General Assessment of Organic Solar Cell Efficiency
Organic solar cell efficiency is generally moderate; which implies that it is neither extremely low nor high.
There are various factors which can be used to explain the efficiency range of organic solar cells (OSCs).
First is the fact that OSC technology is relatively immature, and has been achieved as a result of numerous efforts to both improve and diversify renewable energy technologies.
Therefore, the advent of organic solar cells can be attributed to the implementation of principles of sustainable development and circular economy The technology has been modified over the years alongside other sustainable initiatives like wind farms . hydroelectric facilities, geothermal . wave power . artificial intelligence . Smart house development, biorefineries . and breeder nuclear reactor technology.
Also, the materials used to produce organic solar cells may vary. Differences in chemical composition and photoelectric behavior can affect the efficiency of these cells.
It is common to find organic solar cell efficiency values of around 10%. However, laboratory-engineered OSC units have achieved much higher efficiencies of 18-25%, under controlled conditions [3].
The importance of Eff i ciency in Organic Solar Cells
As implied above, the conditions and composition of organic solar cells are important to determine their efficiency.
While it cannot be said that organic solar cells are extremely efficient, the technology is currently capable of performing above average for electricity generation purposes.
Energy efficiency is very important in all energy systems. This is because it determines how much energy will be effectively utilized by these systems, without wastage.
Energy management systems, heat exchangers. electric generators . and cogeneration facilities are all examples of systems whose performance depends entirely on their energy efficiency.
Because energy efficiency affects how m u ch energy is utilized without wastage, it is directly proportional to energy conservation [5]. This means that efficiency determines the economic and environmental sustainability of energy conversion and consumption.
For all types of solar panels and PV cells (including organic solar cells), efficiency determines how much electricity can be generated from a given amount of solar energy [2]. This is also the photoelectric conversion rate.
It implies that when a solar system has efficiency of 10%, this is an indication that it effectively utilizes 10% of the total solar energy that is available, and so forth.
Conventional Silicon Solar Cell Vs Organic Solar Cell Efficiency
Organic solar cell efficiency is currently lower than that of conventional silicon solar cells.
The average efficiency of organic solar cells is 11%, while that of silicon solar cells is 15%.
These values are based on the range of efficiencies for different types of organic solar cells. as well as the range for silicon solar cells.
Organic solar cell efficiency ranges from 10-19%, while silicon solar cell efficiency ranges from 15-20%. In outstanding cases, OSC efficiency can reach 25%, while some silicon solar cells have recorded up to 50% efficiency.
Such cases generally involve PV cells that have undergone chemical engineering in a laboratory, for purposes of experiment; research and development. These have not generally been adopted for practical purposes.
The higher efficiency of silicon solar cells can be attributed to their maturity compared to organic solar cells. Silicon PV technology has undergone more modifications and improvements than OSC technology, and has also advanced due to large-scale adoption.
Lastly, the effectiveness and complexity of the photoelectric conversion process makes silicon solar cells more efficient.
Generally, photoelectric conversion in silicon PV cells is more effective and less complicated than in organic solar cells, as it involves only inorganic materials and processes, and does not require much buildup of charges to transmit the electricity generated.
It is expected that organic solar cell efficiency will increase as the technology undergoes modifications.
The following table summarizes the efficiency comparison between organic and silicon solar cells;
Factors that Affect Organic Solar Cell Efficiency
Factors that affect organic solar cell efficiency are; composition, environmental conditions, and photoelectric conversion mechanism.
These factors affect the instantaneous efficiency of organic solar cells. Other factors that affect efficiency in general include level of technological maturity and adoption.
1). Composition (as a Factor Affecting Organic Solar Cell Efficiency)
Organic solar cells are generally composed of an organic semiconductor layer that is sandwiched between two metallic layers [1].
The metallic layers act as electrodes for transmission (by conduction) of the electricity that is generated by the organic semiconductor. They often include a layer of indium tin oxide (ITO) alongside a calcium, magnesium, or aluminum layer.
The organic semiconductor layer is usually polymeric. Examples of materials used in this layer include poly(3-nethyl-thiophene), and Poly(ortho phenylenediamine) [6].
These materials differ in their efficiencies, and affect the performance of the organic solar cell.
