Crystallization Control And Voltage Improvement Strategies In Printable Mesoscopic Perovskite Solar Cells

Developing new energy resources to replace traditional fossil energy resources is the necessary way to solve the energy crisis and environmental pollution,and it is also a strategic requirement for national development.Developing solar cell technology is one of the effective methods.In recent years,thanks to the excellent photoelectric properties of metal halide perovskite materials,the power conversion efficiency（PCE）of perovskite solar cells（PSCs）has broken through to 25.8%,demonstrating broad application prospects.Among them,the hole-conductor-free printable mesoscopic perovskite solar cells（p-MPSCs）based on the inorganic scaffolds of mesoporous titania（mp-TiO₂）,mesoporous zirconia（mp-ZrO₂）,and porous carbon electrode have attracted much attention due to their excellent stability and and ease of large-area fabrication.However,due to the complex mesoporous structure,the crystallization of perovskite materials is difficult to control and the devices are accompanied by severe open-circuit voltage(V_OC)loss,which limits the further improvement of PCE.In this thesis,a series of studies have been conducted on the crystallization control of perovskite and the reduction of V_OC loss in p-MPSCs.Based on the two-step deposition process and supported by perovskite crystallization control,the V_OC loss are reduced by gradient self-doping strategy and in-situ sulfidation post-treatment strategy.The main contents are as follows:Two-step method is used to control the critical parameters for the crystallization of formamidinium lead triiodide（FAPbI₃）perovskite material in the mesoporous structure,including the temperature of the mesoporous film substrate and precursor solution,the thickness of mp-ZrO₂ layer,and component engineering.The pre-heated substrate and precursor solutions significantly increase the V_OC and short-circuit current density(J_SC)of the devices,indicating that the deposition process of lead iodide（PbI₂）is critical for the crystalline quality of perovskite.The decomposition of the PbI₂-dimethyl sulfoxide（DMSO）intermediate phases is promoted and the residual PbI₂ is reduced by decreasing the thickness of mp-ZrO₂ layer.Finally,cesium iodide（CsI）is introduced into the PbI₂precursor solution to inhibit the formation of the non-perovskite phase,and a PCE of 13.9%is achieved.However,the pore filling and crystalline uniformity of perovskite still need to be improved.To address the issue of poor crystallization and phase impurity of FAPbI₃ perovskite in the two-step method,the morphology and crystallinity of PbI₂ are finely controlled by introducing nonhalide lead precursor,lead acetate（PbAc₂）,which controls the crystallization and pore filling of perovskite in the mesoporous structure.The low crystallinity,smaller grain size and appropriate porosity of PbI₂ films improve the pore filling of perovskite in the mesoporous films,enhance the crystallinity of perovskite and promote the charge carrier extraction at the perovskite/TiO₂ interface.Further,by introducing cesium bromide（CsBr）in the mixed lead precursor,the FAPbI₃ perovskite phase can be obtained at 70℃or even at room temperature and a PCE of 16.24%is achieved.This demonstrates the possible advantage of mesoscopic structure in stabilizing the FAPbI₃ perovskite phase.Based on the unique device structure of p-MPSCs,the transport of photogenerated holes generated in the mp-TiO₂ layer to the carbon electrode is dominated by the diffusion-assisted charge carrier movement,while the driving force for the oriented transport of charge carriers is insufficient,which leads to severe nonradiative recombination loss,resulting in low V_OC of the devices.Exploiting the differences in crystallization and pore filling of perovskite in different porous layers,an optimized two-step method is developed to construct the gradient component difference,that is,the work function differences of perovskite in different layers based on p-MPSCs,which reinforce the built-in electric field inside the device,thus promoting oriented transport of the photo-induced carriers and reducing the recombination loss.Based on this strategy,an average V_OC improvement of more than 60 mV and a PCE of 17.68%are obtained,while the stability of the device remains to be improved.In order to balance the efficiency and stability of FA-based p-MPSCs,a facile post-treatment method for in-situ sulfidation of perovskite layers is developed based on the above strategy of control the work function of perovskite to reduce V_OC loss.Through the post-treatment,lead sulfide（PbS）is formed in-situ in p-MPSCs,which broadens the spectral absorption range of perovskite material and significantly enhances the light absorption of the devices.In addition,the introduction of PbS significantly increases the work function of perovskite,enhances the built-in electric field inside the device and accelerates carrier transport and extraction,resulting in a PCE exceeding 19%.In addition,the hydrophobic property of PbS and its lattice matching with perovskite significantly enhanced the storage stability of the devices,with no significant performance degradation during 235 days of storage.