Liquid Crystalline Perylene Diimide Outperforming Nonliquid Crystalline Counterpart: Higher Power Conversion Efficiencies (PCEs) in Bulk Heterojunction (BHJ) Cells and Higher Electron Mobility in Space Charge Limited Current (SCLC) Devices

In this work, we propose the application of liquid crystalline acceptors as a potential means to improve the performances of bulk heterojunction (BHJ) organic solar cells. LC-1, a structurally-simple perylene diimide (PDI), has been adopted as a model for thorough investigation. It exhibits a broad temperature range of liquid crystalline (LC) phase from 41 °C to 158 °C, and its LC properties have been characterized by differental scanning calorimetry (DSC), polarization optical microscopy (POM), and X-ray diffraction (XRD). The BHJ devices, using P3HT:LC-1 (1:2) as an organic photovoltaic active layer undergoing thermal annealing at 120 °C, shows an optimized efficiency of 0.94 %. By contrast, the devices based on PDI-1, a nonliquid crystalline PDI counterpart, only obtain a much lower efficiency of 0.22%. Atomic force microscopy (AFM) images confirm that the active layers composed of P3HT:LC-1 have smooth and ordered morphology. In space charge limited current (SCLC) devices fabricated via a spin-coating technique, LC-1 shows the intrinsic electron mobility of 2.85 × 10^–4 cm²/(V s) (at 0.3 MV/cm) which is almost 5 times that of PDI-1 (5.83 × 10^–5 cm²/(V s)) under the same conditions for thermal annealing at 120 °C.