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 high-performance 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 (COOP-EDOT-2HT-BDT) were synthesized and characterized. The compound COOP-2HT-EDOT-BDT showed higher molar extinction coefficient (eabs max = 1.06 × 105 L mol−1 cm−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 (eabs max = 0.96 × 105 L mol−1 cm−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.