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Major advances in organic polymers for plastic solar cells

Major advances in organic polymers for plastic solar cells

30-Jun-10
A team of researchers from the University of Alberta and the National Institute for Nanotechnology has extended the operating life of an unsealed plastic solar cell, from mere hours to eight months. The research groups' development of an inexpensive, readily available plastic solar cell technology hit a wall because of a chemical leeching problem within the body of the prototype. A chemical coating on an electrode was unstable and migrated through the circuitry of the cell. The team developed a longer lasting, polymer coating for the electrode. Electrodes are key to the goal of a solar energy technology, extracting electricity from the cell. Prior to the polymer coating breakthrough the research team's plastic solar cell could only operate at high capacity for about ten hours. When the team presented their paper the plastic solar cell had performed at high capacity for 500 hours. But it kept on working for another seven months. The team says the unit eventually stopped working when it was damaged during transit between laboratories. A major advance developed in the synthesis of organic polymers for plastic solar cells:
• reduced reaction time by 99%, from 48 hours to 30 minutes, and
• Increased average molecular weight of the polymers by a factor of more than 3
This finding has been announced by Professor Guillermo Bazan and a team of postgraduate researchers at UC Santa Barbara. The reduced reaction time effectively cuts production time for the organic polymers by nearly 50% (since reaction time and purification time are approximately equal in the production process) in laboratory as well as commercial environments. The higher molecular weight of the polymers, reflecting the creation of longer chains of the polymers, has a major benefit in increasing current density in plastic solar cells by as much as a factor of more than four. Over polymer batches with varying average molecular weights, produced using varying combinations of the elements of the new methodology, the increase in current density was found to be approximately proportional to the increase in average molecular weight. The methodology will greatly accelerate research in this area by making possible the rapid production of different batches of polymers for evaluation. The team plans to take advantage of this approach to generate new materials that will increase solar cell efficiencies and operational lifetimes, and to reevaluate previously-considered polymer structures that should exhibit much higher performance than they showed initially. To make these gains, the team replaced conventional thermal heating with microwave heating, modified reactant concentrations, and varied the ratio of reactants by only 5% from the nominal 1:1 stoichiometric ratio normally employed in polymerization reactions. The reduction in synthesis time should also make it easier to optimize the chemical structure as the research moves forward and will ultimately reduce the manufacturing cost.
Experiments conducted by Greg Scholes and Elisabetta Collini of University of Toronto offers new insights into the way molecules absorb and move energy. The team whose work is devoted to investigating how light initiates physical processes at the molecular level and how humans might take better advantage of that fact - looked specifically at conjugated polymers which are believed to be one of the most promising candidates for building efficient organic solar cells. Conjugated polymers are very long organic molecules that possess properties like those of semiconductors and so can be used to make transistors and LEDs. When these conductive polymers absorb light, the energy moves along and among the polymer chains before it is converted to electrical charges. One of the biggest obstacles to organic solar cells is that it is difficult to control what happens after light is absorbed: whether the desired property is transmitting energy, storing information or emitting light. The experiment suggests it is possible to achieve control using quantum effects, even under relatively normal conditions. The team found that the ultra fast movement of energy through and between molecules happens by a quantum-mechanical mechanism rather than through random hopping, even at room temperature. Scholes and Collini's discovery opens the way to designing organic solar cells or sensors that capture light and transfer its energy much more effectively. It also has significant implications for quantum computing because it suggests that quantum information may survive significantly longer than previously believed. Scientists at the US Department of Energy's (DOE) Argonne National Laboratory have refined a technique to manufacture solar cells by creating tubes of semiconducting material and then "growing" polymers directly inside them. The method has the potential to be significantly cheaper than the process used to make today's commercial solar cells. Because the production costs of today's generation of solar cells prevent them from competing economically with fossil fuels, Argonne researchers are working to re-imagine the solar cell's basic design. Most current solar cells use crystalline silicon or cadmium telluride, but growing a high-purity crystal is energy- and labor-intensive, making the cells expensive. The next generation, called hybrid solar cells, uses a blend of cheaper organic and inorganic materials. To combine these materials effectively, the researchers created a new technique to grow organic polymers directly inside inorganic nanotubes. At its most basic level, solar cell technology relies on a series of processes initiated when photons, or particles of light, strike semiconducting material. When a photon hits the cell, it excites one electron out of its initial state, leaving behind a "hole" of positive charge. Hybrid solar cells contain two separate types of semiconducting material: one conducts electrons, the other holes. At the junction between the two semiconductors, the electron-hole pair gets pulled apart, creating a current. In the study, the team had to rethink the geometry of the two materials. If the two semiconductors are placed too far apart, the electron-hole pair will die in transit. However, if they're packed too closely, the separated charges won't make it out of the cell. In designing an alternative, scientists paired an electron-donating conjugated polymer with the electron acceptor titanium dioxide (TiO2). Titanium dioxide readily forms miniscule tubes just tens of nanometers across�10,000 times smaller than a human hair. Rows of tiny, uniform nanotubes sprout across a film of titanium that has been submerged in an electrochemical bath. The next step required the researchers to fill the nanotubes with the organic polymer�a frustrating process as the polymer ends up bending and twisting, which leads to inefficiencies both because it traps pockets of air as it goes and because twisted polymers don't conduct charges as well. Also, since the polymer doesn't like titanium dioxide, it pulls away from the interface whenever it can. Trying to sidestep this problem, the team hit on the idea of growing the polymer directly inside the tubes. They filled the tubes with a polymer precursor, turned on ultraviolet light, and let the polymers grow within the tubes. Grown this way, the polymer doesn't shy away from the TiO2. In fact, tests suggest the two materials actually mingle at the molecular level; together they are able to capture light at wavelengths inaccessible to either of the two materials alone. This "homegrown" method is potentially much less expensive than the energy-intensive process that produces the silicon crystals used in today's solar cells. These devices dramatically outperform those fabricated by filling the nanotubes with pre-grown polymer, producing about 10 times more electricity from absorbed sunlight. The solar cells produced by this technique, however, do not currently harness as much of the available energy from sunlight as silicon cells can. Darling hopes that further experiments will improve the cells' efficiency.
 
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