New Ultrathin Solar Cells
Pros and Cons
While “conventional” crystalline Silicon photovoltaic cells have an efficiency of about 15% to 20%, and multi-junction semiconductor PV cells well into to the 40% range, organic solar cells (OPV) cannot measure up in this regard. They are however, extremely light weight, cost efficient in production (roll to roll processing), flexible, and withstand mechanical deformation such as stretching or compressing. Just like Silicon thin film solar cells, they can be put on almost everything like consumer electronic devices, or have the potential to be integrated into windows and other building materials.
Drawbacks are their comparably low efficiency and short lifetime of about one year. Like other PV cell technologies, their efficiency has constantly been increasing over the past decade; from a very modest 3% in 2004 to a 7.9% in 2009, developed by Solarmer Energy, CA. Early this year, a research group at the University of California Los Angeles reached a maximum efficiency of 10.6%, but partners for commercialization are still needed. And in April 2012, the German company Heliatek caught up with a very close 10.7%. The efficiency record of 11% for organic thin-film PV cells as of today though was being announced shortly thereafter by Mitsubishi Chemical Corp.
Efforts are not only being made to increase longevity and efficiency, but also to optimize the weight-efficiency ratio. A research cooperation between the Johannes Kepler University Linz, Austria and the University of Tokio, Japan, developed the lightest OPVs so far, as published in the April 2012 edition of “Nature Communications”. With a thickness of 1.9 µm (or roughly 0.000 074 in), and high flexibility and bendability those organic thin film PV cells can be wrapped around a human hair, which has a diameter of about 35 µm. They consist of polymer-based material on plastic foil substrates, showing an efficiency of 4.2% and withstanding extreme mechanical deformation. The authors claim to have developed the highest power output per weight, 10 Watt per gram, which is 25 times more than can be found in the high efficiency multi-junction cells. Together with the already proven cost effective roll to roll (R2R) production technique, they are very promising for applications in aircrafts (satellites), robotics, electronic skin, textiles, etc. The R2R manufacturing process is a common term for applying a thin layer of (in this case photovoltaic) material onto a substrate, enabling large scale production.
Really That Green?
Another important issue, which will inevitably get more attention in the future, is the recyclability of solar cells. The less (thinner) material is used, the less needs to be recycled (“R”educe beforehand); of course, factors like lifetime (old ones need to be replaced too often) and the amount (due to poor efficiency) of the even ultra thin films result eventually in increased use of material. There is no satisfying process or regulation right now for recycling Silicon based PV cells or thin film cells (some types containing carcinogenic materials such as Cadmium). Silicon based solar cells contain materials like glass, aluminum, semiconductor components, which are all valuable materials for recycling.
The big amount of solar modules that already have been installed over the past couple of years and that are still being installed in a growing number will reach their end of life in about 15 to 20 years the latest, so this will become an impending issue more sooner than later.