We’ve raved about solar cells previously: here, and here, the technology has taken several quantum leaps over the past decade. Paintable crystalline and printable solar cells seem to be the way of the future, the fight now is for real solar efficiency. Solar panels that can be simply printed have inched a step closer with the development of an energy efficient, organic, small-molecule solar cell. The solar cell, which was developed by a team from the University of California, Santa Barbara, has energy efficiencies of 6.7 per cent, which rivals the best polymer-based solar cells. Most polymer-based designs have reached the 6 to 8 range for efficiency.
“These results provide important progress for solution-processed organic photovoltaics and demonstrate that solar cells fabricated from small donor molecules can compete with their polymeric counterparts,” the authors, including Nobel Prize winner Professor Alan Heeger, wrote in Nature Materials.
A combination of polymer design, morphology control, structural insight and device engineering has led to power conversion efficiencies – PCEs – reaching the 6–8% range for conjugated polymer/fullerene blends. Solution-processed small-molecule BHJ [SM BHJ] solar cells have received less attention, and their efficiencies have remained below those of their polymeric counterparts. The researchers have reported an efficient solution-processed SM BHJ solar cells based on a new molecular donor. The high efficiency was obtained by using remarkably small percentages of solvent additive during the film-forming process, which lead to reduced domain sizes in the BHJ layer. These results provide important progress for solution-processed organic photovoltaics and demonstrate that solar cells fabricated from small donor molecules can compete with their polymeric counterparts.
Materials scientist Dr Chris McNeill from Monash University says the paper marks an incremental improvement in the field, “but an important increment”.
Organic solar cell devices are under intense investigation in academic and industrial laboratories worldwide because of their potential to allow mass production of flexible and cost-effective solar devices. Although they have similar properties to the silicon panels that lie on many Australian roofs, they can be manufactured more cheaply, and their lightweight and flexible characteristics will also allow them to be used in a variety of applications and over uneven surfaces.
Professor Paul Meredith, of the Centre for Organic Photonics and Electronics at the University of Queensland, says most attention to date has been on the development of organic polymer-based solar cells. However these tend to be very difficult to control and produce uniformly in terms of size and properties.
“If I was to make a bucket of these polymers and I was to look at every molecule, I would invariably see a large distribution of different shapes and sizes,” he said.
“With these new small-molecules, if you get the synthesis right, then every molecule is virtually identical, and this gives you much better control over the manufacturing of the solar cells.”
Professor Meredith says the next step is to build a commercial-sized module that maintains this level of power conversion.
Panels on roofs are about 10 square centimetres, he says, while the active area of the small-molecule solar cell in the Nature Materials paper is 0.196 square centimetres.
“The problem with organic solar cells currently is when we try to scale up to larger devices the best [power conversion] we’ve come up with is a couple of per cent,” Professor Meredith said.
He says there remains a question as to whether the small molecule solar cell can maintain its conversion efficiency in large area cells and if it can be made on an industrial scale.
But he says it is “a beautiful molecule … very elegant.”
“The thing that surprises me is it organises itself so well in order to have such good electrical properties,” he said.
Dr McNeill says the main benefit of organic solar cells is they can be manufactured cheaply in a reel-to-reel printing process similar to that used by newspapers.
“If you can manufacture a module that is lightweight, efficient and low cost, people will take that up,” he said.
“With organic solar cells you don’t need to do any changes to the infrastructure of the roof because you can literally roll it out, stick it on and plug it in.”