In addition to composition, the design of solar cells also determines their efficiency, with conventional designs being generally more efficient than thin-film models.
2). Environmental Conditions
The efficiency of organic solar cells is affected by environmental conditions like radiation intensity, temperature, altitude, wind energy influence, humidity, and air quality .
Radiation intensity determines how much solar energy is available to generate electricity . This directly influences efficiency.
Other environmental factors may have an indirect effect on efficiency. Factors like humidity and air quality, can affect the ability of the solar cell’s surface to interact with and absorb solar energy.
Regions that are susceptible to high humidity, particulate matter pollution, or desertification . may have low photoelectric efficiency, due to moisture or dust accumulating on the surface of solar panels [4].
Similarly, in regions with high temperature or unfavorable atmospheric chemical composition, organic solar cells are at risk of impairment and may not perform optimally.
Organic solar cell technology can be said to have a cause-effect relationship with the environmental, since solar energy is renewable and can help to reduce the use of energy resources that prevent environmental sustainability by causing greenhouse emissions . climate change . global warming . and other forms of environmental degradation .
3). Conversion Mechanism (as one of the Factors Affecting Organic Solar Cell Efficiency)
Conversion mechanism refers to the process by which solar energy is converted to electricity by an organic solar cell.
The conversion mechanism of OSCs is based on the photoelectric effect, like conventional solar PV cells.
However, organic solar cells utilize a more complex process for photoelectric conversion. This either involves organic electronics (with polymeric semiconductors), or hybrid organic-inorganic electronics (with dye-sensitized photovoltaic units).
The above mechanisms do not apply a unidirectional or simple pathway of photon absorption and electricity generation like silicon solar cells. This relative complexity can reduce the efficiency of conversion and the performance of the solar cell.
Recent developments include efforts to adopt the photoelectric model of silicon semiconductors in OSCs. This can enhance efficiency.
Conclusion
Organic solar cell efficiency ranges from 10-19%, with an average of 11%.
Factors that effect the eff i ciency of organic solar cells include;
Environmental Conditions
Conversion Mechanism
Organic Solar Cells and Photovoltaics: Structure, Functions, Price
Scientists all around the world are developing new technologies to make efficient use of solar energy. From roof-top solar panels to solar lights, there are numerous devices to help people generate electricity from sun rays.
Organic photovoltaics is the most recent development in this sector. over, these high-potential solar cells are the game-changers in how solar electricity is generated.
Although organic solar cells offer numerous benefits, many people are unaware of this innovative technology. In this blog, you will get familiar with the organic solar photovoltaic cells, their function, pricing, and various aspects of this recent solar technology.
What are Organic Photovoltaics Solar Cells?
Organic solar cells use organic electronics and carbon-based materials as semiconductors to generate electricity from solar energy.
They are also referred to as polymer solar cells or plastic solar cells. Although they follow the same process as traditional silicon solar panels for generic electricity, there is one significant difference between the two.
Unlike silicon solar panels, organic photovoltaics have a flexible structure. Therefore, they can easily fit multiple spaces and are also suitable for making solar power Windows.
Although this new technology requires more research and development, the wide range of applications is increasing its popularity and reliability already.
over, organic solar cells are less costly to produce. It means that more people will be able to install them without worrying about their budget.
Structure of Organic Photovoltaics Solar Cells
OPV or organic photovoltaics have a flexible structure due to carbon-rich compounds. As a result, they enhance PV cell functions like bandgap, colour, and transparency.
To create an OPV structure, organic compounds that easily dissolve in ink are printed on thin plastic layers.
These solar power generic cells are relatively less efficient and durable than conventional solar panels. However, they are less expensive too. It certainly results in their high production volume.
In addition, the plastic-based material of the OPV is easily applicable to a variety of surfaces and areas.
How do Organic Photovoltaics Solar Cells Function?
The function of organic photovoltaics is similar to polycrystalline and monocrystalline silicon solar cells.
They generate solar electricity with the photovoltaic effect. It means, they directly convert the sun’s rays into electricity at the atomic level.
- In the first step of the photovoltaic effect, the organic solar cells absorb sunlight in the form of energy known as photons.
- The cells then break the photons to loosen electrons.
- These free electrons flow to electron acceptors to create a direct current.
