Stanford study could lead to paradigm shift in organic solar cell research

Organic solar cells have long been touted as lightweight, low-cost alternatives to rigid solar panels made of silicon. Dramatic improvements in the efficiency of organic photovoltaics have been made in recent years, yet the fundamental question of how these devices convert sunlight into electricity is still hotly debated.

Now a Stanford University research team is weighing in on the controversy. Their findings, published in the Nov. 17 issue of the journal Nature Materials, indicate that the predominant working theory is incorrect, and could steer future efforts to design materials that boost the performance of organic cells.

“We know that organic photovoltaics are very good,” said study coauthor Michael McGehee, a professor of materials science and engineering at Stanford. “The question is, why are they so good? The answer is controversial.”

A typical organic solar cell consists of two semiconducting layers made of plastic polymers and other flexible materials. The cell generates electricity by absorbing particles of light, or photons.

When the cell absorbs light, a photon knocks out an electron in a polymer atom, leaving behind an empty space, which scientists refer to as a hole. The electron and the hole immediately form a bonded pair called an exciton. The exciton splits, allowing the electron to move independently to a hole created by another absorbed photon. This continuous movement of electrons from hole to hole produces an electric current.

In the study, the Stanford team addressed a long-standing debate over what causes the exciton to split.

“To generate a current, you have to separate the electron and the hole,” said senior author Alberto Salleo, an associate professor of materials science and engineering at Stanford. “That requires two different semiconducting materials. If the electron is attracted to material B more than material A, it drops into material B. In theory, the electron should remain bound to the hole even after it drops.

“The fundamental question that’s been around a long time is, how does this bound state split?”

Some like it hot

One explanation widely accepted by scientists is known as the “hot exciton effect.” The idea is that the electron carries extra energy when it drops from material A to material B. That added energy gives the excited (“hot”) electron enough velocity to escape from the hole.

But that hypothesis did not stand up to experimental tests, according to the Stanford team.

“In our study, we found that the hot exciton effect does not exist,” Salleo said. “We measured optical emissions from the semiconducting materials and found that extra energy is not required to split an exciton.”

So what actually causes electron-hole pairs to separate?

“We haven’t really answered that question yet,” Salleo said. “We have a few hints. We think that the disordered arrangement of the plastic polymers in the semiconductor might help the electron get away.”

In a recent study, Salleo discovered that disorder at the molecular level actually improves the performance of semiconducting polymers in solar cells. By focusing on the inherent disorder of plastic polymers, researchers could design new materials that draw electrons away from the solar cell interface where the two semiconducting layers meet, he said.

“In organic solar cells, the interface is always more disordered than the area further away,” Salleo explained. “That creates a natural gradient that sucks the electron from the disordered regions into the ordered regions. ”

Improving energy efficiency

The solar cells used in the experiment have an energy-conversion efficiency of about 9 percent. The Stanford team hopes to improve that performance by designing semiconductors that take advantage of the interplay between order and disorder.

“To make a better organic solar cell, people have been looking for materials that would give you a stronger hot exciton effect,” Salleo said. “They should instead try to figure out how the electron gets away without it being hot. This idea is pretty controversial. It’s a fundamental shift in the way people think about photocurrent generation.”

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Scientists develop heat-resistant materials that could vastly improve solar cell efficiency

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Scientists have created a heat-resistant thermal emitter that could significantly improve the efficiency of solar cells. The novel component is designed to convert heat from the sun into infrared light, which can than be absorbed by solar cells to make electricity – a technology known as thermophotovoltaics. Unlike earlier prototypes that fell apart at temperatures below 2200 degrees Fahrenheit (1200 degrees Celsius), the new thermal emitter remains stable at temperatures as high as 2500 F (1400 C).

“This is a record performance in terms of thermal stability and a major advance for the field of thermophotovoltaics,” said Shanhui Fan, a professor of electrical engineering at Stanford University. Fan and his colleagues at the University of Illinois-Urbana Champaign (Illinois) and North Carolina State University collaborated on the project. Their results are published in the October 16 edition of the journal Nature Communications.

A typical solar cell has a silicon semiconductor that absorbs sunlight directly and converts it into electrical energy. But silicon semiconductors only respond to infrared light. Higher-energy light waves, including most of the visible light spectrum, are wasted as heat, while lower-energy waves simply pass through the solar panel.

“In theory, conventional single-junction solar cells can only achieve an efficiency level of about 34 percent, but in practice they don’t achieve that,” said study co-author Paul Braun, a professor of materials science at Illinois. “That’s because they throw away the majority of the sun’s energy.”

