Characterization of ZnS-layer-inserted bulk-heterojunction organic solar cells by ac impedance spectroscopy

A fluorine-doped tin oxide (FTO)/zinc sulfide (ZnS)/[6,6]-phenyl C 61 butyric acid methyl ester (PCBM): regioregular poly(3-hexylthiophene) P3HT)/poly(3,4-ethylenedioxylenethiophene): poly(4-styrene sulfonic acid) (PEDOT:PSS)/Au type organic solar cell (FTO/ZnS/PCBM:P3HT/PEDOT:PSS/Au, ZnS cell) with a 1 cm2 active area was first developed using transparent ZnS prepared on a FTO electrode by a chemical bath deposition method. The ZnS inserted solar cells were investigated by photocurrent-voltage (I-V) and ac impedance spectroscopy (IS) measurements. In photo I-V measurements, the ZnS cell exhibited scattered power conversion efficiencies (η) of 0.7%-1.2% when the FTO/ZnS electrode was not immersed in various aqueous solutions before the fabrication of the ZnS cells. In contrast, the solar cells with a surface-modified FTO/ZnS electrode by immersing in 0.1M Na2 S solutions at pH 7-9 containing hydrosulfide ions (HS-) exhibited reproducible η of 1.5%-1.7%. The electric resistance components in the cell consisting of five layers were separately estimated by the IS measurement. The larger of the solar cell using HS- modified ZnS was explained by the fact that the charge transport resistance at the ZnS/PCBM:P3HT interface became smaller due to this surface modification. When a continuous light irradiation was carried out for the modified cells, the resistance at the ZnS/PCBM:P3HT interface increased, being accompanied by the decrease in the η. That is, the cell performance was controlled mainly by charge transport velocity at the ZnS/PCBM:P3HT interface.