Quantum dot funnels increase solar-cell fill factor
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uantum dot funnels—which are devices comprising graded layers comprising quantum dots of different sizes, and therefore bandgaps—offer the possibility of funneling energy toward a suitable acceptor. In the September 14 issue of Nano Letters (DOI: 10.1021/ nl201682h; p. 3701), I.J. Kramer, E.H. Sargent, and colleagues at the University of Toronto constructed a new solar cell designed around this concept where photoelectrons are efficiently transferred from their source to an electron acceptor. PbS quantum dot funnels were fabricated on TiO2 electrodes. The resulting solar cells define a path to achieving further improvements in the fill factor of colloidal quantum dot photovoltaics—a crucial determinant of the devices’ performance. The researchers created three funnel types, two of which were graded by their
bandgap to promote or discourage charge-carrier collection, and one control. These were graded, antigraded, and ungraded, respectively. The researchers started with a base layer of glass, coated in SnO2/F and then applied a commercially available TiO2 paste. Multilayer spin coating of solutions of quantum dots generated a graded funnel with three layers of 4.3 nm diameter CQDs (colloidal quantum dots), one layer of 4.2 nm, and one layer of 4.0 nm. Ungraded controls comprised five layers of 4.3 nm CQDs, while antigraded funnels consisted of three layers of 4.3 nm, one layer of 4.5 nm, and one layer of 4.7 nm diameter CQDs. Each CQD layer was 25 nm thick. Finally, the prepared devices were coated with layers of thin films of gold and silver. Ungraded control funnels provided a fill factor of 49%, graded
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(a) Spatial band diagrams of ungraded, graded, and antigraded colloidal quantum dot (CQD) solar cells. Color coding corresponds to larger bandgaps (more blue/violet) and smaller bandgaps (more yellow/red). (b) Detailed band alignment for the TiO2 and PbS CQD materials used, also employing the same color coding scheme. Reprinted with permission from Nano Lett. 11 (9) (2011), DOI: 10.1021/nl201682h; p. 3701. © 2011 American Chemical Society.
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NEWS & ANALYSIS RESEARCH/RESEARCHERS funnels 54%, and antigraded funnels only 37%, where the fill factor provides a measure of the efficiency of the cell. That a 5% increase in fill factor was achieved on changing the structure from ungraded to graded is key to future applications, and the researchers predict the CQD fun-
Thin-film heterostructures of Fe- and Co-BaTiO3 exhibit interface multiferroicity at room temperature
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UPCOMING IMI-SEE CO-SPONSORED EVENT
aterials possessing coupled roomtemperature ferromagnetic (FM) and ferroelectric (FE) order are currently the subject of intense research for use in spintronic memories that store information through charge and spin. Singlephase materials displaying such multiferroic order are exceedingly rare in nature, so attention has shifted to artificially grown thin-film FM and FE hetero-
nels can improve systems with high base fill factors. Additi
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