When Electrons Leave Holes in Organic Solar Cells Jean-Luc Bredas Science 343, 492 (2014); DOI: 10.1126/science.1249230
If you wish to distribute this article to others, you can order high-quality copies for your colleagues, clients, or customers by clicking here. Permission to republish or repurpose articles or portions of articles can be obtained by following the guidelines here. The following resources related to this article are available online at www.sciencemag.org (this information is current as of January 30, 2014 ): Updated information and services, including high-resolution figures, can be found in the online version of this article at: http://www.sciencemag.org/content/343/6170/492.full.html A list of selected additional articles on the Science Web sites related to this article can be found at: http://www.sciencemag.org/content/343/6170/492.full.html#related This article appears in the following subject collections: Physics, Applied http://www.sciencemag.org/cgi/collection/app_physics
Science (print ISSN 0036-8075; online ISSN 1095-9203) is published weekly, except the last week in December, by the American Association for the Advancement of Science, 1200 New York Avenue NW, Washington, DC 20005. Copyright 2014 by the American Association for the Advancement of Science; all rights reserved. The title Science is a registered trademark of AAAS.
Downloaded from www.sciencemag.org on January 31, 2014
This copy is for your personal, non-commercial use only.
PERSPECTIVES chemically if not physically, to fertilize their eggs using the male’s sperm. In this scenario, selection could well have led to the evolution of male pheromones that alter female physiology in a way that enhances male fitness, even if they shorten female life span. Given that males are rare and do not know which female in their environment they might mate with, releasing pheromones that influence all females in their vicinity might be the best way to maximize fitness. But why don’t female worms evolve ways to avoid such costly consequences of perceiving males? This might be a case of the “rare enemy” effect. If males are seldom encountered by hermaphrodites, then selection might be too weak to favor what could be costly measures for hermaphrodites to fight off male attempts at manipulation. What about the observation that male mortality in flies increases merely by perceiving the presence of females? Here, too, sexual conflict might be the proximate explanation. Interestingly, the detrimental effects
from exposure to female pheromones can be largely alleviated by allowing the male fruit fly to mate with multiple females (5). Is this a cost of false expectations? An alternative possibility is that the very traits that males have evolved to manipulate females can be costly to the male. Males produce toxic proteins in their seminal fluid that promote egg-laying in the female but also reduce her life span (1). It may be that perceiving females are sufficient to drive production of these or other toxic molecules, and failure to ejaculate them during mating could result in deleterious consequences for the male flies— the evolutionary equivalent of friendly fire. Perhaps sexually antagonistic coevolution has shaped sensory pathways—the signals produced by one sex, and the receptors and neural pathways that transduce these signals in the other sex. Although additional studies are needed to definitively test this idea, these same pathways have evolved to modulate longevity and fecundity in response to subtle and complex changes in a multitude of envi-
ronmental parameters, including nutrients, temperature, pathogens, and population density. How humans age might thus be a consequence not only of the way we treat our body, and the genetic lottery handed to us by our parents; it might also be affected by the social interactions—some cooperative, some conflictual—played out by males and females over millions of years. References 1. T. Chapman et al., Nature 373, 241 (1995). 2. D. Gems, D. L. Riddle, Nature 379, 723 (1996). 3. C. Shi, C. T. Murphy, Science 343, 536 (2014); 10.1126/ science.1242958. 4. T. J. Maures et al., Science 343, 541 (2014); 10.1126/ science.1244160. 5. C. M. Gendron et al., Science 343, 544 (2014); 10.1126/ science.1243339. 6. L. Partridge, P. Harvey, Nature 316, 20 (1985). 7. S. Libert et al., Science 315, 1133 (2007). 8. E. D. Smith et al., BMC Dev. Biol. 8, 49 (2008). 9. G. Arnqvist, L. Rowe, Sexual Conflict. Monographs in Behavior and Ecology, J. Krebs, T. H. Clutton-Brock, Eds. (Princeton Univ. Press, Princeton, NJ, 2005). 10. W. R. Rice, Nature 381, 232 (1996). 10.1126/science.1250174
APPLIED PHYSICS
Ultrafast spectroscopy reveals how the charge carriers in organic solar cells separate at interfaces and avoid substantial energy loss.
