Semiconducting carbon single-walled nanotubes as a cu-free, barrier-free back contact for CdTe solar cell

Copper diffusion from the back contact degrades the performance of CdTe solar cells over time and increases the levelized cost of electricity production from CdTe photovoltaics. Recently, carbon single-wall nanotubes (SWNTs) were shown to be a Cu-free, stable alternative that preserves the device efficiency (Phillips et al., Nano Letter, 2013). Large diameter tube samples containing a mixture of semiconducting (s-SWNT) and metallic (m-SWNT) species were used in the previous work, and the mechanisms leading to a low back barrier for majority carrier flow were not clear. The good performance of the back contact was ascribed to the interaction between the s-SWNTs in the film and the polycrystalline facets of the CdTe surfaces. In that case, the s-SWNT species had small bandgaps (~0.6-0.8 eV). Here, in an attempt to develop a more detailed understanding of the SWNT/CdTe back contact, we employed SWNT samples that are predominantly semiconducting (95%) and of larger bandgap (~1.1-1.3 eV). The power conversion efficiency of these unoptimized devices was 11.5 % with a s-SWNT back contact, as compared to 11.2% with a standard Cu/Au back contact.