Use of solar photovoltaics to generate electricity in terms of solar parks and solar panels on rooftops is growing but even decades after photovoltaics were first developed, the slabs of silicon remain bulky, expensive, and inefficient. Fundamental limitations prevent these conventional photovoltaics from absorbing more than a fraction of the energy in sunlight. Standard silicon solar cells mainly capture the visual light from violet to red. Since 1961, it has been known that there is an absolute theoretical limit, known as the Shockley-Queisser Limit, to how efficient traditional solar cells can be in their energy conversion. For a single-layer cell made of silicon—the type used for the vast majority of today’s solar panels—the upper limit is about 32 per cent.

Hot Solar Cells

Photovoltaics to thermophotovoltaics

However, there are some possible avenues to increase the overall efficiency by using multiple layers of cells. In this regard, a widely studied method is solar thermophotovoltaics (STPVs) wherein sunlight is first converted to heat before generating electrical power.

In fact, theory predicts that in principle this method, which involves pairing conventional solar cells with added layers of high-tech materials, could more than double the theoretical limit of efficiency, potentially making it possible to deliver twice as much power from a given area of panels. Scientists have built a different sort of solar energy device that uses inventive engineering and advances in Materials Science to capture far more of the sun’s energy. The trick is to first turn sunlight into heat and then convert it back into light, but now focussed within the spectrum that solar cells can use. By converting heat to focussed beams of light, a new solar device could create cheap and continuous power. While various researchers have been working for years on socalled solar thermophotovoltaics, this device is the first one to absorb more energy than its photovoltaic cell alone, demonstrating that the approach could dramatically increase efficiency. The device is still a crude prototype, operating at just 6.8 per cent efficiency but with various enhancements it could be roughly twice as efficient as conventional photovoltaics.

Advantages

The use of such a system could offer a number of advantages over conventional photovoltaics whether based on silicon or other materials:

• The photonic device is producing emissions based on heat rather than light implying that it would be unaffected by brief changes in the environment such as clouds passing in front of the sun.
• If coupled with a thermal storage system, it could in principle provide a way to make use of solar power on round-the-clock basis.
• The biggest advantage is the promise of continuous on-demand power.
• Additionally, due to the way the system harnesses energy (that would otherwise be wasted as heat), it can reduce excessive heat generation that can damage some solar-concentrating systems.

Technology behind STPVs

While current photovoltaic panels efficiently convert part of the solar spectrum directly into electricity, they become significantly less efficient as they get hotter—an inevitable side effect of absorbing sunlight. Unlike traditional photovoltaics, which maintain their efficiency by dispersing the heat away from the panel or cooling the panel in some way, hot solar cell panels will be built from materials that can operate efficiently at temperatures far higher than the typical panel and will integrate with a solar thermal collector that absorbs the unused portion of the light spectrum and converts it into heat. The aim is to make a photovoltaic device that can operate at temperatures as hot as the inside of a brick oven. This is definitely highrisk research, as solar cells have never been run this hot and they will need to be both reliable and efficient at that temperature for a long time. However, the potential payoffs are huge.

The key step in creating the device was the development of an absorberemitter which essentially acts as a light funnel above the solar cells. The absorbing layer is built from solid black carbon nanotubes that capture all the energy in sunlight and convert most of it into heat. As temperatures reach around 1,000°C, the adjacent emitting layer radiates the energy as light, now mostly narrowed to bands that the photovoltaic cells can absorb. The emitter is made from a photonic crystal, a structure that can be designed at the nanoscale to control the wavelengths of light flowing through it. Another critical advance was the addition of a highly specialized optical filter that transmits the tailored light while reflecting nearly all the unusable photons. This ‘photon recycling’ produces more heat, which generates more of the light that the solar cell can absorb, improving the efficiency of the system.

The panels will use technology from concentrated solar power—a different method for capturing solar energy used in several large solar power plants—to transfer the heat to high-temperature fluids that can be used to power a steam turbine and generate electricity. These fluids can also be easily stored so that the heat energy can be dispatched when the sun is not shining or whenever electrical demand rises; this method of storing solar energy is more cost-effective than storing energy in batteries. The current high cost of storing solar electricity in batteries, combined with the natural variation of available sunlight, will weaken the economic drive for photovoltaic market growth. Hot solar project addresses both these challenges by taking the best elements of photovoltaic panels and combining them with the best elements of concentrated solar power.

