YSM Issue 90.1

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NEWS materials science SOLAR CELLS Organic solar cells reach new heights in efficiency ►BY JOE KIM The old solar cell revolution has come to a halt. The types of solar cells that are now widespread were commercialized more than 50 years ago. Despite scientific improvements and increased attention to solar energy, the cost of conventional solar cells remains high due to the high cost of silicon, which converts solar energy into electric energy by producing electrons when light hits the silicon layer in the solar cell. Understanding the negative effects of fossil fuels and the necessity for cheaper renewable energy, scientists have worked on engineering new solar cell designs using different materials. Currently, in contrast to the inorganic solar cells that dominate the field, organic polymer solar cells have been getting much attention due to their mechanical flexibility, large area, light weight, and low costs in mass-scale production. Organic polymer solar cells differ in that carbon-based polymer is used, and not the conventional silicon. The main hurdle that many researchers face in developing organic polymer solar cells is their low efficiency. Although slow improvements have been made, the most common design, single-junction cells with only one electron-producing active layer, has only an eight to ten percent power conversion efficiency—far less than the commercially required efficiency of over 20 percent. Recently, researchers in professor Andre Taylor’s transformative materials and devices lab broke through the 10 percent boundary by incorporating multiple cocrystalline squaraines into the solar cell’s active layer. These cocrystalline squaraines are organic fluorescent dyes that absorb light and produce electrons, which are then picked up by the interlayer to produce a current. Tenghooi Goh, a recent PhD graduate, explained that their new design, which contains a greater number of electron donors, allows the solar cell to absorb a wider range of wavelengths, or types of light. Typical polymer cells only absorb specific types of light and waste the light outside that range, resulting in decreased efficiency. However, Goh said that incorporating more materials is extremely complicated, as undesirable interactions between incompatible chemicals can occur. “Mixing things are not as simple and direct as it may suggest. In a lot of times, if you mix two incompatible things together…they actually drive down the efficiency instead of having a positive effect,” Goh said. Even with the setback, Goh stated this was a path to a fundamental breakthrough in organic polymer solar cell efficiency. Thus, the researchers focused on minimizing this potential destructive interaction. Previously, Goh and Taylor’s team successfully combined squaraine and high efficiency polymer component, thanks to certain properties of the system. Chromophores, or lightsensitive molecules, transfer energy between an acceptor and donor molecule through Förster resonance energy transfer (FRET). Goh referred to FRET as “a mechanism like photosynthesis, with electrons jumping over non-conducting gaps.” Therefore, FRET stabilized the final mixture by allowing energy to be transferred between materials, ultimately helping the team mix together potentially incompatible components so that the wide wavelength of light was preserved. The energy transfer between two squaraines, ASSQ and DPSQ, along with high performance electron donating compounds in active layers, helped stabilize the final mixture in the active layer. The team found that photovoltaic efficiency increased by over 25 percent on average between solar cells with and without ASSQ and DPSQ incorporated. In addition, multiple FRET pairs with rapid and efficient energy transfer were observed through spectroscopy techniques, which measures rapid changes in the absorbance of certain wavelengths of light. Such observations corresponded with the abovementioned explanation validating that FRET was indeed responsible for the stable combination of components. The solar cells have their shortcomings—Goh pointed out that the organic polymer solar cells are not free from environmental consequences such as harmful solvent waste. Even so, this research can provide greater benefits by lowering costs of production, leading to more widespread usage of solar cells and decreased fossil fuel usage. The efficiency of Goh’s team’s solar cell was recorded to be up to 10.7 percent, widely considered a milestone in efficiency of organic polymer solar cells. Furthermore, he added that this is before efficiency optimization by researching and modifying the layers surrounding the active cell. Goh is very hopeful about future research. “In the future, if we combine the pinnacle of different research together, we can probably reach greater efficiency.” PHOTOGRAPHY BY JOSHUA MATHEW ►Research in Andre Taylor’s lab focuses on organic solar cells, which consist of thin films with unique light-absorbing and energy-transfer properties that allow them to harness solar energy more efficiently. 10 Yale Scientific Magazine December 2016 www.yalescientific.org
