Interfacial Modification And Performance Of TiO₂-based Perovskite Solar Cells

As a new member,perovskite solar cells?PSCs?with excellent optical and electrical properties have attracted tremendous attention.The PCE of PSCs have successfully exceeded 23%from the initial 3.9%.The PSCs based on perovskite materials obtained a higher photoelectric conversion efficiency?PCE?because of the advantages of longer carrier diffusion length,lower recombination rate and stronger light absorption.In order to further improve the efficiency of the PSCs,more refined optimization and control of the device is required.In the perovskite devices,there are multiple interfaces among layers.The performance of the device may be affected by interface defects or energy level mismatch,which directly leads to the recombination of carriers.By introducing suitable modification of the interface,the defect density at the interface can be effectively reduced.Therefore,the electron collection efficiency is improved,and thus the improved photoelectric performance is achieved.In addition,the shape and size of perovskite crystals directly affect the optical and electrical properties of perovskite solar cells,the film optimization has become the key to PCE of the PSCs.In this paper,we focused on improving the interface of the electron transport layer/perovskite light absorbing layer and improving the quality of the perovskite film.The interfaces between the perovskite layer and the electron transport layer were improved by the method of sol-gel dip-coating and chemical bath deposition?CBD?,and the coating and pores filling of the perovskite light absorbing layer in mesoporous scaffold were also improved.In the process of CBD,the anatase TiO₂ nanocuboids grown on the surface of rutile TiO₂ electron transport layer by introducing fluorine ions,forming an anatase/rutile mixed phase miscibility heterogeneous structure that can promote the separation of electrons and holes at the interface.In addition,the annealing treatments of perovskite films with a mixed solvent of isopropyl alcohol?IPA?and N-methyl-2-pyrrolidone?NMP?can effectively increase the perovskite grains and improve the film quality,thus further improving the photoelectric properties and stability of the corresponding device.The main contents are as follows:?1?The surface of TiO₂ nanorods were modified by TiO₂ sol-gel dip-coating method.The effects of TiO₂ nanoparticles on the properties of PSCs were investigated by analyzing the morphology and structure information.By changing the times of dip-coating,we achieved a TiO₂ nanoparticle/nanorod mesoporous scafford layer which was favorable for the deposition of perovskite film.The dense TiO₂ nanoparticles can also passivated the surface of the TiO₂ nanorods,reduced the density of defect states of the electron transport layer,and finally improved the ability to collect electrons at the interface.A PCE of 10.55%was obtained,which was 45%higher than the bare TiO₂ nanorod based device.?2?The surface of TiO₂ nanorods were modified by CBD method,and the effects of TiO₂ nanoparticles modified layer on the device performance were investigated.This method allowed for the preparation of large batches of substrates,effectively reduced the experimental period.Meanwhile,the nanoparticles prepared by the CBD method can palyed the role of hole blocking layer and passivated surface defects of the electron transport layer.The PCE of device with CBD interface modified can reached 12.97%,which was about 1.3 times that of the bare TiO₂ nanorod based device.?3?The TiO₂ nanoparticles were eached after the introduction of F ions during the process of CBD,resulting in the formation of nanocuboids with exposed the{001}crystal plane.It was shown that the anatase TiO₂ nanocuboids with exposed{001}facets can adsorbed more perovskite materials,improved the pore filling of perovskite into the scaffold layer and generated more carriers.TiO2 nanocuboids also can increased the specific surface area of the nanorods and provide more channels for carrier transfer.After testing and analysis,we found that a large number of heterojunctions were formed between the anatase phase of TiO₂ nanocuboids and rutile nanorods.The studies have shown that heterojunction can effectively separated electron and hole more at the interface,and then explorde the effect of this mixed-phase heterojunction on the device photoelectric performance.Based on the TiO₂ nanorod/nanocuboid mixed-phase structure,the PCE reached 15.56%,which was equivalent to 1.2 times of that the conventional CBD interface modified method.Meanwhile,the short-circuit current density(J_sc)improved significantly and reached to 21.25 mA/cm².?4?The perovskite film was thermal annealed through the IPA solvent and obtained a high quality film.IPA is a weakly coordinated solvent for a poor solubility of PbI₂ that provides a moist environment for the reaction and a longer diffusion distance for the precursor.Therefore,the grain size of perovskite was increased by IPA solvent annealing.By adding a certain amount of NMP into the IPA solvent,the effects of mixed solvent on the morphology and properties of the perovskite were investigated,and the optimal annealing conditions was IPA/NMP volume ratio of 19:1.The PCE of the device prepared under this condition reach 16.98%,which was 10%higher than that of conventional thermal annealing.The FF of the device has also increased from 0.67 to 0.73.Under the synergistic effect of the weakly coordinated IPA solvent and the high-boiling aprotic NMP solvent,the grain size was twice than that of the conventional annealing.Meanwhile,the film was dense and flat with a high crystallinity.Finally,to observe the long-term stability,the devices were stored in the air atmosphere under the dark condition with a relative humidity of 40%.We found that the best device remained a PCE of 15.4%after 60 days with the relatively slow degradation.It was confirmed that the mixed solvent annealing reduced the grain boundaries by increasing the grain size,effectively alleviated the decomposition reaction of perovskite in the air environment,and finally improved the stability of device.