- The electrical current then transfers to the solar inverter.
- The solar inverter converts DC power into AC power for residential usage.
Different Types of Organic Photovoltaic Solar Cells Available in the Market
Polymer-based organic solar cells are categorised into three groups according to their production method. Have a look!
- Single-layer organic cells: In this type, the external circuit connects to two electrodes through a conductor. The difference in the functions creates an electrical field in the layers of the organic cells.
- Bilayer organic cells: This type of organic photovoltaics consists of multiple layers of cathode, acceptor, ITO, donor, and substrate. Bilayer organic solar cells split excitons for increased efficiency.
- Bulk heterojunction organic cells: In this type of organic solar cell, there are two transparent electrodes and one active layer to trap the solar energy.
Power Generation From Organic Photovoltaics Cells
OPV and PV follow a similar process of power generation. However, the low efficiency of OPV results in less power generation due to insufficient absorption of sun rays.
Nonetheless, they’re cheaper than other conventional solar cells. Hence, there’s definite future scope.
What is the Pricing of Organic Photovoltaic Solar Cells?
Organic photovoltaics technology is a revolutionary development in the sector of solar power generation.
The OPV harnesses solar energy to domestic power establishments at a highly affordable price. Although this technology is new and requires extensive research for development, the average cost of organic solar cells varies between INR 2,485/m2 to INR 7,456/m2.
Pros and Cons of Organic Photovoltaics Solar Cells
Organic photovoltaics offer the following benefits:
- The soluble organic molecules of organic solar cells facilitate an easy and less costly manufacturing process.
- The organic solar cells have adaptive and flexible structures, resulting in a large area of application. over, these lightweight structures are appropriate for use in Windows and doors that receive abundant sunlight.
- Manufacturers have a vast supply of building block materials for organic photovoltaics.
Cons of Organic solar cells:
- The efficiency of organic photovoltaics is comparatively lower than a conventional silicon solar cell. Generally, silicon solar cells offer 18-20% efficiency in the conversion of sun rays into usable electricity. On the other hand, an organic cell’s efficiency is estimated at around 8-12%.
- The organic materials of OPV degrade much faster than silicon. Therefore, OPVs are a little less durable.
Why Isn’t Solar Energy Popular?
Sun offers a sustainable source of energy to every part of the world. However, solar energy is not as widespread a process of generating electricity as it should be.
The following factors play a vital role in limiting the use of organic photovoltaics and PV system for solar power generation:
- Solar panels require a huge investment. Therefore, many economically backward groups find difficulty in investing in solar panels.
- The efficiency of organic photovoltaics is low. People need to install multiple organic solar panels to generate sufficient units of electricity. This requires enormous space.
- The generation of solar power directly depends upon the availability of bright sunlight. Conditions like storms and clouds can rob people’s access to continual electricity from the Sun.
That being said, applying for a solar subsidy provided by the government and EMI solutions provided by solar companies can nullify the financial constraints.
Conclusion
Organic photovoltaics is a promising system for generating sustainable energy. Many researchers are developing new ways to increase its efficiency.
It is highly expected that in the coming days, this technology will gain popularity.
over, the abundant material availability of organic photovoltaics makes it affordable. Almost everyone will be able to harness the benefits of sunlight to fulfil their daily power requirement.
FAQs
Q. How long do organic photovoltaics Solar Cells last?
Organic photovoltaics solar cells generally show less than 30% degradation in two months when exposed to harsh climatic conditions.
However, multiple searches are underway to increase the durability of these organic cells.
Q. What leads to the low efficiency of organic solar cells?
Organic cells are highly prone to recombination due to the increased attraction between carbon-based materials and electrons. This results in low efficiency of the organic solar compounds.
Q. What is the meaning of efficiency in organic photovoltaics?
The efficiency of solar cells means the amount of solar energy they can convert into usable electricity.
Therefore, an 8% efficiency means that the solar panel can convert 8% of the total sunlight it receives into an electric current.
