Effects Of Defect Suppression On The Performance And Stability Of Perovskite Solar Cells

Photovoltaic cells are one of the main ways to use solar energy and have become an important part of the national new energy development strategy.Among them,solar cells（PSCs）based on organic-inorganic hybrid perovskite light-absorbing materials have developed rapidly.Currently,stability is a key factor restricting the commercialization of PSCs,and the stability of PSCs is directly related to the defects existing on the surface and inside of perovskite light-absorbing layer films.Therefore,passivation or reduction of defects existing on the surface and inside of perovskite films and reduction of non-radiative recombination of excitons are not only effective means to improve cell efficiency but also enhance stability.In this thesis,the methods of suppressing surface and bulk defects of perovskite films are developed,and efficient and stable PSCs are obtained by passivation through defects and improving the quality of perovskite films.The detailed research contents are as follows:（1）Fabrication of efficient and stable PSCs by using organic Lewis bases to passivate the surface defects of perovskite absorber films.Three Lewis base passivants with double coordination sites were selected,including 2,2’-bipyridine（BPY）,4-hydroxy-1,5-naphthyridine-3-carbonitrile（DQCN）,thiophene Acyl trifluoroacetone（TTFA）as passivants,and their effects on defect passivation are compared with the tranditional zwitterion passivants phenethylammonium iodide（PEAI）and piperazine hydroiodide（PI）.The results show that the coordination sites of BPY,DQCN and TTFA can form"N-Pb"or"O-Pb"coordination bonds with Pb²⁺on the perovskite surface,which can effectively reduce the Pb²⁺defect concentration,and thereby reducing the nonradiative recombination,and enhancing carrier lifetime.These passivant molecules are anchoring on the perovskite surface and have little effect on the perovskite film grain and surface morphology.Solar cells with an inverted configuration of ITO/Ni O_x/passivated MAPb Cl_xI_3-x/C₆₀/BCP/Ag have been fabricated,and it was found that the photoelectric conversion efficiency（PCE）of the devices using PEAI,PI,BPY,DQCN and TTFA as passivants were 19.80%,18.21%,17.18%,16.80%and 18.49%,respectively,which are enhanced by 20.9%,11.2%,4.9%,2.6%and 12.9%compared to the PCE（16.38%）of the control device（perovskite without passivation）.Under relative humidity of±12%and low oxygen environment,storage for 40 days without encapsulation,the control device,PEAI,PI passivated devices can only maintain 44.6%,61.2%and 55.6%of the initial PCE,while the passivated devices based on BPY,DQCN and TTFA can still maintain 86.8%,83.3%,and 87.1%of their initial PCE,showing significantly enhanced stability.The adsorption energy of these passivating molecules on the perovskite surface is further calculated by simulation,combined with the perovskite surface contact angle test,it was found that compared with the zwitterion passivants PEAI and PI,BPY,DQCN and TTFA contained bulky aromatic substances.The group passivants can effectively improve the surface hydrophobicity of perovskite thin films and has a large adsorption energy,which should be the reason why its device stability is better than that of zwitterion passivants post-treatment devices.（2）Using single crystal precursor solution to improve the quality of the perovskite thin films and fabricate efficient and stable PSCs.The synthesized single crystals instead of the traditional stoichiometric mixture of lead halide and ammonium salt were used to prepare the perovskite precursor solution.The perovskite film was prepared by a one-step anti-solvent method.The results show that,compared with the traditional method,the perovskite films prepared from the single crystal precursor solution have less Pb I₂ residue,and they are more uniform composition with larger grains,and fewer grain boundaries,and smaller surface roughness.These films have stronger fluorescence.These results indicate that the perovskite films prepared from single-crystal precursor solutions have better quality and fewer defects,which can significantly reduce non-radiative recombination loss of carriers.The efficiency of the device prepared based on this method reaches 17.11%,which increased by 4.8%compared to 16.33%prepared by the stoichiometric solution.Under the relative humidity of±12%and low oxygen environment,and stored for 21 days without encapsulation,the device efficiency of the stoichiometric solution can only keep 73.7%of the initial PCE,while the device efficiency of the new method can keep 94.6%of the initial PCE.This proves that reducing the bulk defects of perovskite films can not only improve the device efficiency,but also significantly enhance its environmental stability.Above results show that passivation of surface defect by organic Lewis bases and reduce of bulk defect by using single crystal perovskite to prepare solution can not only improve solar cell efficiency but also significantly enhance device stability.