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Stable quantum dots promise better solar cells
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C. Sealy
Fig. 1 (Left) Production of graphene by shear exfoliation of graphite in the solvent N-methyl-pyrrolidone using a Silverson LM5 high shear mixer. In this lab-scale experiment, 5 l of graphene suspension was produced. [Credit: CRANN, Trinity College Dublin.] (Top right) Transmission electron microscope image of nanosheets of shear exfoliated graphene. The scalebar is 100 nm. [Credit: CRANN, Trinity College Dublin.] (Bottom right) Atomic resolution, scanning transmission electron microscope image of part of a nanosheet of shear exfoliated graphene. The bright blobs are carbon atoms. [Credit: CRANN/SuperSTEM.]
E-mail address: [email protected] http://dx.doi.org/10.1016/j.nantod.2014.06.008 1748-0132/$ — see front matter
Stable quantum dots promise better solar cells Cordelia Sealy
Two recent advances in nanoscale semiconductor particle preparation and processing are propelling solar cells to new levels of efficiency. The easy processing, bandgap tunability, and multiexciton generation potential of chalcogenide colloidal quantum dots (CQDs) are attractive for optoelectronic devices like photovoltaics, light emitting diodes, photodetectors, and field-effect transistors. But chalcogenides like
PbS and organic semiconductors are notoriously prone to oxidation, which results in the loss of n-type characteristics upon exposure to air and a drop in performance. Now, researchers from the University of Toronto and Dalhousie University in Canada, King Abdullah University of Science and Technology in Saudi Arabia, and Huazhong University of Science and Technology in China have found a means of stabilizing or passivating the surface of n-type
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Figure 1 Inverted quantum junction solar cells fabricated with air-stable n-type colloidal quantum dots (CQDs). (Left) Air-stable n-type CQD resist the bonding of oxygen on CQD surface, where gray spheres represent Pb atoms, yellow spheres S atoms, blue spheres I, and red spheres O. (Right) Schematic of inverted quantum junction solar cell based on the combination of p-type and n-type CQDs, blue spheres representing n-type CQD with iodide ligands, yellow spheres p-type PbS CQD layer with bromide ligands, and red spheres P+ -type CQD layer with MPA ligands. The bottom layer is TiO2 layer on FTO substrate, and top contact is MoO3 /Au/Ag. Credit: Zhijun Ning, University of Toronto.
CQDs to render them stable in air [Z. Ning et al., Nat. Mater. (2014), http://dx.doi.org/10.1038/nmat4007]. Using density functional theory, the researchers determined that iodide ligands would be able to protect CQDs from the effects of oxygen exposure without having to resort to full encapsulation. Led by Edward H. Sargent of the University of Toronto, the researchers synthesized CQDs via a wet solution reaction, before cleaning and protecting with a solution phase ligand exchange reaction. ‘‘The key new practical outcome [is] that the n-type character of the new nanoparticles [is] preserved well after
397 air exposure,’’ explains first author Zhijun Ning. ‘‘Iodide is almost a perfect ligand.’’ The air-stable n-type CQDs are used to create an ‘upside-down’ p—n quantum junction solar cell that remains operational in air for up to four days without degradation and demonstrates solar power conversion efficiencies of up to 8% (Fig. 1). ‘‘By judiciously optimizing CQDs and inverted quantum junction solar cells structure, we are optimistic that over 10% efficiency is achievable in the coming years,’’ says Ning. Meanwhile, a team led by Moungi G. Bawendi of Massachusetts Institute of Technology has already achieved an efficiency of 8.55% with devices based on solution-processed ZnO/PbS quantum dots (QDs) [C.-H.M. Chuang et al., Nat. Mater. (2014), http://dx.doi.org/10.1038/nmat3984]. Bawendi’s approach is to use different ligands — in this case tetrabutylammonium iodide (TBAI) and 1,2ethanedithiol (EDT) — in a similar way to passivate the QD layers and tune their bandgaps into alignment. The result is devices that achieve an accredited efficiency of 8.55% and are stable in air for over 150 days without encapsulation. Moreover, both the ZnO and PbS quantum dots can be solution-processed in air and at room temperature. ‘‘Every part of the cell, except the electrodes for now, can be deposited at room temperature, in air, out of solution. It’s really unprecedented,’’ says first author ChiaHao M. Chuang in a statement. QD-based solar cells are now making rapid progress in efficiency and are looking increasingly attractive because of their promise of low-cost, versatility, light weight, and large area devices. ‘‘[Sargent’s work] is a nice effort to improve the performance lead chalogenide-based QD solar cells,’’ says Prashant V. Kamat of the University of Notre Dame. ‘‘The 8% efficiency is one of the highest efficiency reported by this group. [Bawendi’s work] shows 8.55% efficiency on similar QD solar cells and has also succeeded in demonstrating long-term stability.’’ However, Kamat cautions that the efficiency of this type of device will have to reach at least 15% to compete with other thin film technologies. E-mail address: [email protected] http://dx.doi.org/10.1016/j.nantod.2014.06.007 1748-0132/$ — see front matter
Nanotwins make diamond harder Cordelia Sealy
Diamond, that hardest of materials, may have just got even harder thanks to a new synthesis method that creates a unique crystalline structure. Although the hardness of diamond means that is has long been used for cutting tools, its
poor thermal stability has limited its application at high temperatures. Now Yongjun Tian from Yanshan University and colleagues from Jilin University in China and the University of Chicago have succeeded in improving both the hardness
C. Sealy
Fig. 1 (Left) Production of graphene by shear exfoliation of graphite in the solvent N-methyl-pyrrolidone using a Silverson LM5 high shear mixer. In this lab-scale experiment, 5 l of graphene suspension was produced. [Credit: CRANN, Trinity College Dublin.] (Top right) Transmission electron microscope image of nanosheets of shear exfoliated graphene. The scalebar is 100 nm. [Credit: CRANN, Trinity College Dublin.] (Bottom right) Atomic resolution, scanning transmission electron microscope image of part of a nanosheet of shear exfoliated graphene. The bright blobs are carbon atoms. [Credit: CRANN/SuperSTEM.]
