Development of ZnTe Contacts for Cd 1-x Mg x Te Thin-Film Solar Cells for Tandem Applications
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1165-M09-05
Development of ZnTe Contacts for Cd1-xMgxTe Thin-Film Solar Cells for Tandem Applications Joel N. Duenow, Ramesh G. Dhere, Jian Li, Wyatt K. Metzger, Anna Duda, Timothy A. Gessert National Renewable Energy Laboratory, 1617 Cole Blvd., Golden, CO 80401
ABSTRACT Polycrystalline Cd1-xMgxTe (CMT) thin films are a potential absorber material for twojunction thin-film tandem solar cell applications because the desired top cell bandgap range of 1.6 to 1.8 eV is readily obtained using CMT with only small resultant changes in the lattice constant from that of CdTe. Tandem devices require the top cell to have a transparent back contact to transmit the sub-bandgap spectrum to the bottom cell. Sputtered Cu-doped ZnTe (ZnTe:Cu) thin films, which offer potential as a transparent back contact interface layer, have been used successfully in CdTe devices. We apply ZnTe:Cu back contacts to CMT devices to continue development toward a transparent top cell. We describe the effects of depositing ZnTe:Cu at different temperatures and with different Cu contents on the net acceptor concentration, minority carrier lifetime, and device performance. We present here the highest reported CMT device efficiency of 9.6% at a bandgap of 1.57 eV.
INTRODUCTION Two-junction tandem solar cells are composed of a wide-bandgap top cell overlying a narrower-bandgap bottom cell. The two cells may be grown monolithically or mechanically stacked. Theoretical calculations for two-junction cells have shown that the optimal bandgap for the top cell is between 1.6 and 1.8 eV [1]. Polycrystalline Cd1-xMgxTe (CMT) thin films are of particular interest as an absorber material for thin-film tandem solar cell applications because available CdTe knowledge may shorten development time compared to that of lesser-studied materials systems. The desired top cell bandgap can be readily achieved using a substitution of 5–10% Mg for Cd with only small resultant changes in the lattice constant from that of CdTe. In previous work with CMT, cell efficiencies up to 8% and open-circuit voltages up to 840 mV were achieved for CMT bandgaps of ~1.6 eV when using Cu/Au back contacts [2,3]. Effective two-junction tandem photovoltaic (PV) devices require the back contact to the top cell to be transparent to the sub-bandgap radiation absorbed by the bottom cell. In this study, we work toward implementing a transparent back contact interface layer on the CMT top cell. We chose ZnTe:Cu because it can be made reasonably transparent at the layer thicknesses required to fabricate high-performance PV devices. Its Cu source is important for establishing an ohmic contact to and doping the CMT absorber layer. ZnTe:Cu offers the additional advantage that it can be deposited by the industrially relevant radio frequency (RF) sputtering process. This work focuses on the top cell for a two-junction tandem device. Modeling shows [1] that the optimal bottom cell bandgap is ~1.1 eV, enabling already established Cu(In,Ga)Se2 or CuInSe2 to serve as the bottom cell absorber layer.
EXPERIMENT
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