Low band gap polymer / PC₇₁BM composities for photovoltaic applications

We report here two novel 2,7-fluorene- And 2,7-carbazole -based conjugated polymers PFDCTBT and PCDCTBT containing ladder-type heptacyclic structures with forced planarity. PFDCTBT and PCDCTBT simultaneously possess excellent solubilities for solution-processability, low band gap with suitable position of HOMO/LUMO energy levels, and high hole mobilities. Both polymers are soluble in common organic solvents. The HOMO levels being estimated to be -5.32 eV for PFDCTBT and -5.38 eV for PCDCTBT are at ideal range to assure better air-stability and greater attainable V_oc in the final device.10 It is found that the hole mobility of the PCDCTBT/PC₇₁BM composite (4×10^-4 cm²/Vs) is higher than that of the PFDCTBT/PC₇₁BM (5×10^-5 cm²/Vs) blend under the same fabrication condition. Without extensive optimization, the preliminary photovoltaic performance based on PFDCTBT already showed a J _sc of 9.5 mA/cm², a V_oc of 0.77 V, a fill factor (FF) of 0.38, leading to a decent PCE of 2.8%. More encouragingly, the device using PCDCTBT as the p-type material delivered superior performance with a J_sc of 10.7 mA/cm², a V_oc of 0.80 V, a FF of 0.43, improving the PCE to 3.7%.