Luminescent and photovoltaic properties of poly(9,9-dioctylfluorene-co-bithiophene) in organic electronic devices

We studied the luminescent and photovoltaic properties of poly(9,9-dioctylfluorene-co-bithiophene) (F8T2) based on ITO/PEDOT:PSS/F8T2/Bphen/LiF(0 or 1 nm)/Al and ITO/PEDOT:PSS/F8T2:PCBM/Bphen/Al. A stable and bright yellow emission was obtained from polymer F8T2, and the electroluminescence power reached 45 μW at a 15 V driving voltage. Polymer F8T2 shows a broad absorption band from 400 to 500 nm, and has a shorter absorption edge at about 560 nm compared to that of the typical electron donor P3HT (650 nm). The photoluminescence quenching of F8T2 occurs with only a small fraction of blended PCBM due to the effective exciton dissociation at the interface between F8T2 and PCBM. Polymer solar cells (PSCs) using F8T2:PCBM as the active layer show a low power conversion efficiency (PCE) of 0.10% with an open circuit voltage (V oc) of 0.91 V and short circuit current density (J sc) of 0.23 mA/cm2. The PSCs using F8T2:P3HT:PCBM as the active layer have a V oc of 0.85 V and J sc of 3.02 mA/cm2, improving the PCE by about 0.90%. We attribute the improved cell performance to the higher number of photons harvested by P3HT molecules.