The Research On The Performance Of Formamidine Perovskite Solar Cell Passivated With Organic Salts

As one of the most promising new-generation photovoltaic devices,organic-inorganic metal halide perovskite solar cells have achieved a photoelectric conversion efficiency（PCE）of>25%in just ten years.The excellent photoelectric properties of perovskite materials are mainly derived from adjustable bandgap,high optical absorption coefficient,low exciton binding energy and long carrier diffusion length.Among many perovskite materials,the formamidine lead iodide（FAPbI₃）perovskite has become the focus of research in the photovoltaic field due to its suitable band gap and high theoretical conversion efficiency.The efficiency of the perovskite perovskite cells based on FAPbI₃material has reached 25.2%,but the defects of thin film and energy level mismatch limit the further breakthrough of efficiency.Besides,the stability of formamidine-based solar cells is poor,and they are prone to chemical degradation and structural degradation in light,humidity and high temperature environments.Based on the current development status and trends in the field of perovskite solar cells,this paper focuses on the photoelectric conversion efficiency and stability of formamidine-based perovskite cells.The performance of the charge transfer layer and perovskite active layer was improved by optimizing the preparation technology of the formamidine-based cell.Small organic salts are used to improve the efficiency and stability of devices.The main research contents of this paper are as follows:The performance optimization of the charge transport layer and the active layer of the formamidine-based perovskite solar cells was carried out.The effects of the thickness of SnO₂layer on the device performance was studied.The best photoelectric performance is obtained at a speed of 3700 rpm and SnO₂:H₂O is 1:6,short-circuit current and filling factor of the devices are reduced due to the excessive thickness of the SnO₂layer,which is mainly due to the reduced absorbance and the increased interfacial contact resistance.The impact of surface modification of SnO₂layer on device performance was studied,the SnO₂layer is subjected to plasma treatment for 30s at 100%power and 30%air flow,which can not only ensure the complete spread of the subsequent perovskite solution,but also improve the charge transport performance of the SnO₂layer.The oxidation conditions of the hole transport layer have been optimized,the best photoelectric conversion efficiency of 18.9%is achieved when the concentration of Co salt is 4 mg/mL and the air oxidation time is 8h.In order to reduce the defects of the perovskite film,DMSO was used to improve the crystal quality of the perovskite.DMSO as an additive is beneficial to increase the grain size of the film,the average grain size of the perovskite film increases to 1.46μm with the addition amount of 10%.The influence of MACl on the quality of the perovskite film and the stability of the device were studied.The results show that 10%MACl can not only stabilizeαphase,improve the quality of perovskite film,but also increase the photoelectric conversion efficiency to 19.25%.The effect of excessive PbI₂on the film quality was explored,the best passivation effect of perovskite film is obtained when the excess PbI₂was5%.The influence of the atmosphere of the second annealing of the perovskite film on the efficiency was studied.The second step annealing in air with humidity of 15%can achieve a photoelectric conversion efficiency of more than 20%,and the device has good repeatability.The appropriate amount of moisture assists can improve the crystal quality of perovskite film,and finally improve the photoelectric performance of the device.In order to improve the stability of the formamidine device,a small organic salt,namely 4-chlorobenzamidine hydrochloride（CBAH）as a passivation agent,was introduced to the surface of the perovskite film,and formamidine perovskite devices without Cs⁺and Br^-doping were prepared by a one-step method.The influence of CBAH on the properties of perovskite film and device performance was explored.Based on the analysis of the surface morphology and crystallization properties of perovskite as well as the electronic characteristics of XPS,it is found that CBAH interacts with excessive PbI₂ on the surface of perovskite,and the para-substitute structure of CBAH helps oriented crystallization to form nanorods,which effectively fills the grain boundaries and reduces the number of defects in the perovskite film.The optical properties and carrier dynamics analysis of the perovskite film shows that the fluorescence intensity of the CBAH-treated perovskite film decreases more slowly,indicating that the defect density of CBAH-treated films is reduced and the energy loss induced by non-radiation recombination is reduced.Analysis of the electrical and photovoltaic performance of the device shows that the passivation layer formed by directional crystallization helps to enhance the extraction and transport of holes,increase the open-circuit voltage and fill factor,and achieve a photoelectric conversion efficiency of 21.75%.At the same time,the hydrophobic properties of the passivation layer and the repair of the defects of the perovskite film are beneficial to improve the illumination,thermal and moisture stability of the device.These results broaden the selection of organic salts for interface optimization.In order to achieve efficient and stable formamidine-based perovskite solar cells,a small organic salt,namely cyclopropyl formamidine hydrochloride（CPAH）,was used to construct 2D/3D perovskite.Different from the currently reported molecules,the steric hindrance effect of cyclopropyl is conducive to the formation of 2D perovskite structure,and the relatively short-chain organic cations of CPAH can reduce the insulating effect of long chains to ensure effective charge transfer.The morphology,crystallization properties and XPS electronic properties of the perovskite films at different etching depths prove that the interaction between CPAH and PbI₂,and 2D perovskite is formed with annealing treatment.Based on the carrier dynamics analysis of the perovskite film,it is found that the 2D/3D perovskite film has a longer fluorescence lifetime,which indicates that the 2D perovskite layer can repair the traps in the perovskite films and suppress charge nonradiative recombination.The analysis of the energy level structure of 2D/3D perovskite film shows that the construction of 2D/3D structure achieves a more appropriate energy level gradient,which accelerates the extraction and transmission of holes and reduces the energy loss.The photoelectric conversion efficiency of the 2D/3D formamidine-based device is22.81%,and the open-circuit voltage is 1.16V,the optimal voltage loss is 0.34 eV,which is one of the best results of formamidine-based perovskite solar cell without Br and Cs doping.The stability of unencapsulated devices was tested under high temperature,high humidity and continuous illumination conditions.The results shows that the photoelectric conversion efficiency of CPAH-treated devices remained above 70%after storage in different environments for 150 hours,compared with the control device（<50%）,the illumination,thermal and moisture stability has been significantly improved.