Compositional calibration curves were established by measuring the XRF spectra of a series of thiourea/RbBr mixture films with known S, Rb, and Br composition (Figure ). Calibration samples were spin coated on the same substrates as the OMIEC samples to avoid introducing new fluorescence that would complicate comparisons. Using these calibration data, XRF peak area ratios were converted into quantitative elemental compositions (Figure ).