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Faculty Spotlight: Enrique Gomez– “Improving Solar Cell Efficiency”

Gomez Lab

Gomez Lab. Photo by Abigail Johnson.

Although solar energy has the potential to solve many of our current energy woes, its feasibility as a large-scale energy provider is limited by factors such as cost, energy storage, maintenance and, perhaps most significantly, efficiency.

Penn State’s Enrique Gomez, assistant professor of chemical engineering in the College of Engineering, is making huge strides toward improving organic solar cell efficiency.

Organic solar cells use organic electronic materials, which employ synthetic conductive polymers or molecules for light absorption and charge transport.

A more cost-effective alternative to inorganic solar cells, organic cells are able to absorb a large amount of light with a small amount of material. They can also be processed in new ways that save both time and money.
However, they are less efficient and less stable than inorganic cells.

Enter Gomez.

His recent research tackles the inefficiency of organic solar cells and provides the foundation for the creation of solar cells that are both cheap and efficient.

In an article published in the June 2013 issue of Nano Letters, Gomez and his colleagues demonstrate the potential for conjugated block copolymers to enhance performance in organic solar cells.

Gomez Lab

Gomez Lab. Photo by Abigail Johnson.

Most organic solar cells are made with polymer-fullerene blends, and because of the material’s structure, producers have very little control over what happens at the interface between the polymer and fullerene.

This means that electrons may hop back and forth between molecules instead of traveling to an external circuit. As a result, much of the energy that is put into the system through sunlight is lost.

In an attempt to gain more control over this crucial interface, Gomez and his team created solar cell devices made strictly from polymeric substances and then added heat to control the microstructure.

They heated the devices at varying temperatures for varying amounts of time in an effort to compare the energy conversion efficiency of a polymer blend (P3HT/PFTBT) to a block copolymer (P3HT-b-PFTBT).

Under optimal thermal conditions, the devices composed of P3HT/PFTBT blends exhibited a maximum power conversion efficiency of 1.0 percent. Those composed of P3HT-b-PFTBT block copolymers, however, achieved average power conversion efficiencies of 2.7 ± 0.4 percent with short-circuit currents of 5.0 ± 0.3 mA/cm2, an almost three fold improvement in device performance. The best overall efficiency was recorded at 3.1 percent with an open-circuit voltage of 1.23 V.

Gomez Lab

Gomez Lab. Photo by Abigail Johnson.

According to the researchers, “This device performance is remarkable for solar cells based on donor-acceptor block copolymers and for nonfullerene solution-processed organic solar cells.”

In order to determine the basis for this enhanced performance, the research team carried out resonant soft X-ray scattering and grazing-incidence X-ray scattering measurements. Scattering data from polymer blends show little structure, while those from P3HT-b-PFTBT block copolymers exhibit signs of self-assembly into block copolymer lamellar microdomains. This structure occurs when polymer blocks of similar length form vertically-oriented alternating bands.

Based on this data, the researchers attributed the improvement in device performance to the self-assembly of block copolymers into well-defined mesostructures. Further measurements examined the molecular order in the block copolymer devices using conventional X-ray diffraction and grazing-incidence wide-angle X-ray scattering. These methods revealed that one of the molecules, P3HT, flips to an orientation that it doesn’t usually exhibit. This change, the researchers predict, likely enhances efficiency. The unique structure of the block copolymers tested demonstrates the potential to improve solar cell efficiency by controlling the donor-acceptor interface and molecular orientations.

“Our achievement has been has to demonstrate that you could use a block copolymer structure,” Gomez said. “So now, suddenly you have the potential to control that interface to… not only study fundamentally what the properties of the interface might be, but also to try to exploit the ability to control that interface to maximize performance.”

Gomez, the project’s principal investigator, completed his Ph.D. at the University of California, Berkeley. His lab at Penn State focuses on understanding the role of structure on various properties of soft matter.

The article, “Conjugated Block Copolymer Photovoltaics with near 3% Efficiency through Microphase Separation,” is co-authored by Changhe Guo, a graduate student in Gomez’s lab, Matthew D. Witman, an undergraduate in the lab, and by collaborators from Rice University, Lawrence Berkeley National Laboratory, and Argonne National Laboratory.

The full article can be found online at http://pubs.acs.org/doi/abs/10.1021/nl401420s.

View Gomez's PSIEE faculty detail webpage.

by Abigail Johnson, PSIEE writing intern


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