Oxide Materials with Near Ultra Violet Broadband Absorption for Potential Photovoltaic Applications

For optimal energy conversion in photovoltaic devices based on crystalline silicon (c-Si), the most important requirement is that the total amount of energy of the incident photons is used. The major problem in c-Si photovoltaic cell is the efficiency of the energy conversion which takes place through the cell. In conventional operation, the physics imposes the so-called Shockley–Queisser [1] limit, and only around 29% of the solar spectrum could be converted into electrical energy. However, this limit is estimated to be improved up to 38.4% by modifying the solar spectrum by quantum cutting phosphors that convert one photon of high energy into two photons of lower energy[2]. Quantum cutting conversion from a visible-UV photon, shorter than 500 nm, in two or more infrared photons has been reported in some trivalent lanthanide ions pairs. One of them is always Yb³⁺. The Yb³⁺ ion is suitable as an acceptor and emitter because the energy of the only excited level of Yb³⁺ (~1.2 eV) well-match the bandgap of c-Si (~1.1 eV) and the luminescent quantum efficiency of Yb³⁺ could reach a value of 100 %. However, since absorption transitions of trivalent lanthanides as a donor are due to the forbidden f-f transitions, the absorption line-widths are sharp and the absorption cross-section is not large. In order to overcome this problem, absorption broadbands of the allowed f-d transitions of Ce³⁺ and Eu²⁺ or that of ZnO host has been considered to be used as the sensitizer of Yb³⁺ luminescence in order to absorb efficiently the high energy region of the solar spectrum. In this work we focused on the host absorption of some perovskite-like compounds: SrTiO₃ (STO), Bi₄Ti₃O₁₂ (BIT) and (Na_0.5Bi_0.5)TiO₃ (NBT). All of them are considered wide-band semiconductors. In addition, trivalent lanthanide can substitute Bi³⁺ in the last two compounds. As we show, Yb³⁺ doped STO, BIT and NBT are candidates as wavelength converters for near UV photons to IR photons. We establish the optimal Yb³⁺ concentration in each compound for near UV to IR photon conversion by luminescence spectroscopy. The compounds were further characterized by scanning electron microscopy, X-ray diffraction, IR spectroscopy and UV-Vis diffuse reflectance spectroscopy. The excitation spectra show broadbands in the near UV region when Yb³⁺ emission at 980 nm is observed.  Because there is no spectral overlap between the absorption band of STO, BIT and NBT and that of Yb³⁺, the resonant energy transfer by the multipolar interaction and exchange interaction may be discarded. One possibility is the energy transfer by the quantum cutting mechanism as it was suggested for CeO₂:Yb³⁺ [3]. A similar phenomenon ocurrs between Tb³⁺ and Yb³⁺. In this case also a non-resonant energy transfer mechanism operates.  

Acknowledgement. The authors are indebted to the CONACyT (Mexico) for their financial support to carry out this work (Project CB-2010-01 154962, and INFR-2011-1-163250). G. López-Pacheco and G. Santillán-Reyes are grateful to CONACyT for the M. Sc. fellowships granted. Also special thanks to LDRX (T-128) UAM-I for XRD measuremets.

 

References

[1] W. Shockley and H. J. Queisser, J. Appl. Phys. 32, 510 (1961).

[2] T. Trupke, M. A. Green, and P. Wurfel, J. Appl. Phys. 92, 1668 (2002).

[3] J. Ueda and S. Tanabe, J. Appl. Phys. 110, 073104 (2011)