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Solar cells developed from trees
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Materials Today Volume 16, Number 4 April 2013
Solar cells developed from trees Recyclable solar cells have been created using natural substrates derived from biomass such as trees. This research could lead to a jump in recyclable, sustainable and renewable solar energy technology. The organic cells, developed on optically transparent cellulose nanocrystal (CNC) substrates, act in the same way as leaves; by trapping sunlight and turning it into energy. The transparency of the substrates allows light to pass through them before being absorbed by an extremely thin layer of the organic semiconductor. The study, by scientists at Georgia Institute of Technology and Purdue University in the US, and reported in the journal Scientific Reports [Zhou et al., Sci. Rep. (2013) doi:10.1038/srep01536], demonstrated that the cells – which have a low surface roughness of only about two nanometers – were able to achieve a power conversion efficiency of 2.7%, much higher than previously reached for cells on substrates derived from renewable raw materials. To fabricate the solar cells, it was crucial that the team could achieve a film that had a
surface roughness of a couple of nanometers. Previously, substrates tended to have a much larger roughness, creating problems in the fabrication process for the solar cells. Another advantage of the organic cells is that they can be recycled in water once they have reached the end of their life, with the substrate easily dissolving and the cells being separated out into their major components. The production of typical substrates, made
from glass or plastic, is much less environmentally friendly. Although we are still at an early stage in the development of solar technologies fabricated from natural products (as well as cellulose nanomaterials being touted as the next big thing for high-value nanoparticles) the research does offer an important proof of concept validation. Also, as they are extracted from plants and are therefore abundant; there are plans for large-scale production, which could happen over the next five years. Bernard Kippelen, who led the study, said ‘Our next steps will be to work toward improving the power conversion efficiency over 10 percent, levels similar to solar cells fabricated on glass or petroleum-based substrates.’ To manage this, they hope to optimize the optical properties of the solar cell’s electrode. It is hoped that efficient and recyclable organic solar cells fabricated on CNC substrates could find a range of applications in photovoltaic technology and clean energy production, as well as in disposable electronics. Laurie Donaldson
outcome: ‘‘If you have too much current going through the dendrites while the battery is being charged, the battery can catch fire.’’ This had been thought to be an unavoidable characteristic of lithium batteries, the main limiting factor in their performance, and an obvious safety concern. But Garcı´a’s team from Purdue have shown that, at least on paper, it is possible to control dendrite growth, leading to Li-ion batteries with improved performance and reliability. Their analytical theory may allow researchers to actively predict the early stages of dendrite formation. The work, which appeared in the Journal of the Electrochemical Society [J. Electrochem. Soc. (2013) doi:10.1149/1.057304jes], identified the various ways that lithium-ion batteries can fail during recharge, and suggested possible solutions to limit dendrite formation. For example, dendrites grow on
very specific locations on the anode; one option may be to engineer the anode’s surface, so that instead of nucleating into beads (and then dendrites) the lithium wets the electrode surface evenly. A potential outcome to this particular solution is a lithium battery that could charge in minutes rather than hours. Dendrites grow faster when exposed to the high voltages needed for fast recharging, which has limited the recharging speed of Li-ion batteries. Uniformly distributed lithium deposits may be less susceptible to these high voltages, so may finally make fast charging a reality. This paper represents the first analytical approach to the issue of dendritic growth in lithium batteries, and if the suggestions are adapted by other research groups, may lead to a new generation of more reliable, safer Li-ion batteries. Laurie Winkless
The solar cell. Image courtesy of Georgia Tech.
