Organic solar panel

  Efficient Organic solar panel : Solar energy revolution

The fight against climate change might be gaining pace, but green energy silicon solar cells are reaching the limits of what they can do. Organic semiconductors offer a viable alternative to silicon-based photovoltaic panels at a lower cost and with greater flexibility. A new class of materials called non-fullerene acceptors increased organic solar cell's efficiency. Researchers have made a breakthrough in understanding organic semiconductors, hinting at more efficient and versatile solar cells. For years, silicon has dominated the solar energy landscape. Its efficiency and durability have made it the go-to material for photovoltaic panels. However, silicon-based solar cells are rigid and expensive to produce, limiting their potential for curved surfaces. Work is underway to perfect a new substance that might be cheaper, easier to handle and even more efficient for next-generation solar cells.

There is an enormous fission reactor in our planet’s sky. In just one hour, this reactor bathes the Earth’s surface in enough energy to supply all humanity’s electricity needs for a whole year. The problem is, the Sun’s energy arrives as solar radiation but we need to turn it into electricity. The most direct way to make the conversion right now is with solar panels, but there are other reasons why they’re the great hope of renewable energy. Their key component, silicon, is the second most abundant substance on Earth after oxygen; since panels can be put where the power is needed, on homes, factories, commercial buildings, ships, road vehicles, there’s less need to transmit power across landscapes; and mass production means solar panels are now so cheap the economics of using them are becoming inarguable. According to the International Energy Agency’s 2020 energy outlook report, solar panels in some locations are producing the cheapest commercial electricity in history. Even that traditional bug-bear “what about when it’s dark or cloudy?” is becoming less problematic thanks to transformative advances in storage technology. But silicon solar panels are reaching the practical limits of their efficiency because of some quite inconvenient laws of physics. Commercial silicon solar cells are now only about 20% efficient .

Organic semiconductors, these carbon-based materials offer a viable alternative at a lower cost and with greater flexibility. “They can potentially lower the production cost for solar panels because these materials can be coated on arbitrary surfaces using solution-based methods, just like how we paint a wall,” explained Wai-Lun Chan, associate professor of physics & astronomy at the University of Kansas. For all these advantages, organic solar cells have struggled to match the efficiency of their silicon counterparts. While silicon panels can convert up to 25% of sunlight into electricity, organic cells have typically hovered around 12% efficiency. This gap has proved to be a significant obstacle to widespread adoption. But these organic semiconductors aren’t just about cost savings. They boast an ability to be tuned to absorb specific wavelengths of light, opening up a plethora of new possibilities. “These characteristics make organic solar panels particularly suitable for use in next-generation green and sustainable buildings,” noted Chan. Imagine transparent and coloured solar panels, seamlessly integrated into architectural designs.

Led by graduate student Kushal Rijal, the team experimented with a sophisticated technique called time-resolved two-photon photoemission spectroscopy. This method allowed them to track the energy of excited electrons to less than a trillionth of a second. The researchers believe this unusual energy gain stems from a combination of quantum mechanics and thermodynamics. At the quantum level, excited electrons can appear to exist on multiple molecules simultaneously. Coupled with the second law of thermodynamics, this quantum behaviour reverses the direction of heat flow. “For organic molecules arranged in a specific nanoscale structure, the typical direction of the heat flow is reversed for the total entropy to increase,” Rijal explained. “This reversed heat flow allows neutral excitons to gain heat from the environment and dissociates into a pair of positive and negative charges. These free charges can in turn produce electrical current. Despite entropy being a well-known concept in physics and chemistry, it’s rarely been actively utilized to improve the performance of energy conversion devices,” emphasized Rijal."

Recent developments have rejuvenated the excitement around organic semiconductors. A new class of materials called non-fullerene acceptors (NFAs) pushed organic solar cell efficiency closer to 20%, narrowing the gap with silicon. The research team set out to understand why NFAs perform so much better than other organic semiconductors. Their investigation led to a surprising discovery: in certain circumstances, excited electrons in NFAs can gain energy from their surroundings instead of losing it. This finding flies in the face of conventional wisdom. “This observation is counterintuitive because excited electrons typically lose their energy to the environment like a cup of hot coffee losing its heat to the surrounding,” Chan explained. Beyond improving solar cells, the team believes that their findings are applicable in other areas of renewable energy research. They think the discovered mechanism will lead to more efficient photo catalysts to convert carbon dioxide into organic fuels. 

