ScienceDaily (Oct. 17, 2008) —
Researchers have created a new material that overcomes two of the major
obstacles to solar power: it absorbs all the energy contained in
sunlight, and generates electrons in a way that makes them easier to
capture.

Ohio State University chemists and their colleagues combined
electrically conductive plastic with metals including molybdenum and
titanium to create the hybrid material.

"There are other such hybrids out there, but the advantage of our
material is that we can cover the entire range of the solar spectrum,"
explained Malcolm Chisholm, Distinguished University Professor and
Chair of the Department of Chemistry at Ohio State.

Sunlight contains the entire spectrum of colors that can be seen
with the naked eye — all the colors of the rainbow. What our eyes
interpret as color are really different energy levels, or frequencies
of light. Todays solar cell materials can only capture a small range
of frequencies, so they can only capture a small fraction of the energy
contained in sunlight.

This new material is the first that can absorb all the energy contained in visible light at once.

The material generates electricity just like other solar cell
materials do: light energizes the atoms of the material, and some of
the electrons in those atoms are knocked loose.

Ideally, the electrons flow out of the device as electrical current,
but this is where most solar cells run into trouble. The electrons only
stay loose for a tiny fraction of a second before they sink back into
the atoms from which they came. The electrons must be captured during
the short time they are free, and this task, called charge separation,
is difficult.

In the new hybrid material, electrons remain free much longer than ever before. 

To design the hybrid material, the chemists explored different
molecular configurations on a computer at the Ohio Supercomputer
Center. Then, with colleagues at National Taiwan University, they
synthesized molecules of the new material in a liquid solution,
measured the frequencies of light the molecules absorbed, and also
measured the length of time that excited electrons remained free in the
molecules.

They saw something very unusual. The molecules didn’t just fluoresce
as some solar cell materials do. They phosphoresced as well. Both
luminous effects are caused by a material absorbing and emitting
energy, but phosphorescence lasts much longer.

To their surprise, the chemists found that the new material was
emitting electrons in two different energy states — one called a
singlet state, and the other a triplet state. Both energy states are
useful for solar cell applications, and the triplet state lasts much
longer than the singlet state.

Electrons in the singlet state stayed free for up to 12 picoseconds,
or trillionths of a second — not unusual compared to some solar cell
materials. But electrons in the triplet state stayed free 7 million
times longer — up to 83 microseconds, or millionths of a second.

When they deposited the molecules in a thin film, similar to how
they might be arranged in an actual solar cell, the triplet states
lasted even longer: 200 microseconds.

"This long-lived excited state should allow us to better manipulate charge separation," Chisholm said.

At this point, the material is years from commercial development,
but he added that this experiment provides a proof of concept — that
hybrid solar cell materials such as this one can offer unusual
properties.

The project was funded by the National Science Foundation and Ohio States Institute for Materials Research.

Chisholm is working with Arthur J. Epstein, Distinguished University
Professor of chemistry and physics; Paul Berger, professor of
electrical and computer engineering and physics; and Nitin Padture,
professor of materials science and engineering to develop the material
further. That work is part of the Advanced Materials Initiative, one
Ohio States Targeted Investment in Excellence (TIE) programs.

The TIE program targets some of societys most pressing challenges
with a major investment of university resources in programs with a
potential for significant impact in their fields. The university has
committed more than $100 million over the next five years to support 10
high-impact, mostly interdisciplinary programs.

Co-authors on the PNAS paper from Ohio State included: Gotard
Burdzinski, a postdoctoral researcher; Yi-Hsuan Chou, a postdoctoral
researcher; Florian Fiel, a former postdoctoral researcher; Judith
Gallucci, a senior research associate; Yagnaseni Ghosh, a graduate
student; Terry Gustafson, a professor; Yao Liu, a postdoctoral
researcher; Ramkrishna Ramnauth, a former postdoctoral researcher; and
Claudia Turro, a professor; all of the Department of Chemistry. They
collaborated with Pi-Tai Chou and Mei-Lin Ho of National Taiwan
University.