Exciton times for photovoltaics

Organic semiconductors have promised flexibility for electronic devices for many years, but there remain obstacles in their development into inexpensive TV screens that roll up, roofing tiles that double as solar panels and solar-powered fabrics. Now, Madalina Furis of the University of Vermont and her colleagues have turned to a colorful old friend, the blue dye molecule, which they say will lift the barriers on an "electron superhighway" [Furis et al, Nature Commun, 2015, 6, online; DOI: 10.1038/ncomms9201]

Excitons are neutral species and are not pushed along by a voltage, nevertheless they bounce from one tightly stacked phthalocyanine molecule to the next carrying their payload of solar-driven energy with them until it is released as an electric current. In order to work efficiently and effectively the excitons in organic thin film photovoltaic materials must be able to diffuse sufficient distance before they break down and they release their energy as an electric current.

Furis and colleagues have carried out scanning laser microscopy with linearly polarized light and exploited photoluminescence to optically probe the molecular structure of phthalocyanine crystals in a thin film. The new imaging technique reveals the structural defects and grain boundaries in the crystal that act as bumps and potholes to exciton traffic. Recently, the US Department of Energy identified "determining the mechanisms by which the absorbed energy (exciton) migrates through the system prior to splitting into charges that are converted to electricity" as an important part of the research needed to boost photovoltaic research and development. "One of today's big challenges is how to make better photovoltaics and solar technologies," echoes Furis, who directs UVM's program in materials science, "and to do that we need a deeper understanding of exciton diffusion."

Such insights should allow the team to remove the roadblocks to open up an electron superhighway in the thin film by very carefully controlling how the thin films are deposited in the first place to smooth out the bumps and fill in the potholes. The team used a novel "pen-writing" technique with a hollow capillary, working alongside Randy Headrick, to make films with jumbo-sized crystal grains and "small angle boundaries" to create smooth sliproads on to the highway.

Though the team focused on phthalocyanine in their paper, the new imaging technique they have developed could be used to explore many other types of organic materials. "We view our work as one of the few attempts to understand these small molecule semiconductors from the perspective of condensed matter physics rather than physical chemistry," Furis told Materials Today. "What that means is we employ a series of experimental tools and concepts that have been useful in understanding long-range order in inorganic systems in the past, towards a  better understanding  of excitonic phenomena that extend beyond the single molecule or nearest neighbor molecules  in these systems."

David Bradley blogs at Sciencebase Science Blog and tweets @sciencebase, he is author of the bestselling science book "Deceived Wisdom".