Insights into the working mechanism of cathode interlayers in polymer solar cells via [(C8H17)4N]4[SiW12O40]

Abstract

A low-cost (<$1 per g), high-yield (>90%), alcohol soluble surfactant-encapsulated polyoxometalate complex [(C₈H₁₇)₄N]₄[SiW₁₂O₄₀] has been synthesized and utilized as a cathode interlayer (CIL) in polymer solar cells (PSCs). A power conversion efficiency of 10.1% can be obtained for PSCs based on PTB7-Th (poly[[2,6′-4,8-di(5-ethylhexylthienyl)benzo[1,2-b;3,3-b]-dithiophene][3-fluoro-2[(2-ethylhexyl) carbonyl] thieno [3,4-b]-thiophenediyl]]):PC₇₁BM ([6,6]-phenyl C71-butyric acidmethyl ester) due to the incorporation of [(C₈H₁₇)₄N]₄[SiW₁₂O₄₀]. Combined measurements of current density–voltage characteristics, transient photocurrent, charge carrier mobility and capacitance–voltage characteristics demonstrate that [(C₈H₁₇)₄N]₄[SiW₁₂O₄₀] can effectively increase the built-in potential, charge carrier density and mobility and accelerate the charge carrier extraction in PSCs. Most importantly, the mechanism of using [(C₈H₁₇)₄N]₄[SiW₁₂O₄₀] as the CIL is further brought to light by X-ray photoemission spectroscopy (XPS) and ultraviolet photoemission spectroscopy (UPS) of the metal/[(C₈H₁₇)₄N]₄[SiW₁₂O₄₀] interface. The findings suggest that [(C₈H₁₇)₄N]₄[SiW₁₂O₄₀] not only decreased the work function of the metal cathodes but also was n-doped upon contact with the metals, which provide insights into the working mechanism of the CILs simultaneously improving the open circuit voltage, short circuit current and fill factor in the PSCs.