When Electrons Leave Holes in Organic Solar Cells Jean-Luc Bredas1,2
O
rganic solar cells convert sunlight into electricity by exploiting the electronic properties of electrically and optically active organic materials. Since the initial report of an organic solar cell reaching a power conversion efficiency near 1% (1), many efforts have brought the efficiency of carefully optimized devices in the 10 to 12% range (2). These efficiencies remain, however, well below the thermodynamic limit for single-junction organic solar cells, estimated to be >20% (3). Part of the failure in reaching the full potential of these devices is the lack of a comprehensive mechanistic picture of energy harvesting and carrier generation, transport, and recombination, particularly as a function of materials properties and activelayer morphology. On page 512 of this issue, Gélinas et al. (4) present a major step forward in the characterization of the charge-separation mechanism in organic solar cells.
An established view of organic πconjugated materials is that the primary photoexcitations are excitonic in nature. The photoexcited electron is not free to move on
hν
h e
OMe O
N
1
School of Chemistry and Biochemistry, and Center for Organic Photonics and Electronics, Georgia Institute of Technology, Atlanta, GA 30332–0400, USA. 2Department of Chemistry, King Abdulaziz University, 21589 Jeddah, Saudi Arabia. E-mail: [email protected]
492
its own; it remains bound to the hole (positive charge carrier) that forms on the molecular orbital from which the electron was excited. The binding energies of these electron-hole pairs, or excitons, are generally more than one order of magnitude greater than the thermal energy at room temperature. The formation of strongly bound excitons is in contrast to the situation in inorganic materials, such as crystalline silicon, where photoexcitations lead to immediate separation of elec-
PC71BM
R1
S
R2
S N
S
F
S
Si
R2 S
N S F
p-DTS(FBTTh2)2
N S
S
R1
31 JANUARY 2014 VOL 343 SCIENCE www.sciencemag.org Published by AAAS
Charges at the interface. In this illustration of the donor-acceptor interface and the ultrafast process of charge separation of hole and electron probed by Gélinas et al., the ovals represent the donor molecules [p-DTS(FBTTh 2) 2] and the circles the fullerene acceptor molecules (PC71BM). The modeling carried out by Gélinas et al. suggests that the electron wave function is delocalized over several fullerene molecules.
CREDIT: V. ALTOUNIAN/SCIENCE
PERSPECTIVES tron and hole at room temperature and generation of free charge carriers. The large exciton binding energies in π-conjugated materials are related not only to their low dielectric constants (which offer less screening between charges and lead to stronger interactions) but also to the presence of strong electron-vibration and electronelectron interactions (5). Because excitons are neutral species unable to carry a current, the efficiency of organic solar cells depends critically on charge-separation processes at heterojunctions between an electron donor (D; typically a π-conjugated polymer or molecule) and acceptor (A; typically a fullerene derivative), such as those displayed in the figure. These D-A heterojunctions are required to produce a driving force to dissociate excitons into spatially separated charges and are generally considered as an important factor limiting efficiency compared to inorganic p-n junction solar cells [the efficiency of crystalline silicon solar cells is on the order of 25% (2)]. Understanding the energy-harvesting mechanisms in organic solar cells, thus requires the ability to characterize the elementary charge-generating processes as a function of materials choice and in the presence of multiple nano- and mesoscale morphological variations. From an electronic-structure standpoint, when an exciton appears at the D-A interface, the exciton state can evolve into a charge-transfer (CT) state and eventually into a charge-separated state (5). A CT state is a D-A interfacial state for which a hole on a donor molecule or polymer segment is located next to an electron on an acceptor (fullerene) molecule. In the CT state, the electron and hole are still electrostatically bound to one another; as a result, there is limited electronic polarization of the surrounding molecules because the CT exciton is neutral. For the electron and hole to separate, they have to overcome their Coulomb attraction; this is facilitated by an increased electronic polarization of the surrounding molecules, which stabilizes the separated charges. Gélinas et al. developed ultrafast spectroscopic tools that resolve the electron-hole separation in the femtosecond regime. They exploit the signature of the electric field that is generated between the electron and the hole as they separate. This field alters the molecular orbital energies of the surrounding molecules and thus their optical transition energies, i.e., it leads to an electro-absorption (EA) signal (6). Gélinas et al. could measure the EA signals with
If you wish to distribute this article to others, you can order high-quality copies for your colleagues, clients, or customers by clicking here. Permission to republish or repurpose articles or portions of articles can be obtained by following the guidelines here. The following resources related to this article are available online at www.sciencemag.org (this information is current as of January 30, 2014 ): Updated information and services, including high-resolution figures, can be found in the online version of this article at: http://www.sciencemag.org/content/343/6170/492.full.html A list of selected additional articles on the Science Web sites related to this article can be found at: http://www.sciencemag.org/content/343/6170/492.full.html#related This article appears in the following subject collections: Physics, Applied http://www.sciencemag.org/cgi/collection/app_physics
Science (print ISSN 0036-8075; online ISSN 1095-9203) is published weekly, except the last week in December, by the American Association for the Advancement of Science, 1200 New York Avenue NW, Washington, DC 20005. Copyright 2014 by the American Association for the Advancement of Science; all rights reserved. The title Science is a registered trademark of AAAS.
