Small biomolecule dopant retinals: Electron blocking layer in P3HT:PCBM type organic solar cells

We present a comparative study of the photophysics and electron/hole properties of all-trans retinal-benzimidazole type molecules decorated with different electronic moieties (such as -OCH3, -N(CH3)2, -F, -CF3) in organic photovoltaic (OPV) devices in solution and on solid thin films. Steady-state spectra of synthesized dyes give large Stokes shifts (6887-13152 cm−1) in studied solvents. Decay times of these dyes were found to be substituent dependent giving a bi-exponential decay for fluorine containing retinals. Trans to cis photoisomerization rate constants of synthesized dyes were found to be about 3.3-16.4 × 10−6 s−1. Using a cyclic voltammetry measurements, HOMO and LUMO energy levels of fluorine-substituted dyes shift to lower values as compared to that of unfluorinated derivatives. We compared unusual electron blocking behavior of methoxy- and N,N-dimethylamino-substituted derivatives (Ret-I and Ret-II, respectively) in bulk heterojunction solar cells (BHJ-SCs) incorporating an active layer of P3HT:PCBM doped with Ret derivatives at various weight ratios. Hole mobility values for fluorine containing retinals were found to be about 1.0 × 10−4 and 7.1 × 10−4 cm2/V s for Ret-III and Ret-IV dyes, respectively.