Jul 16

Nanowires and graphene: Keys to low-cost, flexible solar cells

0 comments

Overview

 

MIT researchers have made major strides toward developing solar cells that are inexpensive, efficient, flexible, and transparent using a design that combines two special components. Microscopic fibers called nanowires rapidly carry electrons liberated by solar energy through the solar cell to a flexible, transparent electrode made of graphene, a form of carbon that occurs in one-atom-thick sheets. Using a novel approach involving solutions and relatively low temperatures, the researchers were able to attach those two components together, “growing” nanowires directly on graphene in their lab. Tests with assembled solar cells showed that incorporating the nanowires pushed up device efficiency and that there was no performance penalty for replacing a conventional high-cost, brittle electrode with their version made of abundant, inexpensive carbon.

In the past decade or so, scientists and engineers have begun making photovoltaic solar cells out of organic—that is, carbon-containing—materials rather than inorganic silicon. The potential benefits are many. Conventional silicon solar cells require expensive, meticulous, step-by-step processing and careful handling to be sure they’re not damaged during transport and installation. In contrast, organic solar cells can be manufactured using inexpensive, continuous “roll-to-roll” processing in which individual layers are deposited sequentially on large areas of material. The finished organic cells are lightweight, flexible, and transparent, so they’re easy to handle and can be installed on roofs, windows, electronic device displays, and other surfaces, both flat and curved.

However, two problems have slowed development of this promising technology. First, organic solar cells can’t yet convert visible light into electricity as efficiently as their silicon counterparts can. And second, researchers haven’t been able to find a flexible, transparent, low-cost material for the electrode that carries current out of the cell. Silvija Gradečak, the Thomas Lord Associate Professor in Materials Science and Engineering, and a team of collaborators from four MIT departments have come up with methods of dealing with both of those problems.

The efficiency challenge

The challenge with conversion efficiency, says Gradečak, is getting the right geometry inside the organic cell. Any solar cell requires two materials—a “donor” material that absorbs incoming solar energy and gives off energized electrons, and an “acceptor” material that picks up those electrons and carries them to the electrode, where they exit the device as electrical current. In the usual organic solar cell, two polymers act as the donor and acceptor materials, and they need to be intertwined to provide lots of interfaces for the jumping electrons. The difficulty is controlling the nanometer-scale structure inside the organic cell to achieve those interfaces while providing pathways for the rapid movement of electrons to maximize the current coming out of the device.

To solve that problem, Gradečak has been working to make a “hybrid” solar cell by replacing one of the organic polymers with an inorganic material that will move the electrons more efficiently. But she adds a special twist: She makes the inorganic material into nanowires, microscopic fibers that are a few billionths of a meter in diameter and millions of times longer. Each nanowire is a single crystal, with an extensive surface area and no defects to interfere with the flow of electrons. And Gradečak’s group—the Laboratory for Nanophotonics and Electronics— has an unmatched ability to grow nanowires at any length, diameter, and density desired.

With this approach, there’s no need to worry about interconnecting regions of polymers. The electron-donor material can surround a “forest” of tall, solid nanowires—an overall structure that’s predictable and stable and maximizes contact between the two materials (see the figure below). Assembling the solar cell involves growing the nanowires up from a transparent electrode, infiltrating that forest with the polymer or other electron-donor material, and topping it off with a second electrode. When the solar cell is in use, light enters through the transparent electrode, and electrons knocked loose from the donor material move into nearby nanowires. The electrons travel rapidly through the nanowires to the transparent electrode, out along an external circuit, and back to the second electrode.

 

 

Incorporating nanowires into solar cells

 

In this solar cell design, tall, thin nanowires grow up from a transparent electrode and are surrounded by a light-absorbing polymer or other electron-donor material. A second electrode tops off the system. Light enters through the transparent electrode and energizes electrons in the polymer. The electrons move into the nanowires and flow to the transparent electrode and then out of the device into an external circuit. After powering, say, a light bulb, the electrons return to the second electrode and rejoin the “holes” they left behind. Using nanowires in place of the usual second polymer increases the stability and predictability of the structure and allows the electrons to move more quickly to the surface of the device.

To test their design, Gradečak and her collaborators grew nanowires on a transparent electrode and then deposited a solution containing the donor material on top. Images with a scanning electron microscope (SEM) showed that the solution infiltrated deep into the nanowire array, making good contact with the nanowire surfaces and leaving few voids to reduce performance. And in experiments with fully assembled devices, the presence of the nanowires pushed up efficiency by as much as 35%, depending on the donor material used.