Downloaded from www.sciencemag.org on January 31, 2014
This copy is for your personal, non-commercial use only.
PERSPECTIVES chemically if not physically, to fertilize their eggs using the male’s sperm. In this scenario, selection could well have led to the evolution of male pheromones that alter female physiology in a way that enhances male fitness, even if they shorten female life span. Given that males are rare and do not know which female in their environment they might mate with, releasing pheromones that influence all females in their vicinity might be the best way to maximize fitness. But why don’t female worms evolve ways to avoid such costly consequences of perceiving males? This might be a case of the “rare enemy” effect. If males are seldom encountered by hermaphrodites, then selection might be too weak to favor what could be costly measures for hermaphrodites to fight off male attempts at manipulation. What about the observation that male mortality in flies increases merely by perceiving the presence of females? Here, too, sexual conflict might be the proximate explanation. Interestingly, the detrimental effects
from exposure to female pheromones can be largely alleviated by allowing the male fruit fly to mate with multiple females (5). Is this a cost of false expectations? An alternative possibility is that the very traits that males have evolved to manipulate females can be costly to the male. Males produce toxic proteins in their seminal fluid that promote egg-laying in the female but also reduce her life span (1). It may be that perceiving females are sufficient to drive production of these or other toxic molecules, and failure to ejaculate them during mating could result in deleterious consequences for the male flies— the evolutionary equivalent of friendly fire. Perhaps sexually antagonistic coevolution has shaped sensory pathways—the signals produced by one sex, and the receptors and neural pathways that transduce these signals in the other sex. Although additional studies are needed to definitively test this idea, these same pathways have evolved to modulate longevity and fecundity in response to subtle and complex changes in a multitude of envi-
ronmental parameters, including nutrients, temperature, pathogens, and population density. How humans age might thus be a consequence not only of the way we treat our body, and the genetic lottery handed to us by our parents; it might also be affected by the social interactions—some cooperative, some conflictual—played out by males and females over millions of years. References 1. T. Chapman et al., Nature 373, 241 (1995). 2. D. Gems, D. L. Riddle, Nature 379, 723 (1996). 3. C. Shi, C. T. Murphy, Science 343, 536 (2014); 10.1126/ science.1242958. 4. T. J. Maures et al., Science 343, 541 (2014); 10.1126/ science.1244160. 5. C. M. Gendron et al., Science 343, 544 (2014); 10.1126/ science.1243339. 6. L. Partridge, P. Harvey, Nature 316, 20 (1985). 7. S. Libert et al., Science 315, 1133 (2007). 8. E. D. Smith et al., BMC Dev. Biol. 8, 49 (2008). 9. G. Arnqvist, L. Rowe, Sexual Conflict. Monographs in Behavior and Ecology, J. Krebs, T. H. Clutton-Brock, Eds. (Princeton Univ. Press, Princeton, NJ, 2005). 10. W. R. Rice, Nature 381, 232 (1996). 10.1126/science.1250174
APPLIED PHYSICS
Ultrafast spectroscopy reveals how the charge carriers in organic solar cells separate at interfaces and avoid substantial energy loss.