Outstanding performance of organic solar cell using tin oxide
Organic solar cells have a photoactive layer that is made from polymers and small molecules. The cells are very thin, can be flexible, and are easy to make. However, the efficiency of these cells is still much below that of conventional silicon-based ones. Applied physicists from the University of Groningen have now fabricated an organic solar cell with an efficiency of over 17 percent, which is in the top range for this type of material. It has the advantage of using an unusual device structure that is produced using a scalable technique. The design involves a conductive layer of tin oxide that is grown by atomic layer deposition. The scientists also have several ideas to further improve the efficiency and stability of the cell. The results have been described in the journal Advanced Materials on 31 March.
Credit: Loi Lab, University of Groningen
Organic solar cells have a photoactive layer that is made from polymers and small molecules. The cells are very thin, can be flexible, and are easy to make. However, the efficiency of these cells is still much below that of conventional silicon-based ones. Applied physicists from the University of Groningen have now fabricated an organic solar cell with an efficiency of over 17 percent, which is in the top range for this type of material. It has the advantage of using an unusual device structure that is produced using a scalable technique. The design involves a conductive layer of tin oxide that is grown by atomic layer deposition. The scientists also have several ideas to further improve the efficiency and stability of the cell. The results have been described in the journal Advanced Materials on 31 March.
In organic solar cells, polymers and small molecules convert light into charges that are collected at the electrodes. These cells are made as thin films of different layers—each with its own properties—that are stacked onto a substrate. Most important is the photoactive layer, which converts light into charges and separates the electrons from the holes, and the transport and blocking layer, which selectively directs the electrons towards the electrode.
Stability
‘In most organic solar cells, the electron transport layer is made of zinc oxide, a highly transparent and conductive material that lays below the active layer,’ says David Garcia Romero, a PhD student in the Photophysics and Optoelectronics group at the Zernike Institute for Advanced Materials at the University of Groningen, led by Professor Maria Antonietta Loi. Garcia Romero and Lorenzo Di Mario, a postdoctoral researcher in the same group, worked on the idea of using tin oxide as the transport layer. ‘Zinc oxide is more photoreactive than tin oxide and, therefore, the latter should lead to a higher device stability,’ he explains.
Although tin oxide had shown promising results in previous studies, the best way to grow it into a suitable transport layer for an organic solar cell had not yet been found. ‘We used atomic layer deposition, a technique that had not been used in the field of organic photovoltaics for a long time,’ says Garcia Romero. However, it has some important advantages: ‘This method can grow layers of exceptional quality and it is scalable to industrial processes, for example in roll-to-roll processing.
Scalable
The organic solar cells that were made with tin oxide deposited by atomic layer deposition on top show a very good performance. ‘We achieved a Champion efficiency of 17.26 percent,’ says Garcia Romero. The fill factor, an important parameter of solar cell quality, showed values up to 79 percent, in agreement with the record values for this type of structure. Furthermore, the optical and structural characteristics of the tin oxide layer could be tuned by varying the temperature at which the material is deposited. A maximum power conversion was reached in cells with a transport layer that was deposited at 140 degrees Celsius. This same result was demonstrated for two different active layers, meaning that the tin oxide improved efficiency in a generic way.
‘Our aim was to make organic solar cells more efficient and to use methods that are scalable,’ says Garcia Romero. The efficiency is close to the current record for organic solar cells, which stands around 19 percent. ‘And we haven’t optimized the other layers yet. So, we need to push our structure a bit further.’ Garcia Romero and his co-author Lorenzo di Mario are also keen to try making larger area cells. These are typically less efficient but are needed to step towards real-world applications and panels.
Improvement
The new solar cell with an impressively high fill factor is a good starting point for further development. Garcia Romero: ‘It may be a bit early for industrial partners to take this on; we need to do some more research first. And we hope that our use of atomic layer deposition will inspire others in the field.’ ‘We always strive to understand what is happening in a material and in a device structure,’ adds Professor Loi. ‘Here, we think that there might be room for improvement. In that process, our tin oxide transport layer is a great initial step.’ This class of solar cells may make an important extra contribution to the energy transition because of their mechanical properties and their transparency. ‘We expect that they will be used in a totally different way than silicon panels,’ says Loi. ‘We need to think broader and out of the box at the moment.’
Lorenzo Di Mario, David Garcia Romero, Han Wang, Eelco K. Tekelenburg, Sander Meems, Teodor Zaharia, Giuseppe Portale en Maria A. Loi: Outstanding Fill Factor in Inverted Organic Solar Cells with SnO2 by Atomic Layer Deposition. Avanced Materials, online 31 maart 2023.
