TUCSON, Ariz. — Photovoltaics – the high-tech approach to converting
sunlight directly into electricity – could be low cost and widely practical
if based on organic “self-assembling” thin film technologies, say scientists
at the University of Arizona.
UA chemists and optical scientists have been funded by two separate new
grants totaling more than $1 million to develop organic molecules that “self
assemble,” or self-organize, from liquid into efficient solar cell coatings.
Neal R. Armstrong, Bernard Kippelen, David O’Brien, Seth Marder and Jean-Luc
Brédas together have previously pioneered breakthroughs in such related
areas as organic light-emitting diode and holographic storage technologies.
They are now applying their discoveries and new materials to unconventional
photovoltaics (PV) — organic solar cell thin films. They are designing,
synthesizing and characterizing molecules that will self-organize from
solution into coatings about 100 nanometers thick, or about one-thousandth
the thickness of a human hair. Molecules in the layer must be very highly
ordered to efficiently transport electrical charge.
“What you’d really like is a solar panel array that would come on a flexible
plastic substrate which would be extremely inexpensive and which you could
roll out on your roof like wall paper,” said chemistry Professor Neal R.
Armstrong. “It would be efficient enough at energy conversion to
economically generate power.”
Armstrong is principal investigator on a 3-year, $490,000 grant from the
Department of Energy’s National Energy Research Laboratory (NREL). The grant
is aimed at developing new organic “liquid crystal” PV materials that could
be inexpensively wet-processed into large area panels.
Ninety-nine percent of the photovoltaic market today is based on single
crystal silicon, an efficient and reliable but expensive material for solar
cells. Silicon PV powers satellites and space missions, but cost, mainly,
limits how much it does in meeting the world’s energy demands.
According to Department of Energy statistics, Americans currently pay 6-to-7
cents per kilowatt-hour for conventionally generated electricity and
20-to-30 cents per kilowatt-hour for solar-generated electricity.
“That sounds discouraging, but the cost of solar-generated power was about
90 cents per kilowatt-hour 10 to 15 years ago,” and can be expected to drop
even more, Armstrong noted.
Efficient new inorganic thin films such as cadmium telluride and
cadmium-indium-galleium-selenide are entering the PV market, he said, and
their efficiencies and potentially lower cost are impressive. But the heavy
metals, tellurium and selenium, in these PVs raise environmental concerns,
both during their manufacture and ultimate disposal.
New organic solar cells would potentially be a less-toxic, Earth-friendly
way to tap energy from the sun, he emphasized, because most of the target
organic materials are environmentally benign when processed and when
discarded in devices.
Armstrong was among hundreds of scientists world-wide interested in organic
PV thin films when he joined the UA in the late 1970s. But until recently,
these films were only one to one-and-a-half percent efficient at converting
solar power to electrical power. There also were serious concerns about the
long term stability of such thin organic films. Not surprisingly, funding
agencies from the late 1980s to the mid-1990s weren’t interested in organic
solar cell research.
“The discouraging thing was that these materials could be tailored to absorb
most of the solar spectrum, they were very cheap, and they were easy to
spread as a thin film on a transparent electrode. But their electrical
properties were very bad. Even if you could generate a lot of electrical
charge inside a thin film you couldn’t transport it, and you couldn’t
harvest it efficiently,” Armstrong said.
“But the last decade has seen a big change in how people feel about organic
technologies in general, and, specifically, about how you make them better
electrical materials,” he added.
Optical sciences Associate Professor Bernard Kippelen agreed. “Ten years
ago, if you went to the Department of Energy and said you wanted to make an
organic solar cell, they would have been very skeptical. But a lot of
research has been done in the emerging field of organic electronics in the
past 10 years, and it has completely changed the way people think.”
Kippelen is principal investigator on a 3-year, $580,000 grant from the
Office of Naval Research that involves Armstrong, Marder and Brédas . Their
goal for Kippelen’s grant is to develop self-assembling polymer (plastic)
liquid crystals for lightweight, flexible and shock-resistant light-emitting
diodes and devices needed by the military, as well as for PV use.