Those results confirm the viability of their hybrid approach. “By combining the organic and inorganic materials, we bridge the advantages of both worlds,” says Gradečak. “We get solar cells that can be processed at a large scale using roll-to-roll methods, but they can still have reasonable power conversion efficiencies.”

Electrode options

Gradečak envisioned one more improvement, namely, a new material for the transparent electrode. In flexible solar cells now being designed, the transparent electrode is generally made of indium tin oxide (ITO)—not a good choice because it’s fairly brittle and indium is expensive and relatively rare. So Gradečak went looking for a better option.

One material that has recently grabbed the attention of researchers worldwide is graphene, a form of carbon that occurs in one-atom-thick sheets and has remarkable characteristics: It’s not only cheap and abundantly available but also highly conductive, flexible, robust, and transparent. To explore its viability as a transparent electrode, Gradečak teamed up with one of the world’s leading experts on graphene, Jing Kong, MIT’s ITT Career Development Associate Professor of Electrical Engineering and director of the Nanomaterials and Electronics Group in the Research Laboratory of Electronics.

The first question was whether they could grow nanowires on graphene while preserving the special properties of each component. “It turns out that this is not a trivial thing to do,” says Gradečak. The usual way to grow nanowires made of zinc oxide—their material of choice—is to deposit a “seed” layer of zinc oxide a few nano-meters thick on a piece of silicon, ITO, or other substrate and then immerse the structure in a solution containing zinc and oxygen ions. The zinc oxide nanowires quickly grow straight up from the surface. Because zinc oxide has a crystalline structure, the nanowires grow from individual crystals in the seed layer to form a forest of tall, skinny wires.

But when Gradečak and her colleagues tried to deposit the necessary seed layer on graphene, the zinc oxide solution separated into droplets rather than forming an evenly distributed coating. The problem is that the structure of graphene is extremely stable, with the carbon atoms in each sheet tightly connected with one another in a hexagonal pattern. As a result, graphene repels water, so the zinc oxide solution beads up instead of spreading out. Other researchers have tried to grow nanowires on graphene by first roughening up the graphene surface using an oxygen plasma, but that approach destroys the graphene, and the properties of the nanowires are not well known.

To preserve the integrity of the materials, Gradečak and her team tried a different approach: using an “inter-layer” between the graphene and the zinc oxide. They identified two commercially available polymers that would do the trick. The polymers would “wet” the graphene, covering the entire surface with a thin coating on which the zinc oxide could disperse. Better still, the polymers are electrically conductive and chemically compatible with zinc oxide, and they won’t interact strongly with the graphene, so it should remain both transparent and conductive.

To test this approach, the team grew zinc oxide nanowires on graphene with the polymer interlayer and on ITO, the standard transparent electrode material, under identical conditions. The results are shown in the SEM images below. The top images show nanowires grown on ITO, viewed from two angles; the bottom images show them grown on graphene with the polymer interlayer. The images confirm that the nanowires grown on the two materials are comparable in their uniformity, shape, and alignment. “So we can grow nanowires on graphene, and the quality of the nanowires is equal to or better than those grown on ITO,” says Gradečak.

Growing nanowires on graphene

 

 

 

 

These scanning electron microscope (SEM) images show zinc oxide nanowires grown on indium tin oxide, or ITO (top), the conventional electrode material used in flexible solar cells, and on graphene (bottom), a flexible, transparent form of ubiquitous, inexpensive carbon. To grow nanowires on graphene, the MIT team had to develop a new deposition process that involves first laying down a thin layer of a carefully selected polymer. The SEM images confirm that their approach yields nanowires that are similar in size, shape, and alignment to those grown on ITO, a material that is both costly and brittle.

The final step was to test the performance of complete solar cells, which team members assembled as shown in the schematic diagram below. First they stacked three monolayers of graphene on quartz, their sample surface. Then they deposited the polymer interlayer followed by the zinc oxide seed layer, the nanowires, the electron-donor material, and finally a second electrode of gold on a thin layer of molybdenum oxide. To test their options, they used this approach to assemble devices with each of their polymer interlayers—called PEDOT and RG-1200—and with two versions of the donor material: a polymer called P3HT and lead-sulfide quantum dots, which are tiny chunks of light-absorbing material that can be tuned to absorb a wide range of wavelengths of light. For comparison, they grew all the same devices on ITO electrodes.

