Microscopic rake doubles efficiency of low-cost solar cells

A scanning electron microscope image shows the rigid pillar-like bristles of the FLUENCE rake, which is used to apply light-harvesting polymers to a solar cell. The distance between the pillars is 1 micrometer, about one-hundredth the diameter of a human hair.

Researchers from the Department of Energy's SLAC National Accelerator Laboratory and Stanford University have developed a manufacturing technique that could double the electricity output of inexpensive solar cells by using a microscopic rake when applying light-harvesting polymers.

When commercialized, this advance could help make polymer solar cells an economically attractive alternative to those made with much more expensive silicon-crystal wafers.

In experiments, solar cells made with the tiny rake double the efficiency of cells made without it and are 18 percent better than cells made using a microscopic straightedge blade.

The research was led by Zhenen Bao, a chemical engineering professor at Stanford and a member of the Stanford Institute for Materials and Energy Sciences (SIMES), which is run jointly by SLAC and Stanford. The team reported its results August 12 in Nature Communications.

"The fundamental scientific insights that come out of this work will give manufacturers a rational approach to improving their processes, rather than relying simply on trial and error," Bao said.

"We also expect this simple, effective and versatile concept will be broadly applicable to making other polymer devices where properly aligning the molecules is important."

The Problem With Polymers

Although prices for silicon-based solar cells are dropping, it still takes five to 15 years before they produce enough electricity to offset their purchase and installation. Silicon solar cells also require a large amount of energy to manufacture, which partly offsets their value as renewable energy sources.

Polymer-based photovoltaic cells are much cheaper because they're made of inexpensive materials that can be simply painted or printed in place. They are also flexible and require little energy to manufacture. While small, lab-scale samples can convert more than 10 percent of sunlight into electricity, the large-area coated cells have very low efficiency -- typically converting less than 5 percent, compared with 20-25 percent for commercial silicon-based cells.

Polymer cells typically combine two types of polymers: A donor, which converts sunlight into electrons, and an acceptor, which stores the electrons until they can be removed from the cell as usable electricity. But when this mixture is deposited on a cell's conducting surface during manufacturing, the two types tend to separate as they dry into an irregular assortment of large clumps, making it more difficult for the cell to produce and harvest electrons.

The SLAC/Stanford researchers' solution is a manufacturing technique called "fluid-enhanced crystal engineering," or FLUENCE, which was originally developed to improve the electrical conduction of organic semiconductors.

In the current work, as the polymers are painted onto a conducting surface, they are forced through a slightly angled rake containing several rows of stiff microscopic pillars. The rake is scraped along the surface at the relatively slow speed of 25-100 micrometers per second, which translates to 3.5-14.2 inches per hour. The large polymer molecules untangle and mix with each other as they bounce off and flow past the pillars, ultimately drying into tiny nanometer-sized crystals of uniform size with enhanced electrical properties.

Simulations and X-rays

The researchers used computer simulations and X-ray analyses at two DOE Office of Science User Facilities -- SLAC's Stanford Synchrotron Radiation Lightsource (SSRL) and Lawrence Berkeley National Laboratory's Advanced Light Source (ALS) -- to customize the FLUENCE rake for making solar cells.

"At SSRL, the team used X-ray diffraction to measure the degree to which the polymers formed crystals and X-ray scattering to determine how clearly the two polymers segregated themselves," said Mike Toney, SSRL Materials Sciences group leader and a co-author on the paper. "These are bread-and-butter techniques for which we've developed some novel approaches at SSRL in recent years."

To achieve the polymer patterns they wanted for the solar cells, the researchers made the pillars in the rake much shorter and more densely packed than those used earlier for organic semiconductors. They were 1.5 micrometers high and 1.2 micrometers apart; for comparison, a human hair is about 100 micrometers in diameter.

Close, But Not Too Close

"Ideally, the two types of photovoltaic polymers should be close enough to each other for electrons to move quickly from donor to acceptor, but not so close that the acceptor gives back its electrons before they can be harvested to electricity," said Yan Zhou, a Stanford researcher on Bao's team.

"Our new FLUENCE rake achieves this happy medium. Because we understand what's happening, we can tune the rake design and processing speed to alter the final polymer structures."

Future research will be aimed at applying the FLUENCE technique to other polymer blends and adapting it to rapid industrial-scale roll-to-roll printing processes -- which can reach speeds of 50 miles per hour -- that promise the lowest solar-cell manufacturing costs.

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Story Source:

The above post is reprinted from materials provided by SLAC National Accelerator Laboratory. Note: Materials may be edited for content and length.

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Journal Reference:

1. Ying Diao, Yan Zhou, Tadanori Kurosawa, Leo Shaw, Cheng Wang, Steve Park, Yikun Guo, Julia A. Reinspach, Kevin Gu, Xiaodan Gu, Benjamin C. K. Tee, Changhyun Pang, Hongping Yan, Dahui Zhao, Michael F. Toney, Stefan C. B. Mannsfeld, Zhenan Bao. Flow-enhanced solution printing of all-polymer solar cells. Nature Communications, 2015; 6: 7955 DOI: 10.1038/ncomms8955

One step closer to low cost solar cells

The flexible transparent electrode - “Flextrode.”

