Power from plastic

Solar power is on the rise. In 2008, the amount of energy generated by solar cells rose by 80%. The vast majority of solar cells are based on silicon. According to consultancy Solarbuzz, they account for more than 90% of the market, with thin film solar cells – based on cheaper substrates such as plastic – taking up only 7%. But the European Photovoltaic Industry Association says that thin film solar cells could account for around 20% of the market by the end of 2010, thanks to their lower price, rising efficiencies and manufacturing flexibility.

Organic solar cells are much cheaper than conventional solar cells, but have two main disadvantages: they convert sunlight to electricity much less efficiently – typically around 3% – and they are relatively flimsy and short-lived. And while a few organic solar cell technologies are already commercial – Solterra’s Quantum Dot and Konarka’s Power Plastic are two examples – many research teams are working on alternative technologies, or trying to boost system performance further.

Better protection
IMEC, the Belgian electronics research centre – in collaboration with materials specialist Cytec – is looking to develop a commercially viable method to make organic devices last ‘beyond five years’. The team will attack the problem in two ways: by stabilising the structure of the photoactive substances on the cell surface; and by developing a barrier encapsulation technology to protect the surface from damage by oxygen and water.

It has already made progress in the first of these tasks. The blend of conjugated polymers and fullerene acceptor molecules in the active layer tends to undergo phase segregation over time, which is accelerated at higher temperature. ‘Fullerenes tend to crystallise and aggregate into lumps,’ says Tom Aernouts, team leader for organic photovoltaics at IMEC. ‘This will affect device performance.’

Adding bulky side chains to the polymer backbone helps to make the conjugated polymer more soluble. The higher molecular weight also makes the molecule heavier and less mobile and more temperature stable, Aernouts says. The side chains can be crosslinked with the fullerene, to further stabilise the mixture. Standard cells have lifetimes of ‘a few hundred hours’ before their performance decreases by 20%. Aernouts says that some of IMEC’s new compounds have seen a tenfold increase in lifetime.

IMEC’s efforts to develop a barrier layer are an attempt to replace silicon oxides, which work well but are brittle and do not ‘flex’, with the PET substrate.

‘The big challenge is to find barrier layers that are transparent,’ Aernouts says.

The two-year project combines Cytec’s expertise in interfacial engineering and coating technology with IMEC’s experience in organic solar cell processing and analysis.

In separate research, IMEC has demonstrated a potentially cheap way of mass-producing organic solar cells. It has used spray-coating – which is commonly used in industrial coating – to deposit an active layer and a metal top contact layer on a plastic substrate. The active layer is a solution of two substances: a conductive polymer known as P3HT and a fullerene acceptor molecule, PCBM. When deposited by spray-coating, it showed power conversion efficiencies above 3%, which is comparable to that of spin-coated devices. For the metal top contact, IMEC spray-coated a solution of silver nanoparticles, and sintered it at 150°C – compatible with processing on flexible substrates.

The spray method has several advantages: it can cover larger areas, so could be used to make larger cells; and it is easy to scale up, so is more suitable for mass production.

‘R&D on organic solar cells has now reached the stage where we can consider low-cost, highvolume manufacturing,’ says Aernouts. ‘This is essential for the uptake of this technology by industry.’

These two strands of IMEC’s research – its spray coating technique, and its attempts to extend solar cell lifetimes – are complementary, Aernouts adds. ‘The end point will be to combine them. This could happen within two years.’

As well as having a much lower price, organic solar cells could be produced much more flexibly. Instead of relying on a silicon foundry, plastic solar cells could be produced as easily as a newspaper or magazine.

US company Solterra has already begun to build a commercial plant in Jeddah City in Saudi Arabia, which will supply its Quantum Dot solar cells to the booming construction market there. It expects to begin mass production early in 2010. ‘Several solar module manufacturers are waiting to transfer our printed cells into large area devices,’ says ceo Steve Squires. ‘We expect to start shipping at the end of Q1.’

At the same time, Konarka has developed a line of solar panels, for applications such as battery chargers, handbags and restaurant umbrellas, that will be available to product developers by the end of this year. Its Power Plastic 20 Series includes the Power Plastic 120 (1 Watt), 320 (3W) and 620 (7W) products. Panels vary in size and are available with or without integrated connectors – so are ready to use or can be integrated into a manufacturer’s device or product. ‘Manufacturers from new and existing markets will integrate these solar panels into their products,’ says Rick Hess, president and ceo. ‘Initial applications will address portable and remote power needs.’

In 2010, Konarka plans to launch products for higher and lower voltage applications.

Cells by the metre
Solterra and Konarka may have stolen a lead in their commercialisation plans, but other research teams are developing their own methods to print organic solar cells.

CSIRO, the Australian research organisation, calls it ‘solar cells by the metre’. It is working with a number of partners, including BP Solar, roofing supplier Bluescope Steel and Merck, to develop a ‘reel to reel’ printing process for organic solar cells.

Securency International – whose products include polymer banknotes for countries such as Australia and Thailand – will carry out the printing. Peter Batchelor, Victoria’s minister for energy and resources, said at the launch of the project: ‘The production of these film-like solar cells will literally be as easy as printing money.’

The ability to print solar cells onto thin film substrates will make energy generation far more portable: solar cells could easily be incorporated into building components, such as roofs or panels, enabling efficient local energy generation, says CSIRO.

By 2020, it expects to be producing 100,000km/ year of polymeric solar cells, which could generate as much energy as a conventional coal-fired or nuclear power station.