Benzothiadiazole Substituted Semiconductor Molecules for Organic Solar Cells: The Effect of the Solvent Annealing Over the Thin Film Hole Mobility Values

We have synthesized and characterized two low molecular weight organic molecules, namely, CS01 and CS03 having the benzo[c][1,2,5]thiadiazole-4,7-diamino core but differing in the number of aromatic rings at the amino groups. The molecules, when processed to make thin organic films, display absorbance up to the near-IR region (∼750 nm) and good hole mobility values. Upon mixing each organic semiconductor molecule with the fullerene derivative PC71BM, we monitored a strong quenching of the fluorescence emission. We assigned such a process to efficient charge transfer from the CS01 and CS03 molecules to the fullerenes. Moreover, fueled by this observation, we prepared organic solar cells and obtained, as a first attempt, efficiencies over 2% under 1 sun light simulated solar radiation. Furthermore, the film optimization through a careful solvent annealing process increased further the efficiencies up to 4.80% for CS01 and 5.12% for CS03. The observed increase in efficiency is due to a better morphology obtained through solvent annealing of the thin films. However, an in-depth analysis reveals that the solvent annealing led to a better hole mobility, but the electron mobility remains similar.