Application Of Polyoxometalate-based Functional Material Additive Engineering In Perovskite Photodetectors

With the continuous growth of the modern population,the traditional fossil energy has been unable to meet the needs of human life,and solar energy is expected to replace fossil fuels to cope with the energy crisis.Perovskite materials have higher photoelectric conversion efficiency than other materials.In the past decade,the rapid development of the performance of perovskite-based photovoltaic devices has attracted the attention of researchers.The quality of perovskite thin films in devices seriously affects their performance.Therefore,how to improve the quality of perovskite thin films in photovoltaic devices is still one of the important topics that needs our in-depth research.In order to improve the quality of perovskite films,we need to find a material that can not only improve light absorption,but also increase the carrier transfer rate.Polyoxometalates show good performance in the absorption of visible light and near-ultraviolet light.In addition,the appropriate energy level bandgap of polyoxometalates can adjust the energy level structure of the device,and their good absorbance can also promote the transport rate of photogenerated carriers.Based on this,introducing polyoxometalates into perovskite photovoltaic devices can effectively improve device performance.In consideration of improving the dispersion of polyoxometalates,compounding polyoxometalates with other materials,the obtained polyoxometalates-based composite can adjust the size and morphology of perovskite grains to a certain extent while giving full play to the advantages of polyoxometalates,play a synergistic effect on the improvement of device performance.Based on the above considerations,we prepared two kinds of polyoxometalates-based composite in which polyoxometalates are compounded with metal organic frameworks（MOF）and graphene derivatives,and introduced these two composite into the perovskite active layer of perovskite photodetectors,thus improving the efficiency and stability of the devices.The specific research and work are as follows:1.The SiW₁₁@ZIF-8 is obtained by encapsulating Keggin type POMs{SiW₁₁}into the pore of the ZIF-8 through electrostatic action,and it is added to the perovskite precursor solution to improve the performance of the perovskite photodetector.The composite has good stability,and the doping of POMs will not destroy the structure of ZIF-8 itself.The energy level of SiW₁₁@ZIF-8 matches with the layers in the perovskite photodetector,which improves the carrier transmission rate in the device and inhibits the non-radiative recombination in the perovskite film.At the same time,due to the good light absorption of SiW₁₁@ZIF-8,the utilization of light in the device is also enhanced.On the other hand,the doping of the composite improves the crystallinity of perovskite grains and passivates the grain boundaries of the perovskite layer.Compared with the perovskite layer of traditional devices,the average crystal size of perovskite in the optimized device has increased from254.50 nm to 721.47 nm.At the same time,the optical response current of the device increased from 14.41μA to 41.95μA at the additional voltage of 3 V.The optimized device showing good stability and it can maintain more than 90%efficiency after 700 hours in the air without packaging.This work proves the potential application of POM@MOF in the field of perovskite photodetectors.2.By combining Keggin type POMs{SiW₉Co₃}with graphene derivative r GO,a polyoxometalates-based composite SiW₉Co₃@r GO was obtained and used to modify perovskite photodetectors.Graphene derivatives have unique electronic conductivity,flexibility and mechanical properties.Adding them to the perovskite precursor solution can passivate the grain boundaries of the perovskite film and promote the carrier transfer inside the device.Due to the good absorption of POMs in the ultraviolet and visible regions,they can also greatly promote the absorption when applied to photodetectors.Compared with the blank device without doping,the average grain size of perovskite increased by 381%,and the optical response current increased by about 226%.On the other hand,due to the special network structure of graphene material,it can effectively prevent the adverse effects of water erosion on the long-term stability of the device.Therefore,when the optimized device is placed in the air humidity of 45±5%,the indoor temperature of 25℃and without packaging,the device can still maintain more than 90%efficiency after nearly 700 hours.This work broadens the application field of POMs,and also proves that polyoxometalates-based composites have good application prospects in the field of optoelectronics.