A Solar Sandwich

 

It’s a fact that more energy, in the form of sunlight, strikes Earth in one hour than all of the energy consumed by humans in an entire year.

So why not use it? With oil and gas supplies running out, energy demands expected to increase 50% by 2030 and the earth heating up, solar power seems like a no-brainer. Even in New Zealand where much of our electricity comes from hydro-generation, solar power would ease the pressure on our rivers and lakes, secure power supplies in times of drought and save huge amounts of energy wasted in transmission.

Unfortunately, as MacDiarmid Institute Principal Investigator, Dr Tim Kemmitt, points out “in theend it comes down to dollars per Watt”. As long as solar power is more expensive than conven­tional energy sources it won’t take off and at the moment high production costs and expensive materials are thwarting its progress.

Dr Kemmitt is a chemist and solar technol­ogy researcher at Industrial Research Limited. He is going all out for cheap solar power – cheap materials, cheap processes, cheap everything. Last year he won an award from the Sustainable Energy Association of NZ for the most innovative approach to raising the standard of the photovoltaics industry, both commercial and domestic.

The award recognised his work developing a highly efficient, cost effective solar cell prototype using “quantum dots”, tiny fluores­cent particles that emit unprecedented amounts of electricity when exposed to sunlight.

The structure of the cell is a bit like a sandwich. It has a thin sheet of conductor on the bottom from which sprouts a layer of nanotubes (like microscopic bean sprouts). On top are sprinkled quantum-dots. The solar sandwich is topped off with another layer of conductor, transpar­ent this time to let the sun through to the active ingredients.

“So what happens” explains Tim, “is your photon of light comes in here [through the trans­parent conductor], hits one of these quantum dots, flicks out an electron (a little package of electricity) and it goes through the nanotube to the bottom electrode”.

Connect the conducting layers on the top and bottom with a wire and stick an electronic device in between, and you’ve got a complete circuit driven by sunlight.

Like most sandwiches the filling is the most important part. The quantum dots settle in amongst the nanotubes, a bit like snow settling in long grass, and the nanotubes transport all the electricity produced by them down into the circuit. Not only are the quantum dots cheap to make; they also produce more electricity per photon of sunlight than any known bulk material. So the potential is for unprecedented efficiency.

Dr Kemmitt is working with fellow MacDiarmid researcher and quantum dot expert, Dr Richard Tilley at Victoria University who synthesizes quantum dots straight out of solution using cheap, readily available materials.

“Do you make the bed of nanotubes too?” I ask.

“Yeah”. he says casually “That’s just done from hydrother­mal treatment.” He explains how the atoms on the surface of a sheet of metal, titanium for example, start to rearrange themselves into tiny tubes when put in the oven with a selection of chemicals.

He shows me an electron microscope image of nanotubes grown at different temperatures. “They start to look like tubes here,” he says pointing to a honeycomb-like structure. “They grow longer and longer and eventually take over the whole thing.” They look like something alive and organic, like vines sprouting and growing into a tangled maze over the substrate. They’re not alive of course. Titanium dioxide is a common mineral used in paint and moisturiser to give it its white colour.

One of the team’s key strengths is the simple, cheap, quick production techniques they have developed. “If you’ve got a big plant making things,” he explains “you want to be able to fly through operations at a hundred miles an hour.”

These days most commercial solar cells are made from crystalline silicon or other semi-conductors, which have to be grown atom by atom in high-vacuum conditions, an expensive time-consuming procedure. Dr Kemmitt’s techniques seem more like cooking than high-tech engineering and are ideally suited to large scale production.

The technique for growing nanotubes took ages to perfect. “What often happens is you’ll make a whole lot and they’ll just fall off the surface” says Dr Kemmitt. But in the right condi­tions they basically grow themselves.

One of Dr Kemmitt’s greatest strengths in terms of materials research is in using solutions for making surfaces rather than high energy sputtering or physical deposition. The quantum dots, for example, are dissolved in solution and then poured over the bed of nanotubes. “You’ve got to be able to get a solution spread on there in a way that’s flat and then you can just put it in the oven and dry it out” Dr Kemmitt explains. The liquid dries off and leaves the quantum dots stuck all over the surface. It’s cheap and easy.

Dr Kemmitt is collaborating with researchers at Oxford University in the UK to develop similar solution deposition methods to apply their new low cost transparent conductor materials to the surface of his solar cells.

Ultimately Dr Kemmitt’s aim is commercialisation. What we’re going for” he says “is cheap solar panels to stick on people’s rooves…If industry is interested, that’s everything really as far as we’re concerned.”

A problem with some of the more innovative new solar technologies is that manufacturers lack the infrastructure and experience to produce them. To tackle this, Dr Kemmitt’s group have formed a partnership with a company that designs manufacturing plants. “So they make stuff that makes stuff,” he says. They can make the production facilities and take care of commercialisation from there on. “They are an ideal partner.” Dr Kemmitt says “They can sell or license out manufacturing plants to lots of different people so everybody can start making them and hopefully we’ll just carry on and improve the product.”

The technology is still in early stages but all the potential is there for a successful product. He hopes to be doing field trials within four or five years.

“Obviously if it all works to plan”, he says “there should be a lot more interest overseas but at the moment we’re just sticking to NZ and that’s the way it should be really because this is about creating benefit for NZ.”

AMN-5 the fifth international conference on advanced materials and nanotechnology presented by the MacDiarmid Institute

Dr Kemmitt is the Chair of the Organising Committee for the AMN-5 Conference coming up in Feburary 2011 – the fifth two-yearly conference hosted by the MacDiarmid Institute. In typically understated tone he says it was accidental that he was chosen, but after eight years in the Institute he is happy to be able to give something back in this way. He sees the AMN conferences as vitally important events for New Zealand science.

“I think it’s a showcase for NZ materials research science across the board. It gets all these overseas people here and they can see what’s going on in NZ. We’ve had some really high profile Nobel Laureates come and see what the breadth of the research is.”

“Are they impressed?”

“Oh I think so, yes. Some of them come back again year after year. So it’s really important that the MacDiarmid Institute puts on its best stuff for it.”

Dr Kemmitt says it’s a bit early to give highlights for the conference but he is keen to ensure there is a balance from across the research themes and MacDiarmid Centres. In 2011 it will be the 100th year of superconductivity, an area of particular expertise in NZ, so there might be a sub-theme of that, he suggests.