Jin Zhang, professor of chemistry at the University of California, Santa Cruz and his team combined the two methods and created a TiO₂thin film doped with nitrogen and sensitized with cadmium selenide (CdSe) quantum dots (QDs) via a linking molecule, thioglycolic acid (TGA). When tested, the new nanocomposite material performed better than predicted.

CdSe QDs linked to TiO₂/N nanoparticles using TGA were found to significantly increase the photocurrent and power conversion of the films compared to standard TiO₂/N films without QD sensitization. The incident photon-to-current conversion efficiency (IPCE) is 6% at 400 nm for TiO₂/N-TGA-CdSe solid-state solar cells and 95% for TiO₂/N-TGA-CdSe films near 300 nm in a Na₂S electrolyte, which is much higher than that of undoped TiO₂ with QD sensitization or TiO₂/N without QD sensitization. The power conversion efficiency (η) was found to be 0.84% with a fill factor (FF%) of 27.7% with 1100 nm thick TiO₂/N-TGA-CdSe thin films. The results show that combining nitrogen doping with the QD sensitization of TiO₂ thin films is an effective and promising way to enhance the photoresponse in the near-UV and visible region, which is important for potential photovoltaic (PV) and photoelectrochemical applications.

The group’s findings were reported in the Journal of Physical Chemistry C. Lead author of the paper was Tzarara Lopez-Luke, a graduate student visiting in Zheng’s lab who is now at the Instituto de Investigaciones Metalurgicas, UMSNH, Morelia, Mexico.

We have discovered a new strategy that could be very useful for enhancing the photo response and conversion efficiency of solar cells based on nanomaterials. We initially thought that the best we might do is get results as good as the sum of the two, and maybe if we didn’t make this right, we’d get something worse. But surprisingly, these materials were much better.

—Jin Zhang

Zhang’s team characterized the new nanocomposite material using a broad range of tools, including atomic force microscopy (AFM), transmission electron microscopy (TEM), Raman spectroscopy, and photoelectrochemistry techniques.

The hybrid material offered a combination of advantages. Nitrogen doping allowed the material to absorb a broad range of light energy, including energy from the visible region of the electromagnetic spectrum. The quantum dots also enhanced visible light absorption and boosted the photocurrent and power conversion of the material. The nanocomposite’s IPCe was as much as three times greater than the sum of the IPCEs for the two other materials, Zhang said.

We think what’s happening is that it’s easier for the charge to hop around in the material. That can only happen if you have both the quantum dot sensitizing and the nitrogen doping at the same time.

—Jin Zhang

The nanocomposite material could be used not only to enhance solar cells, but also to serve as part of other energy technologies. One of Zhang’s long-term goals is to marry a highly efficient solar cell with a state-of-the-art photoelectrochemical cell. Such a device could, in theory, use energy generated from sunlight to split water and produce hydrogen. The nanocomposite material could also potentially be useful in devices for converting carbon dioxide into hydrocarbon fuels, such as methane.

Sources of funding for this research included the US Department of Energy, the National Science Foundation of China, and the University of California Institute for Mexico and the United States (UC-MEXUS).

Research collaborators included Abraham Wolcott, Li-ping Xu and Shaowei Chen at UCSC; Zhenhai Wen and Jinghong Li at Tsinghua University in Beijing, China; and Elder De La Rosa of the Centro de Investigaciones en Optica, A.C., in Leon, Guanajuato, Mexico.

Resources

• Tzarara López-Luke, Abraham Wolcott, Li-ping Xu, Shaowei Chen, Zhenhai Wen, Jinghong Li, Elder De La Rosa, and Jin Z. Zhang, “Nitrogen-Doped and CdSe Quantum-Dot-Sensitized Nanocrystalline TiO₂ Films for Solar Energy Conversion Applications” J. Phys. Chem. C, ASAP Article 10.1021/jp077345p S1932-7447(07)07345-1