Fig. 4. (a) Definition of the functional groups used for the spiro-R series. (b) Cyclic voltammograms for HTM series recorded in 0.1 M n-NBu₄PF₆ DCM solutions at room temperature. (c) Simulated and experimental HOMO and LUMO energies for the spiro-R series and spiro-OMeTAD as a reference material.