Investigation of Europium B-Diketonate complexes as spectral converters for solar cells

Abstract

Photovoltaic technologies for solar energy conversion are promising routes to renewable energy generation. A major difficulty in improving photovoltaic energy conversion efficiency lies in the spectral mismatch between the energy distribution of photons in the incident solar spectrum and the bandgap of semiconductor materials. Luminescent downshifting (LDS) layers and luminescent solar concentrators (LSCs) offer a passive approach to utilise the short-wavelength portion of the solar spectrum by absorbing short-wavelength photons and re-emitting at longer wavelengths, where the external quantum efficiency of the photovoltaic device is high. β-Diketonate-based europium(III) complexes are promising candidates for use as spectral converters due to their high quantum yields, extended absorption window, large Stokes shift between the absorption and emission spectra and narrow emission bands in the red region.

Two Eu(III) complexes have been synthesised, and their photophysical properties investigated: Eu(tta)3DPEPO and Eu(hfac)3DPEPO, where tta is 2-Thenoyltrifluoroacetone, hfac is hexafluoroacetylacetone, and DPEPO is bis[2-(diphenylphosphino)phenyl]ether oxide. The photophysical properties of the complexes were examined in the solution phase and in polymethylmethacrylate (PMMA) films. Both complexes exhibited favourable photophysical properties with total quantum yields in the range of 0.75 to 0.85.

Co-doping europium complexes with light-harvesting organic chromophores were considered as a practical approach to cut the cost of the metal used and to extend the harvesting window in the UV region. The use of Förster resonance energy transfer to enhance ultraviolet excitation of the Eu(III) complexes in PMMA films co-doped with organic molecules was investigated. The absorption spectra of the chosen donors, DPEPO, indole, biphenyl, and p-terphenyl, complement those of the complexes, making them useful in harvesting high-energy photons in the 250–320 nm region. Examination of the excitation spectra of the europium complexes in the presence and absence of the donors showed that, in each case, an enhancement in UV excitation of the Eu(III) complex was observed, except for Eu(hfac)3DPEPO co-doped with indole.

The energy transfer efficiencies of the different donors were determined. DPEPO had the highest energy transfer efficiency for each complex, 33% and 38% for Eu(tta)3DPEPO and Eu(hfac)3DPEPO, respectively. Preliminary solar testing experiments of Eu(tta)3DPEPO and Eu(tta)3DPEPO co-doped with DPEPO were conducted to evaluate their practical performance as spectral converters on c-Si solar cells. Results showed an improvement in the short-circuit current density and the external quantum yield of the device.

The role of an optically active host material in LDS layers and LSCs applications was considered in terms of their photophysical properties. The photoactive nature of Poly(N-vinylcarbazole) (PVK) and its unique photophysical properties were discussed. The use of PVK as an active host for Eu(hfac)3DPEPO and Eu(tta)3DPEPO was investigated. It was determined that energy transfer to the complexes occurs from the carbazole monomer excited state, not from the emitting excimer species. Thus, in PVK doped with the europium complexes, each complex acts as a trap for the migrating monomer excitons.

Energy transfer from PVK to Eu(hfac)3DPEPO was found to occur with an efficiency of ~15% and appeared to be independent of complex concentration over the range of 2.5 to 16 wt%. However, absorption by the PVK matrix competes with direct excitation of the complex, resulting in an overall depletion in excitation intensity rather than the desired enhancement. It was predicted that if the energy transfer efficiency could be increased to 50% ere would be a substantial enhancement of the excitation intensity over the majority of the wavelength range. In contrast, for Eu(tta)3DPEPO, the use of PVK as a matrix was found to give a beneficial enhancement in excitation intensity. The energy transfer efficiency from PVK to the complex was found to be ~20%, similar to that found for energy transfer in Eu(hfac)3DPEPO. The effect of varying the PVK matrix concentration on energy transfer to Eu(tta)3DPEPO was investigated. The effect of adding co-dopant organic chromophores to Eu(tta)3DPEPO in the PVK matrix was also investigated. A multistep energy transfer process can be proposed to explain the enhancement in the excitation intensity by DPEPO and biphenyl.

In summary, the thesis reports the synthesis and photophysical characterisations of two europium complexes in the solution phase and PMMA for spectral converters applications. Co-doping the europium complexes with organic chromophores was investigated, and energy transfer process efficiencies were determined and analysed.

The most efficient system was coupled to a c-Si solar cell for solar testing and evaluation.

The role of an optically active polymer in transferring energy to the europium complexes and to the Eu(tta)3DPEPO co-doped with organic molecules was discussed, and energy transfer processes were proposed and analysed.