The dwindling resources for conventional energy sources make renewable energy an exciting and increasingly important avenue of research. However, even seemingly new and green forms of energy production, like silicon-based solar cells, are not as cost effective as they could be. An OIST research team led by Yabing Qi is investigating solar cells based on organic materials that have electrodes both flexible and transparent, enabling the fabrication of these solar cells at a low cost.

In a recent paper published in the journal Organic Electronics, Qi and his research group characterized the electrodes made with new materials, including plastic, conductive material and zinc oxide. They also successfully identified methods by which to clean the electrodes to restore their conductivity and work function after an extended period of storage, thus contributing to the optimization of making these new solar cells.
Traditional silicon-based solar cells are expensive to make because of the cost of the raw materials and stringent fabrication requirements. Silicon-based solar cells are also rigid and opaque, meaning their usage and placement are limited. Qi and colleagues work with flexible conductive materials that are also transparent. The fabrication of the "Flextrodes," as these flexible transparent electrodes have been named, is more cost effective and potentially easier to fabricate using a method called roll-to-roll coating, due to their flexible nature. For example, the main component for fabricating Flextrodes is PET, the same inexpensive and readily available plastic that comprises disposable drink bottles. In addition, their use and placement is potentially much more diverse than the silicon cells. For example, they may even be placed on windows since the organic solar cells can be made partially transparent.
Since these Flextrodes are a relatively new technology, basic surface science studies had not been conducted. In their recent paper, Qi and colleagues looked at their work function, surface conductivity and chemical states. They also observed that after an extended period of storage, Flextrodes had an insulating layer of contaminants on the surface that greatly reduced their efficiency and function. The researchers were able to show that two common cleaning methods, one using UV ozone treatment, the other using oxygen plasma treatment, were both effective in removing the contaminants and restoring function to the Flextrodes in a timely and cost-efficient way. The research demonstrated that these methods could easily be integrated into the solar cell fabrication process to regenerate ready-to-use Flextrodes.
Qi is excited about the future of these low-cost organic solar cells. He explains that unlike conventional silicon-based solar cells, "the organic materials available to make the cells are virtually limitless." His lab is working on design and optimization of these new solar cells. The possibility of this technology being available for widespread public use may be just around the corner. Perhaps the next window decoration you put up will be one composed of organic solar cells, providing not just nice aesthetics, but clean energy as well.
This work was done in collaboration with Plasticphotovoltaics.org and Prof. Frederik C. Krebs and his laboratory at Technical University of Denmark, who kindly provided the Flextrode for this study.

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Story Source:
The above story is based on materials provided by Okinawa Institute of Science and Technology - OIST. Note: Materials may be edited for content and length.

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Journal Reference:

1. Yuichi Kato, Min-Cherl Jung, Michael V. Lee, Yabing Qi. Electrical and optical properties of transparent flexible electrodes: Effects of UV ozone and oxygen plasma treatments. Organic Electronics, 2014; 15 (3): 721 DOI: 10.1016/j.orgel.2014.01.002

Towards 'printed' organic solar cells and LEDs

A flexible organic solar cell from TREASORES project undergoing mechanical testing: the cell is repeatedly flexed to a 25 mm radius whilst monitoring its performance. Such cells have shown lifetimes in excess of 4000 hours

In order to make solar energy widely affordable scientists and engineers all over the world are looking for low-cost production technologies. Flexible organic solar cells have a huge potential in this regard because they require only a minimum amount of (rather cheap) materials and can be manufactured in large quantities by roll-to-roll (R2R) processing. This requires, however, that the transparent electrodes, the barrier layers and even the entire devices be flexible. The EU-funded project "TREASORES" (Transparent Electrodes for Large Area Large Scale Production of Organic Optoelectronic Devices), which started in November 2012 with an overall budget of more than 14 Mio Euro and is led by Empa researcher Frank Nüesch, aims at developing and demonstrating technologies to facilitate R2R production of organic optoelectronic devices such as solar cells and LED lighting panels.