 

Solar cell with nanowires and graphene electrode

 

 

The figure below presents sample results from experiments with the quantum dot devices. The curves show how much current can be extracted at different voltages under the standard illumination used in such tests. The red curve shows results when the polymer interlayer is PEDOT, the blue curve when it is RG-1200. The black curve presents data from the device grown directly on ITO. The performance of the devices with the different interlayers is similar to one another and—more significantly—to that of the ITO-based device. In other tests, the devices with nanowires, quantum dots, and graphene electrodes achieved power conversion efficiencies of 4.2%—less than the efficiency of general-purpose silicon cells but competitive for specialized applications.

 

Performance of devices made with nanowires and quantum dots on graphene and ITO electrodes

 

 

 

 

This diagram presents results from experiments with devices made with zinc oxide nanowires and lead-sulfide quantum dots. The red and blue curves show results from devices grown on graphene electrodes with a polymer interlayer of PEDOT and of RG-1200, respectively. The black curve shows results from a device grown directly on an ITO electrode. Measurements of current extracted at differing voltages are similar for all three devices, confirming the viability of using the inexpensive graphene electrode without sacrificing device performance.

Gradečak is pleased with those results. “We’ve shown that the polymers we use to make devices on graphene don’t interrupt the flow of electrons out of the device,” she says. “Our results show that we can replace the ITO electrode with graphene without sacrificing device performance.” Moreover, their simple low-temperature, solution-based method of depositing nanostructured materials on graphene without altering its structure or properties may be useful in fabricating novel versions of other optoelectronic devices, such as light-emitting diodes, lasers, and photodetectors.

The researchers are continuing to improve the performance of their solar cells, for example, by optimizing the size and spacing of the nanowires to maximize surface area, the amount of infiltrated electron-donor material, and the interface between the two. And they’re beginning to assemble their devices on other surfaces, including aluminum foil and lightweight plastics. “In our graphene-based solar cells, all the active components—the nanowires, polymers, and quantum dots—as well as the electrodes are flexible,” says Gradečak. “We’re now beginning to deposit them on flexible substrates, and that’s extremely exciting.”

This research was supported by Eni S.p.A. under the Eni-MIT Alliance Solar Frontiers Program. Eni is a Founding Member of the MIT Energy Initiative. Further information can be found in:

H. Park, S. Chang, J. Jean, J.J. Cheng, P.T. Araujo, M. Wang, M.G. Bawendi, M.S. Dresselhaus, V. Bulović, J. Kong, and S. Gradečak. “Graphene cathode-based ZnO nanowire hybrid solar cells.” Nano Letters, vol. 13, pp. 233–239, 2013.

J. Jean, S. Chang, P.R. Brown, J.J. Cheng, P.H. Rekemeyer, M.G. Bawendi, S. Gradečak, and V. Bulović. “ZnO nanowire arrays for enhanced photocurrent in PbS quantum dot solar cells.” Advanced Materials, vol. 25, pp. 2790–2796, 2013.

