A solar panel is a solar panel, right? Not exactly. They’re kind of like cars. While all car have wheels, a steering wheel, and pedals to control it, there are a wide variety of engines, horse power, type of tires and brakes that all determines how efficient the car can be and how it handles the road. The same is true for solar panels. There are different materials and technologies being researched that can have a great impact on the amount of energy generated and the associated cost. Past few years have seen a lot of researches in these areas which could significantly help the wider acceptance of solar energy.
The cost of solar power has been dropping dramatically over the past decade, while at the same time solar efficiency has been rising. If you look back to 1977, the cost per watt of solar energy was around $77, but today its around $ 0.13. And it’s continuing to drop. Let's take a quick look at how solar evolved, so that we can understand the latest developments in some context.
The photoelectric effect was first observed in 1839. William Coblentz received the first patent in solar cell in 1913. However, it took a while before solar energy caught the attention of others. In 1954 Bell Labs invented the first practical silicon solar cell which had an efficiency around 6%. In 1957, Hoffman Electronics was able to increase that efficiency by 8% and then to 10% by 1959. By the time we get into the early 1960’s, solar cells had achieved about 14% efficiency. Today, most solar panels are somewhere between 15% and 20% efficient. With some of the higher efficiency models being in the low 20% range. The LG panels are about 21.1% efficient. And SunPower has a panel that’s almost 23% efficient. So commercially available solar cells have gone from about 10% efficiency in 1959 to 23% efficiency today.
To understand the efficiency of solar cells, it is important to understand the materials used and how they are manufactured. The primary material in solar panels is silicon, which can be formed in three ways: i) mono-crystalline, ii) polycrystalline and iii)thin-film panels. Mono-crystalline solar panels have the highest efficiency with current ratings between 15-22.2%, and a life span of around 25-30 years. Silicon wafers cut from bars made out of silicon gives us monocrystalline solar cell where the structure of the crystals is similar. Due to the single crystal structure, electrons have more room to move and a better chances of creating a better flow of electricity. To make polycrystalline panels, fragments of silicon are melted together to form the wafers. Polycrystalline solar panels have average efficiency between 12-18% with a 23-27 years life span. Since the mixture contains various crystal structures, it creates resistance in the flow of electricity and thus it is less effective. However, since the silicon need not be treated to an extend to be allowed set into a single crystalline structure reduces the cost of production. Finally, thin-films have the least efficiency between 9-14% and a lifespan close to 20 years. Thin films are achieved by applying a very thin layer to plastic to create flexible solar panels. Due to lower efficiency and lower cost, it is generally used in areas where there is large space available or to be used on the surface of other devices.
Currently we are stuck at 23% efficient solar panels, which are not far off from the theoretical maximum efficiency of a single material. The theoretical maximum efficiency known as the Shockley-Queisser limit for silicon panels is around 30%. However, scientists are trying to increase the efficiency by using various materials. The number of researches around perovskite have grown in the past few years. The structure of perovskite makes them highly effective at converting light photons into usable electricity, thus making it a suitable alternative for silicon. Perovskite are suitable to beat the efficiency level achieved by silicon based solar cells, Also, perovskite is man made material from abundantly available natural resources, which will help in keeping the cost of perovskite solar cells lower when it is mass produced. Perovskite cells can be made through a process called solution processing, where you can use inkjet printers to deposit materials on plastic sheets, So perovskite solar cells are another form of thin-film solar, but with much higher efficiency.
However, perovskite is not without it's own challenges. Some of the challenges are shorter lifespan, durability and toxicity. Perovskite cells are more sensitive to air and moisture. This puts their durability in question and thus reducing its lifespan. Also lead is used in making perovskite which makes it more toxic. However, since solar cells are encased within plastic and glass panels, careful handling and better recycling infrastructure should take care of these challenges. Still perovskite cells should match up with the 20-25 years life cycle of current solar cells or provide enough energy to match up or overtake conventional solar cells during its lifetime.
