Nanotechnology

[FreeGrass]

Kicking around solar cell materials

August 15, 2010

Sheffield's David Lidzey working with Athene Donald of the University of Cambridge and experts from Cardiff University and Nick Terrill at the Diamond Light Source, the UK's synchrotron facility in Didcot, have investigated the structure of solar cell materials to help them improve photovoltaic efficiencies. The research into understanding the structure of plastics used in new-types of low-cost solar cell based on fullerenes could improve their efficiency significantly.

Photovoltaic semiconductor devices have the potential to generate low-cost renewable-electricity simply by absorbing and converting the energy of sunlight. Current silicon-based devices are expensive and fragile so researchers have been looking for alternative organic-based materials with the right optical and electronic properties. A photovoltaic film made from plastic would be far less expensive than silicon as well as being flexible and easier to fabricate to fit particular devices, roof profiles, or other areas where renewable energy might be needed.

The development of plastic photovoltaic devices will inevitably rely on being able to control morphology of thin organic semiconductor films at the nanoscale where charge-generation and charge-extraction occur. Specifically, the researcher say, "The power conversion efficiency in a conjugated polymer-functionalized fullerene bulk heterojunction organic photovoltaic (OPV) device [turns out to be] dependent both on the electronic properties of the constituent materials and on the nanoscale morphology of the active semiconductor layer thin-film."

Earlier research has investigated thin film structure after casting the materials from solution. But, members of the Sheffield-led collaboration wanted to understand the dynamic processes that occur as the semiconductor thin-film blend is first being deposited. This, they hoped, would give them insights into the structure and morphology of the film in real-time as the solution dried. Such dynamic information would allow them to uncover the basic mechanisms of film formation and so provide new clues to the design and production of improved materials.

"This is the first time we have unravelled the nanostructure evolution from random, long chains into densely packed nanocrystals," says Sheffield team member and post-doctorate researcher Tao Wang. "The results will direct us to better control our device fabrication process for high efficient large-area plastics solar cells."

The data obtained using the very bright synchrotron X-rays generated by Diamond to carry out grazing incidence X-ray scattering (GI-XS) combined with the equally powerful technique of spectroscopic ellipsometry (carried out at Sheffield) has given the team just such information. They investigated molecular self-organization in thin film formation from a solution of a polymer blend of poly(3-hexylthiophene) (P3HT) and the all-carbon material fullerene in the form of [6,6]-phenyl C61-butyric acid methyl ester (PCBM). Both are required in their photovoltaic material for electrical charge generation.

The data showed that the drying process and consequent crystallisation take place in three main stages, with the second stage of rapid polymer crystallisation being the most important. "We illustrate the evolution of the extinction coefficient from a solution to a solid, semi-crystalline state," the team explains. "We show that once the solvent fraction in the film falls below 50%, the P3HT undergoes rapid crystallization via heterogeneous nucleation; a process that is complete in seconds." A higher degree of crystallinity is associated with more rapid electrical charge-conduction and therefore more efficient device operation. As such, the team adds that the mechanistic insights into film formation revealed by their study demonstrate how the deposition processes might be optimized for large-area organic photovoltaic manufacture. The team is now working on manipulating the crystallisation of various polymers.

As the University of Sheffield launches Project Sunshine, aimed at bringing together scientists from various disciplines to tackle the challenge of solar energy, Lidzey enthuses about his team's work: "This has been a very exciting experiment for us - we have used Diamond Light Source to carry out some important science on a technologically important class of materials," he says." This has allowed us to understand a process that has so far remained unexplored in these materials. This information will quickly feed into our solar-cell research programme."

"Operational devices based on this material system have been around for some time," Lidzay told SpectroscopyNOW. "However, lots of groups are trying to understand how the structures that self-assemble in the active layers of devices confer efficient (or otherwise) operation. Once we understand this better, we can apply knowledge to the design and processing of new photovoltaic materials that chemists are currently synthesizing to make even more efficient devices."