The solar cells developed by the Flexible Electronics group at CSIRO in Clayton, Victoria, together with Paul Mulvaney's team from the University of Melbourne, were fabricated in air using liquids that contain semiconductor nanoparticles. By using nanoparticle inks, the researchers are able to use a layer-by-layer process to easily fabricate solar cells with defined electronic structures and efficiencies of up to 7%. "This milestone efficiency highlights the feasibility of developing good solar cells through solution-processed techniques that are also cost-effective," project leader Jacek Jasieniak told nanotechweb.org.

Conventional solar cells are made using highly pure and therefore relatively expensive chemicals, at typically very slow rates and at quite high temperatures of above 400 °C. The new solar cells are for the most part produced using nanoparticle solutions made from standard reagent grade chemicals, in air at lower temperatures of around 300–350 °C. What's more, because the solution-processing technique is versatile, it allows the researchers to tune the composition and thus the electronic properties of the final cells.

More sophisticated devices
The method can also be easily adapted to other materials, which means that complex, multi-layered devices, such as "tandem" solar cells, could be made in the future. Making such sophisticated devices is more difficult with conventional techniques.

"Our solar cells are solution-processed, compatible with roll-to-roll processing, require only very short heating steps of just a few seconds, are made predominantly in air as opposed to vacuum or argon, utilize relatively cheap starting materials with scalable protocols and already posses quite respectable efficiencies," said Jasieniak. "All these factors suggest that our cells could be significantly cheaper to make compared to existing thin-film solar cells on the market today."

The researchers begin by first synthesizing nanoparticles between 1–30 nm in size using conventional metal salt based methods in liquids. The nanoparticles exist as isolated spheres dispersed in these liquids and have a well-defined surface chemistry that needs to be modified to render the particles compatible with subsequent multi-layer deposition steps. The modified nanoparticles are dispersed in appropriate liquids to form nanoparticle inks that can be used to build the solar cells.

"At present, we use a simple deposition technique called spin coating to deposit high-quality thin film layers of cadmium telluride nanoparticles with a total thickness of 100–200 nm atop a transparent conductive electrode," explained Jasieniak. "Each layer is chemically treated to induce further changes to the surface chemistry and then thermally annealed in air to make the nanocrystals grow to around 50–150 nm in size. We repeat this process to build up the required final thickness of around 300–1000 nm."

The researchers then deposit thin films from different nanoparticle inks, for example, cadmium sulphide, zinc oxide and zinc selenide, to make sure that they have the appropriate electronic structure for making their solar cells. After the final layer, they evaporate a 100 nm thick aluminium layer onto the structures.

When light is shone onto the device, it is transmitted through the glass and the transparent conductive electrode, then gets absorbed and converted to electrical charges in the cadmium telluride layers. These charges are finally collected at the transparent conductive electrode and the reflective aluminium top electrode.

The solution-processed technique, and others like it, will allow scientists to innovate and develop new kinds of solar panels – for example, semi-transparent windows that can partially collect solar energy, solar panel roofs instead of add-on panels and integrated clothing charging units are some possibilities.

The team, who published its work in Nano Letters, is now busy working on scaling up its fabrication methods to be able to print the solar cells through roll-to-roll techniques.