Investigation of the property change of polymer solar cells by changing counter anions in polyviologen as a cathode buffer layer

Polyviologen (PV) derivatives are known as materials for lowering the work function of cathodes, thereby reducing the electron injection/collection barrier at the cathode interface of polymer solar cells (PSCs). In order to demonstrate the effect of the size of counter anions in PV derivative on the photovolatic properties, we introduce different types of counter anions such as bromide (Br), hexafluorophosphate (PF6), and p-toluene sulfonate (OTs), in PV derivative. The effective work function of the Al electrode is gradually increased by increasing the size of counter anion, indicating the larger counter anion leads to the larger reduction of a Schottky barrier. The power conversion efficiency (PCE) value of the devices is also improved by increasing the size of counter anion. The device (ITO/ PEDOT/P3HT:PCBM/PV/Al) with the thin layer of PV derivative bearing a counter anion of OTs as a cathode buffer layer demonstrates the PCE of 3.90%, with a open circuit voltage (Voc) of 0.64 V, a short circuit current density (Jsc) of 11.39 mA/cm2, and a fill factor (FF) of 53.5%, respectively. This is better than the device with PV derivative having a counter anion of PF6 (PCE=3.73%, Jsc=11.14 mA/cm2, Voc=0.64 V, FF=52.1%) or Br (PCE=3.62%, Jsc=10.95 mA/cm2, Voc=0.64 V, FF=51.6%). Here, our results show that it is possible to improve the performance of PSCs and to tune the electron injection/collection barrier height at the cathode interface by choosing different counter anions without complicated synthesis.