CdSe and 800 mV for ZnTe/CZT. Transmission of 80% of the low energy light to be utilized by the CIGS bottom cell has also been demonstrated.