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

• Main Authors: Khanal, Rajendra R., Phillips, Adam B., Zhaoning Song, Plotnikov, Victor V., Carter, Chad W., Stayancho, John M., Heben, Michael J.
• Format: Conference Proceeding
• Language: English
• Subjects: Annealing; back contact; carbon single wall nanotube; CdTe; efficiency; Gold; open circuit voltage; Performance evaluation; Photonic band gap; Photovoltaic cells
• ISSN: 0160-8371
• DOI: 10.1109/PVSC.2014.6925398

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.