Low Work Function Lacunary Polyoxometalates as Electron Transport Interlayers for Inverted Polymer Solar Cells of Improved Efficiency and Stability

Abstract

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 (TiO) 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) of TiO 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-C butyric acid methyl ester (PTB7:PCBM)-based PSCs exhibited an efficiency of 8.07%, which represents a 21% efficiency enhancement compared to the reference TiO cell. Similar results were obtained in POM-modified devices based on poly(3-hexylthiophene) (P3HT) with electron acceptors of different energy levels, such as PCBM or indene-C bisadduct (ICBA), 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 TiO 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/TiO interfaces.

Polyoxometalates: Tailoring metal oxides in molecular dimension toward energy applications

“…Last but not least, dual functions of light‐induced charge transfer and energy storage could be developed in some polyoxometalates, since many polyoxometalates exhibit combined virtues of light responsiveness and charge storage properties . As a result, the polyoxometalates could be an effective source for photobattery, which essentially consists of a lithium‐ion battery and a solar cell in one compartment.…”

“…Last but not least, dual functions of light-induced charge transfer and energy storage could be developed in some polyoxometalates, since many polyoxometalates exhibit combined virtues of light responsiveness and charge storage properties. 197,397,398 As a result, the polyoxometalates could be an effective source for photobattery, which essentially consists of a lithium-ion battery and a solar cell in one compartment. The single unit is photo-rechargeable and enjoys device and packaging simplicity as well as reduced weight and lower cost compared with separate energy conversion and storage devices.…”

A silanol-functionalized polyoxometalate with excellent electron transfer mediating behavior to ZnO and TiO2 cathode interlayers for highly efficient and extremely stable polymer solar cells

“…•16H2O were given elsewhere. 44 To a well stirred suspension of α-K9[PW9O34]•16H2O (10g, 3.48 mmol) in dry MeCN (200 mL) under argon solid nBu4NBr (6 g A, 18.6 mmol) were added and then tBuSiCl3 (2.1 g, 10.1 mmol) the mixture was stirred overnight at 0°C. After separation of the white solid (KCl+KBr), white crystals of (nBu4N)3[PW9O34(tBuSiOH)3] formed upon slow evaporation of the resulting solution in an open vessel at room temperature.…”

“…37,38 They were recently used by our group and others as cathode interlayers in PSCs with either the conventional or the inverted architecture due to their unique properties, such as facile processing from water or alcohol solutions, adequate electron mobility, and high transparency in the visible range. [39][40][41][42][43][44] However, their possible aggregation when forming a solid film from aqueous solutions may be extremely harmful to device performance and stability; consequently, it is rather unlikely for pristine POMs to be suitable for large-area production of PSCs. As yet, works to exploit appropriate concepts to simultaneously achieve superior electron transfer properties of POMs while also avoiding aggregation for successful application in PSCs are limited.…”

“…As yet, works to exploit appropriate concepts to simultaneously achieve superior electron transfer properties of POMs while also avoiding aggregation for successful application in PSCs are limited. 43,44 In order to overcome these drawbacks and utilize the unique advantages of POMs we synthesized a silanol functionalized POMbased complex containing the PW9O34 POM cluster with the lacunary Keggin structure namely (nBu4N)3 [PW9O34(tBuSiOH)3] (chemical formula C61H139.5N3.5PO37Si3W9) (referred to as tBuSiOH-POM), which is inserted into the vicinity of the metal oxide/organic interface to allow superior electron transfer to TiO2 and ZnO CILs. The effective anchoring of tBuSiOH-POM onto metal oxide substrates through silanol linkers results in the formation of a uniform POM film.…”