New Posts
  • An updated analysis from OpenAI shows how dramatically the need for computational resources has increased to reach each new AI breakthrough. In 2018, OpenAI found that the amount of computational power used to train the largest AI models had doubled every 3.4 months since 2012. The San Francisco-based for-profit AI research lab has now added new data to its analysis. This shows how the post-2012 doubling compares to the historic doubling time since the beginning of the field. From 1959 to 2012, the amount of power required doubled every 2 years, following Moore’s Law. This means the doubling time today is more than seven times the previous rate. This dramatic increase in the resources needed underscores just how costly the field’s achievements have become. Keep in mind, the above graph shows a log scale. On a linear scale (below), you can more clearly see how compute usage has increased by 300,000-fold in the last seven years. The chart also notably does not include some of the most recent breakthroughs, including Google’s large-scale language model BERT, OpenAI’s large-scale language model GPT-2,  or DeepMind’s StarCraft II-playing model AlphaStar. In the past year, more and more researchers have sounded the alarm on the exploding costs of deep learning. In June, an analysis from researchers at the University of Massachusetts, Amherst, showed how these increasing computational costs directly translate into carbon emissions. In their paper, they also noted how the trend exacerbates the privatization of AI research because it undermines the ability for academic labs to compete with much more resource-rich private ones. In response to this growing concern, several industry groups have made recommendations. The Allen Institute for Artificial Intelligence, a nonprofit research firm in Seattle, has proposed that researchers always publish the financial and computational costs of training their models along with their performance results, for example. In its own blog, OpenAI suggested policymakers increase funding to academic researchers to bridge the resource gap between academic and industry labs
  • StarckGate is happy to work together with Asimov that will be aiming to radically advance humanity's ability to design living systems. They strive to enable biotechnologies with global benefit by combining synthetic biology and computer science. With their help we will able to grasp the following domains better Synthetic Biology Nature has evolved billions of useful molecular nanotechnology devices in the form of genes, across the tree of life. We catalog, refine, and remix these genetic components to engineer new biological systems. Computational Modeling Biology is complex, and genetic engineering unlocks an unbounded design space. Computational tools are critical to design and model complex biophysical systems and move synthetic biology beyond traditional brute force screening. Cellular Measurement Genome-scale, multi-omics measurement technologies provide deep views into the cell. These techniques permit pathway analysis at the scale of a whole cell, and inspection down at single-nucleotide resolution. Machine Learning We are developing machine learning algorithms that bridge large-scale datasets with mechanistic models of biology. Artificial intelligence can augment human capabilities to design and understand biological complexity.
  • The use of AI (artificial intelligence) in agriculture is not new and has been around for some time with technology spans a wide range of abilities—from that which discriminates between crop seedlings and weeds to greenhouse automation. Indeed, it is easy to think that this is new technology given the way that our culture has distanced so many facets of food production, keeping it far away from urban spaces and our everyday reality. Yet, as our planet reaps the negative repercussions of technological and industrial growth, we must wonder if there are ways that our collective cultures might be able to embrace AI’s use in food production which might include a social response to climate change. Similarly, we might consider if new technology might also be used to educate future generations as to the importance of responsible food production and consumption. While we know that AI can be a force for positive change where, for instance, failures in food growth can be detected and where crops can be analyzed in terms of disease, pests and soil health, we must wonder why food growth has been so divorced from our culture and social reality. In recent years, there has been great pushback within satellite communities and the many creations of villages focussed upon holistic methods of food production. Indeed, RegenVillages is one of many examples where vertical farming, aquaponics, aeroponics and permaculture are part of this community's everyday functioning. Moreover, across the UK are many ecovillages and communities seeking to bring back food production to the core of social life. Lammas is one such ecovillage which I visited seven years ago in Wales which has, as its core concept, the notion of a “collective of eco-smallholdings working together to create and sustain a culture of land-based self-reliance.” And there are thousands of such villagesacross the planet whereby communities are invested in working to reduce their carbon footprint while taking back control of their food production. Even Planet Impact’s reforestation programs are interesting because the links between healthy forests and food production are well known as are the benefits of forest gardening which is widely considered a quite resilient agroecosystem. COO & Founder of Planetimpact.com, Oscar Dalvit, reports that his company’s programs are designed to educate as much as to innovate: “With knowledge, we can fight climate change. Within the for-profit sector, we can win this battle.” Forest gardening is a concept that is not only part of the permaculture practice but is also an ancient tradition still alive and well in places like Kerala, India and Martin Crawford’s forest garden in southwest England where his Agroforestry Research Trust offers courses and serves as a model for such communities across the UK. But how can AI help to make sustainable and local farming practices over and above industrial agriculture? Indeed, one must wonder if it is possible for local communities to take control of their food production. So, how can AI and other new tech interfaces bring together communities and food production methods that might provide a sustainable hybrid model of traditional methods and innovative technology? We know already that the IoT (internet of things) is fast becoming that virtual space where AI is being implemented to include within the latest farming technology. And where businesses invested in robotics are likewise finding that there is no ethical implementation of food technology, we must be mindful of how strategies are implemented which incorporate the best of new tech with the best of old tech. Where AI is helping smaller farms to become more profitable, all sorts of digital interfaces are transmitting knowledge, education and the expansion of local farming methods. This means, for instance, that garden maintenance is continued by others within the community as some members are absent for reasons of vacation or illness. Together with AI, customer experience is as much a business model as it is a local community standard for communication and empowerment. The reality is that industrial farming need not take over local food production and there are myriad ways that communities can directly respond to climate change and the encroachment of big agriculture. The health benefits of local farming practices are already well known as are the many ways that smartphone technology can create high-yield farms within small urban spaces. It is high time that communities reclaim their space within urban centers and that urban dwellers consider their food purchasing and consumption habits while building future sustainability which allows everyone to participate in local food production. As media has recently focussed upon AI and industrial farming, we need to encourage that such technology is used to implement local solutionsthat are far more sustainable and realistic instead of pushing big agriculture.

Proudly created by Starckgate 

© 2020 by Starckgate

  • White Facebook Icon
  • White Twitter Icon
  • White Instagram Icon