When Electrons Leave Holes in Organic Solar Cells Jean-Luc Bredas1,2
O
rganic solar cells convert sunlight into electricity by exploiting the electronic properties of electrically and optically active organic materials. Since the initial report of an organic solar cell reaching a power conversion efficiency near 1% (1), many efforts have brought the efficiency of carefully optimized devices in the 10 to 12% range (2). These efficiencies remain, however, well below the thermodynamic limit for single-junction organic solar cells, estimated to be >20% (3). Part of the failure in reaching the full potential of these devices is the lack of a comprehensive mechanistic picture of energy harvesting and carrier generation, transport, and recombination, particularly as a function of materials properties and activelayer morphology. On page 512 of this issue, Gélinas et al. (4) present a major step forward in the characterization of the charge-separation mechanism in organic solar cells.
An established view of organic πconjugated materials is that the primary photoexcitations are excitonic in nature. The photoexcited electron is not free to move on
hν
h e
OMe O
N
1
School of Chemistry and Biochemistry, and Center for Organic Photonics and Electronics, Georgia Institute of Technology, Atlanta, GA 30332–0400, USA. 2Department of Chemistry, King Abdulaziz University, 21589 Jeddah, Saudi Arabia. E-mail: [email protected]
492
its own; it remains bound to the hole (positive charge carrier) that forms on the molecular orbital from which the electron was excited. The binding energies of these electron-hole pairs, or excitons, are generally more than one order of magnitude greater than the thermal energy at room temperature. The formation of strongly bound excitons is in contrast to the situation in inorganic materials, such as crystalline silicon, where photoexcitations lead to immediate separation of elec-
PC71BM
R1
S
R2
S N
S
F
S
Si
R2 S
N S F
p-DTS(FBTTh2)2
N S
S
R1
31 JANUARY 2014 VOL 343 SCIENCE www.sciencemag.org Published by AAAS
Charges at the interface. In this illustration of the donor-acceptor interface and the ultrafast process of charge separation of hole and electron probed by Gélinas et al., the ovals represent the donor molecules [p-DTS(FBTTh 2) 2] and the circles the fullerene acceptor molecules (PC71BM). The modeling carried out by Gélinas et al. suggests that the electron wave function is delocalized over several fullerene molecules.
CREDIT: V. ALTOUNIAN/SCIENCE
PERSPECTIVES tron and hole at room temperature and generation of free charge carriers. The large exciton binding energies in π-conjugated materials are related not only to their low dielectric constants (which offer less screening between charges and lead to stronger interactions) but also to the presence of strong electron-vibration and electronelectron interactions (5). Because excitons are neutral species unable to carry a current, the efficiency of organic solar cells depends critically on charge-separation processes at heterojunctions between an electron donor (D; typically a π-conjugated polymer or molecule) and acceptor (A; typically a fullerene derivative), such as those displayed in the figure. These D-A heterojunctions are required to produce a driving force to dissociate excitons into spatially separated charges and are generally considered as an important factor limiting efficiency compared to inorganic p-n junction solar cells [the efficiency of crystalline silicon solar cells is on the order of 25% (2)]. Understanding the energy-harvesting mechanisms in organic solar cells, thus requires the ability to characterize the elementary charge-generating processes as a function of materials choice and in the presence of multiple nano- and mesoscale morphological variations. From an electronic-structure standpoint, when an exciton appears at the D-A interface, the exciton state can evolve into a charge-transfer (CT) state and eventually into a charge-separated state (5). A CT state is a D-A interfacial state for which a hole on a donor molecule or polymer segment is located next to an electron on an acceptor (fullerene) molecule. In the CT state, the electron and hole are still electrostatically bound to one another; as a result, there is limited electronic polarization of the surrounding molecules because the CT exciton is neutral. For the electron and hole to separate, they have to overcome their Coulomb attraction; this is facilitated by an increased electronic polarization of the surrounding molecules, which stabilizes the separated charges. Gélinas et al. developed ultrafast spectroscopic tools that resolve the electron-hole separation in the femtosecond regime. They exploit the signature of the electric field that is generated between the electron and the hole as they separate. This field alters the molecular orbital energies of the surrounding molecules and thus their optical transition energies, i.e., it leads to an electro-absorption (EA) signal (6). Gélinas et al. could measure the EA signals with
