Simplifying Organic Solar Cell Configuration for Better Efficiency

A team of researchers from Purdue University has proposed a theory that opposes a basic assumption about the operation of organic solar cells created from cheap plastics and suggests an innovative technique to produce solar technology in an economical manner.

Doctoral student Aditya G. Baradwaj (at left), professor Muhammad A. Alam (center) and doctoral student Ryyan Khan, operate a solar simulator. They are members of a research team led by former doctoral student Biswajit Ray and also involving assistant professor Bryan Boudouris. (Purdue University image/John Underwood)

So far, organic solar cells have been difficult to promote on a commercial level due to their various inefficiencies, but with this new technique, the creation of a whole new range of solar technology capable of competing with regular silicon cells is possible.

"These solar cells could provide a huge cost advantage over silicon," said Muhammad Ashraful Alam, Purdue University's Jai N. Gupta Professor of Electrical and Computer Engineering.

It is possible to manufacture plastic solar cells using a roll-to-roll technique, just as in the case of newspaper printing.

Bryan Boudouris, an assistant professor of chemical engineering stated that "this has been the hope for the last 20 to 25 years."

The flexibility of organic solar cells makes them an ideal choice for several new applications where the traditional rigid silicon cells, such as photovoltaics that are installed in buildings, can no longer be used. Furthermore, the organic solar cells are likely to be cheaper, requiring less energy to manufacture than their silicon counterparts.

"Now it appears there is no fundamental reason why organic cells have to be less efficient than silicon," Alam said.

The research details can be found in the August 17th Proceedings of the National Academy of Sciences. Former Purdue doctoral student Biswajit Ray, who has now graduated from the University, headed the research team.

"Biswajit had the courage, conviction, and persistence to do a series of difficult experiments, and he was ably supported at various stages by doctoral students Aditya Baradwaj and Ryyan Khan," Alam said.

The paper was authored by Ray, Baradwaj, Khan, Boudouris and Alam.

The initial hurdle to successful creation of highly efficient organic solar cells lies in the basic operation of the organic photovoltaic technology. When the semiconducting material is illuminated with light, the electrons begin to travel from one energy level to another. The atomic structure is such that the electrons in the semiconductor are present in an energy region referred to as the "valence band", while the material lies in the dark.

However, when light illuminates the material, electrons are forced to absorb energy, thereby raising them to a higher level of energy referred to as "conduction band." When the electrons travel towards the conduction band, “holes” are left behind in the valance band, and electron-hole pairs are formed in the plastic solar cells, known as excitons.

"Because the electron is negatively charged and the hole is positively charged, they like each other so much that they orbit around each other," Ray said. "You have to keep these two separated or they will recombine and you will not generate current."

This "charge separation" can be preserved by inserting many structures known as bulk heterojunctions. Manufacturing this design in a large-scale, high-speed, and reproducible manner is quite a mammoth task.

"You can think of these heterojunctions almost like knives cutting through the material to separate the electrons and holes," Alam said. "These heterojunctions have to be distributed throughout so that no matter where the electron-hole pair is generated you can cut them."

This particular requirement restricts the efficiency of organic solar cells, as highlighted in research from nearly 20 years ago. However, Ray used comprehensive computational modeling to reveal that there was a flaw in the basic hypothesis regarding the organic solar cells, thereby removing the need for heterojunctions.

"He kept saying he didn't need to invoke excitons in order to explain many of the experimental results," Alam said. "It turns out that the original experiment was misinterpreted."

The design of organic cells is the reason behind this misinterpretation. There are two metal contacts in the cells, with one located on top and the other at the bottom of the device. Both are made of different kinds of metal. This arrangement causes the incoming sunlight to produce an intense electric field at the bottom of the cell, which enables the electron-hole pairs to voluntarily rejoin.

Ray suggested swapping the contacts around so that the electric field is no longer produced at the bottom of the cell but instead, at the top, thereby allowing for higher charge separation.

"He inverted the structure and explained that the inefficiency is taking place because the electron-hole pairs are not staying separated," Alam said.

Initial simulations revealed that this new configuration did function better, and provided higher efficiency and charge separation. This theory was later validated in practice through laboratory experiments conducted by Ray, Baradwaj and Khan.

Ray stated that based on the research findings, the design of organic solar cells can be simplified. In addition, the manufacture of traditional organic solar cells would require two types of polymers to be mixed together while the innovative new design would require only one polymer.

"Currently, you have to design the solar cells according to how well two organic materials mix together in order to produce these numerous heterojunctions," Boudouris said. "But if you only needed one polymer instead of two, the manufacturability on the large scale could be very much improved, so this is an exciting development."

Additionally, the research findings also revealed that cells manufactured using purer polymers are likely to result in highly efficient solar cells. Alam stated that this research will probably pave the way towards a better understanding of the physics behind the functioning of organic solar cells.

Most of the experiments were conducted within the Purdue Solar Energy Utilization Laboratory, which was formed with funds from the National Science Foundation (NSF). NSF is an organization that assists specialists from different fields to collaborate. The Purdue research is an ongoing process, and Khan will be focusing on creation of a new class of solar cells that do not depend upon bulk heterojunctions.

The research was funded by the U.S. Department of Energy through DOE’s Center for Re-Defining Photovoltaic Efficiency through Molecule Scale Control, the National Science Foundation’s Network for Computational Nanotechnology, and the U.S. Air Force Office of Scientific Research.

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