medicine NEWS THE FLU SEASON CRAVINGS PARADOX The connection between metabolism and disease outcome ►BY MARY CHUKWU www.yalescientific.org PHOTOGRAPHY BY CHUNYANG DING ►Yale researchers in Professor Ruslan Medzhitov’s lab found connections between metabolism and infection that may lead to new approaches to nutrition for the critically ill. As winter settles in, perhaps the only seasonal “foods” more iconic than hot chocolate and s’mores are cough drops and tea. Why do some people want to weather colds holding steaming bowls of comforting soup, while others suffer queasy stomachs and leave dinner plates untouched? The seemingly paradoxical appetite changes associated with sickness are a well-recognized and evolutionarily ancient trait, but up until now scientists could only speculate about their cause and purpose. Yale researchers led by Ruslan Medzhitov, professor of immunobiology at the Yale School of Medicine, have found that the answer to this puzzle lies with the causes of infection. The key insight of their research is that not all sicknesses are created the same. Fighting viral versus bacterial infections requires completely opposite nutritional needs. The findings of their research hold the potential to transform how healthcare approaches nutrition in illness. The researchers began by tracing out how anorexia, or loss of appetite, affects disease outcome in bacterial infections. Researchers infected mice with Listeria monocytogenes, a common cause of food poisoning. Mice that were fed during infection died; those who did not eat lived. Similarly, the researchers infected mice with influenza virus, the cause of the flu, and again varied caloric intake. Now the fortunes were reversed: fed mice survived viral infection while those that did not eat mostly died. These trends held true not only for infection but also when the experiments were repeated for bacterial and viral inflammation—whole body immune activation. The real story linking nutrition and disease, however, emerged in the search for the nutrient causing the outcomes. Between protein, fat, and glucose, only glucose was required to induce the complete effects of caloric intake. Moreover, blocking glucose utilization rescued bacteria-infected mice while killing virus-infected mice. Interestingly, glucose does not change the number of pathogens or the strength of the immune response. Instead, glucose shapes the role of metabolism in the body’s tolerance of the immune response. Metabolism refers to all the chemical reactions needed to sustain life; use of the immune system comes at a price. “Depending on the type of inflammation or infection, there are different metabolic processes that are necessary to survive the critical illness condition,” Medzhitov explained. Specifically, bacterial and viral infections generate different kinds of tissue damage depending on the presence of glucose. While glucose dominates metabolism in the fed state, during a fasting state the body begins breaking down fat stores to utilize molecules called ketone bodies for energy. Bacterial infections cause an immune response that releases reactive oxygen species (ROS), or “free radicals,” which kill pathogens as well as damage host cells. Glucose exacerbates this negative side effect while ketone bodies from starvation are protective. A parallel exists for viral infection. In response to the cellular damage caused by viral infection, the unfolded protein response (UPR) targets and eliminates cells producing abnormal proteins. This process is regulated by glucose—without glucose, this stress-induced response leads to excessive cell damage. “We discovered that by blocking glucose utilization in viral infections we prevented a normal response against viruses and instead caused a response that ended up being destructive for the host,” said Andrew Wang, first author and clinical fellow in medicine. Shockingly enough, the battleground where glucose decided life and death was not in the heart or the lungs, as may be expected for a pulmonary illness like the flu, but rather in the brain. PET scans and tissue studies revealed damage at critical sites in the brain following bacterial and viral infection. Absence of glucose increased neuronal damage in viral infections while for bacteria the opposite was true. The damage was once again linked to either overactivity of the UPR or overproduction of ROS. Medzhitov and his team hope that these new findings can improve nutrition for the acutely ill. Currently, all ICU patients are fed intravenous liquid food that is 30 to 60 percent carbohydrates, but Medzhitov’s study suggests that these formulations need to be adjusted based on the cause of illness. The Yale team is currently preparing clinical trials that test that premise. Future studies by the team will continue using mouse models to study different types of infections, including parasitic and fungal. Their work will contribute to new holistic models of healthcare treatment. “The general implication is that there is a certain wisdom to the body’s reactions and our preferences during illness, and many of these are protective,” Medzhitov said. In sickness at least, we can all benefit from listening to our bodies. December 2016 Yale Scientific Magazine 11
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YSM Issue 90.1

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