Researchers are working on multi-layered solar cells by combining various combination of materials. These materials when combined together in theory should be able to absorb a wide spectrum of light of varying wavelengths. Thus they should be able to capture more energy than conventional silicon only solar cells. There are a few companies which are working to produce solar cells which are a combination of silicon and perovskite cells and capable of achieving 28% efficiency. The higher efficiency is due to silicon absorbing red band and perovskite absorbing blue band of the visible light spectrum.
National Renewal Energy Laboratory, USA (NREL), achieved the biggest breakthrough in solar efficiency using this multi layered approach in 2020. They created a record by achieving 47.1% efficiency from a specially fabricated solar cell in the lab. This efficiency was achieved under concentrated illumination. Under more realistic conditions it achieved 39.2% efficiency. Instead of using two layers which is the current norm in hybrid solar cells, NREL created six different material combinations. They ordered these six materials in 140 layers within 1/3rd the thickness of a human hair. However, high the efficiency achieved maybe, it is still not production ready due to technological constraints and high cost associated with the production of such a cell using current technology. We will have to wait for few years before such cells become mainstream. However, the research has paved a way where we can breach the 30% efficiency barrier of single material and solar energy can be utilized for powering most of our needs.
Austrian PV manufacturing company Energetica, will be producing solar cells with gapless technology which will be increasing the density of the solar cells per module by reducing the gap between them. Although the efficiency of the solar cells is not increased, but the combined output of the modules will be higher than normal solar cells.
Another innovation is in having an intelligent solar panel which is capable of tracking the sun and realigning itself accordingly. As per the manufacturer of such solar panels, the energy output is 40% higher than stationary solar cells. It does make sense, a self aligning solar cells will get much more sunlight as compared to stationary solar cell throughout the day.
Environmental factors also affect the output of solar cells, for example, shadows. Current solar panels don't work if they get partially obstructed by shadows, thus restricting their applications everywhere. Researchers from National University of Singapore have developed a prototype of a device called the Shadow Effect Energy Generator (SEG), which generated power from the shadows. The way the technologies works is by generating and harvesting a small amount of electricity from the difference in contrast between the shadow and the illuminated sections of the device. If the device is in full shadow or full light, it’s not generating a voltage, but the closer you get to 50% coverage the more voltage it produces. The working prototype was able to generate about 1.2 V, enough energy to power a digital watch during their demonstration. It is still a long way from being practical and very much in lab, but it’s a very unique concept worth thinking about.
Another concept which is being researched is allowing solar panels to generate electricity even when it is raining. This concept was demonstrated by in a laboratory at Soochow University in China, in 2018. The device places two transparent polymer layers on top of a solar photovoltaic cell. A static electricity charge is generated due to the friction created by raindrops which roll off the panels. Researchers have also created similar devices to work on solar panels. They are known as triboelectric nanogenerators (Tengs). The design is significantly simpler and more efficient. One of the polymer layers plays the role of the electrode for both the Teng and the solar cell. Other scientists in China are also using Tengs on solar cells for harvesting some power from the wind. To help focus more light on the solar cell, the top layer of the Teng is also grooved . However, for such solution to be accepted, the power the device generates from falling rain needs to be significantly higher to start making an overall difference to a solar panel’s output. A hybrid device will give an important advantage in making it more compact and efficient.
Another area that is being researched is artificial photosynthesis. Photovoltaic cells have limitations due to variance in sunlight received through out the day and also due to the climate. The idea is to use PV cells to create electricity directly and simultaneously using the photons from the sunlight to produce a fuel from sunlight that can be stored conveniently and used when sunlight is not available. With the development of catalysts able to reproduce the major parts of photosynthesis, water and sunlight would ultimately be the only needed sources for clean energy production. The only by-product would be oxygen, and production of a solar fuel has the potential to be cheaper than gasoline.
Source: Berkeley Lab