A combination of organic thin films and carbon nanotubes could make large-scale solar energy generation commercially viable, says Emmanuel Kymakis, an engineer based at the Technological Educational Institute in Crete, Greece.
In silicon-based photovoltaics, power efficiencies have reached 25%, but the downside is that fabricating mono-crystalline silicon cells is both technically difficult and energy-intensive, leading to high manufacturing costs and lengthy payback times.
A different approach is to use polymer films. Despite the relatively poor efficiencies achieved so far (currently around 5%), there is widespread interest in organic solar cells owing to the low production costs of lightweight, flexible and large area thin films.
Combine polymer films with carbon nanotubes, and the performance rises dramatically, as Kymakis and his colleague Gehan Aramatunga first showed in 2002. In a review article published recently in the journal Nanotechnology Law & Business, Kymakis discusses the use of nanotubes in photovoltaic devices.
Carbon nanotubes are well-suited to use in solar cells since they not only efficiently transport electrons, but also provide a high electric field at the polymer/nanotube interface. This favours dissociation of the electron-hole pairs that form when sunlight hits a photovoltaic cell, and thus facilitates charge collection.
In time, efficiencies as high as 15–20% should be achieved, says Kymakis. He adds: “The lifetime problem is a very important issue that must be tackled in conjunction with efficiency. Nevertheless, the success of organic LEDs with lifetimes greater than 40,000 hours encourages the scientific community. Once higher efficiencies are reached, organic solar cell technology can compete directly with silicon.”
The challenges that must be met before organic photovoltaic cells can become commercially viable are efficiencies of 5–10 %, lifetimes greater than 5 years, and costs of less than €0.75 per peak watt.
But even now, there is commercial interest in using organic solar cells for use in low-power devices. The cells can be painted onto flexible substrates and act as portable power generators for wearable electronic devices.
Emmanuel Kymakis – The Impact of Carbon Nanotubes on Solar Energy Conversion, Emmanuel Kymakis, Nanotechnology Law & Business 3, 405 (2006) (subscription required for full article).
Structure of an organic polymer and carbon nanotube-based photovoltaic cell. Image courtesy: Emmanuel Kymakis.
Article first published in Nanomaterials News.