Transparent electrodes with superior performance
The TREASORES project recently completed its mid-term review and has already achieved some major milestones. The international team that comprises researchers from 19 labs and companies from five European countries has, for instance, developed an ultra-thin transparent silver electrode that is cheaper than, and outperforms, currently used indium tin oxide (ITO) electrodes. The researchers could also demonstrate a record efficiency of 7 % for a perovskite-based solar cell using such novel transparent electrodes. What's more, their first fully R2R-produced solar cells already achieved commercially acceptable lifetimes when tested "in the field." The next step, says Nüesch, is to scale up and improve the most promising technologies identified so far, say, to produce barrier materials and transparent electrodes in larger quantities, i.e. in rolls of more than 100 meters in length.
In its second half, the TREASORES project will also continue to develop other promising technologies such as transparent and flexible electrodes based on woven fabrics, nanowires and carbon nanotubes (CNTs). "We are working on the most crucial issues in large-scale organic optoelectronics. Our new low-cost electrode substrates already outperform existing conductive oxide electrodes in many ways," says Nüesch. "But we must further improve the resulting device yields from large-scale production by reducing the defect density of the substrates."
The new materials have been thoroughly tested using special instruments for mechanical, electrical, and optical testing and their performance in practical devices has been characterized e.g. for lifetime and quality of illumination. Silver nanowires were used to produce flexible electrodes with a sheet resistance of below 20 Ohms/square -- a measure for the electrical conductivity of thin films -- and an optical transmission of 80%. Copper nanowires were even better, yielding a sheet resistance of below 10 Ohms/square and an optical transmission of 90% on glass. They clearly outperformed current ITO electrodes, which typically have sheet resistance values of 100 Ohms/square and above for such high transparency. Solar cell devices with an energy conversion efficiency of over 3% have been made on these substrates with copper electrodes. CNT electrode performance likewise made significant progress during the first half of the project, reaching a sheet resistance of 74 Ohms/square with an optical transmission of 90%. The organic solar cells that were produced with these electrodes reached an energy conversion efficiency of 4.5%.
"Ironing" the rough electrode surface
All these electrode technologies suffer, however, to some extent from waviness or roughness and require a flattening layer to allow defect-free deposition of optoelectronic device stacks. That's why the researchers set out to develop yet another electrode technology, which uses thin silver (Ag) films sandwiched between two metal oxide (MO) layers. These films turned out to be much flatter. MO/Ag/MO electrode stacks provide a sheet resistance of 6 Ohms/square with an optical transmission of 85% and allowed the construction of more efficient optoelectronic devices compared to the other electrode technologies, which is due, at least in part, to the low peak-to-valley roughness of about 20 nm. With these "ultra-flat" electrodes record efficiencies of up to 7% were obtained for organic solar cells using commercially available materials for light harvesting. Using the very same electrode materials, the team achieved 17 lm/W for the production of white light organic LEDs (OLEDs) and more than 20 lm/W for organic light-emitting electrochemical cells (OLECs). Although not quite record values for flexible OLED and OLEC devices, Nüesch stressed that "all electrodes were produced by an R2R process in an industrial environment or with industrially relevant processes on large areas of the polymer substrate. We can thus say that the processes we used are robust and reproducible."

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The above story is based on materials provided by Empa Swiss Federal Laboratories for Materials Science and Technology. Note: Materials may be edited for content and length.

Solar cells: Increased pressure creates a happy union

By tailoring the interface between the two sections of a solar cell, A*STAR researchers have produced a high-performance solar cell from the abundant and cheap materials of copper (II) oxide and silicon.

For solar energy to become environmentally friendly and cost effective, the two main components of 'heterojunction' solar cells ― the n- and p-type layers ― need to be fabricated from nontoxic, abundant materials. Copper (II) oxide, also known as cupric oxide, holds promise as a p-type semiconductor since it meets both these criteria and also has an ideal bandgap for absorbing sunlight and a high light absorption.
On paper, copper oxide and silicon are a perfect pair for producing high-performance solar cells. In practice, however, their performance has been disappointing because of the tendency of holes and electrons to recombine in them ― a process known as charge recombination. This recombination limits the production of electricity in a solar cell since it reverses the generation of electrical charges from light. One cause of this problem is the poor quality of the interface between copper oxide and silicon as the result of silicon oxide on the silicon surface.
Now, Goutam Dalapati from the A*STAR Institute of Materials Research and Engineering at Singapore and co-workers have used conventional procedures to produce high-performance solar cells that employ cupric oxide as the p-type material and silicon as the n-type material. They realized this by increasing the pressure during the deposition stage of fabrication, which they found enhances both the crystal and interface quality, thereby reducing the charge recombination rate.
Using a sequence of analytical techniques -- Raman spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy and high-resolution transmission electron microscopy -- they showed that the interface quality was limited by the formation of a copper-rich oxide layer as well as by the production of a silicon oxide layer on silicon surface. Dalapati explains that the team were surprised by the formation of this copper-rich layer as the cupric oxide target contained an equal mix of copper and oxygen. But the scientists also discovered that they could minimize this layer by increasing the pressure during deposition and the annealing time. Using this tactic, the team successfully produced a high-quality solar cell that had a low charge recombination rate.
Dalapati notes that "to develop cost-effective, environmentally friendly photovoltaic devices using Earth-abundant nontoxic cupric oxide, it is essential that we can increase the efficiency further." This is possible, he adds, "by reducing, or even eliminating, the copper-rich interfacial layer and the silicon oxide insulating layer."

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Story Source:
The above story is based on materials provided by The Agency for Science, Technology and Research (A*STAR). Note: Materials may be edited for content and length.

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Journal Reference:

1. Saeid Masudy-Panah, Goutam Kumar Dalapati, K. Radhakrishnan, Avishek Kumar, Hui Ru Tan, Elumalai Naveen Kumar, Chellappan Vijila, Cheng Cheh Tan, DongZhi Chi. p-CuO/n-Si heterojunction solar cells with high open circuit voltage and photocurrent through interfacial engineering. Progress in Photovoltaics: Research and Applications, 2014; DOI: 10.1002/pip.2483