Effects of Mild Plasma Exposure on the Active-Layer in Polythiophene:Fullerene Bulk-Heterojunction Solar Cells

In order to investigate the optoelectronic impact of plasma treatment on the active-layer in polymer based bulk-heterojunction photovoltaics, we fabricated solar cells with poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) (1:0.8 wt%) blend as the active-layer. It was observed that even a short (5 s exposure) air or argon plasma treatment can effectively change the wettability of several polar organic solvents on the P3HT:PCBM layer, while the devices realized from the treated active-layers still remain functional with a slight reduction in the effective built-in electric field. Other structural properties of the active-layer, like surface-morphology, film-thickness, UV-Vis absorbance and photoluminescence (PL) remained unaffected. Time-resolved PL spectroscopy showed that bi-exponential fluorescence decay parameters of the P3HT:PCBM layer were also not altered. Our results indicate that a mild plasma exposure induces some charge traps, only on the surface of the active-layer, without affecting its bulk structural and photophysical properties. Hence, a short plasma exposure of the active-layer is promising, as a largely non-destructive technique for the realization of solution processable multilayer heterojunction and tandem photovoltaic architectures from species soluble in dissimilar solvents.