Future of STPVs

A photovoltaic device that converts sunlight into heat to generate power has achieved greater efficiency than previous such devices, thanks to the design of nanomaterials in the light-absorbing layer. The system converts solar heat into usable light, thus increasing the device’s overall efficiency. Solar thermal photovoltaics can exceed photovoltaics output with a direct comparison of the same cells, for a sufficiently high-input power density, lending this approach to applications using concentrated sunlight. The new record for solar means thermal photovoltaics using a solar simulator, selective absorber, selective filter, and photovoltaic receiver, which reasonably represents actual performance that might be achievable outdoors. The next steps include finding ways to make larger versions of the small, laboratory-scale experimental unit, and developing ways of manufacturing such systems economically.

There are some downsides to the approach, including the relatively high cost of certain components. It also currently works only in a vacuum. But the economics should improve as efficiency levels climb and the researchers now have a clear path to achieving the same. Scientists believe they can further tailor the components now that they have an improved understanding of what is needed to get to higher efficiencies. Researchers are also exploring further ways to take advantage of strength of solar thermophotovoltaics. Because heat is easier to store than electricity, it should be possible to divert excess amounts generated by the device to a thermal storage system, which could then be used to produce electricity even when the sun is not shining. If the researchers can incorporate a storage device and ratchet up efficiency levels, the system could one day deliver clean, cheap, and continuous solar power. In addition to converting a portion of the sunlight directly into electricity, the solar cells will use the remainder of the light to heat high-temperature fluids that can drive a steam turbine or be stored for later use.

Printed Solar Cells

Conventional, silicon-based solar panels are rigid and bulky. The future of solar energy depends on a union of new and old technologies. The most efficient are the perovskitebased cells. The latest of these, with just a few years of research behind them, convert 22 per cent of incident solar power to electrical power. This is more efficient than solar cells made from multicrystalline silicon. But perovskite cells cannot be rolled out commercially yet because they degrade under high humidity and heat. Taking into consideration the importance and growth of solar photovoltaic (PV) power generation in present-day scenario of harsh reality and sincere efforts towards a sustainable environment and depleting conventional energy resources, coming up of printed solar cells will boost the use of solar energy. If photovoltaic (PV) devices that turn light into electricity could be mass produced with printing presses, as if they were newspapers or banknotes, they could be affordable and ubiquitous. Small, thin, and flexible PV devices that are lightweight and translucent on films are already being manufactured. These use little material and can generate electricity in low light, even indoors. The thin, flexible solar cells could offer an affordable solution to meeting the needs of increasing energy demands around the world. Integrating them into phones and watches, as well as walls and windows, could transform the world’s energy generation, reduce pollution, and mitigate climate change.

Printable solar cells that are flexible and lightweight are the need of the hour in order to make best use of solar cells. Printed PV devices are typically made from many layers of material on a substrate of conductive glass or plastic. Each layer has a function: semiconductors or sensitizers absorb visible light, and other materials carry electrical charges to electrodes. Many types of printed PV devices are being developed; some feature organic semiconductors such as polythiophenes and others use lightabsorbing dyes, including rutheniumbased polypyridines. In quantum-dot solar cells, nanoparticles absorb light. Other examples feature semiconductors with a chalcogen element (sulphur, selenium, or tellurium) or contain organic– inorganic light absorbers with a structure similar to that of the mineral perovskite. All of these are classed as thin-film solar cells. At the moment, printable solar cells are made by printing a specially developed ‘solar ink’ onto a plastic film, similar to the way plastic bank notes are printed. There is a need to develop new materials and processes to enable the production of thin, flexible solar cells based on printable solar inks. These inks are deposited onto flexible plastic films using a range of processes, including spray coating, reverse gravure, slot-die coating, and screen printing. Some techniques for printing PV devices have been demonstrated in the lab over areas of about 10 sq. cm. These include feeding ink through a slit (slot-die printing), spray-coating the substrate, passing the substrate over a rotating cylinder (gravure printing), and moving a blade over the substrate through an ink supply. Interestingly, each technique has a downside. Such drawbacks mean that printed solar cells are less than half as efficient as the best non-printed equivalents.

