Simplified organic solar cells boost efficiency while lowering costs

Technology News |
By eeNews Europe

The team’s research findings claim that commercialization of organic solar cells has been hindered by inefficiencies and point toward a potential path to create a new class of solar technology able to compete with standard silicon cells.

Plastic solar cells might be manufactured using a roll-to-roll process similar to newspaper printing.

"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.

Because organic solar cells are flexible they could find new applications that are unsuitable for rigid silicon cells such as photovoltaics integrated into buildings, and they have the potential to be lower-cost and less energy-intensive to manufacture than silicon devices. However, a critical bottleneck has prevented development of organic solar cells efficient enough to compete with silicon solar technology.

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

The primary bottleneck to more efficient organic solar cells is rooted in the fundamental workings of the organic photovoltaic technology. As the semiconducting material is illuminated with light, electrons move from one energy level to another. Due to the atomic structure, the electrons in the semiconductor occupy a region of energy called the ‘valence band’ while the material is in the dark. But shining light on the material causes the electrons to absorb energy, elevating them into a region of higher energy called the ‘conduction band’. As the electrons move to the conduction band they leave behind ‘holes’ in the valance band, generating so-called electron-hole pairs in the plastic solar cells called 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," said Ray. "You have to keep these two separated or they will recombine and you will not generate current."

The ‘charge separation’ is maintained by inserting numerous structures called bulk heterojunctions, a design that has been a challenge to manufacture in a high-speed, large-scale and reproducible manner.

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

The requirement limits the efficiency of organic solar cells, a bottleneck established by research more than two decades ago. However, Ray showed through detailed computational modeling that a fundamental assumption about the organic solar cells was incorrect, eliminating the need for heterojunctions.

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

The misinterpretation arises from the design of organic cells. The cells have two metal contacts, one on top and one on the bottom of the device, and each is made of different types of metal. Because of this configuration, incoming sunlight generates an electric field concentrated at the bottom of the cell, which allows the electron-hole pairs to readily recombine. Ray proposed flipping the contacts so that the electric field forms at the top of the cell instead of at the bottom, allowing for better charge separation.

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

Simulations showed flipping the configuration allowed for better charge separation and higher efficiency, and then laboratory experiments by Ray, Baradwaj and Khan validated the new concept.

The findings also suggest the design of organic solar cells can be simplified, representing a major potential innovation, Ray said. Whereas conventional organic solar cells are made by mixing two types of polymers, the new design requires 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," said Boudouris. "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."

Findings also suggest that producing the cells out of purer polymers could result in more efficient solar cells, and the research likely will lead to a better understanding of the physics of how organic solar cells operate, Alam said.


Collection-limited Theory Interprets the Extra-ordinary Response of Single Semiconductor Organic Solar Cells   
Biswajit Ray, Aditya G. Baradwaj, M. Ryyan Khan1, Bryan W. Boudouris, and Muhammad A. Alam
Proceedings of the National Academy of Sciences (August 17, 2015)

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