This ideal has recently taken a step closer to fruition thanks to researchers at the CSIRO and the University of Melbourne, who have developed a new method for making printable solar cells using nanocrystal-based inks.

Nanocrystals, also called quantum dots, are semiconducting particles with a diameter of 1-10 nanometres – approximately 10,000 times smaller than the diameter of a strand of human hair. Due to their extremely small size these nanocrystals can be suspended in a liquid to make ‘solar inks’. These inks can then be deposited onto a variety of surfaces by methods such as printing or spray deposition.

Once the solar ink has been deposited onto a surface it is then heated so that the liquid evaporates and what remains is a thin layer of nanocrystals. This heating step is vital as it causes the crystals to sinter together, increasing their size and greatly enhancing their electrical properties.

What the project’s lead researchers, PhD student Brandon MacDonald and CSIRO postdoctoral fellow Dr Jacek Jasieniak have discovered, is that during this heat treatment there is a tendency for defects and ‘pinholes’ to form within the nanocrystal thin-film. As a result, the two electrodes of the solar cell can come into direct contact, causing a short circuit. To prevent this from occurring, Mr MacDonald and Dr Jasieniak developed a new layer-by-layer processing method.

In this approach, several layers of nanocrystals are deposited, with the described heat treatment performed following each layer. This way, defects which are formed in the thin-film can be filled in by subsequent depositions. The end result is a dense, uniform layer of solar cells.

This layer-by-layer deposition method has been used to make solar cells based on cadmium telluride (CdTe) nanocrystals.

CdTe is well established as a photovoltaic (PV) material, both on the laboratory scale where power conversion efficiencies (PCE) of up to 16.7 per cent have been achieved, and on the commercial scale, where it is used by First Solar, one of the world’s largest solar cell manufacturers.

By pairing the positively doped, ‘p-type’ CdTe with negatively doped, ‘n-type’ zinc oxide, nanocrystal solar cells with power conversion efficiencies of up to 10 per cent have been demonstrated. This is a significant improvement over earlier reports on CdTe nanocrystal cells, which previously had not exceeded power conversion efficiencies of 5 per cent. Achieving a PCE of 10 per cent is commonly viewed as one of the major milestones on the path to commercialisation.

In addition to using a low-cost, solution-based deposition method, these cells offer a number of advantages over conventional silicon and thin-film PV. One is that nanocrystal solar cells require very little material, as the CdTe layer is approximately 0.5 microns thick. This is a substantial reduction over conventional CdTe cells, which are typically 3-10 microns thick, and silicon cells, which are hundreds of microns thick.

Another important advantage of the layer-by-layer method is the relatively low processing temperatures that are required – between 300 and 350° Celsius. In addition to reducing the energy input required for manufacturing, this temperature range is compatible with the use of flexible substrates such as metal foils or certain plastics.

Using a flexible and lightweight substrate increases the opportunity for a wide range of potential applications beyond traditional rooftop installations or large-scale arrays. For example, flexible solar cells can be incorporated into products such as backpacks or battery chargers to provide a portable power source. Nanocrystal solar cells can also be manufactured in a manner that makes them semi-transparent, allowing them to be incorporated into products such as ‘solar windows’, where a portion of the light is absorbed by the solar cell to produce power while the remainder of the light is able to pass through and provide illumination.

For the development of the layer-by-layer assembly method for nanocrystal solar cells, Mr MacDonald and Dr Jasieniak have received widespread recognition. Mr MacDonald was awarded the 2010/11 DuPont Young Innovator Award, which recognises a tertiary student that has designed an innovative technology, product or service. He beat postgraduate students from across Australia and New Zealand to claim the $20,000 research funding prize.

Mr MacDonald was also named as one of 16 Fresh Scientists for 2011, a highly competitive nationwide program that provides young researchers with the opportunity to present their work to the general public.

Dr Jasieniak has recently been named a recipient of a 2011 Fulbright Fellowship, an award that will allow him to spend a year furthering his study of solar cells in the group of Nobel Prize winner Professor Alan Heeger at the University of California, Santa Barbara, in the United States.

The layer-by-layer assembly method for CdTe nanocrystal solar cells has been patented by its inventors, and discussions are currently underway with investors interested in helping to move this promising technology from benchtop to rooftop.

Brandon MacDonald is a PhD student whose work is supported by the University of Melbourne, the Bio21 Institute, a research centre specialising in medical—agricultural and environmental biotechnology—and the CSIRO Future Manufacturing Flagship.