Solar Cells of the Future: System for Increasing the Efficiency of Organic Solar Cells
Organic solar cells are cheaper to produce and more flexible than their counterparts made of crystalline silicon, but do not offer the same level of efficiency or stability. A group of researchers led by Prof. Christoph Brabec, Director of the Institute of Materials for Electronics and Energy Technology (i-MEET) at the Chair of Materials Science and Engineering at FAU, have been working on improving these properties for several years. During his doctoral thesis, Andrej Classen, who is a young researcher at FAU, demonstrated that efficiency can be increased using luminescent acceptor molecules. His work has now been published in the journal Nature Energy.
The sun can supply radiation energy of around 1000 watts per square meter on a clear day at European latitudes. Conventional monocrystalline silicon solar cells convert up to a fifth of this energy into electricity, which means they have an efficiency of around 20 percent. Prof. Brabec’s working group has held the world record for efficiency in an organic photovoltaic module of 12.6% since September 2019. The multi-cell module developed at Energie Campus Nürnberg (EnCN) has a surface area of 26 cm². “If we can achieve over 20% in the laboratory, we could possibly achieve 15% in practice and become real competition for silicon solar cells,” says Prof. Brabec.
Flexible application and high energy efficiency during manufacturing
The advantages of organic solar cells are obvious – they are thin and flexible like foil and can be adapted to fit various substrates. The wavelength at which the sunlight is absorbed can be ‘adjusted’ via the macromodules used. An office window coated with organic solar cells that absorbs the red and infrared spectrum would not only screen out thermal radiation, but also generate electricity at the same time.
One criterion that is becoming increasingly important in view of climate change is the operation period after which a solar cell generates more energy than was required to manufacture it. This so-called energy payback time is heavily dependent on the technology used and the location of the photovoltaic (PV) system. According to the latest calculations of the Fraunhofer Institute for Solar Energy Systems (ISE), the energy payback time of PV modules made of silicon in Switzerland is around 2.5 to 2.8 years. However, this time is reduced to only a few months for organic solar cells according to Dr. Thomas Heumüller, research associate at Prof. Brabec’s Chair.
Loss of performance for charge separation
Compared with a ‘traditional’ silicon solar cell, its organic equivalent has a definite disadvantage: Sunlight does not immediately produce charge for the flow of current, but rather so-called excitons in which the positive and negative charges are still bound. “An acceptor that only attracts the negative charge is required in order to trigger charge separation, which in turn produces free charges with which electricity can be generated,” explains Dr. Heumüller.
A certain driving force is required to separate the charges. This driving force depends on the molecular structure of the polymers used. Since certain molecules from the fullerene class of materials have a high driving force they have been the preferred choice of electron acceptors in organic solar cells up to now. In the meantime, however, scientists have discovered that a high driving force has a detrimental effect on the voltage. This means that the output of the solar cell decreases, in accordance with the formula that applies to direct current – power equals voltage times current.
Andrej Classen wanted to find out how low the driving force has to be to just achieve complete charge separation of the exciton. To do so, he compared combinations of four donor and five acceptor polymers that have already proven their potential for use in organic solar cells. Classen used them to produce 20 solar cells under exactly the same conditions with a driving force of almost zero to 0.6 electronvolts.
Increase in performance with certain molecules
The measurement results provided the proof for a theory already assumed in research – a ‘Boltzmann equilibrium’ between excitons and separated charges, the so-called charge transfer (CT) states. “The closer the driving force reaches zero, the more the equilibrium shifts towards the excitons,” says Dr. Larry Lüer who is a specialist for photophysics in Brabec’s working group. This means that future research should concentrate on preventing the exciton from decaying, which means increasing its excitation ‘lifetime’.
Up to now, research has only focused on the operating life of the CT state. Excitons can decay by emitting light (luminescence) or heat. By skilfully modifying the polymers, the scientists were able to reduce the heat production to a minimum, retaining the luminescence as far as possible. “The efficiency of solar cells can therefore be increased using highly luminescent acceptor molecules,” predicts Andrej Classen.
