Systematic Investigation of Core and Endcap Selection on the Development of Functional it-Conjugated Materials

We investigate the optoelectronic properties of four new pi- conjugated materials, which feature either a 3,4-ethylenedioxythiophene (EDOT) or an EDOT-flanked benzothiadiazole aryl core with two N-annulated perylene diimide (NPDI) or theobromine endcaps. Endcap selection was an important factor for the preferred molecular geometry, where the use of NPDI endcaps induced a dragonfly-type conformation and the use of theobromine endcaps promoted a corkscrew-like conformation. Electrochemically, the choice of aryl pi- conjugated core was found to have a strong influence on the molecule's highest occupied molecular orbital (HOMO) energy level, while the choice of endcap had a more profound effect on the lowest unoccupied molecular orbital (LUMO) energy level. Although the optical absorption profiles were generally dominated by endcap contributions, using the larger EDOT-flanked benzothiadiazole aryl core was found to decrease the optical energy band gap. Proof-of-concept organic photovoltaic (OPV) devices, using a PM6:Y6 bulk heterojunction (BHJ) photoactive layer, were fabricated using the four new pi-conjugated materials as the cathode interlayer (CIL). The molecules featuring the EDOT aryl core (A and B) enabled OPV device power conversion efficiencies (PCEs) by around 12%, comparable to control devices using PDINN as the CIL, while those featuring the EDOT-flanked benzothiadiazole aryl core (C and D) obtained PCEs of 10.5%. The observed differences in OPV device performance were attributed to superior solubility and subsequent CIL film formation in the case of A and B, compared to C and D, which, in turn, led to improved contact between the Ag top electrode and the BHJ photoactive layer.