研究目的
To investigate the role of isopropyl alcohol (IPA) solvent in the synthesis of organic-lead-halide perovskite CH(NH2)2PbIxBr3?x [FAPbIxBr3?x] thin films including the effect of I/Br composition ratio by the two-step reaction of an amorphous (a-)PbIxBr2?x layer and FAIxBr1?x solution diluted in IPA.
研究成果
1. The effect of Br incorporation into FAPbI3 perovskite films is to promote the densification of perovskite network, resulting in the increase in the free volume as a void and film thickness. In addition, prominent sub-gap absorption tail in the 3:2 film compared to that of 3:1 film suggests that the defect originates from the increased grain boundary of perovskite phase for larger Br compositional ratio. 2. The role of IPA solvent in the synthesis of FAPbIxBr3?x perovskites from an a-PbIxBr2?x thin layer and a solution of FAIxBr1?x in IPA solvent was investigated using SE combined with characterization by XRD and FTIR analysis. IPA played a significant role to promote crystallization of the a-PbIxBr2?x film through the removal of coordinated CPH from the a-PbIxBr2?x network, accompanied by the formation of grain boundaries, voids, and surface roughness. The diffusion of FAIxBr1?x into the voids and/or grain boundaries resulted in the simultaneous formation of large amounts of free volume and an increase in the film thickness, resulting in a decrease in the refractive index. The red shift of absorption edge from 3.4 to 1.73 eV is attributed to a phase transition from 0D to 3D [PbX6]4? octahedral clusters. These factors are the main contributors to the formation of the FAPbIxBr3?x perovskite network.
研究不足
The instability of MAPbI3 with respect to humidity, temperature, and light soaking is a serious problem hindering long-term reliability of the photovoltaic performance.
1：Experimental Design and Method Selection:
Spectroscopic ellipsometry (SE) was used to investigate the role of IPA solvent in the synthesis of FAPbIxBr3?x perovskites. An optical dispersion model was developed to extract the complex refractive index N (=n + ik), optical transition, and film thickness of FAPbIxBr3?x perovskites by SE analysis at different I/Br composition ratio as a function of immersion time in a solution of FAIxBr1?x diluted in IPA.
2：Sample Selection and Data Sources:
A-PbIxBr2?x thin films with a film thickness of around 150 nm were prepared on glass by spin coating a mixture of FAIxBr1?x and PbIxBr2?x powder (PbI2:PbBr2 = 3:1 and 3:2 molar ratio) in a DMF/CHP (95:5, v/v) cosolvent at 3000 rpm for 30 s.
3：List of Experimental Equipment and Materials:
Phase-modulated spectroscopic ellipsometer (UVISEL, Horiba Jobin Yvon), X-ray diffraction (Bruker D8 ADVANCE ECO), Fourier-transform infrared spectroscopy (Shimadzu spectrometer, IRTracer-100), field emission scanning electron microscopy (FESEM; S4800, Hitachi High Technologies, Japan) with energy-dispersive X-ray spectroscopy (EDX) (Bruker XFlash 5030/Quantax 400), and atomic force microscopy (AFM; NanonavieII/SPI-3800, Hitachi High-Tech Science, Japan).
4：Experimental Procedures and Operational Workflow:
A-PbIxBr2?x films were immersed in a solution of FAIxBr1?x (FAI:FABr = 3:1 molar ratio) in IPA solvent for various times (tim) at 25°C. Finally, the FAPbIxBr3?x films were rinsed with IPA to remove the residual solution and contaminants from the film surface.
5：Data Analysis Methods:
The refractive index n and extinction coefficient k of the corresponding FAPbIxBr3?x films were determined using a phase-modulated spectroscopic ellipsometer. The ellipsometric angles, Ψ and Δ, which determine the complex reflection coefficient ratio, ρ = tanΨ e iΔ, were also measured for 71 points in the range of 1.5–5.0 eV (0.05 eV step) with an integration time of 200 ms at each photon energy.
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