Stability of organic photovoltaics
In the second part of our coverage of the event in San Diego last week, we cover highlights from presentations and personal discussions at the event. The first part can be read here.
Over the last three years significant developments have been made with efficiency of organic based PV cells. Plextronics demonstrated in 2007 an organic PV cell with 5.4% conversion efficiency. Konarka and others are also actively working on the technology. Organic PV can potentially be one of the cheapest PV technologies, but importantly also offers many other benefits (even enabling premium pricing) such as being lightweight, flexible, conformal, rollable, environmental etc.
However, there have been scattered reports on degradation of organic PV. Matthew Reese of the National Renewable Energy Lab (NREL) based in Colorado presented the results of their six week study of the effect of air, light and electrodes on organic PV stability. Firstly they discovered that using P3HT PCBM as the active organic layer and silver for the electrodes resulted in significant degradation. The most stable electrodes were found to be Calcium Aluminium. Using Ca/Al for the electrodes and combined with the organic active layer they made several devices which were stored in dark environments and light environments, and tested these against the active layer with no electrodes.
The results showed that the organic layer itself is not inherently unstable, and on its own under constant illumination caused only a limited reduction in performance. However, with the electrode attached there were differences. Versions with the electrode in air and stored in the dark were found to degrade by just under 50% over ten days.
However, versions in air and under illumination would rapidly degrade in tens of minutes and fail within a single sun illumination. Ironically, the light/heat kills the PV device. Reese concluded that the metal organic interface could be a significant contributor to degradation when photactivated. NREL is working on proprietary solutions with companies.
At IDTechEx we feel this is a significant issue facing developers of organic PV, more so than efficiency. We note that this has been a key reason why OLED displays have been slow to take off - versions today are sandwiched in glass but the true advantage is when flexible displays will be viable, otherwise their other uniques are less compelling perhaps when faced with cheaper alternatives which are also sandwiched in glass.
Flexible OLEDs may still be at least five years from wide scale commercialization. So too is this challenge with organic PV, where silicon versions are expected to become much cheaper (see yesterdays article), and have much higher efficiencies, but tend to be heavy, rigid, fragile, bulky etc.
Material supply and sustainability of thin film CIGS and CdTe Photovoltaics
Vasilis Fthenakis of the Brookhaven National Lab presented on the sustainability and availability of Indium (In). As we have covered in earlier articles on Printed Electronics World, at least two universities have independently warned of the cost of these materials reaching points where they become unusable for large area applications. Vasilis thought that with current growth rates supply should be fine, but agrees it is a problem if growth is higher. He explained that the industry's reaction has been to improve extraction efficiencies, with Indium extraction significantly improving over the last two years. The goal is that these materials should be about 50 cents per watt.
The Lab has also looked at the risks of some of the materials over the life cycle of the PV cell. Cadmium (Cd), a poison, was found not to leak from broken cells and in fire testing to over one thousand degrees Celsius held at several hours, Cd emissions where negligible. He estimates that in the total life cycle Cd emissions are 0.02g/GWh, about 100 to 360 times lower than the best coal firing powerplants.
The energy payback time (the time taken for the PV cell to recoup the energy that was expended in its production) is 1.1 years for CdTe, CIGS slightly higher, but silicon higher still ranging from 1.7 to 2.7 years.
Konarka, USA, inkjet print organic PV
Konarka has demonstrated manufacturing of organic bulk heterojunction solar cells using inkjet printing. "Demonstrating the use of inkjet printing technology as a fabrication tool for highly efficient solar cells and sensors with small area requirements is a major milestone," commented Rick Hess, president and CEO at Konarka.
Mitsubishi Chemical Corporation, Japan, develop organic PV
Applied Materials, USA, announce gigawatt silicon PV order
Applied Materials got involved with polycrystalline silicon PV and amorphous silicon PV about two and a half years ago. It now has 1,000 employees working on the sector. The company sells equipment for companies to make PV cells and panels. Over the last few years, it has achieved $700 Million in sales for PV equipment and has just announced an order for what it claims to be the world's first thin film gigawatt fab. Charles Gay, Corporate VP and General Manger of their solar business, sees one of the bottle necks to industry growth as being a lack of sufficiently skilled people across the value chain.
Solar cell production by company
The largest global solar cell producers are shown as follows, in order of production capacity:
1. Q-cells, Germany
2. Sharp, Japan
3. Suntech, China
4. Kyocera, Japan
5. First Solar, USA and Germany
6. MOTECH, Taiwan
Of this list First Solar is the only company to work on a non silicon platform (they use CdTe). In 2007, the company produced 207 MW of cells versus 60 MW in 2006.