Reference: “The role of exciton lifetime for charge generation in organic solar cells at negligible energy-level offsets” by Andrej Classen, Christos L. Chochos, Larry Lüer, Vasilis G. Gregoriou, Jonas Wortmann, Andres Osvet, Karen Forberich, Iain McCulloch, Thomas Heumüller and Christoph J. Brabec, 31 August 2020, Nature Energy.DOI: 10.1038/s41560-020-00684-7
What can organic solar cells bring to the table?
OSCs expand the potential applications of solar technology, but there are still challenges to be overcome before large-scale deployment.
When you picture solar power, chances are you conjure up images of large solar panels spanning the length of a rooftop or a large solar farm out in a field. But what if you could put a solar panel in the sunroof of a hybrid car, on a tent or within the Windows of an office building? What if you could power a vaccine refrigerator in a remote place with a flexible solar panel that could be shipped in a mailing tube? These are just a few possible applications of a relatively new technology known as organic solar cells (OSCs) — new, at least, when compared with silicon solar technology, which has been around since the 1950s.
Like traditional silicon solar technology, OSCs turn the sun’s energy into usable electricity. But they are far more versatile than conventional solar photovoltaics. OSCs are lightweight and flexible and can be made to be semitransparent or in various colors. These qualities give them potential applications for textile, vehicle and building-integrated solar cells, and for creating power in areas where it does not exist.
Unique applications
While additional funding and research are needed to bring OSCs to the commercial market, experts agree they will play an important role in the future of solar technology. That said, they won’t replace or compete head-to-head with silicon solar cells. We shouldn’t expect to see expansive fields of OSCs, like those that generate gigawatts of power at silicon solar farms, says Seth Marder, a chemistry professor at Georgia Tech. Silicon solar is suitable for providing large-scale solar power, while OSCs have other unique strengths that guide its real-world applications.
Two unique features of OSCs are their thinness and flexibility. While a typical silicon solar cell is about as thick as the average width of a human hair, most OSCs are roughly a thousand times thinner. Because of their thinness and flexibility, OSCs can be fabricated on curved surfaces and flexible backings. For example, they can be patched or integrated into the fabric of tents, backpacks and even clothing. Most of these products are still under development and occupy a niche market, but they demonstrate the innovative creativity that OSCs provide. With OSC technology, the possibilities for where solar cells can be used has been greatly expanded beyond just rooftops and solar farms.
If 10 years ago you had told me we would have organic solar cells of 18% efficiency, I would have laughed.
OSCs also can be made transparent, semitransparent or in various colors. As a result, there are many potential applications for architectural use. For example, transparent OSCs could be integrated into Windows to generate energy from sunlight that otherwise might warm a room and contribute to higher air conditioning costs. Franky So, a materials science and engineering professor at North Carolina State University, offers yet another application: OSCs could be used in sunroofs to help power electric and hybrid vehicles.
Additionally, low up-front investment and potentially low product shipping costs make OSC technology accessible to communities in developing countries that lack access to an electrical grid and the financial means to build one. OSCs have a unique ability to bring power where power does not exist, explains Malika Jeffries-EL, an associate professor of chemistry at Boston University. In these instances, OSC technology could provide essential electricity in the smaller quantities needed for tasks such as lighting, charging cell phones and refrigerating medications and vaccines.
Another selling point of OSCs is that they are less energy intensive to manufacture than are silicon solar cells. Extremely hot furnaces — upwards of 2,700 degrees Fahrenheit — are needed to generate high purity silicon for silicon solar cells. By comparison, large-scale OSCs can be manufactured by simply printing the layers of the cell onto a backing in a process similar to that used to print newspapers. Because this process consumes less energy, OSCs have a significantly shorter energy payback time than silicon cells. In other words, OSCs require a shorter amount of time to generate the amount of energy it took to manufacture them.
How it works
The first organic solar cell was developed in 1958, but it wasn’t until the 2000s that OSCs saw a significant increase in efficiency. This improved OSC technology emerged from the field of organic light-emitting diodes, commonly known as OLEDs. OLED technology is used for many television and phone screens on the market today. In an OLED screen, a layer of organic molecules (molecules composed primarily of carbon and hydrogen atoms) emits light when an electric current is applied. OSCs work in essentially the opposite way — the layer of organic molecules generates an electric current when exposed to light.
