Clear route to power

A thin layer of organic photovoltaic film applied to the windows and rooftops of buildings around the world has the potential to generate solar energy

A thin layer of organic photovoltaic (PV) film applied to the windows and rooftops of homes and businesses around the world has the potential to generate solar energy at radically lower costs.

That, at least, is the hope of the Carbon Trust, which recently launched a £5m development project to further the potential of organic PV technology to deliver electricity.

The project, led by Cambridge University and The Technology Partnership (TTP) aims to increase the efficiency and lifespan of organic PV technology. The team will also devise a manufacturing technique to produce organic PVs in an inexpensive and energy-efficient way.

Organic PV power sources have become increasingly popular because they have the potential to be much less expensive to produce than their silicon-based counterparts. However, the drawback is that plastic cannot accommodate an electric current as reliably as silicon.

Most silicon solar cells are able to generate electricity with more than 10 per cent efficiency, while current laboratory polymer cells have only been able to achieve two to five per cent efficiency — and it will be a greater challenge to achieve results of this sort outside the lab.

‘Five per cent efficiency is a good goal to start with,’ said project researcher Neil Greenham of Cambridge’s physics department. He added that a later goal would be to make plastic’s efficiency comparable to, or better than, silicon.

For solar cells to increase electricity production, researchers must improve the material’s design. To do this, they need a better understanding of what happens when light is converted to electrons on polymer films.

Greenham’s team is the first to develop a working solar cell made of a polymer blend.

The plastic PV material consists of a film made of two different types of polymers — one that releases electrons when it is struck by photons of light, and another material that accepts the freed electrons.

Once the charges are split up — with negative charged particles on one material and positive charged particles on the other — Greenham said the challenge is to transport them out of the device to metal electrodes that carry electric current for external use.

‘Each material needs to have a pathway to the appropriate electrodes,’ said Greenham. ‘It’s a challenge because you have to control the deposition process to get the right methodology.

‘We need to get better control of how these materials rearrange themselves.’

The fabrication of PV cells in the laboratory is often a delicate process. It begins by putting a drop of a polymer blend on a glass substrate and spinning it until it spreads into a thin film.

‘Because polymers don’t like to mix, they try to phase separate,’ explained Greenham. ‘If you control that, you can get the pathways that you need for getting the electrons from one polymer and the positively charged particles on the other polymer out of the device.’

The researchers are now working on developing a way to fabricate these materials on a mass scale by using printing techniques to deposit the cells.

‘We’d like to move from glass substrates to flexible plastic substrates, and that has some advantages for the production because you can manufacture on a roll-to-roll web up to one metre wide,’ said Greenham.

Manufacturing will be much cheaper and more energy-efficient than procedures used to fabricate thin film silicon solar cells, because these are created with high temperature deposition.

‘That means quite a lot of energy is used in the manufacturing process,’ said Greenham. ‘In terms of carbon savings, it takes two to three years of the silicon solar cell’s life just to pay back the energy that was used to make it.

‘So by going to a low-temperature process, then the energy costs will be greatly reduced,’ he added.

Once made, the sheets of PV film will be able to sit on a wide range of surfaces. Simple applications could include chargers for mobile telephone and laptop computers.

The project group is aiming to deploy more than 1GW of these cells by 2017 to deliver carbon dioxide savings of more than one million tonnes a year.