Solar cells based on cadmium telluride (CdTe) currently occupy a market position second only to silicon-based devices, and have recently achieved significant efficiency improvements. However, their manufacturing costs must continue to decrease if they are to remain competitive. One approach to cost reduction involves inverting the conventional device orientation to enable the use of opaque metal-foil substrates. However, such designs have previously offered low efficiencies. Now, L. Kranz and colleagues at the Swiss Federal Laboratories for Materials Science and Technology have demonstrated an inverted-cell fabrication technique that results in flexible-substrate cells with efficiencies well over 10%, an important threshold for industrial production. They reported their results in the August 13 issue of the online journal Nature Communications (DOI: 10.1038/ncomms3306).
Conventional CdTe solar cells are grown on glass substrates, with sunlight entering the device through the substrate (the “superstrate” configuration). Reversing this configuration would enable the use of opaque substrates such as flexible metal foils, and enable roll-to-roll manufacturing techniques. However, previous attempts at this approach have foundered on the inability to dope CdTe with Cu, as is required to achieve high hole density. The efficiencies of inverted devices have therefore remained stuck at around 8%.
To overcome this problem, the researchers investigated a high-vacuum evaporation and annealing technique. Starting with three different types of substrates (glass, 50-µm-thick Mo foil, and 30-µm-thick steel foil with a 60/230-nm-thick Ti/TiN impurity diffusion barrier), they first deposited a 600-nm-thick Mo electrical back contact by dc sputtering and layers of MoO3 (150 nm) and Te (50 nm) by vacuum evaporation. Next, they used high-vacuum evaporation to deposit 4–6 µm of CdTe, followed by a standard recrystallization step. They then deposited a carefully controlled layer of Cu through high-vacuum evaporation followed by annealing at 400°C to promote diffusion into the CdTe. The cells were completed with a CdS layer and an i-ZnO/ZnO:Al bilayer front contact. Using secondary ion mass spectroscopy, the researchers found that the CdTe layer was successfully doped with Cu. For the optimal Cu concentration (equivalent to a submonolayer of approximately 1 Å thickness), the glass-substrate cells displayed efficiencies of up to 13.6%, while the Mo-foil-substrate and steel-foil-substrate cells achieved 11.5% and 10.9%, respectively.
These results suggest that roll-to-roll manufacturing of CdTe solar cells with efficiencies approaching those of conventional CdTe cell configurations may be possible, offering significant manufacturing cost reductions and potentially positioning CdTe as an even more notable competitor to silicon-based solar cells.
