Study On Film Formation And Humidity Stability Of High Performance Perovskite Solar Cells

Organic–inorganic hybrid perovskites have spurred scientists'great interests due to the wide absorption spectra,high absorption coefficients,long diffusion length and low nonradiative recombination.Perovskite solar cells?PSCs?employ the organic-inorganic hybrid perovskites as the absorber show great advantages for potential application,such as high efficiency,solution processing and low cost.However,the perovskite fabrication process causes undesirable film morphologies with low coverage upon the carrier transporting layer,leading to severe recombination and poor photoelectric conversion efficiency?PCE?.Moreover,the decomposition of perovskite under humidity hinders the long-term stability of PSCs.Here,we studied the effect of perovskite coverage on the carrier transport and devices performance by controlling the film formation process.Except chasing high performance,we carefully investigated the degradation mechanism of single-cation perovskites and mixed perovskites,and improved the humidity stability by compositional and interfacial engineering to fabricate high-efficiency and stable PSCs.The main results and conclusions were as follows:?1?The low coverage of the perovskite film on the mesoporous TiO₂ film would cause the direct contact of TiO₂ and the hole transport layer,leading to severe recombination.By controlling the concentration of PbI₂ solutions in the sequential deposition process,the coverage of the perovskite layer on the mesoporous TiO₂ film was effectively improved.The increase of the coverage enhanced light absorption and scattering,resulting in enhanced light harvesting.Meanwhile,due to the decreased recombination,the diffusion coefficient was also increased,which facilitated carriers transport and collection.As a result,the PCE of as-fabricated devices was significantly improved,with the photocurrent increased from 15.2 mA cm^-2 to 20.6 mA cm^-2.?2?Formamidinium lead iodide?FAPbI₃?perovskites suffer from serious phase transition from black?-phase FAPbI₃ into yellow non-perovskite?-phase and further decomposition under the ambient condition.Incorporating phenylethylammonium iodide?PEAI?retarded the degradation due to the steric effect and hydrophobicity of the phenrthyl group,which helped prevent moisture from interacting with perovskites.Moreover,PEA⁺assembled at the grain boundaries raised the phase transition energy,hindered the lattice distortion and enhanced the phase stability.The excess I^-counterion also passivated the surface defects of perovskites and improved the device performance from 14.1 to 17.7%.After stored in the ambient air for 16 days,the device still retained90%of its initial PCE,indicating the improved humidity stability by PEAI incorporation.?3?We unveiled the susceptibility of the mixed perovskites to high-level humidity and provided further understanding for its moisture-induced degradation process.A rapid degradation after several hours'storage in an environment with a relative humidity?RH?of 70%was indicated by the appearance of CsPbI₃ phase along with needle-like morphology.To address this issue,we rationally introduced a feasible interfacial modification with PEAI as a molecular ligand onto the compositional perovskite film.The hydrophobicity of PEA⁺increased the contact angle of water on the film surface and hindered the interaction between moisture and perovskites.Excess iodide compensated the absence of iodide at the unsaturated lead atoms,effectively reduced the recombination and enhanced the photovoltaic performance of its derived PSCs?from 13.1%to 17.1%?.Finally,the device with PEAI treatment retained 60%of its initial PCE after kept in 70%RH for 100 h,demonstrating the effectiveness of the PEAI treatment for PSCs'stability improvement under high-level humidity.