The Study Of Photoactive Layer For Highly Efficient And Stable Perovskite Solar Cells

Renewable energy would be one of the solutions to address the issues of the energy crisis and air pollution.Therefore,it is urgent to explore efficient ways to improve the utilization of renewable energy and to increase the proportion of renewable energy in electricity generation.Among different types of renewable energy,solar is a clean,abundant,and widespread energy source,which can be converted into electricity by the photovoltaic devices.Perovskite solar cells(PSCs)as the third generation of photovoltaic technology has unique features of high conversion efficiency,low cost,and scalable fabrication,which has been a hot topic in the field of photovoltaic research.The perovskite structured photoactive layer was used as the photo absorption component in PSCs.Upon the strike of the photon on the perovskite layer,photoexcited electrons are generated and then transported to the interface between perovskite and charge transport layer,which are then collected by the electrode.The film quality,crystallinity,and interface of the perovskite photoactive layer have a significant impact on the charge carrier dynamics such as light absorption,carrier separation and transfer.In this research work,we focused on the fabrication,photoelectric property,and stability of the perovskite photoactive layer.First,we used the additive strategy to modulate the crystallization of perovskite film to achieve high quality and well-crystallized perovskite layers;Next,post-treatments were employed to optimize the morphology and passivate the trap states of the perovskite layer;Besides,due to the poor stability of perovskite layer,were used the composition engineering to improve the chemical and phase stability,aim to enhance the durability of PSCs.We systematically studied the fabrication,interface,and application of the perovskite photoactive layer,to improve both photovoltaic performance and stability.The innovative achievements of this work are summarized below.1.We used graphitic carbon nitrides(g-C₃N₄)prepared by the thermal decomposition of urea to modify the perovskite layer and investigated the influence of g-C₃N₄ concentration on the photovoltaic and stability of perovskite.We employed XRD,SEM,and XPS to determine the crystal structure and chemical composition and measured the morphology,crystallinity,and conductivity of g-C₃N₄.The results indicate that the induced g-C₃N₄can retard the crystallization process,which not only leads to the increase of perovskite grain size but also passivate the trap states.In addition,g-C₃N₄ can significantly increase the photoelectric conversion efficiency of perovskite solar cells,which facilitates the charge carrier transport in the perovskite layer.2.Due to the large ion size of FA compared with MA,FAPbI₃ has a much better thermal and environmental stability but shows inferior phase stability.In order to address the phase stability issue of FAPbI₃ perovskite,we investigated the impact of the fabrication method and the composition on the stability of FAPbI₃ perovskite.We found the two-step deposition method is favorable to the crystallization of FAPbI₃ perovskite and we discovered that the?-FAPbI₃ can be stabilized by employing the PbI₂ complex.Three types of PbI₂ complex,PbI₂(DMSO)?PbI₂-DMSO and PbI₂-NMP were fabricated,with the solvent intercalated between the(001)face of the PbI₂ complex.The induced internal strain can facilitate the crystallization of FAPbI₃ perovskite.In addition,the perovskite composition was modulated by introducing a small percentage of MABr to reduce the tolerance factor,which can significant stabilize the?-FAPbI₃.Combining the fabrication and composition strategy,we further optimized the annealing temperature and annealing time and fabricated highly efficient and stable PSCs.3.With the high performance and stability of FAPbI₃ perovskites,we further explored the application of FAPbI₃.We discovered a novel chemical reaction between FAPbI₃ and methylamine(MA)gas.When the FAPbI₃ film was placed under the exposure of methylamine(MA)gas,it turned into a bleached transparent film immediately.We characterized the bleached transparent film by measuring the transmittance,crystal structure,and electrical property.This transparent film showed high transmittance in the visible light range,and strongly absorbed UV light below 400 nm range.We further measured the conductivity of the transparent film and found the excited charge carrier can be effectively transferred to the Ti O₂ electron transport layer.We used the bleached FAPbI₃ film to fabricate a transparent solar cell device,which showed a short-circuit current density(J_sc)of 0.47m A/cm²,open-circuit voltage(V_oc)of 0.75 V,fill factor of 58.6%,and power conversion efficiency of 0.21%.4.In order to further increase the conversion efficiency of PSCs,a tandem solar cell with FAPbI₃ based PSC working as the top cell and crystalline silicon as the bottom cell was fabricated and studied.The tandem solar cell can effectively increase the conversion efficiency by improving the utilization of the solar spectrum to reduce thermal loss.We tuned the bandgap of FAPbI₃ based perovskite by tailoring the composition for obtaining a bandgap that can match with the bottom silicon solar cell.Through the theoretical calculation,the FAPbI₃ perovskite with a bandgap of 1.65-1.70eV can generate a similar current with the silicon solar cell.Therefore,MABr was added to tune the bandgap;at the same time,the additive of MABr can stabilize?-FAPbI₃.When the concentration of MABr reached FAI:MABr=25:75,a bandgap of 1.65eV can be obtained for the mixed composition FAPbI₃perovskite.With the optimized perovskite layer,the top cell can reach a conversion efficiency of 14.18%,and the silicon bottom cell obtained an efficiency of 6.7%,generating a conversion efficiency of 20.88%for the four-terminal perovskite/silicon tandem solar cell.