To print thin, pinhole-free layers over more than one sq. m will require intelligent, more-precise equipment, and laser processing. An alternative approach would be to develop PV materials that work with existing industrial printing methods. Printing requires that materials can be formed into liquid, solution, or paste. For PV devices, this means using either solutions of chemicals (polymers, dyes or hybrid perovskite, for instance) or dispersions of nanoparticles (such as quantum dots). But many of these can degrade over days to weeks if not properly sealed and more-stable alternatives such as silicon are harder to print. A balance must also be struck between the efficiency of a device and the environmental impacts of its manufacture. The most efficient thin-film solar cells include toxic or rare materials, such as cadmium, ruthenium, and lead, as well as hazardous organic solvents. Indium, another rare element, is a common ingredient in transparent conductive films for PV devices and its use is expected to rise.

Status of Development

Inkjet solar cells are solar cells manufactured by low-cost, low-tech methods that use an inkjet printer to lay down the semiconductor material and the electrodes onto a solar cell substrate. Until recently, inkjet printers have not been used in the printed electronics industry. Industry has decided to move towards inkjet printing because of its low cost and flexibility of use. One of these is the inkjet solar cell. The first instance of constructing a solar cell with an inkjet printer was by Konarka in 2008. In 2011, the Oregon State University was able to discover a way to create Copper indium gallium selenide (CIGS) solar cells using an inkjet printer. In the same year, MIT (USA) was able to create a solar cell using an inkjet printer on paper. The use of an inkjet printer to make solar cells is very new and is still being researched. Mass production at low cost is what the solar industry sorely needs. The main advantage of printing solar cells with an inkjet printer is the low cost of production. The reason it is cheaper than other methods is because no vacuum is necessary which makes the equipment cheaper. Also, the ink is a low-cost metal salt blend reducing the cost of the solar cells. There is very little waste of material in comparison to other methods such as vapour phase deposition when using inkjet printers to lay down the semiconductor material. However, the efficiency of inkjet solar cells is too low to be commercially viable. Even if the efficiency gets better, the materials used for the solar cells could be a problem. We need to develop new printable solar cells that are flexible, lightweight, and extremely thin such that they can cover most surfaces.

Key Challenges

Flexible solar panels face several challenges enumerated as follows:

• Currently, printable solar cells have only reached about 10 per cent efficiency whereas traditional silicon solar PV cells are close to 25 per cent efficiency. The huge success of silicon panels has become a hurdle for emerging technologies. The manufacturers of silicon-based PV devices share materials, equipment, and practices with sibling industries, such as computing and this maturity of the silicon industry means there is little urgency to develop alternatives.
• Some solar cells are based on harmful substances such as heavy metals and use of hazardous solvents and others that are quick to degrade and inefficient at converting light into electricity.
• Printers used in the publishing, computing, and electronics industries struggle to print PV materials which are needed to be built with nanometre precision over many square metres.
• Changing materials into viscous pastes alters their physical and electrical properties.
• Printing layers that are nanometres to micrometres thick—uniformly and without pinholes, and over many square meters—is difficult.
• Capital investment and product commercialization are perceived as risky, given that printable PV devices are still being developed.
• The life span of the printed solar cells is also only six months so researchers are working to increase their efficiency, weather-resistance and life span to reach commercial viability.

For all these reasons, printable solar cells are yet to find a foothold in electricity markets. Printed solar cells would not become widespread until they are cheaper and safer to make. Researchers and businesses must work together to improve the efficiency, environmental impact, and stability of these cells, scale up their manufacture, and plan their market penetration.

Future of Printed Solar Cells

Printable solar cells offer exciting potential for generating electricity more flexibly and at a lower cost, wherever the sun shines. Early printable PV devices should target weaknesses in silicon-based technologies, such as their poor performance in low light and their lack of portability. The next wave should complement silicon solar cells and, ideally, be integrated with them. For example, silicon–perovskite devices would harvest a greater fraction of incoming sunlight than silicon devices alone could do. If printed technologies can capture 5 per cent of the PV market, their advantages should ensure that they play an ever-increasing part in meeting the growing demand for renewable energy. Paper thin solar cells or eventually direct 3D printing will allow creating solar cells on blinds, in windows, curtains, and almost anywhere in the home. New developments in printed solar cells could allow solar energy to create electricity almost anywhere, including walls, windows, roller blinds, shade umbrellas, and even tents! Therefore, printing of solar cells is very promising and could bring a great future for the use of solar power.

Acknowledgement: The use of information retrieved through various references/sources from the Internet in this article is highly acknowledged.

Dr S S Verma, Department of Physics, Sant Longowal Institute of Engineering & Technology (SLIET), Longowal, Distt.Sangrur, Punjab, India.