Solar cells more efficient

UC Riverside researchers find a way to use the infrared region of the sun's spectrum to make solar cells more efficient

By Iqbal Pittalwala on July 27, 2015

RIVERSIDE, Calif. - When it comes to installing solar cells, labor cost and the cost of the land to house them constitute the bulk of the expense. The solar cells - made often of silicon or cadmium telluride - rarely cost more than 20 percent of the total cost. Solar energy could be made cheaper if less land had to be purchased to accommodate solar panels, best achieved if each solar cell could be coaxed to generate more power.

A huge gain in this direction has now been made by a team of chemists at the University of California, Riverside that has found an ingenious way to make solar energy conversion more efficient. The researchers report in Nano Letters that by combining inorganic semiconductor nanocrystals with organic molecules, they have succeeded in "upconverting" photons in the visible and near-infrared regions of the solar spectrum.

"The infrared region of the solar spectrum passes right through the photovoltaic materials that make up today's solar cells," explained Christopher Bardeen, a professor of chemistry. The research was a collaborative effort between him and Ming Lee Tang, an assistant professor of chemistry. "This is energy lost, no matter how good your solar cell. The hybrid material we have come up with first captures two infrared photons that would normally pass right through a solar cell without being converted to electricity, then adds their energies together to make one higher energy photon. This upconverted photon is readily absorbed by photovoltaic cells, generating electricity from light that normally would be wasted."

Bardeen added that these materials are essentially "reshaping the solar spectrum" so that it better matches the photovoltaic materials used today in solar cells. The ability to utilize the infrared portion of the solar spectrum could boost solar photovoltaic efficiencies by 30 percent or more.

In their experiments, Bardeen and Tang worked with cadmium selenide and lead selenide semiconductor nanocrystals. The organic compounds they used to prepare the hybrids were diphenylanthracene and rubrene. The cadmium selenide nanocrystals could convert visible wavelengths to ultraviolet photons, while the lead selenide nanocrystals could convert near-infrared photons to visible photons.

In lab experiments, the researchers directed 980-nanometer infrared light at the hybrid material, which then generated upconverted orange/yellow fluorescent 550-nanometer light, almost doubling the energy of the incoming photons. The researchers were able to boost the upconversion process by up to three orders of magnitude by coating the cadmium selenide nanocrystals with organic ligands, providing a route to higher efficiencies.

"This 550-nanometer light can be absorbed by any solar cell material," Bardeen said. "The key to this research is the hybrid composite material - combining inorganic semiconductor nanoparticles with organic compounds. Organic compounds cannot absorb in the infrared but are good at combining two lower energy photons to a higher energy photon. By using a hybrid material, the inorganic component absorbs two photons and passes their energy on to the organic component for combination. The organic compounds then produce one high-energy photon. Put simply, the inorganics in the composite material take light in; the organics get light out."

Besides solar energy, the ability to upconvert two low energy photons into one high energy photon has potential applications in biological imaging, data storage and organic light-emitting diodes. Bardeen emphasized that the research could have wide-ranging implications.

"The ability to move light energy from one wavelength to another, more useful region, for example, from red to blue, can impact any technology that involves photons as inputs or outputs," he said.

The research was supported by grants from the National Science Foundation and the U.S. Army.

The research was conducted also by the following coauthors on the research paper: Zhiyuan Huang (first author), Xin Li, Melika Mahboub, Kerry M. Hanson, Valerie M. Nichols and Hoang Le.

Tang's group helped design the experiments and provided the nanocrystals.

The UCR Office of Technology Commercialization has filed a provisional patent on the technology.

Archived under:Science/Technology, chemistry, Christopher Bardeen, CNAS, College of Natural and Agricultural Sciences, electricity, green, Ming Lee Tang, nanocrystals, nanoparticles, National Science Foundation, Office of Technology Commercialization, press release, solar, solar cells, solar energy, solar panels, upconversion

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Fine-tuned molecular orientation is key to more efficient solar cell

Polymer solar cells are a hot area of research due to both their strong future potential and the significant challenges they pose. It is believed that thanks to lower production costs, they could become a viable alternative to conventional solar cells with silicon substrates when they achieve a power conversion efficiency - a measure that indicates how much electricity they can generate from a given amount of sunlight - of between 10 and 15 percent. Now, using carefully designed materials and an "inverted" architecture, a team of scientists has achieved efficiency of 10 percent, bringing these cells close to the threshold of commercial viability.

Polymer-based solar cells offer a number of potential advantages. They are made of polymers that are inexpensive and flexible, and can be deposited on glass or plastic substrates, allowing the construction of large-scale structures. They are cheaper to manufacture, and more environmentally-friendly, than their silicon counterparts. Unfortunately, they have lower power efficiency due to their structure and also tend to degrade more quickly.

In the research published in Nature Photonics, a collaboration including Itaru Osaka and Kazuo Takimiya of the RIKEN Center for Emergent Matter Science managed to create a type of polymer solar cell called a bulk-heterojunction solar cell - where the electron donor and acceptor layers are mixed together - with a power conversion efficiency of 10%, close to what will allow these materials to be commercially viable.

According to Osaka, "While private firms have been able to develop cells with similar efficiency, they have done so using proprietary technology, so that it was not possible to know why things were working the way they were. We began experimenting with a substance called PNTz4T, which we had previously developed, and were able initially to achieve a power conversion efficiency of about 8%, with a fairly thick active layer of about 300 nanometers. Surprisingly, though, we found that when we used an inverted architecture, where the light enters through a transparent negative electrode, in our case made of zinc oxide, we found that the cell with the inverted architecture had better efficiency, which is abnormal for cells of the type we built. We believe that it is due to the alignment of molecules inside the mixed layers."

The researchers analyzed the composition of the materials using the SPring-8 synchrotron facility in Harima, and found indeed that in the inverted model, the orientation of the molecules within the active layer was very commonly "face-on," an orientation well suited to the transport of electron holes through the material. Takamiya says, "We surmised that this was the secret to the success in the experiment. It turns out that by trying something that might seem unusual, we got a surprising result, and through this were able to understand something about what makes cells more or less efficient."

According to Professor Hideyuki Murata of the Japan Advanced Institute of Science and Technology, who participated in the research, "This is an exciting result because we now have an understanding of how we can move forward to create polymer solar cells with greater efficiency. We hope that researchers around the world will be able to make use of these results to create commercially viable cells."