AVS 56th International Symposium & Exhibition
    Thin Film Monday Sessions
       Session TF1+PV-MoA

Paper TF1+PV-MoA6
Effects of Cu Doping Level in ZnTe:Cu Back Contacts on Cu Diffusion and CdTe Solar Cell Performance

Monday, November 9, 2009, 3:40 pm, Room A8

Session: Chalcogenide Photovoltaics
Presenter: J.N. Duenow, National Renewable Energy Laboratory
Authors: J.N. Duenow, National Renewable Energy Laboratory
R.G. Dhere, National Renewable Energy Laboratory
S.E. Asher, National Renewable Energy Laboratory
W.K. Metzger, National Renewable Energy Laboratory
J. Li, National Renewable Energy Laboratory
T. Moriarty, National Renewable Energy Laboratory
T.A. Gessert, National Renewable Energy Laboratory
Correspondent: Click to Email
CdTe photovoltaic (PV) cells require a low-resistance back contact for optimal device performance. Cu-doped ZnTe (ZnTe:Cu) has been implemented successfully as an ohmic back-contact interface layer to CdTe, enabling PV cells with efficiencies of 14%. ZnTe:Cu offers the additional advantage of being easily manufactured by vacuum processing, which enables highly controlled, reproducible films. Although CdTe devices have been made without Cu-containing contacts, devices with Cu contacts have generally exhibited higher performance. Controlled Cu diffusion from the back contact is known to be critical for optimal performance. Sufficient Cu is required for doping the CdTe absorber to provide a strong electric field in the junction region. Excessive Cu, however, can lead to poorer collection by narrowing the depletion region to the extent that few minority carriers are generated within a diffusion length of the depletion region. Cu may also adversely affect the junction by diffusing into the CdS window layer.

Previous studies of Cu diffusion from ZnTe:Cu back contacts have been performed at NREL using a fixed doping level of Cu in ZnTe, but altering Cu diffusion by changing the deposition temperature or film thickness. In this study, we instead deposit ZnTe:Cu films from targets containing a range of Cu amounts—from 0.45 to 5 wt.%—while holding the temperature and film thickness constant. Capacitance-voltage, red-light-biased quantum efficiency, and secondary-ion mass spectrometry depth-profile measurements will indicate the net acceptor concentration in the CdTe and the degree of Cu diffusion as a function of the Cu density in the back-contact interface layer. Minority-carrier lifetime measurements will be used to correlate the effects of Cu density to the carrier recombination rate.