Research On Passivation Of Perovskite Solar Cells By Organic Ammonium Salts With Enhanced Efficiency And Stability

Organic-inorganic hybrid perovskite solar cells have become a rapidly-evolving photovoltaic technology attracting great interest due to the numerous advantages besides their impressive efficiency,among them are materials and optical tunability and versatility,striking optical properties,or low-temperature processability.Currently,the certified record power conversion efficiency for single-junction perovskite solar cells has reached 25.8%,which is comparable to the well-established photovoltaic technologies,such as silicon-based solar cells.However,the solution-based fabrication processes of perovskite films inevitably result in massive defects at grain boundaries and on the surface of perovskite films,which provide the sites for carrier non-radiative recombination.The presence of extensive defects not only weakens the performance of the device but also hinders the long-term stability of the solar cells.Organic ammonium salts have emerged as one of the most promising additives or surface ligands candidates for improving the performance and long-term stability of perovskite solar cells.The ammonium molecules show great potential to have multi-functionalities and salient advantages via rational tailoring of their molecular structures.Thus,two organic ammonium salts were synthesized and used for the surface and bulk passivation of perovskite films,respectively.In order to obtain perovskite solar cells with high efficiency and good stability,it is very important to optimize the process parameters of the perovskite absorbing layer.In this chapter,the effect of annealing temperature of PbI2 film on its morphology and crystallinity in the first step of the two-step method is studied,and then the effect of annealing humidity of perovskite film on its film formation and crystallinity is studied in the second step.We found that photovoltaic devices based on PbI2 annealing temperature of 60 ℃ and annealing humidity of 30-40%RH for perovskite films have the best performance and stability.On the basis of the preparation of better control group devices,the passivation effect of organic ammonium salts in perovskite solar cells was explored.A large number of defects,such as vacancy defects and uncoordinated Pb2+,exist on the surface of perovskite films.To this end,we designed and synthesized methylhydrazine iodide(MHyI)as a surface passivator and spin-coated a thin layer on the surface of the perovskite film.It was found that the MHyI molecules not only filled the iodine vacancies and inhibited the generation of A-site cation vacancies,which greatly reduced the defects,but also distributed on the surface of the perovskite to facilitate the carrier transport.The results show that the addition of MHyI passivation layer on the perovskite layer of the perovskite solar cell improves the Voc and FF,increases the efficiency to 23.19%with negligible hysteresis,and improves the hydrophobicity of the passivated perovskite film.The resulting device was able to maintain 87%of the original efficiency after 30 days of aging in the external environment.Grain boundaries act as a deep energy level defect in perovskite solar cells,and although they do not impede carrier transport,they produce rapid nonradiative recombination at the grain boundaries and can also act as a channel for ion migration in the film,which is detrimental to the long-term stability of the device.For this reason,we designed and synthesized fluoropolymer ammonium salts(FPEI·HI)as additives in PbI2 precursor solutions.We found that the fluorine and amine groups in the FPEI·HI molecule can interact with cation and halogen anion in perovskite through hydrogen bonding,respectively,and the hydroxyl group in the FPEI·HI molecule can interact with uncoordinated lead through Lewis acid-base reaction.Based on the above-mentioned interaction relationships,we obtained large grain size and grain boundary-less perovskite films.With this simple grain boundary passivation,the open-circuit voltage and fill factor of the devices are enhanced,while the efficiency of the devices is as high as 24.29%and the devices exhibit no hysteresis due to the anchoring effect of the molecules.Benefiting from the perfluoroalkyl chains in the molecule,the environmental stability of the device is substantially improved,and the unencapsulated device can maintain an initial efficiency of 89%after 2500 h of exposure to ambient conditions(20%-30%RH).