Effect of Alkyl Substituents on DPP-based Small Molecules for Organic Solar Cells

Two diketopyrrolopyrrole (DPP)-based π-conjugated small molecules, DPP4T and α-DPP4T, were synthesized to investigate the effect of alkyl end groups on the physical properties of the molecules. The flexible alkyl end groups resulted in lower phase-transition temperatures and enhanced solubility of α-DPP4T. The UV–vis absorption maxima (λ_max) of films of both small molecules were blue-shifted compared with those of solutions of the molecules. More importantly, a relatively larger shift in λ_max between the solution and film states and an enhanced shoulder peak in the longer-wavelength region were observed in the spectra of the α-DPP4T film, indicating relatively greater intermolecular packing of α-DPP4T molecules. The enhanced device performance (in particular, a higher short-circuit current density and better external quantum efficiency profile) of the α-DPP4T-based device was explained by the improved film morphology and stronger intermolecular interactions of α-DPP4T. After device optimization, a power conversion efficiency of 2.42% was achieved using α-DPP4T.