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Sensitizer takes solar cells to new high
1064 Writing in an accompanying Perspectives, Maria Ibáñez and Andreu Cabot of the Catalonia Energy Research Institute and Universitat de Barcelona in Spain add that the approach also boasts “straightforward scalability, low cost, and ease of technological application” [M. Ibáñez, A. Cabot, Science 340 (2013) 935]. “This is a new twist to solid state chemistry, a term up to now used generally to describe the chemical synthesis of solid materials but that can be used now to describe the precise and controlled chemical reactions which take place in the solid
C. Sealy state, especially at the nanoscale,” adds Victor Puntes of the Institut Català de Nanociència i Nanotecnologia in Spain. “When a great number of wonderful nanoscale materials properties are provided by the surface of nanocrystals, hollow particles are of great interest in energy applications.” E-mail address: [email protected] 2211-2855/$ - see front matter http://dx.doi.org/10.1016/j.nanoen.2013.07.006
Sensitizer takes solar cells to new high Cordelia Sealy Dye-sensitized solar cells (DSSCs) could make low-cost renewable energy possible, but device efficiencies are still lower than current photovoltaic technologies. To boost the performance of organic photovoltaics, light absorption needs to be extended over a wider spread of wavelengths ideally to the near-infrared (IR), where sunlight has as many photons as in the visible range. Researchers from The University of Tokyo in Japan led by Hiroshi Segawa have found a new phosphine-coordinated Ru(II) sensitizer, DX1, that absorbs near-IR light and raises the performance of DSSCs to the best level yet reported [T. Kinoshita, et al., Nature Photonics (2013), http://dx.doi. org/10.1038/nphoton.2013.136]. The new DX1 sensitizer exploits the near-IR, spinforbidden singlet-to-triplet (S–T) transition. The inclusion of a heavy metal from the third row or below in the periodic table in the sensitizer enables spin inversion during the electronic excitation. Instead of photoexcitation promoting a single electron and creating two singly occupied orbitals of opposite spins, the spin inversion creates two singly occupied orbitals with electrons of parallel spin. This creates a triplet excited state directly, explains Segawa. Harnessing spin inversion excitation allows DSSCs to exploit the entire visible spectrum between 400 nm and 800 nm, extending the photo-response characteristics to over 1000 nm. A test DSSC with the DX1 sensitizer generates a photocurrent of 26.8 mA cm 2, the highest photocurrent ever reported for organic photovoltaics, says Segawa, and produces higher incident-photon-to-current efficiency (IPCE) than other conventional sensitizers (Fig. 1). Moreover, when used together with a traditional sensitizer known as N719 in a tandem solar cell, a power conversion of over 12% is achieved under typical simulated sunlight conditions (35.5 mW cm 2). “If wideband technologies are applied to such high voltage solar cells, DSSCs will achieve higher conversion efficiencies,” Segawa told Nano Energy. “We′ve already reached efficiencies of up to 13% with tandem solar cells
Fig. 1 Photograph of a ‘handmade’ tandem solar cell comprising a stack of a red DSSC with N719 and a black DSSC with DX1, the new wideband sensitzer. (Courtesy of Hiroshi Segawa, The University of Tokyo.)
employing the newly enhanced wideband sensitizer DX1. Since traditional sensitizers show narrow spectrum photoresponse, to get efficiencies of over 15–20% we need to use such a wideband sensitizer.” The researchers believe that the concept of using a sensitizer featuring a spin-forbidden transition in the near infrared could be applied more generally to improve the efficiency of other DSSCs and organic thin-film solar cells. E-mail address: [email protected] 2211-2855/$ - see front matter http://dx.doi.org/10.1016/j.nanoen.2013.07.005
C. Sealy state, especially at the nanoscale,” adds Victor Puntes of the Institut Català de Nanociència i Nanotecnologia in Spain. “When a great number of wonderful nanoscale materials properties are provided by the surface of nanocrystals, hollow particles are of great interest in energy applications.” E-mail address: [email protected] 2211-2855/$ - see front matter http://dx.doi.org/10.1016/j.nanoen.2013.07.006
Sensitizer takes solar cells to new high Cordelia Sealy Dye-sensitized solar cells (DSSCs) could make low-cost renewable energy possible, but device efficiencies are still lower than current photovoltaic technologies. To boost the performance of organic photovoltaics, light absorption needs to be extended over a wider spread of wavelengths ideally to the near-infrared (IR), where sunlight has as many photons as in the visible range. Researchers from The University of Tokyo in Japan led by Hiroshi Segawa have found a new phosphine-coordinated Ru(II) sensitizer, DX1, that absorbs near-IR light and raises the performance of DSSCs to the best level yet reported [T. Kinoshita, et al., Nature Photonics (2013), http://dx.doi. org/10.1038/nphoton.2013.136]. The new DX1 sensitizer exploits the near-IR, spinforbidden singlet-to-triplet (S–T) transition. The inclusion of a heavy metal from the third row or below in the periodic table in the sensitizer enables spin inversion during the electronic excitation. Instead of photoexcitation promoting a single electron and creating two singly occupied orbitals of opposite spins, the spin inversion creates two singly occupied orbitals with electrons of parallel spin. This creates a triplet excited state directly, explains Segawa. Harnessing spin inversion excitation allows DSSCs to exploit the entire visible spectrum between 400 nm and 800 nm, extending the photo-response characteristics to over 1000 nm. A test DSSC with the DX1 sensitizer generates a photocurrent of 26.8 mA cm 2, the highest photocurrent ever reported for organic photovoltaics, says Segawa, and produces higher incident-photon-to-current efficiency (IPCE) than other conventional sensitizers (Fig. 1). Moreover, when used together with a traditional sensitizer known as N719 in a tandem solar cell, a power conversion of over 12% is achieved under typical simulated sunlight conditions (35.5 mW cm 2). “If wideband technologies are applied to such high voltage solar cells, DSSCs will achieve higher conversion efficiencies,” Segawa told Nano Energy. “We′ve already reached efficiencies of up to 13% with tandem solar cells
Fig. 1 Photograph of a ‘handmade’ tandem solar cell comprising a stack of a red DSSC with N719 and a black DSSC with DX1, the new wideband sensitzer. (Courtesy of Hiroshi Segawa, The University of Tokyo.)
employing the newly enhanced wideband sensitizer DX1. Since traditional sensitizers show narrow spectrum photoresponse, to get efficiencies of over 15–20% we need to use such a wideband sensitizer.” The researchers believe that the concept of using a sensitizer featuring a spin-forbidden transition in the near infrared could be applied more generally to improve the efficiency of other DSSCs and organic thin-film solar cells. E-mail address: [email protected] 2211-2855/$ - see front matter http://dx.doi.org/10.1016/j.nanoen.2013.07.005