Another Research Group Claims Solar Breakthrough

A team of Northwestern University researchers have developed a new anode coating that, according to the paper they published in the Proceedings of the National Academy of Sciences, improve solar panels, making them convert sunlight into electricity more efficiently.

"This breakthrough in solar energy conversion promises to bring researchers and developers worldwide closer to the goal of producing cheaper, more manufacturable and more easily implemented solar cells," the university's press release reads. "Such technology would greatly reduce our dependence on burning fossil fuels for electricity production as well as reduce the combustion product: carbon dioxide, a global warming greenhouse gas."

How cheaply solar panels can be manufactured, and how efficiently they convert the sun's energy, are key to making the technology cost-competitive with burning fossil fuels. This is one of a series of announcements recently that claim to have made revolutionary advances in the technology that would have commercial applications. While it may be difficult to determine how likely that is from a press release, it at least shows that researchers are actively engaged with the development of next-generation solar power technology. And that is a good thing.

According to the university:

Of the new solar energy conversion technologies on the horizon, solar cells fabricated from plastic-like organic materials are attractive because they could be printed cheaply and quickly by a process similar to printing a newspaper (roll-to-roll processing).

To date, the most successful type of plastic photovoltaic cell is called a “bulk-heterojunction cell.” This cell utilizes a layer consisting of a mixture of a semiconducting polymer (an electron donor) and a fullerene (an electron acceptor) sandwiched between two electrodes -- one a transparent electrically conducting electrode (the anode, which is usually a tin-doped indium oxide) and a metal (the cathode), such as aluminum.

When light enters through the transparent conducting electrode and strikes the light-absorbing polymer layer, electricity flows due to formation of pairs of electrons and holes that separate and move to the cathode and anode, respectively. These moving charges are the electrical current (photocurrent) generated by the cell and are collected by the two electrodes, assuming that each type of charge can readily traverse the interface between the polymer-fullerene active layer and the correct electrode to carry away the charge -- a significant challenge.

The Northwestern researchers employed a laser deposition technique that coats the anode with a very thin (5 to 10 nanometers thick) and smooth layer of nickel oxide. This material is an excellent conductor for extracting holes from the irradiated cell but, equally important, is an efficient “blocker” which prevents misdirected electrons from straying to the “wrong” electrode (the anode), which would compromise the cell energy conversion efficiency.

In contrast to earlier approaches for anode coating, the Northwestern nickel oxide coating is cheap, electrically homogeneous and non-corrosive. In the case of model bulk-heterojunction cells, the Northwestern team has increased the cell voltage by approximately 40 percent and the power conversion efficiency from approximately 3 to 4 percent to 5.2 to 5.6 percent.

The researchers currently are working on further tuning the anode coating technique for increased hole extraction and electron blocking efficiency and moving to production-scaling experiments on flexible substrates.

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