Low-temperature solution-processable vanadium oxide (V2Ox) thin films have been employed as hole extraction layers (HELs) in polymer bulk heterojunction solar cells. V2Ox films were fabricated in air by spin-coating vanadium(V) oxytriisopropoxide (s-V2Ox) at room temperature without the need for further thermal annealing. The deposited vanadium(V) oxytriisopropoxide film undergoes hydrolysis in air, converting to V2Ox with optical and electronic properties comparable to vacuum-deposited V2O5. When s-V2Ox thin films were annealed in air at temperatures of 100 °C and 200 °C, OPV devices showed similar results with good thermal stability and better light transparency. Annealing at 300 °C and 400 °C resulted in a power conversion efficiency (PCE) of 5% with a decrement approximately 15% lower than that of unannealed films; this is due to the relative decrease in the shunt resistance (Rsh) and an increase in the series resistance (Rs) related to changes in the oxidation state of vanadium.

The correlation between cathode work function and open-circuit voltages (V oc) in inverted polycarbazole: fullerene (PCDTBT: PC 70 BM) bulk-heterojunction solar cells has been investigated by post-annealing of tin-indium oxide (ITO) electrodes. ITO work function is seen to change from 4.2 eV 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 ITO work function of 4.2 eV with V oc of 0.89 eV, J sc of 8.06 mA.cm-2 , FF of 64.70 % and PCE of 4.62 %. Together with previously published results, we are able to extract two regimes of V oc dependence on 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 cathode contact design in inverted polymer solar cells.

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