We have discovered that films of carbon single wall nanotubes (SWNTs) make excellent back contacts to CdTe devices without any modification to the CdTe surface. Efficiencies of SWNT-contacted devices are slightly higher than otherwise identical devices formed with standard Au/Cu back contacts. The SWNT layer is thermally stable and easily applied with a spray process, and SWNT-contacted devices show no signs of degradation during accelerated life testing.

No abstract

ALD NiO was deposited on silicon and glass substrates by applying alternative pulses of Ni(amd) (AccuDEP TM Ni, Dow Chemical), and water. The film deposition rate at 200°C was 0.25-0.45 Å/cycle. The material properties of the NiO films were characterized using FESEM, AFM, UV-Vis-NIR spectrometer, and GIXRD. An ultrathin (~3-4 nm) film of the optimized ALD NiO was applied to very thin (500nm) CdTe cells in order to evaluate the potential of the ALD NiO film as a HT-EBL layer. The ALD NiO integrated CdTe cells enhanced Voc and FF, and offered 15% improvement in PCE compared with the control cells.

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.Index Terms-CdTe, carbon single wall nanotube, back contact, efficiency, open circuit voltage, photovoltaic cells