Preparation And Photovoltaic Performance Of Polyoxometalate/Semiconductor Composite Film

The utilization of solar energy has become a hotspot in the world to solve the shortage offossil energy. Among them, an important research topic is to improve light-to-electricityconversion efficiency of the semiconductor materials. Polyoxometalates （POMs） are a kind ofgood electron acceptors. They can transport photogenerated electrons in the semiconductors,which reduces the electron-hole recombination and then improves the photovoltaic response.In this paper, we prepared a series of semiconductors/POMs composite films and investigatedtheir photovoltaic performances.1. We fabricated the nanocomposite films of PW₁₂and TiO₂by the layer-by-layer （LbL）self-assembly method. These films were characterized by UV-vis spectroscopy, IR spectra,and atomic force microscopy. Photocurrent measurements suggested that the photocurrentresponse of the PW₁₂/TiO₂composite film was highly dependent on the deposited number oflayers. Furthermore, the PW₁₂/TiO₂composite film demonstrated the higher photocurrent andphotoelectrooxidation activity for methanol than the TiO₂film, and the power conversionefficiency of the PW₁₂/TiO₂film was improved by50%.2. We prepared the composite thin films of P₂W₁₈andpoly（allylaminehydrochloride）-modified CdS （CdS-PAH） by the LbL self-assembly method.The CdS-PAH/P₂W₁₈composite film displayed a ca.1.5-fold increase in the photocurrentresponse and power conversion efficiency, as compared to the single CdS-PAH film. Thisindicated that P₂W₁₈could act as electron acceptors to efficiently suppress electron holerecombination in CdS and improve the photovoltaic performance. Such a mechanism wasfurther proven by experimental data of fluorescence emission spectra and surfacephotovoltage spectroscopy.3. A composite film containing CuPc and SiMo₁₂was fabricated by the LbLself-assembly method. Under both solar light and visible light irradiation, the photocurrentresponse of the CuPc/SiMo₁₂film was markedly enhanced in comparison with those of theCuPc/PSS film, and the CuPc/SiMo₁₂film displayed a ca.8.7-fold increase in the powerconversion efficiency. Surface photovoltage measurements indicated that the photoinducedelectron transfer occurred between CuPc and POMs. Furthermore, the CuPc/POMs compositefilm exhibited good photoelectrocatalytic performance for the oxidation of hydrazine.4. We prepared the POMs(PW₁₂、P₂W₁₈)/TiO₂composite films and assembled the cells.We study on the photoelectrochemical performance, the electron transport and electron–holerecombination of the different content and type POMs in TiO₂film by measurements ofelectrochemical impedance spectroscopy （EIS）, photocurrent responses and I–V curves. ThePOMs/TiO₂films at low POMs loadings （0.75%） displayed the enhanced photovoltaicperformance. The power conversion efficiency of PW₁₂（0.75%）/TiO₂film andP₂W₁₈（0.75%）/TiO₂film was2.6times and1.6times that of TiO₂film, respectively. EISmeasurements proved that introducing POMs into TiO₂film could reduce electron-holerecombination and facilitate photogenerated electron transfer, which enhanced the light-to-electricity conversion efficiency. However, the excessive content （7.5%） of POMscould almost cause a negative effect on photovoltaic performance due to electron trap fillingsites.5. We fabricated the DSSCs based on both the only TiO₂photoanode and the PW₁₂/TiO₂photoanode. Compared to the TiO₂photoanode, the power conversion efficiency of0.75%-PW₁₂/TiO₂photoanode was improved by33%, while the7.5%-PW₁₂/TiO₂photoanodecould cause a negative effect on photovoltaic performance. The reasons were as follows:（1）the0.75%-PW₁₂/TiO₂film could adsord more dye;（2） The incorporating of PW₁₂into TiO₂photoanode could reduce electron-hole recombination and improve electron transfer. However,electron trap filling sites were present in the7.5%-PW₁₂/TiO₂photoanode.