Cadmium telluride nanocrystal colloids could be used as the photovoltaic “ink” in solar cells, according to new experiments by researchers at the National Renewable Energy Laboratory and the University of Chicago. Devices made using CdTe layers as thin as just 330 nm have a sunlight-to-power conversion of efficiency of 10% while those made with 550 nm thick layers have an efficiency of more than 11%. They also boast an impressive blue light response of nearly 80% external quantum efficiency – something that allows for improved photocurrent from these cells.
Thin-film photovoltaic materials could be alternatives to traditional silicon-based solar-cell materials because they absorb sunlight more efficiently – thanks to the fact that they have direct rather than indirect bandgaps. This means that less material, weight for weight, is needed to absorb the same amount of solar radiation. What is more, thin-film photovoltaics, such as cadmium telluride, can be easily and cheaply deposited onto a wide range of flexible and rigid substrates in solution.
There is a problem, however, in that the power-conversion efficiencies of thin-film materials that have been processed from solution are typically lower than those produced by traditional vapour deposition techniques.
Now, a team led by Dmitri Talapin of Chicago and Joseph Luther at NREL has succeeded in synthesizing CdTe inks from solutions of nanocrystals that have controllable shapes, ranging from spheres to tetrapods, and controllable crystallographic phases: wurtzite and zincblende. The researchers found that the best performing solar-cell devices are those containing tetrapodal-shaped nanocrystals in the wurtzite phase. Following a relatively low-temperature short anneal, these crystals undergo a critical phase change from wurtzite to zincblende that coincides with the small grain soluble nanocrystals growing into large grain, photovoltaic quality, CdTe.
“Rather than depositing the whole CdTe layer at once, we use a layer-by-layer approach to build up a very thin layer of the CdTe and control the grain growth,” explains team member Ryan Crisp, graduate student at the Colorado School of Mines. “We then deposit more nanocrystals and repeat the process until we reach the desired layer thickness.”
As the nanocrystals change phase and sinter (or grow) together, they form polycrystalline films, he adds. These films are unique in that they are exceptionally smooth and uniform (compared with films that are produced by traditional sublimation methods). “This means that further layers have a ‘nice’ surface on which we can deposit without fear of encountering short-circuits caused by irregularities and defects,” he tells nanotechweb.org.
“The crystal grains in our material extend from the top to the bottom in a finished device, allowing us to efficiently extract charge carriers (in this case photoexcited electrons) from it. We are able to do this since the electrons do not encounter many grain boundaries – something that minimizes their chance of being ‘lost’ to defect traps as they travel through the structure.”
Higher-efficiency, lower-cost devices
Solar cells made from the CdTe ink boast a sunlight-to-power conversion efficiency of 10–12%. This value might be further improved by placing the ink on the right type of substrate. “By employing this inexpensive solution-processed ink (instead of the more expensive, and slower throughput thin-film photovoltaic materials produced by sublimation), we can make potentially higher-efficiency, lower-cost devices,” says Crisp. “We explored several device structures and found that the ink-based films perform better in a simple ITO/CdTe/ZnO/Al structure rather than the traditional structure with CdS and ZnTe contacts.”
The main limiting factor to improving device efficiency is increasing the open circuit voltage. “We now plan on improving the quality of the ITO/CdTe interface (used in our highest efficiency device) to do this – and in particular by better controlling the energy levels (that is the band alignment) of the materials at this interface,” adds Crisp.
The new photovoltaic ink is described in ACS Nano