Friday, November 9, 2012

Making Dye-sensitized Solar Panels More Efficient

The solar panels you see on house roofs or in sunny fields, calmly harvesting the radiant energy of the sun, are an icon of the sustainable energy movement. But making them more efficient, durable and affordable is the focus of many researchers, like the engineers from Drexel University and The University of Pennsylvania who are exploring dye-sensitized solar panels, which capture radiation via photosensitive dye and convert it into electricity. The aim is to simplify the electron transfer process inside the solar panels to make them more efficient at converting the radiation into electricity.

Dye-sensitized solar panels at present convert about 11 – 12 percent of the radiant energy that hits them into electricity. The researchers are pushing to make these panels at least as efficient as their silicon counterparts, which currently convert about twice as much radiation as the dye-sensitized panels.

Advantages of Dye Sensitized Solar Cells

Despite their relative inefficiency, dye-sensitized panels have many advantages over silicon cells, chief among them low cost, ease of manufacturing and construction from stable and plentiful materials. Also, the durability of the dye-sensitized panels, combined with their ability to absorb more sunlight per surface area than standard silicon-based solar panels, would make them a smart choice for mainstream use.

There is also the possibility of making dye-sensitized cells flexible. This would open them up to new applications that are not possible for the more rigid silicon panels. Due to the lagging energy conversion rate of dye-sensitized cells, though, they are not as widely used as silicon panels.

“Our ultimate goal is to design and test a highly efficient dye-sensitized solar cell array through computational optimal design, synthesis and integration,” said the project’s lead investigator, Dr. Masoud Soroush.
“We are seeking the combination of electrolyte and electrode materials and cell design that provide the highest power conversion efficiency,” Soroush said. “The final design should have minimum losses in electrical conduction within the photoanode and electrolyte of the cell.”

The group’s strategy involves organizing the erratic movement of radiation-excited, or photogenerated, electrons into a more orderly flow and maintaining that flow through the interior of solar cell by refining the material in its electrolyte substrate.

Nanotube Fire Exits

The present process of energy collection and disbursal in a solar cell works a bit like a chaotic fire drill. Solar radiation hits the photosensitive dye, which excites the electrons and sends them in an electrically charged frenzy through the field of nanoparticles making up the electrode and finally out into the rest of the circuit.

The engineers are trying to direct this rapid exodus of photogenerated electrons by inserting carbon nanotubes, tiny cylindrical graphite-carbon tubes that measure less than 10 nanometers in diameter, to act as corrals for their escape.

“In order for a solar cell to generate an electric current, the photogenerated electrons in the photoanode have to travel through the network of titanium dioxide nanoparticles and they encounter many boundaries between nanoparticles during the transport,” explains Dr. Daeyeon Lee, principal investigator from the University of Pennsylvania. “Due to this random transit path for the electrons, a large fraction of them are lost in the nanoparticle network before they reach the indium tin oxide glass, thus failing to generate electric power.”

Carbon nanotubes, according to Lee, provide continuous pathways for electrons, while also preventing the loss of photogenerated electrons in transit from the solar cell into the exterior circuit. With the addition of the nanotubes, the overall charge collection efficiency of the solar cell is anticipated to increase.

Anti-leak Polymer Substrate

The second prong of the project aims at replacing the electrolyte solution separating the electrodes inside the solar cell with a more effective polymer substance.

The electrolyte serves as an internal pathway for negatively charged ions to carry electrons from the cathode, the negative electrode, to the anode, the positive electrode. Currently, dye-sensitized solar cells use a liquid electrolyte because it is easier for the sponge-like, porous electrode to soak up the liquid for maximum contact. However there is difficulty sealing in the liquid, leading to leakage problems. Into the bargain, the transit of the negatively charged species through a liquid is much less efficient than through a polymer, according to the group.

“Replacing the liquid electrolyte with a polymer will help us make a more efficient solar cell. Unlike the liquid, the polymer will not leak out of the cell and opens the door for making a flexible solar cell,” said Dr. Kenneth Lau, co-principal investigator from Drexel. “The solid polymer is also going to reduce some of the major conversion losses in the cell by closing doors that lead to electron loss that takes place with using a runny liquid.”

Fabrication Challanges

They have also come up with a technique to get the polymer into the sponge-like electrode, a challenge which is one of the main reasons for the use of a liquid electrolyte substrate in current solar cells. “Simply put, we have invented a method for directly making the polymer inside the sponge-like electrode, rather than figuring a way to squeeze an already-made solid polymer into the electrode,” Lau said.

“Our predictive solid-state dye-sensitized solar cell model will allow us to establish important relations between cell performance and cell design and its material parameters,” Soroush said. “We will then use the predictive model to evaluate the cell performance over the entire cell design parameter space. By doing this we will be able to systematically search for and arrive at the design specifications that will optimize the cell’s operation.”

All these moving parts in the concept, including nanotube placement and polymer composition, could make fabrication and testing of prototypes pricey and time-consuming. But with the computational material design program developed by Soroush, the team will avail itself of rapid mathematical modeling to determine the most effective combination of materials and layout. Soroush’s program is unique to Drexel’s research in dye-sensitized solar cells and gives the team a distinct advantage in reaching its goal.


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