Highly luminescent film as enhancer of photovoltaic devices

The most extended Si based conventional photovoltaic cells show low efficiency in the UV region, however this low efficiency can be enhanced by the use of suitable down-shifters or down-converters which transform the wavelength of the incoming radiation into a wavelength for which the Si based cells have high efficiency. In this sense europium(III) luminescent benzoate and phenanthroline (and derivatives) complexes are good candidates for such purposes since they exhibit large absorption at wavelengths below 400 nm and significant emissions at the VIS range. In this work we report the synthesis the crystal structure and the spectroscopic properties of two new Eu3+ and Gd3+ complexes with the ligands 4,7-biphenyl-1,10-phenanthroline (bphen) and benzoate (bz), namely, [Eu2(bphen)2(bz)6] (1) and [Gd2(bphen)2(bz)6] (2). The X-ray single crystal study reveals a dinuclear molecular structure for the Eu3+ complex with two bridging benzoate ligands between the two equivalent Eu3+ ions. The Powder X-ray diffraction study shows that the Eu3+ and Gd3+ compounds are isostructural and allows the use of the Gd3+ compound for the characterization of the excited states of the complexes which were investigated to explain the sensitization process that lead to intense red emission of Eu3+ ions under UV excitation. The photoluminescence study of the lanthanide complexes at 15 K and 297 K revealed very efficient energy transfer processes from the antenna to the emitting Eu3+ ions and a high overall quantum yield. Moreover, the ligands provide rigidity, high stability and solubility to the complex in CH2Cl2, allowing the preparation of poly(methylmethacrylate)-doped films that can be used to cover photovoltaic devices. The analysis of external quantum yield and quantum efficiency (EQE) show its potential application to the solar cell technology with an enhancement EQE in the UV region (280-360 nm) that reaches a 4.5% at 300 nm.