Solar Paints-Technology of the Future
Solar energy is harnessed using technologies such as solar heating, solar photovoltaics, solar thermal electricity, solar architecture and artificial photosynthesis. Scientists have gone a step ahead in developing what is known as 'Solar Paints'. This can revolutionise in lowering carbon footprint and reducing greenhouse gases and it can also act as simple as painting a building. It is renewable, recyclable and features low CO2 energy production. Scientists are trying to find ways and means to develop solar paint that may be printed onto plastic, integrated into tinted windows and other building materials, making the whole structure itself a source of power and delivering a revolution in the industry.
What all it takes is a coat of paint to convert light energy into electricity. A new generation of photovoltaics will bear no resemblance to the rigid solar panels installed on house roofs. Working with semiconductor nano particles synthesized in solution, researchers – analogous to 'electronic ink' – are creating 'solar paints' that can be applied to virtually any structure, similar to regular paint.
Quantum Dots in Solar Paint
The most promising new application for solar power technology is “solar paint”. It’s still in research, and a viable, cost-effective product has not yet been produced. But it is coming. It promises to make solar power much cheaper, and will cover as much surface area as you want. Scientists are taking novel approaches to the subject, experimenting with different materials and chemicals. There are a few commonalities, however. Most importantly the suspended photovoltaic compounds within the paint are called “quantum dots.”
There is no guarantee that anybody’s work will pay off, but the potential is still alluring and it’s sparking some serious creativity. Researchers have engineered viruses to build carbon nano tubes on solar source, boosting efficiency close to 30% in the lab.
Solar paint achieved at nano level
The paint contains nano particles of titanium dioxide which gives whiteness to sunscreen and powdered sugar. The particles are coated with semiconducting cadmium nano crystals, and mixed with water and alcohol, to create a golden yellow paste. The researchers dubbed the product as “Sunbelievable.” They brushed it onto a conductive glass electrode, and attached that to a counter-electrode, to create a complete circuit. When they shined light on the tiny solar cell, it pumped out a small current. The efficiency of the light-to-electricity conversion was only about one percent—much lower than the 10 to 15 percent efficiency of conventional silicon cells.
But the researchers say this paint is relatively cheap, can be made in any color, and doesn’t require a clean room to manufacture, like silicon cells. To convert light energy into electricity through paint is a challenge. This challenge needs to be addressed if it is required to have a transformative photovoltaic technology and meet future energy needs. In recent years semiconductor nano crystal or quantum-dot-based solar cells have drawn significant attention as viable candidates for boosting the energy conversion efficiency beyond the traditional Shockley and Queisser limit of 32% for Si-based solar cells.
Because of the extremely small size of semiconductor quantum dots and high absorption cross section, it is possible to capture nearly the entire incident solar light in the visible region with an extremely thin layer of semiconductor materials. These heterojunction semiconductor solar cells, often referred as ETA (extremely thin absorber) cells, offer new opportunities to develop relatively inexpensive solar cells. One such example utilizes a PbS and TiO2 heterojunction and is reported to exhibit a power conversion efficiency of 5.1%.15 Similarly, Sb2S3-based ETA solar cells have delivered efficiencies greater than 5%.
These recent developments of photo induced charge separation using semiconductor nano crystal-based assemblies and efforts to utilize them in solar cells paves the way to propose transformative research efforts. The other type of quantum dot solar cell employs metal chalcogenide semiconductors as sensitizers which, upon excitation, inject electrons into large band gap semiconductors such as TiO2. The sulphide/polysulfide redox couple, which scavenges holes from the photo anode, is regenerated at the counter electrode. The photo electrochemical cells employing CdS and CdSe have been widely studied, and power conversion efficiency in the 3 to 4% range is often achieved.
Preparation
It typically takes a day or two to prepare quantum dot solar cells in the conventional multi-film architecture. Currently, research is being done on reducing the preparation time of quantum dot solar cells to less than an hour by changing the form to a one-coat quantum dot solar paint. Although the paint form is currently about five times less efficient than the highest recorded efficiency for the multi-film form, the researchers predict that the efficiency can be improved, which could lead to a simple and economically viable way to prepare solar cells.
The new solar paint, which the researchers humorously call “Sun Believable solar paint,” consists of a yellow or brown paste made of quantum dots. The small size of these tiny semiconductor nano crystals makes it possible to capture nearly all incident visible sunlight with an extremely thin layer of dots. The researchers experimented with three types of quantum dots: CdS, CdSe, and TiO2, all of which are powder-like, with water and tert butanol as the solvent.
Quantum dots are semiconductor nano crystals which exhibit size-dependent optical and electronic properties. In a quantum dot sensitized solar cell, the excitation of semiconductor quantum dot or semiconductor nano crystal is followed by electron injection into TiO2 nano particles. These electrons are then transferred to the collecting electrode surface to generate photocurrent. The holes that remain in the semiconductor quantum dot are removed by a hole conductor or redox couple and are transported to a counter electrode.
Solar paint has advantages in simplicity, economics, and stability compared to multi film solar cell architectures. While preparing a quantum dot film as a solar cell usually requires multiple time-intensive steps, solar cells in paint form can simply be brushed on to a surface in one step. If we can optimize the paint preparation, it should be possible for anyone to open a bottle (or a can in the long run) and apply it to a conducting surface. This will decrease the variability between lab to lab or person to person as one encounters in a multi-step process.
Having fewer fabrication steps and ambient preparative conditions should provide an economically viable transformative technology. The researchers experimented with several different combinations and ratios of the quantum dots to make different paint mixtures. They found that a composite of mixed CdS/TiO2 and CdSe/TiO2 nano particles achieve the best performance, particularly when the CdS and CdSe are deposited directly on the TiO2 nano particles as a coating. When coated on a glass electrode, the paint has an overall power conversion efficiency exceeding 1%. Although some multi film quantum dot solar cells have efficiencies greater than 5%, the researchers think that using different quantum dots and further optimization could significantly increase the efficiency of the paint.
Careful control of particle size and better electron transport through TiO2 network should enable the research to maximize the efficiency. The research will also extend the absorption range to near IR by using semiconductors such as PbS and PbSe. Our short term goal is to attain efficiencies greater than 5%, comparable with other semiconductor nano crystal-based solar cells.
The new solar paint is the first step toward developing a solar technology that could potentially have wide-ranging applications. Some uses could include painting electronic devices such as cell phones and computers, in addition to rooftops, windows, and cars. Large-scale applications could be used to build solar farms in deserts.
The goal is to prepare a solar paint that has long shelf life. Additional tests are underway to investigate long-term stability of paints with different compositions. In order to develop a commercial product, the researchers still have to work on two other components of the solar cell paint. The solar paint developed in this study is only one component of the solar cell. The other two components that need further development are a hole conducting layer and a counter electrode network.
Energy Independence is a Paintbrush Away
Solar paint sounds like nothing short of a miracle… the only seemingly bad thing about it is that it isn’t here yet. As with all good things of the future, however, there’s always some kind of catch. Power companies certainly won’t like being edged out by painters, which may or may not affect regulation or legislation. Will the big boys lose their grip? Or will potheads talk about this stuff in 20 years the way they talk nowadays about water-powered cars and Tesla? Both outcomes seem a little extreme, but they at least set the parameters of speculation for something so potentially revolutionary.
Over 99 percent of the energy provided by the sun is wasted each day, leading to our continued burning of fossil fuels, further increasing the effects of global warming. To truly become energy independent all we need to do is wake up and support technical advances such as the solar paint technology. With a little bit of refinement, a sustainable living may just be a paintbrush away.
References:
techie.com, nanowerk.com, phys.org