Composition And Performance Optimization For High-Efficiency Perovskite/Organic Two-Terminal Tandem Solar Cells

Metal-halide perovskites are a kind of semi-conducting material with excellent optoelectronic properties.After more than one decade of rapid development,the world record power conversion efficiency of single-junction perovskite solar cells has skyrocketed to 25.7%,closing to the Shockley–Queisser（SQ）limit efficiency（33.7%）,which hinders their further development.In comparison,perovskite/organic two-terminal tandem solar cells exhibit broader photo-response range and higher SQ limit efficiency（46.1%）.Unfortunately,the current efficiency of perovskite/organic tandem devices still lags behind that of single-junction devices.From the aspect of device configuration,the tandem device consists of three components,including a wide-bandgap perovskite sub-cell,a narrow-bandgap organic sub-cell,and an interconnection layer.Therefore,this study focuses on the optimization of sub-cell performance by component regulation and interface modification.Moreover,through the systematic control of the thickness of intermediate connecting layer components,we have explored the balance mechanism of charge carrier transport in tandem devices,aiming to provide theoretical basis and practical guidance for the design of high-efficiency perovskite/organic tandem solar cells.In ChapterⅡ,we investigated the influences ofβ-GUA（β-guanidinopropionic acid）organic small molecules on the morphology of normal-bandgap FA_0.95Cs_0.05Pb I₃ perovskite and the device performance.Results show that the function of theβ-GUA additive mainly comes from the surface-deep-level defect passivation ability provided by the lone pair electrons on the nitrogen and oxygen atoms.Furthermore,the addition ofβ-GUA modifies the surface morphology of the perovskite film by forming 2D/3D heterojunction structures,ultimately improving the overall device performance.Structural characterization revealed that the 2D/3D mixed phase formed byβ-GUA is embedded in the 3D perovskite grain boundary and is closely connected to the lattice of the 2D perovskite phase,which is mainly n=1 and arranged in a face-on direction.The amino and hydroxyl groups on theβ-GUA molecule act as Lewis bases and coordinate with unsaturated lead ions in the perovskite.The test results showed that the carrier non-radiative recombination in theβ-GUA-modified perovskite devices was significantly suppressed,and the voltage loss（0.14 V）was lower than that of the reference devices（0.176V）,achieving a photoelectric conversion efficiency of 22.2%.Moreover,the working stability of the device was also significantly improved,as it maintained an efficiency of 88%after 400hours of maximum power point tracking,much higher than the reference device’s 10%.The work indicates that the defect passivation at the perovskite grain boundary plays an important role in inhibiting the carrier non-radiative recombination and improving the device performance,providing guidance for us to further develop efficient wide-bandgap perovskite solar cells.In ChapterⅢ,we studied the effect of FA⁺cations on the crystallization kinetics of wide-bandgap perovskite MA_1.06Pb I₂Br（SCN）_0.12 and prepared perovskite/organic tandem devices based on optimized perovskite films.The study found that the introduction of FA⁺ions can improve the solubility of the perovskite precursor solution,thereby promoting the uniform crystallization of the perovskite film and improving the film quality.More importantly,the FA⁺ions can effectively suppress the excessive aggregation of the Pb I₂ phase induced by the SCN^-component,thereby reducing the defect density of the film.After optimization,we obtained a uniformly self-assembled Pb I₂ phase at the perovskite grain boundary,which passivated the surface defects of the film and effectively suppressed the carrier non-radiative recombination.Based on the perovskite film prepared in this work,we achieved a photoelectric conversion efficiency of 17.4%,an open-circuit voltage of 1.19 V,and a fill factor of 78.4%for a single-junction device.Furthermore,by combining this high-performance wide-bandgap front cell with a narrow-bandgap bulk heterojunction（BHJ）organic cell based on PM6:CH1007,we prepared a tandem cell with a high photoelectric conversion efficiency of 21.2%,which was one of the highest efficiencies for perovskite/organic tandem layered solar cells.In this work,the effective passivation strategy of grain-boundary defects of wide-bandgap perovskite was realized,and the related preparation procedure was successfully explored,which laid a solid foundation for our further optimization of the intermediate connecting layer in tandem devices.In ChapterⅣ,we explored the relationship between the design of the interconnection layer and the device performance by studying the effect of the thickness of each component layer（PCBM/BCP/Au/Mo O₃）on the photovoltaic properties of perovskite/organic tandem layered solar cells.The results showed that the carrier transport balance characteristics of the wide-bandgap perovskite sub-cell and the narrow-bandgap BHJ organic sub-cell in the tandem device can be reflected by their relative external quantum efficiency（EQE）intensities.The thickness of the charge transport layer plays a key role in maintaining the carrier modulation ability of the interconnection layer.A thicker electron transport layer will lead to a decrease in the extraction efficiency of the perovskite-side carriers,affect the carrier recombination efficiency,and reduce the EQE intensity of the cell.On the contrary,a thinner electron transport layer will reduce its hole-blocking ability,leading to a degradation in device performance.In addition,we also systematically optimized the thickness of the hole-blocking layer BCP and the hole transport layer Mo O₃.Finally,we successfully prepared perovskite/organic tandem devices using MA_1.06Pb I₂Br（SCN）_0.12 and PM6:Y6 as the active layers of the sub-cells,achieving a efficiency of 20.03%.This work provides a theoretical basis and optimization strategy for designing the interconnecting layer of tandem devices,and lays a foundation for the preparation of efficient perovskite/organic tandem devices with balanced carrier transport properties.In Chapter V,we mainly investigated the effect of the BCP-modified Zn O/BHJ（PM6:CH1007）interface on the performance of narrow-bandgap organic solar cells and successfully fabricated perovskite/organic tandem devices based on the optimized BHJ organic devices.It was found the BCP molecular layer can be used as a passivation material to modify the surface defects of Zn O,suppress defect-induced non-radiative recombination processes,improve carrier extraction efficiency,and thereby reduce the energy loss of the device.We prepared an inverted single-junction organic cell with a photoelectric conversion efficiency of 15.46%based on the optimized ITO/Zn O/BCP/PM6:CH1007/Mo O₃/Ag structure.Additionally,the BCP molecular layer can also block direct contact between Zn O and the organic active layer,avoid the photovoltaic activity of Zn O from damaging the organic layer,and thus improve the stability of the device.Based on the excellent performance of the organic sub-cell,the optimized n-i-p structure perovskite/organic tandem solar cell achieved a high efficiency of 22.43%and an open-circuit voltage of 2.095 V.This work provides a preliminary exploration for optimization strategy of narrow-bandgap organic sub-cells in the tandem devices,and pave the road for the further performance improvement and commercial application of the perovskite/organic monolithic tandem devices in the future.