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Polymer photovoltaics

by Michael Smith
NBS Information Specialist

Cheaper and lighter than its more expensive, cumbersome silicon cousin, polymer photovoltaics (PV) could herald a revolution in the solar power market. However, polymer (also known as plastic or organic) PV technology is relatively new and still being researched by universities, national laboratories and industry worldwide. This article looks briefly at the technology.

What is polymer PV?

The majority of organic materials in day-to-day use are essentially insulators, which don't conduct heat or electricity; however, the specialised molecular structure of organic PV allows the free flow of electrons.

Polymer solar panels differ from most commercial plastics, like polythene, which are essentially insulators. Turning them from a material that prevents conductivity into one that promotes it requires chemists to 'tweak their molecular structure'.

Polymer PVs are a type of flexible solar cell with a stable, thin-film semiconductor deposited on different types of plastic substrate.

Currently, commercial solar cells are made from refined, highly purified silicon crystal. The high cost of these cells and their complex production process has generated interest in developing alternative PV technologies.

Advantages/disadvantages

Compared to silicon-based devices, polymer solar cells are lightweight, potentially disposable and inexpensive to fabricate, flexible, and customisable at a molecular level. The costs however are higher, due to lower efficiency and durability levels. At present, research and development efforts are underway to bring down overall costs of manufacturing and set price levels at less than €1/Watt.

In principle though, these devices could be very inexpensive and cover large areas. They are also considered to have a lower potential for negative environmental impact.

However, there are serious disadvantages in using polymer PV: they only offer about half of the efficiency of hard PV materials, though this figure is improving all the time; the efficiency of the best plastic devices is little more than 8%, whereas silicon solar panels can achieve up to 18%.

Currently, plastic PV is also relatively unstable toward photochemical degradation; the lifetime of plastic PV currently doesn't come anywhere near that of silicon solar panels, which can last around 20 years. However, this may change dramatically when good protective coatings are developed.

For these reasons, and despite continuing advances in semiconducting polymers, the vast majority of solar cells still rely on inorganic materials.

How does the technology work?

Organic technology solar cells are essentially based on the photosynthesis process in plants. The absorption of light in organic cells is done by a 'dye' which substitutes for the silicon in conventional cells. Light causes the dye molecules to become excited and release electrons that are converted to electrical energy.

The absorption of light occurs in dye molecules that are in a highly porous film of titanium dioxide (TiO₂). This causes the electron to be injected into TiO₂ and is conducted to the transparent conductive oxide layer (TCO).

For now, organic semiconductors have the disadvantage that no free charges are formed directly upon absorption of a photon. Instead, an excited molecule (or exciton in the solid state) is created in which the hole and electron are still bound significantly. A second, either more or less electronegative, material is needed to separate the hole and the electron by charge transfer in one or the other direction. Hence, organic photovoltaics are based on the combination of two materials that have a electronic donor‐acceptor relationship.

Uses of polymer PV

David Lidzey from the University of Sheffield, is an expert in organic photovoltaic technology. He says that unlike rigid silicon panels, polymer (or organic) PV is far more flexible making it easier to install, handing it a huge advantage. "If you've got panels that almost roll up like a big sheet of wallpaper then that might be a very good way of powering developing countries."

He also adds that some everyday polymers aren't far from the plastic PV he's researching. "If you look at a crisp packet, what you've got is a plastic film, a few layers of inks and a printed metal layer to keep the materials fresh. Rearrange the order of those layers and you get to a structure that's very similar to the devices we're looking at."

Lidzey also believes that while there is a gulf in efficiency and operational lifetime, this may not be a problem if the cost of plastic PV can be kept down: “The idea is that you might not need to catch up provided you can make them cheap enough.“

Viability

At the moment, an open question remains: to what degree can organic solar cells commercially compete with silicon solar cells and the other thin-film cells? At the moment the silicon solar cell industry has the important industrial advantage of being able to leverage the infrastructure developed for the computer industry.

However, the tide is slowly turning in favour of organic PV technology, and the first trickles of funding are beginning to show. Researchers at the Intelligent Polymer Research Institute (IPRI) at the University of Wollongong in Australia were recently awarded a new Australian Research Council (ARC) Linkage Grant of AUD$320,000 over three years. While this does not seem like much, it is a start in a, previously, largely ignored area.

With this grant, the institute will look at developing a fundamental understanding of how electrical charges are generated under solar irradiation in organic materials, which will be used to increase the efficiency of organic solar cell technology.

The future of polymer PV

Organic PV devices show great promise for decreasing the cost of solar energy to the point where it may become widespread in the near future.

While great progress has been made in the last ten years with respect to understanding the chemistry, physics, and materials science of organic photovoltaics, work remains to be done to further improve their performance. Specifically, novel nanostructures must be optimised to promote charge carrier diffusion; transport must be enhanced through control of order and morphology; and interface engineering must be applied to the problem of charge transfer across interfaces.

New and novel molecular chemistries and materials, however, are beginning to offer hope for revolutionary breakthroughs in the efficiencies of these devices in the (near) future.

State of the art – Konarka 'Power Plastic'

One of the leading lights in developing plastic PV is US-based tech company, Konarka, who are already applying their 'Power Plastic' technology to a wide range of small products. Larger arrays are also being fitted to street furniture, as can be seen on San Francisco's bus shelters.

Konarka Technologies opened a factory in 2008, with the capacity to produce a gigawatt's worth of polymer-fullerene solar cells each year.

The company aims to initially sell the cells for a number of niche applications, such as in laptop-recharging briefcases; put into tents, umbrellas, and awnings; and as window tinting (the cells can be made semi-transparent).

In 2009, Konarka installed a curtain wall to an outside section of its Florida offices as part of a pilot project. Plastic PV, say Konarka, can absorb sunlight from 'all sorts of ranges' allowing it to be installed onto vertical walls.

In late 2010, Konarka announced that its organic-based photovoltaic (OPV) cell was verified by the US National Energy Renewable Laboratory (NREL) as having demonstrated a record-breaking 8.3% efficiency; this figure has risen by around 1% per year since 2005.