Construction Of Efficient And Stable CsPbI₃ Photoelectric Conversion Devices

Recently,inorganic CsPbI₃ perovskite solar cells（PSCs）show excellent photoelectric properties and good thermal stability,which received extensive attention.However,due to the small radius of Cs⁺（1.88?）,the tolerance factor（t）（～0.8472）of CsPbI₃ deviates significantly from the ideal value（0.9-1.0）,resulting in black phase-based CsPbI₃ will spontaneously transform into yellow non-perovskite phase（δ-CsPbI₃）at room temperature,which is still a difficult problem to solve.At present,constructing a two-dimensional（2D,n≤5,where n value is the number of layers of the inorganic plate）or quasi-2D（n>5）CsPbI₃ structure is an effective and promising strategy to pursue higher phase stability.Compared with three-dimensional（3D）CsPbI₃,the large organic cation in 2D or quasi-2D CsPbI₃ can act as barrier molecules to resist water erosion,and produce steric hindrance to prevent or inhibit phase transitions and ion migration.However,high power conversion efficiency（PCE）and long-term stability are considered incompatible in 2D or quasi-2D CsPbI₃ PSCs.Current researches are mainly based on quasi-2D CsPbI₃ PSCs,since the PCE exceeds 15%to meet the commercial application requirements,but the long-term stability is limited.On the contrary,2D CsPbI₃ PSCs have outstanding stability,but their highest PCE reported is only 9.49%.Therefore,optimizing the performance of 2D CsPbI₃ PSCs urgently needs to be addressed.Although CsPbI₃ has a suitable bandgap（<1.73 e V）,it still exhibits a narrow spectral response and can only use ultraviolet-visible light（300 nm-800 nm）,nearly 46%of the infrared light（IR）is directly wasted.Therefore,how to improve the utilization of the entire spectrum is a problem that needs to be faced to optimize the device performance.This thesis focuses on optimizing the PCE of 2D CsPbI₃ PSCs and 3D CsPbI₃ PSCs insufficient spectral utilization,the specific work is as follows:1)By introducing phenylethylammonium cation（PEA⁺）as steric hindrance,the2D Ruddlesden-Popper（RP）（PEA）2（Cs）n-1Pbn I₃n+1 exhibits much stronger phase stability than 3D CsPbI₃.However,an uncontrollable crystallization process leads to poor coverage and unfavorable phase management of（PEA）2（Cs）n-1Pbn I₃n+1 film,resulting in low PCE（<10%）and poor stability of the related PSCs.Here,we propose an underlying surface engineering（USE）method,which improves the wettability of the substrate and promotes the diffusion of the precursor solution to fabricate a high-quality film with high coverage and low defect density.Further characterizations confirm that this method enables a more uniform phase distribution and achieves an orderly arrangement of small-n and large-n phases from bottom to surface in film,which contributes to effective charge transfer to enhance photocurrent transmission and extraction.As a result,the PCE of（PEA）2Cs3Pb4I13 PSCs was boosted from an initial9.03%to a record value of 15.92%,accompanied by enhanced stability.This method also has versatility in other RP and Dion-Jacobson（DJ）types of 2D CsPbI₃ PSCs,paving a broad road for its commercial application in the future.2)Due to quantum and dielectric confinement,2D CsPbI₃ structures usually exhibit large exciton binding energy,and it is difficult to eliminate the energy barriers between inorganic and organic layers in charge tunnel,resulting in poor carrier transport.Here,a class of polycyclic aromatic ammoniums（1-naphthylamine（1-NA））was developed as a spacer for 2D RP CsPbI₃ PSCs.Theoretical simulations and experimental characterizations reveal that the 1-NA spacers with extendedπ-conjugated lengths can reduce the dielectric mismatch,lower the exciton binding energy and promote efficient charge separation compared with traditional single-ring aromatic molecules.Moreover,it also has strong hydrogen bonding（NH...I）and molecular interactions,which can promote perovskite crystallization,yielding high-quality and preferably crystal-vertically oriented films,resulting in lower defect density and directional charge transport.And the 1-NA spacer has a larger contribution in the conduction band,which is also beneficial to carrier transport.As a result,the（1-NA）2（Cs）3Pb4I13-based device exhibits 16.62%excellent performance with enhanced stability.3)In 2D CsPbI₃ PSCs,the insulating properties of the organic spacer are also unfavorable for carrier transport,and seriously reduce the weak interaction of the 2D CsPbI₃ components,which is not conducive to crystal orientation growth and charge transport.Therefore,we comparatively studied the composition and chain length differences of an organic spacer and developed a novel phenylhydrazine（PHA⁺）spacer rich in amino groups（NH）to realize the regulation of crystallization rate and carrier characteristics of 2D CsPbI₃ films.Combined with experimental characterizations,it is demonstrated that the crystallization rate of the films is accelerated due to the enhanced hydrogen bonding（N-H?I）interaction between the PHA⁺spacer and the inorganic layer.It helps to obtain large-grain-sized,dense films with significantly reduced trap state density,resulting in efficient charge carrier transport.Finally,the（PHA）2Cs4Pb5I16-based device achieves high performance of 16.23%and shows good environmental stability.4)To better utilize the IR region in sunlight to photovoltaic devices（PVs）,it has been proposed to improve the performance by upconversion nanoparticles（UCNPs）.However,researchers recently have found that the upconversion（UC）effect is negligible in PVs performance improvement for their ultra-low UC photoluminescence quantum yields of UCNPs solid film,while the real reason for UCNPs in PVs has not been studied.Here,we deeply explored the potential reasons why UCNPs optimize the performance of PVs through experiments and theoretical calculations.Based on the material inorganic perovskitesγ-CsPbI₃,Na YF₄:20%Yb³⁺,2%Er³⁺UCNPs were integrated into different transport layers to optimize the PCE of PVs.Compared with the reference device,the short-circuit current density and PCE of the optimized device reached 20.87 m A/cm~2（20.39 m A/cm~2）and 18.34%（17.72%）,respectively,without sacrificing open-circuit voltage and filling factor.Further experimental characterization confirmed that the improved performance was attributed to enhanced visible light utilization rather than IR.To theoretically explain the statement,the light field distribution in the device was simulated and the absorption of each layer was calculated.The results revealed that the introduction of UCNPs with different refractive indices from other layers induces light field disturbance and enhances the capture of visible light byγ-CsPbI₃.