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The future looks bright for organic solar cells

Researchers in the UK are developing golden window electrodes that will assist in the drive of third generation solar cells, dubbed organic solar cells

First Solar
The largest manufacturer of solar cells in the world today is a second generation technology company called First Solar

Researchers at the University of Warwick have developed a gold plated window as the transparent electrode for organic solar cells. Contrary to what one might expect, these electrodes have the potential to be relatively cheap since the thickness of gold used is only 8 billionths of a metre. This ultra-low thickness means that even at the current high gold price the cost of the gold needed to fabricate one square metre of this electrode is only around USD7. It can also be readily recouped from the organic solar cell at the end of its life and since gold is already widely used to form reliable interconnects it is no stranger to the electronics industry.

 

Organic solar cells have long relied on Indium Tin Oxide (ITO) coated glass as the transparent electrode, although this is largely due to the absence of a suitable alternative. ITO is a complex, unstable material with a high surface roughness and tendency to crack upon bending if supported on a plastic substrate. If that wasn’t bad enough one of its key components, indium, is in short supply making it relatively expensive to use.

 

An ultra-thin film of air-stable metal like gold would offer a viable alternative to ITO, but until now it has not proved possible to deposit a film thin enough to be transparent without being too fragile and electrically resistive to be useful.

 

Now research led by Dr Ross Hatton and Professor Tim Jones in the University of Warwick ’s department of Chemistry has developed a rapid method for the preparation of robust, ultra-thin gold films on glass. Importantly this method can be scaled up for large area applications like solar cells and the resulting electrodes are chemically very well-defined.

 

Dr Hatton says “This new method of creating gold based transparent electrodes is potentially widely applicable for a variety of large area applications, particularly where stable, chemically well-defined, ultra-smooth platform electrodes are required, such as in organic optoelectronics and the emerging fields of nanoelectronics and nanophotonics”.

 

The University of Warwick research team has also had some early success in depositing ultra-thin gold films directly on plastic substrates, an important step towards realising the holy grail of truly flexible solar cells. This innovation is set to be exploited by Molecular Solar, a Warwick spinout company dedicated to commercialising the discoveries of its academic founders in the area of organic solar cells. 

 

The background 

Solar power technology development can be broadly classified into three generations. The first generation began with the invention of the silicon solar cell at Bell Laboratories in 1954. The technology was licensed to Hoffman Electronics who produced the first commercial solar cell in 1955 having an efficiency of 2 per cent for USD24/cell or USD1785/watt. Driven initially by the space program and subsequently by terrestrial applications silicon cell efficiencies and costs have improved considerably over the past 55 years. Today’s silicon solar cells have efficiencies of 20 per cent and cost USD1-2/ Watt. The theoretical limit for the efficiency of an ideal singlejunction silicon solar cell is 30 per cent, but the efficiency of a practical cell is limited to the low 20 per cent range by material imperfections. It has proven difficult to reduce costs further because of the inherently high cost associated with silicon processing. Silicon is a cost effective substrate when incorporating billions of transistors/cm2 but it becomes prohibitively expensive when incorporating the single junction/ large area functionality of a solar cell.

 

Second generation technology utilises thin semiconductor films to overcome the cost limitations of bulk crystalline silicon and are typically supported on glass substrates. The largest thin film companies use Cadmium Telluride (CdTe), amorphous silicon (a-Si) and Copper Indium Gallium Selenide (CIGS). Second generation product sales are currently about USD4b accounting for about 20 per cent of the global solar cell market.

 

Conversion efficiencies for second generation solar cells in production are about half those of silicon cells at 8-12 per cent. Although the costs of thin film semiconductor cells have fallen steadily to less than USD1/Watt there are issues with toxicity and supply which will ultimately limit the cost progress of most second generation technologies. At present it is difficult to predict just where this will occur. We speculate that this may be in the range of USD0.20 -0.40/Watt depending on the ultimate cost of the rare materials used and total life cycle cost including recycling costs for the toxic elements used in some second generation cells.

 

Third generation technology is generally considered to be the production of solar cells employing thin films (< 200 nm) of organic semiconductors as the light harvesting component. These devices have strong potential to be fabricated at exceptionally low cost (<USD0.20/Watt). The cells will be flexible and weigh less than 1 per cent of today’s silicon cells. These advantages are expected to drive new applications that will further accelerate the growth of the solar power market, which has seen 40 per cent/year growth over the last ten years. The use of organic materials that can be mass produced cheaply with minimal environmental impact will also be a critical driver for third generation market growth.

 

Future concerns aside for the moment, the largest manufacturer of solar cells in the world today is a second generation technology company called First Solar. It was founded in 1999 and makes thin film solar cells using CdTe. The first volume production by First Solar was in 2004 with 25MWatts produced. In 2009 their production output was about 1.1GWatt which represents a 9 per cent market share. For 2009 revenue was USD2b and net income was USD640m (32 per cent). Notably, the late John Walton of Wal-Mart fame was an early investor in First Solar. During the past year the Walton estate has sold 330,000 shares in First Solar at USD140/ share equating to a USD46.2m return, illustrating both the pace and the opportunity in the solar power industry.

 

Looking ahead

The challenge to all new photovoltaic (PV) technologies is to reduce the cost of solar electricity generation to be competitive with the cost of conventionally generated electricity at the point of use.

 

When this Grid Parity is achieved an immense nonsubsidised market for PV will emerge. Organic photovoltaic (OPV) technology offers long term opportunities to meet this goal since the basic material is low cost and has no supply constraint. However, the efficiency and lifetime of current OPV systems do not meet the basic requirements for grid-connected electricity generation. The first generation of OPV technology will however be particularly well matched to lucrative, high volume consumer applications for the following reasons:

 

  • Low material cost and sustainability
  • Non-toxic components
  • Unconstrained end-of-life disposability/recyclable
  • High power-to-weight ratio for portability
  • High-speed deposition reducing unit capital investment
  • Flexible substrate enabling compact and versatile product form
  • The potential for significant efficiency improvement through the discovery or engineering of new or tailored compounds 

 

A unique selling point of the Molecular Solar small molecule OPV approach is the potential to realise the highest efficiency and highest operating voltage OPV devices exploiting the relative ease of fabricating multi-stacked cells that can be tailored to extract energy from a large proportion of the solar spectrum. Hence, it is able to closely match the requirements needed for the consumer electronics.

 

A second unique selling point is Molecular Solar’s FLEXIFILMTM substrate technology that provides a flexible low cost alternative to current ITO-glass technology which is expensive and inflexible. The demonstration of the OPV cell technology on the FLEXIFILMTM substrate has very recently been verified. To date, Molecular Solar has filed three patent applications in relation to the photoactive elements, cell structure and substrate electrodes of OPV devices. Furthermore, the ability of the founders to innovate in relation to all aspects of device construction (materials synthesis, device architecture, electrodes, etc) gives them a strong competitive advantage.

 

The third unique selling point of Molecular Solar’s technology is the use of small molecule organic materials – a class of materials which have demonstrated good lifetime performance in commercial OLED applications and amenability to high throughput production methods. Moreover, Molecular Solar’s OPV technology does not utilise doped semiconductor layers, thus simplifying device fabrication and structure. This is a particular advantage over Heliatek's technology, the closest competitor, which is built-around doping the organic semiconductors close to the contacts. Other competitors in third generation technology include Konarka, Plextronics, Global Photonic Energy Corporation, G24 and Solamer.

 

The first companies to discover or engineer solutions that meet commercially viable product efficiency, cost and lifetime targets and protect their intellectual property are most likely to succeed.