Finding a relatively simple way of polymerizing fullerenes – which allows them to be treated like monomers – and including this new material in photovoltaic cells, Roger C. Hiorns, a senior researcher at the Université de Pau et des Pays de l'Adour in France, and his collaborators have opened a route towards plastic solar cells that are much cheaper and easier to fabricate than conventional silicon-based photovoltaic panels.
Scheme and workings of the photovoltaic device based on multi-block copolymers and using nanoscale domains of polymers to create and transport electronic charges out to the electrodes. (Image: Roger C. Hiorns)
Today's solar panels are still too heavy and way too expensive for wide-spread everyday use. Only through generous government subsidies can solar energy compete with conventional (although even more subsidized) fossil-fuel and nuclear energy supply. Silicon-based solar cells (which make up some 95 percent of the solar cell market today) are made from a refined, highly purified silicon crystal, similar to those used in the manufacture of integrated circuits and computer chips. The high cost of these silicon solar cells and their complex production process has generated interest in developing alternative photovoltaic technologies.
Compared to silicon-based devices, polymer solar cells made of conducting plastic material are lightweight, relatively inexpensive to fabricate, flexible, designable on the molecular level, and have little potential for negative environmental impact.
The big question today is to what degree polymer solar cells will be able to commercially compete with silicon solar cells. There are two major issues that need to be solved: 1) The present efficiency of organic solar cells lies at only around 5-8 percent, compared to up to 30 percent for the most efficient silicon cells. 2) Polymer solar cells suffer from huge degradation effects: the efficiency is decreased over time due to environmental effects such as water, oxygen or UV rays.
With the exciting vision of organic solar cells becoming a low-cost electricity source available in any size and shape, as flexible thin films and even coatings, researchers all over the world are working on making organic solar cells commercially attractive. Hiorns' work is one such example.
"We'd like to see the efficiency of polymer solar cells go much higher, but that means really exploring lots of different routes and trying to see if we can't make completely new, novel plastic based devices" Hiorns tells Nanowerk. "So our work is really at the fundamental end of the scale – exploring new routes and making new materials that are really quite different to anything else out there right now."
He provides some background to the current research: Polymer-based photovoltaic devices, discovered in 1992 by N. S. Sariciftci ("Photoinduced Electron Transfer from a Conducting Polymer to Buckminsterfullerene"), have recently started to deliver efficiencies of almost 8 percent after a massive international effort. The photo-active layer is typically made from an annealed composite of electron donor (D) polymers blended with electron acceptors (A). In the dominant physical process, the polymer on irradiation forms an excited electronic state (called an exciton) that can move around 10 nm without disappearing. If it meets an interface of D and A, then it gives a negative charge on the acceptor and a positive charge on the donor. These charges then percolate to the electrodes.
"A big problem is that these charges can get lost on the way because the active material is essentially a poorly organized composite" says Hiorns. "So we thought it would be good if it were organized, with an interface every 10 nm or so, and channels to take the charges to the electrodes (as described here: "Alternatively linking fullerene and conjugated polymers"). The beauty of block copolymers is that you can make domains of about 10 nm, and you can have blocks that act like Donors and blocks that act like Acceptors, and quite a lot of groups have been working on this."
The chemical structure of the new multi-block copolymer, PFDP-block-P3HT. (Image: Roger C. Hiorns)
The originality in Hiorns' and his team's work are two novel approaches:
– to incorporate the team's high fullerene containing polymer PFDP because it's simple to make, has nice chain-ends to do chemistry on, is soluble in common solvents, and could be really quite strong.
The result has been the first example of a block copolymer based on a polyfullerene.
For their system, the researchers chose lengths of polymers to make the right domain sizes.
"We found that our multi-block copolymer – made from the archetypal donor polymer P3HT and our new PFDP – created the right domain sizes when we put it into device structures" says Hiorns. "This means that now we have the two materials, P3HT and PFDP, that are organized, pulling charges in the right way, and channel them to the electrodes."
With their prototype PFDP just mixed into devices with P3HT as a composite, the team's solar cell managed 1.6% efficiency – "pretty amazing for a first try without any development of the prototype PFDP" as Hiorns points out.
Now, with their new, self-organizing system based on multi-block copolymers, Hiorns and his collaborators are expecting to reach much higher efficiencies. The devices are already being tested.