Reducing failure in lithium batteries Scientists at Purdue University have demonstrated that it is possible to control dendritic growth in lithium, the main failure mechanism in lithium-ion batteries. The reliability of lithium-ion batteries came into sharp focus last year when they were linked to fires on-board two Boeing aircraft. It has been suggested that battery failure, caused by an internal short circuit, may have led to these fires. When lithium batteries are recharged, ions move through the electrolyte, separating the battery’s two electrodes. This charge flow also draws material from the electrolyte and results in the formation of lithium dendrites on the positive electrode (anode) of the battery. These dendrites can grow so large that they span the distance between the electrodes; the moment the dendrite reaches the cathode, the battery fails. Prof. R. Edwin Garcı´a, a materials engineer at Purdue, describes a more worrying
108
Materials Today Volume 16, Number 4 April 2013
Solar cells developed from trees Recyclable solar cells have been created using natural substrates derived from biomass such as trees. This research could lead to a jump in recyclable, sustainable and renewable solar energy technology. The organic cells, developed on optically transparent cellulose nanocrystal (CNC) substrates, act in the same way as leaves; by trapping sunlight and turning it into energy. The transparency of the substrates allows light to pass through them before being absorbed by an extremely thin layer of the organic semiconductor. The study, by scientists at Georgia Institute of Technology and Purdue University in the US, and reported in the journal Scientific Reports [Zhou et al., Sci. Rep. (2013) doi:10.1038/srep01536], demonstrated that the cells – which have a low surface roughness of only about two nanometers – were able to achieve a power conversion efficiency of 2.7%, much higher than previously reached for cells on substrates derived from renewable raw materials. To fabricate the solar cells, it was crucial that the team could achieve a film that had a
surface roughness of a couple of nanometers. Previously, substrates tended to have a much larger roughness, creating problems in the fabrication process for the solar cells. Another advantage of the organic cells is that they can be recycled in water once they have reached the end of their life, with the substrate easily dissolving and the cells being separated out into their major components. The production of typical substrates, made
from glass or plastic, is much less environmentally friendly. Although we are still at an early stage in the development of solar technologies fabricated from natural products (as well as cellulose nanomaterials being touted as the next big thing for high-value nanoparticles) the research does offer an important proof of concept validation. Also, as they are extracted from plants and are therefore abundant; there are plans for large-scale production, which could happen over the next five years. Bernard Kippelen, who led the study, said ‘Our next steps will be to work toward improving the power conversion efficiency over 10 percent, levels similar to solar cells fabricated on glass or petroleum-based substrates.’ To manage this, they hope to optimize the optical properties of the solar cell’s electrode. It is hoped that efficient and recyclable organic solar cells fabricated on CNC substrates could find a range of applications in photovoltaic technology and clean energy production, as well as in disposable electronics. Laurie Donaldson
outcome: ‘‘If you have too much current going through the dendrites while the battery is being charged, the battery can catch fire.’’ This had been thought to be an unavoidable characteristic of lithium batteries, the main limiting factor in their performance, and an obvious safety concern. But Garcı´a’s team from Purdue have shown that, at least on paper, it is possible to control dendrite growth, leading to Li-ion batteries with improved performance and reliability. Their analytical theory may allow researchers to actively predict the early stages of dendrite formation. The work, which appeared in the Journal of the Electrochemical Society [J. Electrochem. Soc. (2013) doi:10.1149/1.057304jes], identified the various ways that lithium-ion batteries can fail during recharge, and suggested possible solutions to limit dendrite formation. For example, dendrites grow on
very specific locations on the anode; one option may be to engineer the anode’s surface, so that instead of nucleating into beads (and then dendrites) the lithium wets the electrode surface evenly. A potential outcome to this particular solution is a lithium battery that could charge in minutes rather than hours. Dendrites grow faster when exposed to the high voltages needed for fast recharging, which has limited the recharging speed of Li-ion batteries. Uniformly distributed lithium deposits may be less susceptible to these high voltages, so may finally make fast charging a reality. This paper represents the first analytical approach to the issue of dendritic growth in lithium batteries, and if the suggestions are adapted by other research groups, may lead to a new generation of more reliable, safer Li-ion batteries. Laurie Winkless
The solar cell. Image courtesy of Georgia Tech.
Reducing failure in lithium batteries Scientists at Purdue University have demonstrated that it is possible to control dendritic growth in lithium, the main failure mechanism in lithium-ion batteries. The reliability of lithium-ion batteries came into sharp focus last year when they were linked to fires on-board two Boeing aircraft. It has been suggested that battery failure, caused by an internal short circuit, may have led to these fires. When lithium batteries are recharged, ions move through the electrolyte, separating the battery’s two electrodes. This charge flow also draws material from the electrolyte and results in the formation of lithium dendrites on the positive electrode (anode) of the battery. These dendrites can grow so large that they span the distance between the electrodes; the moment the dendrite reaches the cathode, the battery fails. Prof. R. Edwin Garcı´a, a materials engineer at Purdue, describes a more worrying
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