Research On Regulating The Function Layers And Stability Of Perovskite Solar Cells With Modified Graphitic Carbon Nitride

To date,the certified power conversion efficiency（PCE）of the perovskite solar cells（PSCs）has approached the Shockley-Queisser（S-Q）limit maximum theoretical efficiency,yet the development of PSCs in terms of stability still lags far behind.The instability of PSCs mainly arises from each functional layer and the involved interfaces,especially concentrating in the perovskite absorber layer,2,2’,7,7’-tetrakis（N,N-di-p-methoxyphenylamine）-9,9’-spirobifluorene（Spiro-OMeTAD）-based hole transport layer and interface.The defects at the perovskite films,the side effects of doping Spiro-OMeTAD,and the resulting interface contact problems are the main reasons for stability loss.Graphitic carbon nitride（g-C₃N₄）materials possess excellent electronic structure,physical and chemical properties,and stability,and can be stably dispersed in PSCs solution systems.Therefore,this subject synthesizes the modified g-C₃N₄ with multiple active sites such as in situ non-metal doping,oxide loading,and metal doping,and introduces them into functional layers as additives to improve the crystallization of perovskite,regulate the doping of Spiro-OMeTAD and optimize the interface contact,to achieve high performance and high stability devices.Aiming at the crystallization and defects in perovskite films,the iodine-doped graphitic carbon nitride（g-CNI）was synthesized by in-situ thermal polymerization using dicyandiamide（DCDA）as precursor and ammonium iodide（NH₄I）as heteroatomic doping source.The modified graphitic carbon nitride is used as an additive to modulate triple-cation perovskite film.It was found that the iodine atom and pyridine nitrogen atom could synergistically modulate the triple cation perovskite crystallization and multi-point passivate the defects at the surface and grain boundaries of the films.g-CNI can improve the crystallinity of perovskite films,and obtain the compact perovskite films with large grains and few grain boundaries,as well as reduce the trap states density and increase the carrier lifetime.After doping with g-CNI,the optimal efficiency of PSCs is increased from 16.57%for the control to 18.28%.Importantly,the corresponding unencapsulated devices maintained more than 80%of the initial PCE after 700 h of storage under atmospheric environment（25°C,30%RH）and thermal stress（85°C,N₂ atmosphere）.In view of the side effects caused by Spiro-OMeTAD doping and interfacial contact problems,the composites of CNVx were constructed with graphitic carbon nitride nanosheets loaded with V₂O₅ nanoparticles.The high valence V₂O₅ enabled quantitative oxidation of Spiro-OMeTAD under the inert atmosphere,and increased the film conductivity and hole mobility nearly two times.The pyridine nitrogen sites in CNVx can passivate the defects at the interface with perovskite,thereby enhancing carrier transport and suppressing recombination.CNVx doping enhanced the optimal PCE of PSCs to 21.10%,with a higher fill factor of 0.8.Benefiting from the synergistic effect between the regularly arranged pyridine nitrogen in g-C₃N₄ and the layered crystal structure in V₂O₅,the lithium-ion migration was significantly suppressed in the doped PSCs.Meanwhile,the planarπ-conjugate stacking of amorphous CNVx nanosheet could regulate the crystalline state of Spiro-OMeTAD and suppress the undesired glass transition.The constructed devices based on CNVx obtained better humidity stability and thermal stability,maintaining 82%of the initial PCE even after storing or heating for 720 h,respectively.To solve the problems of component migration in the Spiro-OMeTAD layer and the ion consumption of perovskite at the interface,the preparation of nanosheets was further optimized to avoid the complicated preparation process and low yield of the top-down strategy.The Co（III）ion-grafted ultrathin graphitic carbon nitride（Co（III）-CN）nanosheets with a thickness of 2-3 nm were synthesized using a bottom-up strategy with molecule self-assembly process,and utilized as an additive in Spiro-OMeTAD.The Co（III）-CN with matching redox properties could achieve controllable oxidation of Spiro-OMeTAD under the inert atmosphere,as well as promote the reduction of iodine defects at the interface with perovskite,thus enhancing the electrical properties of Spiro-OMeTAD and effectively suppressing the consumption of ions migration in perovskite.The Co（III）-CN passivated the interface with perovskite and ensured efficient carrier transport,thus the optimal PCE of the doped device reached 23.01%.While passivating the interface,the coordination interaction in theπ-conjugated structure and pyridine nitrogen effectively inhibited lithium ion migration and t BP volatilization,and t BP volatilization time obtains quantitative analysis,which improved the film quality and stability.Benefiting from the excellent heat resistance and structural stability of Co（III）-CN,the constructed films and unencapsulated devices achieved excellent humidity stability and thermal stability.The device stability could be up to 1200 h in atmospheric environment at 30%RH and 720 h in N₂environment at 85°C thermal stress.