Organic solar cells of enhanced efficiency and stability using zinc oxide:zinc tungstate nanocomposite as electron extraction layer

In this work, the enhanced performance of inverted bulk heterojunction (BHJ) organic solar cells (OSCs) using zinc oxide (ZnO): polyoxometalate (POM), in particular sodium metatungstate (Na6H2W12O40), nanocomposite films as electron extraction layers (EELs) is demonstrated. The addition in the precursor solution of ZnO of sodium metatungstate results in the formation of ZnO: ZnWO4 nanocomposite as evidenced by X-ray diffraction, Fourier transform infrared and photoluminescence measurements. The formation of ZnO: ZnWO4 heterointerface reduces the work function of the nanocomposite material leading to a more favorable electron extraction/transport at the organic blend/electron transport layer interface. Additionaly, the amount of zinc interstitial defects is suppressed having a profound positive effect on device stability. As a result, simultaneously improved open-circuit voltage (Voc), short-circuit current density (J(sc)) and fill factor (FF) are obtained in the devices using the ZnO: ZnWO4 nanocomposites. Therefore, both of the inverted BHJ OSCs composed of either poly (3-hexylthiophene) (P3HT):[ 6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) or P3HT: indene-C60 bisadduct (IC(60)BA) photoactive blends show a significant performance enhancement when using the nanocomposite electron extraction layer, exhibiting a 27% and 23%, respectively, improvement in their power conversion efficiency (PCE) values compared to the reference devices based on pristine ZnO. In addition, the devices with the ZnO:ZnWO4 layer exhibit a remarkable stability enhancement retaining 95% of their initial PCE value upon storage for 500 h.