Low Work Function Lacunary Polyoxometalates as Electron Transport Interlayers for Inverted Polymer Solar Cells of...

Effective interface engineering has been shown to play a vital role in facilitating efficient charge-carrier transport, thus boosting the performance of organic photovoltaic devices. Herein, we employ water-soluble lacunary polyoxometalates (POMs) as multifunctional interlayers between the titanium dioxide (TiO₂) electron extraction/transport layer and the organic photoactive film to simultaneously enhance the efficiency, lifetime, and photostability of polymer solar cells (PSCs). A significant reduction in the work function (W_F) of TiO₂ upon POM utilization was observed, with the magnitude being controlled by the negative charge of the anion and the selection of the addenda atom (W or Mo). By inserting a POM interlayer with ∼10 nm thickness into the device structure, a significant improvement in the power conversion efficiency was obtained; the optimized POM-modified poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl][3-fluoro-2-[(2- 33 ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]]:[6,6]-phenyl-C₇₀ butyric acid methyl ester (PTB7:PC₇₀BM)-based PSCs exhibited an efficiency of 8.07%, which represents a 21% efficiency enhancement compared to the reference TiO₂ cell. Similar results were obtained in POM-modified devices based on poly(3-hexylthiophene) (P3HT) with electron acceptors of different energy levels, such as PC₇₀BM or indene-C₆₀ bisadduct (IC₆₀BA), which enhanced their efficiency up to 4.34 and 6.21%, respectively, when using POM interlayers; this represents a 25-33% improvement as compared to the reference cells. Moreover, increased lifetime under ambient air and improved photostability under constant illumination were observed in POM-modified devices. Detailed analysis shows that the improvements in efficiency and stability synergistically stem from the reduced work function of TiO₂ upon POM coverage, the improved nanomorphology of the photoactive blend, the reduced interfacial recombination losses, the superior electron transfer, and the more effective exciton dissociation at the photoactive layer/POM/TiO₂ interfaces.