Carbon Single Wall Nanotubes As a Low-Barrier, Copper-Free Back Contact to CdTe Solar Cells

With a 1.45 eV band gap, a high absorption coefficient, and proven low-cost, high-volume manufacturing, CdTe-based photovoltaic (PV) technology accounted for more than 7% of the 100 GW of solar power generating capacity that was installed worldwide by the end of 2012.  Copper doping of CdTe assists in forming back contacts to CdTe, but Cu is known to diffuse through the device and produce junction-shunting pathways.  Because of the detrimental effects of Cu, several material systems have been investigated for use as the back contact for Cu-free CdTe devices. If long-term degradation of the photovoltaic performance could be eliminated, the levelized cost of electricity would be subtantially reduced without, in this case, significant changes in the device architecture or manufacturing process flow.

Here, we discuss results¹ that demonstrate that carbon single-wall nanotube (SWNTs) films can make a high-performance electrical back contact to CdTe solar cells without the use of Cu.  Back contacts formed with SWNT films showed improved open circuit voltage in comparison to cells fabricated with standard Au/Cu back contacts and, once overcoated with a thin metal layer, the solar-to-electric conversion efficiency was higher as well.  The results are understood by considering that individual SWNTs within the film extend through the film’s thickness and make barrier-less contacts to individual CdTe grains in the active layer of the device.

This work was supported by the U.S. Department of Energy under Award Number DE-SC0006349 and by faculty start-up funds from the University of Toledo, and was conducted in collaboration with the Wright Center for Photovoltaics Innovations and Commercialization (http://www.utoledo.edu/research/pvic/)

(1) Phillips, et al., Nano Lett., 2013, 13 (11), pp 5224–5232