Reducing Non‐Radiative Voltage Losses by Methylation of Push–Pull Molecular Donors in Organic Solar Cells

Abstract

Organic solar cells are approaching power conversion efficiencies of other thin-film technologies. However, in order to become truly market competitive, the still substantial voltage losses need to be reduced. Here, the synthesis and characterization of four novel arylamine-based push-pull molecular donors was described, two of them exhibiting a methyl group at the para-position of the external phenyl ring of the arylamine block. Assessing the charge-transfer state properties and the effects of methylation on the open-circuit voltage of the device showed that devices based on methylated versions of the molecular donors exhibited reduced voltage losses due to decreased non-radiative recombination. Modelling suggested that methylation resulted in a tighter interaction between donor and acceptor molecules, turning into a larger oscillator strength to the charge-transfer states, thereby ensuing reduced non-radiative decay rates.

Donor End‐Capped Alkyl Chain Length Dependent Non‐Radiative Energy Loss in All‐Small‐Molecule Organic Solar Cells

“…Because this phenomenon only exists on the very small Δ E LE–CT (annealed blends at 65 °C, and MPhS‐C2/BTP‐eC9 optimized blends), probably, an encouraging hybridization between triplet CT states and LE states in OSCs blends is formed. [ 43,44 ] In conclusion, the EQE EL of the blend could be much higher than that of the pristine acceptor at a small Δ E LE–CT , even for the optimized blends, illustrating the importance of finely energy level regulation in the reduction of Δ V nr.…”