TechTheDay

In a previous post, we reported how University of Liverpool researchers found a way to make use of a specific ingredient in tofu to improve solar panel manufacture. This month, University of Massachusetts – Amherst researchers are reporting a new kind of lightweight, more efficient, and cheap solar cell technology that is considered a breakthrough in improving the power conversion efficiency of organic solar cells.

Breaking the Electrode Barrier

Organic solar cells have been, for years, faced with a major obstacle when it comes to improving their power conversion efficiency on account of the inherent drawbacks of commonly used metal electrodes. These drawbacks include metal electrode instability and susceptibility to oxidation or corrosion. University of Massachusetts Amherst researchers, however, have developed a more efficient, easy-to-process, and light solar cell that is compatible with virtually any metal for the electrode. This effectively breaks the “electrode barrier” and opens the doors for considerable improvements in solar cells.

So what is it about the traditional metal electrodes that prevents them from becoming more efficient? Why does an efficiency enhancement barrier exist? According to UMass Amherst Polymer Science and Engineering Professor Thomas Russell, “one reason is the trade-off between oxidative stability and work function of the metal cathode.” Oxidative stability refers to the resistance of the metal electrode to oxidation while work function pertains to the level of difficulty electrons go through as they transfer from the solar cell’s photoactive layer to the electrode that delivers power to a device or the battery.

The researchers broke through the “electrode barrier” by synthesizing new polymers with zwitterions on them and applying them to numerous polymer scaffolds  in semiconductors in the inter-layer of solar cells. Zwitterions are neutral molecules (i.e. it has both positive and negative charges) that interact strongly with metal electrodes because of their strong dipoles. Several materials were experimented upon until the researchers found the preferable metal electrodes to use. They employed ultraviolet photoelectron spectroscopy (UPS) in the process of categorizing the several metals they tested.

Zwitterions and Fullerenes

The researchers were able to develop conjugated polymer zwitterions or CPZs as they sought to incorporate any conjugated backbone with zwitterionic functionality. They experimented on various materials until they turned to fullerenes, which are usually used in the photoactive layer of solar cells. Fullerenes are known for their  excellent electron transport so the researchers thought they are likely going to observe improved efficiency with them. They modified the fullerenes with zwitterions and have successfully been able incorporate zwitterion functionality in an efficient manner.

More Efficient, Lightweight, and Low-Cost

All these mean that zwitterion-modified fullerenes are capable of altering electrode work function to pave the way for the production of high efficiency solar cells. The researchers have successfully created fullerenes and polymers that modify the properties of the metals they get in contact with, transforming these metals into more efficient components for a solar power device. Even better, further tests conducted with the fullerene layer showed that there is no need for this layer placed in between the photoactive layer and electrode to be ultra thin. The researchers found that a 60-N fullerene layer worked more effectively compared to a C60-SB type. This leads to lower costs and a simpler manufacturing process.

The researchers were able to achieve Power Conversion Efficiency (PCE) values that are higher than 8.5% for organic photovoltaics regardless of the metal cathode selected. The C60N layer they used in the study was able to reduce the effective work function of gold, copper, and silver electrodes to 3.65 eV.

More details of this research can be found in the magazine “Science” of the American Association for the Advancement of Science in the research titled “Fulleropyrrolidine interlayers: Tailoring Electrodes to Raise Organic Solar Cell Efficiency.” The listed authors include Zachariah A. Page, Volodimyr V. Duzhko, Thomas P. Russell, and Todd Emrick.