Solar cell technology was one of the many topics discussed at the 2010 Spring Meeting of the Materials Research Society, which took place in San Francisco this April. Credit: CHRIS GLADDEN

Solar cell technology is arguably the most economically viable and practical form of 'green' energy generation. It is also a topic that attracted much attention from researchers attending the recent 2010 Spring Meeting of the Materials Research Society (MRS), which took place from 5 to 9 April in San Francisco, USA.

The meeting, which attracted around 4,500 registrants, 42 symposia and 3,892 papers, covered several fields relating to photonics, with talks ranging from optoelectronics to metamaterials and plasmonics. Within photonics, the theme that gathered the greatest interest was undoubtedly solar cells — particularly those based on emerging technologies such as solution-processed, organic or dye-sensitized designs. Indeed, day one of the event kicked off with a series of lectures that included the 'Fred Kavli Distinguished Lectureship In Nanoscience' talk by Harry Atwater from Caltech, USA. Atwater explained how new approaches in photonics, combined with advances in materials science, may push solar energy conversion to the terawatt scale.

In the quest to reduce the cost of photovoltaic device fabrication, solution-processing approaches, which are seen by many as a promising large-scale alternative to traditional semiconductor manufacturing, were much discussed. Susan Habas from the National Renewable Energy Laboratory in the USA spoke about the development of fabrication techniques for photovoltaic devices based on liquid precursors. In particular, she discussed the use of metallization 'inks' as a convenient means of forming contact electrodes. The inks are printed onto heated substrates using an aerosol spray system enclosed within a nitrogen atmosphere, with the thickness of the lines (widths of <50 μm) controlled by the substrate temperature and the ink deposition rate.

“Metal organic decomposition inks, for example, can be deposited by non-contact methods such as ink-jet printing or aerosol spraying, to form the front contacts of solar cells. In addition to silver, low-cost metals such as nickel and copper can be printed with micrometre-scale resolution and with conductivities very close to those of the bulk metal,” Habas explained. “Furthermore, the chemistry of the solution precursors can be precisely tailored to yield the desired material with no post-processing steps. The development of soluble precursors for thin-film absorber layers, conductive oxides and metal contacts, in combination with solution deposition and processing techniques, has the potential to lead us towards fully printable photovoltaic devices.”

One point that was raised time after time, particularly by those attending presentations, was that solar technology should be truly green. The environmental impact of various solar cell materials and manufacturing processes came under fire during several talks. Some presentations specifically tackled this issue, including that of Gopala Krishna and Jean-Christophe Bolsée, both from Hasselt University in Belgium, who described recent progress in 'green' Grätzel solar cells with water-soluble constituents. Grätzel cells, also known as dye-sensitized solar cells, have attracted interest due to their potentially low cost when compared with standard silicon solar cells. However, many researchers are concerned about the environmental impact of the particular liquid electrolytes being used in such cells. Several talks at the meeting focused on all-solid-state designs, in which the liquid electrolyte is replaced with a solid-state hole transport material such as polythiophenes. However, polythiophenes are soluble in organic solvents, which make them toxic and not environmental friendly, according to the Belgian researchers. “To overcome this, we introduced water-soluble polythiophenes as hole conductors and photo-active layers for solar cells,” Bolsée told Nature Photonics. “Solid-state polymer/TiO2 nanocrystalline hybrid solar cells deliver efficiencies of up to 0.7%, which is five times higher than the efficiencies achieved so far using water-soluble polythiophenes.”

Talks on organic solar cells were also aplenty, with a discussion session and several symposia debating the various hurdles in attaining solar cell efficiencies of >10%. Among these interesting talks was a presentation by Karl Leo from the Dresden University of Technology in Germany, who announced a tandem organic photovoltaic device with an efficiency of 7.7%. The 1.1 cm2 device, fabricated at Heliatek, was made using the thermally evaporated small-molecule approach, with initial tests suggesting a lifetime of >10 years.

Other exciting developments for efficient, cost-effective and environmentally friendly solar technology will no doubt feature strongly at the next Spring Meeting of the Materials Research Society, which takes place in San Francisco from 25 to 29 April 2011.