“Even a few years ago, people thought that any organic material for optical
or optoelectronic application was unstable, that it could have only a very
limited lifetime,” Kippelen said. “With all the successful research that has
been done now, people know that if you synthesize the materials correctly,
if you purify them and keep them from water and oxygen, even organic
materials can have very long lifetimes.”
Industry now confidently markets “Organic EL” displays for car stereos and
for cell phones, for example. These devices emit light using 100
nanometer-thick organic films that carry high current densities, have great
stabilities, and are bright and pleasing to look at.
The challenge for PV is to achieve higher electrical “mobility,” or create
films that rapidly carry charge, Kippelen said.
“We don’t want to make predictions that sound overly optimistic, but
theoretically there is no reason that we cannot make solar cells with 20
percent efficiency,” Kippelen said.
Researchers from the University of Cambridge and the Max Planck Institute
reported in Science earlier this month that they have developed a
potentially efficient self-assembled organic thin film PV, proving the
concept works. Their work developed from an earlier collaboration with
Brédas which demonstrated how the right kind of self-assembly would increase
PV efficiency.
Initially, the UA scientists have focused on self-assembling liquid crystals
that Armstrong and chemistry Professor David O’Brien developed from a common
deep blue-green pigment called “phthalocyanine. ” Under the right conditions
– for example, when heated – these disk-shaped molecules line up like a
stack of coins, solidifying as long, rod-like molecular stacks in a well
organized film. Chemistry Professor Seth Marder has developed a set of
complimentary liquid crystalline materials that also self-assemble into
coherent stacks.
The UA team is working to engineer molecules that stack themselves
vertically rather than horizontally on the substrate for higher electrical
mobility. That is no small feat, Armstrong said, “But we feel a 10 percent
conversion efficiency is a realistic goal, based on our own recent work and
the work of several other groups in Europe and Japan.”
Such a film could greatly improve a potentially important type of organic
solar cell, Kippelen said.
Such cells, first proposed in the 1990s by scientists at the Ecole
Polytechnique in Lausanne, Switzerland, already have reached power
conversion efficiencies of 10 percent. But the solar cells are not widely
practical because they contain liquid electrolytes. The electrolytes can
evaporate and decompose as they sit in the sun and can be hard to process
into large area PV arrays.
The dye sensitized organic solar cell has a transparent electrode coated
with a porous network of titanium dioxide nanoparticles – the semiconductor.
By itself, titanium dioxide cannot absorb visible sunlight efficiently. So a
photosensitive dye is added to the network – the sensitizer. The dye absorbs
photons from sunlight and releases electrons that flow as an electrical
current to a counter electrode.
But once photoactive dye molecules have given up electrons, they must be
very quickly recharged. Dye molecules cannot absorb more photons until their
lost electrons are replaced, or “regenerated.”
“We want to replace the liquid electrolyte currently used to regenerate dye
in these organic solar cells with a new charge-transporting film,” Kippelen
said. “That would be the big development.”
“And we can make the titanium dioxide network even more porous so
photoactive dye covers a greater surface area,” thereby increasing the solar
cell’s light absorption potential, he added. ” Further, dyes currently used
do not absorb the full spectrum of the sun, only the visible part, and only
about 45 percent of the light is being harvested. So we also will work on
making dyes that absorb more of the infrared part of the spectrum.”
The Department of Energy and Office of Naval Research are funding organic
solar cell research through new programs, and other basic and applied
research funding sources are increasingly interested in the technology as
well, the UA researchers say. They are confident that their team-oriented
approach is ideal for developing the technology, they add, and that simple,
inexpensive solar cell materials may generate electricty in the
not-too-distant future.
Contact Information
Neal R. Armstrong
520-621-8242 nra@u.arizona.edu
