Materials Synthesis from Atoms to Systems

Enhancing the Efficiency of Solar Energy Conversion in Titania

The smaller shoulder in the red spectrum of the titania valence band near 3 eV binding energy corresponds to the impurity states created by Cr-N codoping that, in comparison with the blue curve, closes the band gap of titania to about 2 eV.

A new method that simultaneously incorporates chromium (Cr) and nitrogen (N) atoms as Cr-N pairs into titania yields a material with an extraordinarily large (> 1 eV) band gap reduction. Reducing the bandgap of the titania semiconductor shifts the absorption of light into the visible part of the spectrum where a large fraction of the solar energy resides and would therefore allow the development of more efficient solar cells.  In this work soft X-ray spectroscopy allowed the energy location and quantum-mechanical origins of the impurity levels responsible for this reduction to be elucidated.

Incorporation of a pair of dopants with different charge states (such as Cr-N), a method known as non-compensated codoping, has multiple benefits that include enhancing the kinetic and thermodynamic solubility of the dopants, resulting in higher substitutional nitrogen contents compared to N-only doping, and easing the spreading or ‘delocalization’ of the photo-generated charge carriers.  Bandgap reduction and carrier delocalization are critical factors for efficient light-to-current conversion and codoping has significant potential to create novel oxide semiconductors for photocatalytic and photovoltaic applications.  

C. Parks Cheney, P. Vilmercati, E. W. Martin, M. Chiodi, L. Gavioli, M. Regmi, G. Eres, T. A. Callcott, H. H. Weitering, and N. Mannella, “Origins of electronic band gap reduction in Cr/N codoped TiO2,” Phys. Rev. Lett. 112, 036404 (2014).   DOI:10.1103/PhysRevLett.112.036404

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