Frontiers of Engineering Reports on Leading-Edge Engineering from the 2005 Symposium (2006) / Chapter Skim
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Organic Semiconductors for Low-Cost Solar Cells
Organic Semiconductors for Low-Cost Solar Cells
Pages 119-130
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From page 119... ...
Although it is possible that this could be done by using carbon sequestration along with fossil fuels or by greatly expanding nuclear power plants, it is clearly desirable that we develop renewable sources of energy. The sun deposits 120,000 TW of radiation on the surface of the earth, so there is clearly enough power available if an efficient means of harvesting solar energy can be developed.
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From page 120... ...
Organic materials have similar intramolecular covalent bonds but are held together only by weak intermolecular van der Waals interactions. The electronic wave function is thus strongly localized to individual molecules, and the weak intermolecular interactions instigate a narrow electronic bandwidth formed in molecular solids.
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From page 121... ...
Excitons in organic semiconductors that are not split eventually recombine either radiatively or nonradiatively, thereby reducing the quantum efficiency of a solar cell. In inorganic semiconductors the attraction between an electron-hole pair is less than the thermal energy kT.
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From page 122... ...
This low charge-carrier mobility puts a constraint on the thickness of organic materials that can be used in a solar cell because recombinative loss increases with increasing thickness. Fortunately, this drawback is offset because only a very thin layer of organic materials is necessary because organic semiconductors are highly absorptive.
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From page 123... ...
do not act as potential charge-carrier recombination sites. PRODUCTION OF HETEROJUNCTION DEVICES The simplest organic solar cells can be made by sandwiching thin films of organic semiconductors between two electrodes with different work functions.
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From page 124... ...
of an electron donor to the LUMO of an electron acceptor in a planar heterojunction cell; (c) a bulk heterojunction solar cell based on semiconducting polymer and C60 derivative.
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From page 125... ...
(6) FIGURE 4 Schematic energy diagrams h+ of the semiconductors and energy levels h+ in a bulk heterojunction solar cell show ing the (a)
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From page 126... ...
. Research is under way to improve exciton transport in organic semiconductors, for example by using resonance energy transfer to funnel excitons directly to an absorber located at the charge-splitting interface or by incorporating phosphorescent semiconductors, which exhibit longer excited-state lifetimes (Liu et al., 2005; Shao and Yang, 2005)
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From page 127... ...
Fortunately, the wealth of chemical synthetic knowledge and the dependence of electronic properties of organic molecules on their molecular structures allow for flexible tuning of the band gap and energy levels of organic semiconductors by chemical synthesis. Significant research on band engineering of this type should yield very promising results in the near future.
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From page 128... ...
2005. Using resonance energy transfer to improve exciton harvesting in organic-inorganic hybrid photovoltaic cells.
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From page 129... ...
2004. Asymmetric tandem organic photovoltaic cells with hybrid planar-mixed molecular heterojunctions.
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