Molecular-scale engineering set the stage for a new organic solar photovoltaic conversion record set earlier this year. Image: imecInvestigated heavily since the 1970s, solar cells have been the great unfulfilled promise for unlimited, almost free energy to power the world. The reasoning is solid: The Earth absorbs almost as much energy per hour than the entire human race uses in a single year.

But feasibility of solar energy has lagged well behind its practicality. Decades of intensive R&D has produced a proliferation of rooftop-mounted solar panels and major solar array installations. Yet solar energy still accounts for much less than 1% of the generated electricity in the U.S. today, and the overall influence on energy use and generation has been relatively modest.

Factors and alternatives
Efficiency, cost and reliability concerns affect nearly every type of solar energy conversion technology. The challenge of converting photonic energy to electricity has stimulated the development of a wide variety of technical schemes, the most popular and successful of which is the crystalline silicon solar cell. The first research-oriented solar cells, based on selenium, recorded just 1% over solar baseline efficiency. Twenty years later, researchers in the U.S. achieved a modest 4.5 to 6% efficiency upon the adoption of silicon. Now, the best of these crystalline cells can achieve 25%, and have dominated the market with a combination of stability, competitive cost and widespread production.

Still, silicon fabrication is relatively expensive and these cells have seen volatile swings in price over the years. Cost is one drawback. So is reliability. The best commercial monocrystalline silicon solar cells may last up to 50 years. But efficiency can degrade a cell up to 0.5% per year. The cells are bulky and fragile. Replacement is inevitable, and comes at a cost.

The grid-scale solar industry has moved to specialized mirror-based solar concentrator installations. But in the laboratory, researchers have increasingly looked toward cheaper, lighter solar collectors that are easier to make. Dye-sensitized solar cells feature a photo-sensitized anode and an electrolyte that can be sandwiched together with a roll-to-roll printing process. This yields an attractive price-to-performance ratio; and the best ones achieve 15% efficiency. But unlike silicon cells, stability is poor.

Another option is the organic solar cell, sometimes known as polymer cell. These devices are built from small organic structures, including molecules and different types of conductive organic plastics. The driver for the technology is its very low cost, ease of manufacture and naturally high optical absorption coefficient of organic materials, which has helped produce conversion efficiencies of 4 or 5% or more using a small amount of material. Plus, organic solar cells are inherently flexible, which has prompted concepts for widespread applications on buildings. However, relatively low efficiency, low strength and poor stability have kept this type of cell confined to the laboratory.

The main disadvantages associated with organic photovoltaic cells are low efficiency, low stability and low strength compared to inorganic photovoltaic cells. A recent breakthrough by researchers at the semiconductor developer imec (Leuven, Belgium), however, may help change its fortunes.

New materials, new theory
The simplest form of an organic solar cell is the single-layer organic electronic materials sandwiched between two metallic conductors. One is an active indium-tin-oxide layer and a backing of aluminum or magnesium. These early cells were poor performers. When these organic photovoltaic sandwiches, or stacks (OPVs), absorb a photon, it creates an excited state attached to a molecule on a polymer chain. It’s basically an electrostatic interaction that is managed by a field created by the dissimilar metal layers. The absorber material, in essence, releases the electron to the acceptor material.

The single-layer organic cell was a poor performer, so multi-layer OPVs are made from two different materials in between conductive layers. The materials are chosen to have much different ionization energies, which further enhances the charge collection efficiency. They also typically use fullerenes as the acceptor material due to its high electron mobility and inherent talent for accepting stable electrons. However, the small absorption overlap with the solar spectrum limits the photocurrent generation in fullerene acceptors, and their deep energy level for electron conduction limits the open-circuit voltage.

Engineers at imec have developed a third option, one that removes fullerenes from the equation. The new structure came about in an unexpected way for researchers.

“We had been working for some years now with the Univ. of Madrid to develop new materials for use in OPVs,” says Tom Aernout, group leader of Organic Photovoltaics at imec. The research group had been investigating a variety of organic molecules in the cyanine family and was able to chemically modify the structure of a specific acceptor already used in OPV prototypes, called subphthalocyanine (SubPC). The process involved reducing the number of “out rings” from four to three, promoting strong absorption at low energy levels. The increase in absorption was enough so that, when paired with another organic acceptor material called subnaphthalocyanine (SubNC), they achieved enough electron mobility to substitute for fullerenes. Even better, the bandgap of the new acceptor was far more conducive to collection of visible wavelengths and worked well with the established donor material sexithiophene.

“These cyanine materials were already a replacement for the fullerenes, but then we looked at how the different energy levels were interacting and that brought in the idea of cascading. We could generate electrons in the donor material and cascade them into the electrode using the cyanine materials, which gave a significant jump in efficiency,” says Aernout.

The three layers are arranged as discrete heterojunctions. In addition to an established exciton dissociation at the central donor-acceptor interface, the excitons generated in the outer acceptor layer of this new OPV are first relayed by energy transfer to the central acceptor, and subsequently dissociated at the donor interface.

Firstly, the implementation of fullerene-free acceptor materials resulted in high open-circuit voltages and useful absorption spectra in the visible. Secondly, high short-circuit currents were achieved by developing a multi-layer device structure of three active semiconductor layers with complementary absorption spectra, and an efficient exciton harvesting mechanism.

The result of this effort is quantum efficiency above 75% between 400 and 720 nm and a record conversion efficiency of 8.4% at a high open-circuit current close to 1 V.