Open-Circuit Voltage in Inverted Polycarbazole:Fullerene Bulk Heterojunction Solar Cells

The correlation between cathode work function and open-circuit voltages (
V
_oc
) in inverted polycarbazole:fullerene (PCDTBT:PC70BM) bulk-heterojunction solar cells has been investigated by postannealing of indium tin oxide (ITO) electrodes. The ITO function is seen to change from 4.2 to 4.5 eV without the need to insert additional interfacial layers with annealing temperature up to 400 °C. The best device performance was obtained at room temperature with the ITO work function of 4.2 eV with a
V
_oc
of 0.89 eV, a
J
_sc
of 8.06 mA·cm
^-2
, a fill factor (FF) of 64.70%, and a power conversion efficiency of 4.62%. Together with previously published results, we are able to extract two regimes of
V
_oc
dependence on the cathode work function: first, a linear relationship when the cathode work function exceeds the lowest unoccupied molecular orbital (LUMO) of PCBM and, second, a constant
V
_oc
regime when the ITO work function reduces below the LUMO level. These results provide general guidelines for the cathode contact design in inverted polymer solar cells.