Quantum 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 colloid-al quantum dot photovoltaics—a crucial determinant of the devices’ performance.

(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 (moer 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:10.1021682h; p 3701.© 2011 American chemical society.

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 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 funnels can improve systems with high base fill factors. Additional theoretical modeling shows that such benefits could apply to high-efficiency photovoltaics, as well as the 2–3% efficiency technologies on which these funnels were tested. The research team suggests that CQD funnel solar cells could enable higher power-per-square-foot densities, lowering the square footage of solar cells needed to power a building.