Synergistic Effect of Polyoxometalates and rGO for Enhancing the Photocurrents of TiO₂-based Composite Film

Abstract

Polyoxometalates possess unique electron-accepting ability while maintaining their structures; this makes them effective electron mediators in semiconductor-based optoelectronic system via retarding the photoexcited electronhole recombination. H 3 PW 12 O 40 (PW 12 ) was introduced into TiO 2 film using layerby-layer method in the presence of reduced graphene oxide, serving as a conductive additive. The composite film had smooth surface and dense packing of TiO 2 nanoparticles, and showed enhanced transient photocurrent compared to the films without PW 12 or reduced graphene oxide (rGO). IV measurement further confirmed that the power conversion efficiency of PW 12 / rGO/TiO 2 film was remarkably improved compared to those of the films without PW 12 or rGO. PW 12 functions as an electron mediator and rGO serves as a conductive agent for enhancing the photocurrent and power conversion efficiency, due to the efficient dissociation of excitons and fast electron transfer. Practical application of solar cells is greatly dependent on the enhancement of photovoltaic response for the purpose of substantial power conversion efficiency. Basically, excitons are generated upon exposure to light in a photovoltaic device.1 Up to date, retarding the recombination of photoexcited electronhole pairs and accelerating the charge transport are reported to play significant roles for enhancing the performance of photovoltaic device. 2 rGO can be prepared by partially removing the oxygencontaining groups of graphene oxide, which possess unique electron conductivity, flexibility, and mechanical property. 3,4 Graphene-based hybrid nanomaterials have been widely applied in the photovoltaic devices, and showing positive effect on the enhancement of energy conversion efficiency. 5,6 Introduction of rGO into TiO 2 composite can effectively lead to superior performance of dye-sensitized solar cells via serving as photoanode. Polyoxometalates (POMs) have diverse structures and variable chemical composition, showing unique electron acceptor while maintaining intact structures. 811 Very recently, POMs have showed the ability to trap photogenerated electrons from the conduction band of semiconductors, and also promote electron transfer to electron collector acting as electron scavenger, leading to enhance the photovoltaic response of TiO 2 , ZnO, phthalocyanine-based composites, etc.12 The combination of POMs with semiconducting perovskites successfully creates a photoconductive system with enhanced photocurrent because of the effective electron-transfer pathway to separate the photoexcited excitons and retard the electronhole recombination.However, smart combination of POMs, rGO, and TiO 2 might show enhanced photocurrent and energy conversion efficiency via the retardance of electronhole recombination and promotion of charge transfer to electron collector.In this paper, layer-by-layer (LbL) method is applied to prepare the rGO/TiO 2 /PW 12 -based composite film on ITO substrate. 1317 Compared to pure TiO 2 -based film, the composite film sh...