Thermophotovoltaic devices are designed to overcome that limitation. Instead of sending sunlight directly to the solar cell, thermophotovoltaic systems have an intermediate component that consists of two parts: an absorber that heats up when exposed to sunlight, and an emitter that converts the heat to infrared light, which is then beamed to the solar cell.

“Essentially, we tailor the light to shorter wavelengths that are ideal for driving a solar cell,” Fan said. “That raises the theoretical efficiency of the cell to 80 percent, which is quite remarkable.”

So far, thermophotovoltaic systems have only achieved an efficiency level of about 8 percent, Braun noted. The poor performance is largely due to problems with the intermediate component, which is typically made of tungsten – an abundant material also used in conventional light bulbs.

“Our thermal emitters have a complex, three-dimensional nanostructure that has to withstand temperatures above 1800 F (1000 C) to be practical,” Braun explained. “In fact, the hotter the better.”

In previous experiments, however, the 3D structure of the emitter was destroyed at temperatures of around 1800 F (1000 C). To address the problem, Braun and his Illinois colleagues coated tungsten emitters in a nanolayer of a ceramic material called hafnium dioxide.

The results were dramatic. When subjected to temperatures of 1800 F (1000 C), the ceramic-coated emitters retained their structural integrity for more than 12 hours. When heated to 2500 F (1400 C), the samples remained thermally stable for at least an hour.

The ceramic-coated emitters were sent to Fan and his colleagues at Stanford, who confirmed that devices were still capable of producing infrared light waves that are ideal for running solar cells.

“These results are unprecedented,” said former Illinois graduate student Kevin Arpin, lead author of the study. “We demonstrated for the first time that ceramics could help advance thermophotovoltaics as well other areas of research, including energy harvesting from waste heat, high-temperature catalysis and electrochemical energy storage.”

Braun and Fan plan to test other ceramic-type materials and determine if the experimental thermal emitters can deliver infrared light to a working solar cell.

“We’ve demonstrated that the tailoring of optical properties at high temperatures is possible,” Braun said. “Hafnium and tungsten are abundant, low-cost materials, and the process used to make these heat-resistant emitters is well established. Hopefully these results will motivate the thermophotovoltaics community to take another look at ceramics and other classes of materials that haven’t been considered.”

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Other authors of the study are Nicholas Sergeant, Linxiao Zhu and Zongfu Yu of Stanford; Andrew Cloud, Hailong Ning, Justin Mallek, Berç Kalanyan, Gregory Girolami and John Abelson of Illinois; and Mark Losego and Gregory Parsons of North Carolina State University.

This article was written by Mark Shwartz, Precourt Institute for Energy at Stanford University.

Related information:

Fan Research Group http://www.stanford.edu/group/fan/

Braun Research Group http://braungroup.beckman.illinois.edu/

Global Climate and Energy Project http://gcep.stanford.edu/

 

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Nanowires Have the Power to Revolutionize Solar Energy

Imagine a solar panel more efficient than today’s best solar panels, but using 10 000 times less material. This is what EPFL researchers expect given recent findings on these tiny filaments called nanowires. Solar technology integrating nanowires could capture large quantities of light and produce energy with incredible efficiency at a much lower cost. This technology is possibly the future for powering microchips and the basis for a new generation of solar panels. Read more

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U of T Engineering breakthrough promises significantly more efficient solar cells

TORONTO, ON – A new technique developed by University of Toronto Engineering Professor Ted Sargent and his research group could lead to significantly more efficient solar cells, according to a recent paper published in the journalNano Letters. Read more

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Solar cells made from black silicon

The Sun blazes down from a deep blue sky – and rooftop solar cells convert this solar energy into electricity. Not all of it, however: Around a quarter of the Sun’s spectrum is made up of infrared radiation which cannot be converted by standard solar cells – so this heat radiation is lost. One way to overcome this is to use black silicon, a material that absorbs nearly all of the sunlight that hits it, including infrared radiation, and converts it into electricity. Read more

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WPI professor wins Catalyst Award for innovative design for grid storage batteries

Yan Wang receives award from the Massachusetts Clean Energy Center to support work on a new design for flow batteries that can be used to store electric energy produced by wind and solar power installations

Worcester, Mass. – An innovative design developed by a researcher at Worcester Polytechnic Institute (WPI) for flow batteries—rechargeable energy systems that can be used to store electric energy produced by wind and solar power installations—has received a 2012 Catalyst Award from the Massachusetts Clean Energy Center. The one-year, $40,000 award will support continued work on the new technology, which promises significantly higher energy and power density and longer cycle life than conventional flow batteries at a significantly lower cost. Read more

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