Electron transport through a structure of nanoparticles (left) and more ordered nanotubes (center) is shown. At right, different wavelengths of light can be absorbed by different-sized quantum dots layered in a “rainbow” solar cell. Image credit: Kongkanand, et al. ©2008 ACS.
In the Notre Dame study, the scientists assembled cadmium selenide (CdSe) quantum dots in a single layer on the surface of nano films and tubes made of titanium dioxide (TiO2). After absorbing light, the quantum dots inject electrons into the TiO2 structures, which are then collected at a conducting electrode that generates photocurrent.
“Anchoring CdSe quantum dots on TiO2 nanotubes allowed us to create an ordered assembly of nanostructures,” Kamat told PhysOrg.com. “This architecture facilitated efficient transport of electrons to the collecting electrode surface and allowed us to achieve efficiency improvement.”
The researchers used four different sizes of quantum dots (between 2.3 and 3.7 nm in diameter) which exhibited absorbent peaks at different wavelengths (between 505 and 580 nm). The group observed a trade-off in performance corresponding with quantum dot size: smaller quantum dots could convert photons to electrons at a faster rate than larger quantum dots, but larger quantum dots absorbed a greater percentage of incoming photons than smaller dots. The 3-nm quantum dots offered the best compromise, but the researchers plan to improve both the conversion and absorption performances in future prototypes.
Besides investigating the quantum dots’ size quantization effect, the researchers also experimented with two different nano architectures – particle films and nanotubes – that act as scaffolds for transporting electrons from the quantum dots to the electrodes. The group found that the hollow 8000-nm-long nanotubes, where both the inner and outer surfaces were accessible to quantum dots, could transport electrons more efficiently than films.
“Usually, silicon-based photovoltaic panels operate with an efficiency of 15-20%,” Kamat said. “Silicon solar cells generate only one electron-hole pair per incident photons, irrespective of their energy. Thus, the higher energy of blue light is simply wasted in terms of heat. The obvious question is, can nanotechnology provide new ways to harvest these higher energy photons more efficiently?
“Semiconductor quantum dots seem to be the answer. They are capable of producing multiple charge carriers when excited with high energy light. If we succeed in capturing these charge carriers, we can expect significantly higher efficiencies. The target is to reach efficiency values greater than 30% using quantum dot rainbow solar cells.”
To achieve this efficiency, Kamat explained that there are two main challenges. The first is organizing the light harvesting nanostructures so that they efficiently absorb light in the visible and near infrared region, and transport electrons within the films. Secondly, the quantum dots should generate multiple charge carriers to be captured to generate photocurrent.