An organic solar cell is made up of multiple layers of materials, one of which is the acceptor layer. When sunlight hits the cell, an electron is released from the layer of organic molecules, and the job of the acceptor is to pass that electron on to the electrode. This process causes a build-up of charge, which is what generates electricity.
With the development of non-fullerene acceptors, the efficiency of OSCs increased sharply. Graph courtesy of Felipe Larrain
Traditionally, the most commonly used acceptors in OSCs were materials based on fullerene — a molecule composed of 60 carbon atoms joined together in a structure that resembles a soccer ball. However, with fullerene acceptors the efficiency of OSCs was limited to around 10 percent. In other words, only 10 percent of the sunlight hitting the solar cell was converted into electricity. Researchers therefore set out to explore new types of acceptor layers as a means to increase OSC efficiency.
The breakthrough that permitted OSCs to achieve higher efficiencies was the development of non-fullerene acceptors (NFAs). With NFAs the efficiency of OSCs increased sharply — up to 18 percent in just a few years. This has brought OSCs to the lower end of the 18 percent to 22 percent efficiency of the average commercially available silicon solar cell. This uptick in efficiency has exceeded the expectations of many experts, some of whom began working in the field when the efficiency of OSCs hovered around just 3 percent. If 10 years ago you had told me we would have organic solar cells of 18 percent efficiency, I would have laughed, Marder says.
Barriers to overcome
There is still much work to be done before OSCs can be widely marketed. One of the biggest challenges is the solvents used in the manufacturing process. Most top-performing OSCs are made using chlorinated solvents, which present both health and environmental hazards. When scaling up OSC manufacturing, you have to consider the exposure of people who will be working in the manufacturing plants, says Bernard Kippelen, a professor of electrical and computer engineering at Georgia Tech. The research to date has focused largely on obtaining increasingly higher efficiencies, but as Kippelen says, we need an approach that goes well beyond just one number. To make OSCs a viable technology, the manufacturing process must be optimized to make it safer and more cost-effective.
Organic solar cells require a shorter amount of time to generate the amount of energy it took to manufacture them.
Another barrier to the mass production of OSCs is the difference between the efficiencies of individual cells tested under ideal lab conditions and the efficiencies that have been demonstrated for larger modules. Individual cells can have high efficiencies, but assembling multiple cells into modules, panels or arrays requires additional electrical connections that will decrease the efficiency. However, as Kippelen points out, such disparities are expected. It takes some time before the increases in cell efficiency are reflected in the efficiencies of modules coming off the manufacturing lines, he says. The same was true of silicon solar cells.
Funding for OSC research is another concern. In the United States, much of the funding for solar cell research comes from government agencies, such as the Department of Energy. However, according to Kippelen, A lot of funding sources kind of dried up to do research on OSC, due to the emergence of a rapidly expanding class of solar cells called perovskites. There has been a lot of excitement around the use of perovskites because their efficiency is even higher than silicon in some cases. However, even as funding for OSCs has decreased in the U.S., China continues to spearhead OSC research and development. The amount of work [on OSC research] in the United States is a tiny fraction of the amount of work in China,” Marder says. People in China are going full blast on this.
Reasons for optimism
Future world energy consumption will continue to rise, especially as developing countries aspire to the same benefits of on-demand energy production that developed countries enjoy. Researchers such as Marder, Kippelen, Jeffries-EL and So say OSC technology has the potential to play a unique and important role in the global transition toward renewable energy. The recent increase in OSC efficiency to 18 percent has many researchers working to advance this technology, and scientists are looking into tandem OSCs (which use two materials that absorb distinct wavelengths of sunlight) to capture even more energy. Some are hopeful that this development could increase OSC efficiency even further — up to 20 percent.
Kippelen calls for a long-term view of OSC technology. Solar technology is going to be around for a long time,” he says, “and I truly believe OSC, with time, will establish itself as a really important technology.
Editor’s note: Kellie Stellmach wrote this story as a participant in the Ensia Mentor Program. She is a graduate student pursuing her Ph.D. in chemistry at Georgia Tech. Her research is unrelated to the field of organic solar cells. While she has taken a class with Seth Marder, the interviews for this story were done before she was his student.
This story first appeared on: