Investigation of supramolecular interactions between liquid crystals and PCBM for improved morphological stability in solar cells

Abstract

Supramolecular interactions between liquid crystals (LCs) of different chemical structures and PC₆₁BM molecules have been studied, showing that 4-cyano-4′-pentylterphenyl (5CT) containing electron-withdrawing cyano substituents on the phenyl ring exhibited the strongest interactions with PC₆₁BM, as revealed via density functional theory (DFT) calculations and experimental analysis based on Fourier transform infrared spectrometry (FTIR), differential scanning calorimetry (DSC) and polarized optical microscopy (POM). In contrast, 4-octyloxy-4′-cyanobiphenyl (8OCB) comprising an electro-donating octyloxyl group and dioctylterthiophene (8TTP8) with thiophene rings and long alkyl groups showed weaker interactions with PC₆₁BM. After electric field treatment at 600 V mm⁻¹ in an air environment, the P3HT:PC₆₁BM:8OCB specimen showed a higher power conversion efficiency (PCE) of 2.9% and a more stable morphology than P3HT:PC₆₁BM:8TTP8 with a PCE of 2.7%. Upon annealing at 150 °C for 1 h, 5CT is most effective in restricting the aggregation and crystallization of PC₆₁BM molecules, thus stabilizing the morphology of P3HT:PC₆₁BM. Moreover, the supramolecular interaction between LCs and PCBM could also influence the thermal stability in the narrow bandgap system of PTB7-Th:PC₇₁BM, with 8TTP8 showing the highest ability to restrict the depression of the PCE value.