Researchers show promising material for solar energy gets its curious boost from entropy. Study solves in part the outstanding performance achieved by a new class of organic semiconductors known as non-fullerene acceptors (NFAs). Researchers discovered a microscopic mechanism that solves in part the outstanding performance achieved by a new class of organic semiconductors known as non-fullerene acceptors (NFAs). Solar energy is critical for a clean-energy future. Traditionally, solar energy is harvested using silicon, the same semiconductor material used in everyday electronic devices. But silicon solar panels have drawbacks: for instance, they're expensive and hard to mount on curved surfaces. Researchers have developed alternative materials for solar-energy harvesting to solve such shortcomings. Among the most promising of these are called "organic" semiconductors, carbon-based semiconductors that are Earth-abundant, cheaper and environmentally friendly.

While organic semiconductors already have been used in the display panel of consumer electronics such as cell phones, TVs and virtual-reality headsets, they have not been widely used in commercial solar panels yet. One shortcoming of organic solar cells has been their low light-to-electric conversion efficiency, about 12% versus single crystalline silicon solar cells that perform at an efficiency of 25%. According to Chan, electrons in organic semiconductors typically bind to their positive counterparts known as "holes." In this way, light absorbed by organic semiconductors often produces electrically neutral quasiparticles known as "excitons." But the recent development of a new class of organic semiconductors known as non-fullerene acceptors (NFAs) changed this paradigm. Organic solar cells made with NFAs can reach an efficiency closer to the 20% mark.

Despite their outstanding performance, it's remained unclear to the scientific community why this new class of NFAs significantly outperforms other organic semiconductors. In a breakthrough study, Chan and his team, including graduate students Kushal Rijal (lead author), Neno Fuller and Fatimah Rudayni from the department of Physics and Astronomy, and in collaboration with Cindy Berrie, professor of chemistry at KU, have discovered a microscopic mechanism that solves in part the outstanding performance achieved by an NFA. The key to this discovery were measurements taken by lead author Rijal using an experimental technique dubbed the "time-resolved two photon photoemission spectroscopy" or TR-TPPE. This method allowed the team to track the energy of excited electrons with a sub-picosecond time resolution (less than a trillionth of one second). "In these measurements, Kushal [Rijal] observed that some of the optically excited electrons in the NFA can gain energy from the environment instead of losing energy to the environment," said Chan. "This observation is counterintuitive because excited electrons typically lose their energy to the environment like a cup of hot coffee losing its heat to the surrounding."

The team, whose work was supported by the Department of Energy's Office of Basic Energy Sciences, believes this unusual process occurs on the microscopic scale thanks to the quantum behaviour of electrons, which allow an excited electron to appear simultaneously on several molecules. This quantum weirdness pairs with the Second law of Thermodynamics, which holds that every physical process will lead to an increase in the total entropy (often known as "disorder") to produce the unusual energy gain process. "In most cases, a hot object transfers heat to its cold surroundings because the heat transfer leads to an increase in the total entropy," said Rijal. "But we found for organic molecules arranged in a specific nano-scale structure, the typical direction of the heat flow is reversed for the total entropy to increase. This reversed heat flow allows neutral excitons to gain heat from the environment and dissociates into a pair of positive and negative charges. These free charges can in turn produce electrical current."

Based on their experimental findings, the team proposes that this entropy-driven charge separation mechanism allows organic solar cells made with NFAs to achieve a much better efficiency. "Understanding the underlying charge separation mechanism will allow researchers to design new nanostructures to take advantage of entropy to direct heat, or energy, flow on the nano-scale," Rijal said. "Despite entropy being a well-known concept in physics and chemistry, it's rarely been actively utilized to improve the performance of energy conversion devices." While the KU team believes the mechanism discovered in this work can be utilized to produce more efficient solar cells, they also think it can help researchers design more efficient photo-catalysts for solar-fuel production, a photochemical process using sunlight to convert carbon dioxide into organic fuels.