Natural Dyes as Candidate Materials for Organic Solar Cells - An Optical, Electronic and Structural Characterisation
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- Institutt for fysikk 
Natural dyes strongly absorbing light in the visible spectrum are appealing ma- terials for the use in organic solar cells, due to their direct availability from nature. An especially interesting potential application arises from a reported reduction in charge carrier trapping in organic semiconductors blended with large-band gap com- pounds. On this account, an optical, electronic and structural characterisation of the natural yellow and red dyes curcumin and bixin was performed, to create a basis for future investigations of their applicability in organic solar cells.Optical absorption spectra measured by UV-Visible Spectroscopy (UV-Vis) on ther- mally evaporated films revealed that bixin undergoes chemical degradation upon evaporation, while curcumin appears to remain unaffected. Cheap availability of high purity curcumin and easy processability allowed for reliable determination of its band gap, both from the absorption spectrum, as well as combined Ultravi- olet Photoemission Spectroscopy (UPS) and Inverse Photoemission Spectroscopy (IPES). The obtained values of 2.3eV and 2.4eV, respectively, are in good agree- ment with each other. The band gap of bixin was found to be larger than expected (2.5eV and 2.8eV, respectively), most likely as a result of the degradation of the molecule and an insufficient film thickness for photoemission measurements free of contributions from substrate contaminants. UPS and IPES spectra furthermore re- vealed that both curcumin and bixin are hole-conducting materials.X-ray diffraction (XRD) measurements on powder performed at the Brazilian Syn- chrotron Light Laboratory (LNLS) in Campinas, Brazil, revealed a higher crys- talline order in curcumin than bixin. A Pawley unit cell refinement using the ob- tained diffraction pattern on curcumin yields the refined lattice parameters a = 12.7419(3) Å, b = 7.2202(3) Å, c = 20.0530(9) Å and β = 94.9801(1)◦, which are in good agreement with those found in literature. After annealing the curcumin powder at 200◦C, the signal intensity is found to be decreased, which could be interpreted as an increase in crystalline order with the majority of crystallites oriented such that their reflections cannot be detected with the used geometry of the setup. A diffraction pattern on bixin powder was successfully measured. In future work this could, in combination with computational density functional theory (DFT) calcula- tions, reveal the crystal structure of bixin for the first time. Evaporated as well as solution-cast samples of both dyes proved to be too thin to produce a useful signal in the XRD measurement.The results show that curcumin has several advantages over bixin in terms of pro- cessability, stability and crystalline order, which are favourable properties for or- ganic solar cell applications. Curcumin can therefore be considered a promising candidate material. A meaningful analysis of bixin allowing for a clear conclusion is complicated by difficulties forming sufficiently thick films, as a result of the lack of cheaply available high purity bixin. The observed decomposition of the molecule upon evaporation does not rule it out as a candidate material, but adds to the need for studying solution-processed films made from a cheaper (potentially less pure) form of it.