Influence of the Location of Electron-Donating 3,4-Ethylenedioxythiophene (EDOT) Moiety in the A-π-D-π-A Type Conjugated Molecules on the Optoelectronic Properties and Photovoltaic Performances

A π D π A type conjugated small molecules play an indispensable role in organic photovoltaics. Understanding the relationship between the molecular structure and performance is a fundamental question for the further rational design of highperformance organic materials. To red-shift the absorption spectrum of benzo[1,2-b:4,5-b']dithiophene (BDT) based A π D π A type compounds, an electron-donating 3,4-ethylenedioxythiophene (EDOT) moiety was introduced into the π-conjugation bridge unit. Two new compounds with EDOT next to the central BDT core (COOP-2HT-EDOT-BDT) or next to the terminal electron acceptor unit (COOPEDOT-2HT-BDT) were synthesized and characterized. The compound COOP-2HT-EDOT-BDT showed higher molar extinction coefficient (ϵ_abs^max= 1.06 × 105 L mol^-1cm^-1), lower optical band gap (E_g= 1.56 eV) and high HOMO energy level (E_HOMO= -5.08 eV) than COOP-EDOT-2HT-BDT (ϵ_abs^max= 0.96 ⊗ 10⁵L mol^-1cm^-1, E_g= 1.71 eV, E_HOMO= -5.26 eV), which is attributed to the intensive interaction between the EDOT unit and the HOMO orbital, as confirmed by the theoretical calculation results. However, the higher power conversion efficiency of 3.58% was achieved for the COOP-EDOT-2HT-BDT: PC₆₁BM-based solar cells, demonstrating that the electron-donating EDOT unit adjacent to the electron-withdrawing end-capped group (COOP) is a better way to achieve high-performance photovoltaic materials.