E-mail address: [email protected] http://dx.doi.org/10.1016/j.nantod.2014.06.008 1748-0132/$ — see front matter
Stable quantum dots promise better solar cells Cordelia Sealy
Two recent advances in nanoscale semiconductor particle preparation and processing are propelling solar cells to new levels of efficiency. The easy processing, bandgap tunability, and multiexciton generation potential of chalcogenide colloidal quantum dots (CQDs) are attractive for optoelectronic devices like photovoltaics, light emitting diodes, photodetectors, and field-effect transistors. But chalcogenides like
PbS and organic semiconductors are notoriously prone to oxidation, which results in the loss of n-type characteristics upon exposure to air and a drop in performance. Now, researchers from the University of Toronto and Dalhousie University in Canada, King Abdullah University of Science and Technology in Saudi Arabia, and Huazhong University of Science and Technology in China have found a means of stabilizing or passivating the surface of n-type
News and opinions
Figure 1 Inverted quantum junction solar cells fabricated with air-stable n-type colloidal quantum dots (CQDs). (Left) Air-stable n-type CQD resist the bonding of oxygen on CQD surface, where gray spheres represent Pb atoms, yellow spheres S atoms, blue spheres I, and red spheres O. (Right) Schematic of inverted quantum junction solar cell based on the combination of p-type and n-type CQDs, blue spheres representing n-type CQD with iodide ligands, yellow spheres p-type PbS CQD layer with bromide ligands, and red spheres P+ -type CQD layer with MPA ligands. The bottom layer is TiO2 layer on FTO substrate, and top contact is MoO3 /Au/Ag. Credit: Zhijun Ning, University of Toronto.
CQDs to render them stable in air [Z. Ning et al., Nat. Mater. (2014), http://dx.doi.org/10.1038/nmat4007]. Using density functional theory, the researchers determined that iodide ligands would be able to protect CQDs from the effects of oxygen exposure without having to resort to full encapsulation. Led by Edward H. Sargent of the University of Toronto, the researchers synthesized CQDs via a wet solution reaction, before cleaning and protecting with a solution phase ligand exchange reaction. ‘‘The key new practical outcome [is] that the n-type character of the new nanoparticles [is] preserved well after
397 air exposure,’’ explains first author Zhijun Ning. ‘‘Iodide is almost a perfect ligand.’’ The air-stable n-type CQDs are used to create an ‘upside-down’ p—n quantum junction solar cell that remains operational in air for up to four days without degradation and demonstrates solar power conversion efficiencies of up to 8% (Fig. 1). ‘‘By judiciously optimizing CQDs and inverted quantum junction solar cells structure, we are optimistic that over 10% efficiency is achievable in the coming years,’’ says Ning. Meanwhile, a team led by Moungi G. Bawendi of Massachusetts Institute of Technology has already achieved an efficiency of 8.55% with devices based on solution-processed ZnO/PbS quantum dots (QDs) [C.-H.M. Chuang et al., Nat. Mater. (2014), http://dx.doi.org/10.1038/nmat3984]. Bawendi’s approach is to use different ligands — in this case tetrabutylammonium iodide (TBAI) and 1,2ethanedithiol (EDT) — in a similar way to passivate the QD layers and tune their bandgaps into alignment. The result is devices that achieve an accredited efficiency of 8.55% and are stable in air for over 150 days without encapsulation. Moreover, both the ZnO and PbS quantum dots can be solution-processed in air and at room temperature. ‘‘Every part of the cell, except the electrodes for now, can be deposited at room temperature, in air, out of solution. It’s really unprecedented,’’ says first author ChiaHao M. Chuang in a statement. QD-based solar cells are now making rapid progress in efficiency and are looking increasingly attractive because of their promise of low-cost, versatility, light weight, and large area devices. ‘‘[Sargent’s work] is a nice effort to improve the performance lead chalogenide-based QD solar cells,’’ says Prashant V. Kamat of the University of Notre Dame. ‘‘The 8% efficiency is one of the highest efficiency reported by this group. [Bawendi’s work] shows 8.55% efficiency on similar QD solar cells and has also succeeded in demonstrating long-term stability.’’ However, Kamat cautions that the efficiency of this type of device will have to reach at least 15% to compete with other thin film technologies. E-mail address: [email protected] http://dx.doi.org/10.1016/j.nantod.2014.06.007 1748-0132/$ — see front matter
Nanotwins make diamond harder Cordelia Sealy
Diamond, that hardest of materials, may have just got even harder thanks to a new synthesis method that creates a unique crystalline structure. Although the hardness of diamond means that is has long been used for cutting tools, its
poor thermal stability has limited its application at high temperatures. Now Yongjun Tian from Yanshan University and colleagues from Jilin University in China and the University of Chicago have succeeded in improving both the hardness