1. Introduction

Perovskite solar cells (PSCs), as an emerging technology in the field of new energy in recent years, use perovskite type organic metal halide semiconductors as light absorbing materials and belong to the third generation of solar cells. Due to its narrow bandgap, perovskite solar cells are able to absorb a wider spectral range of sunlight, thereby achieving higher energy conversion efficiency (PCE).

Among numerous perovskite materials, lead iodide (FAPbI)3Molecular formula CH5N2PbI3）Compared to traditional methyl lead iodide (MAPbI)3）Having a narrower bandgap, superior thermal stability, and stronger light absorption ability, it is regarded as a representative of the third-generation perovskite photovoltaic materials, demonstrating significant application potential.

Figure 1. Schematic diagram of the structure and operation mechanism of perovskite solar cells

II FAPbI3crystal structure

FAPbI3It is a polycrystalline crystalline material that exists in four crystal phases over a wide range of phase transition temperatures (Tc), including the cubic photoactive phase (α - FAPbI)3）Hexagonal non photoactive phase (δ - FAPbI)3）And two low-temperature photoactive phases, namely tetragonal phase (β - FAPbI)3）Orthogonal phase (γ - FAPbI)3）. However, cubic photoactivity is in a thermodynamic metastable state and only forms at phase transition temperatures above 390 K. When the temperature is higher than Tc, the entropy gain caused by the rotational motion of FA cations reduces the Gibbs free energy of the cubic phase to much lower than that of the hexagonal phase, resulting in the formation of the cubic phase. When cooled to room temperature, FA cations preferentially orient in the hexagonal phase, and the cubic phase with higher Gibbs free energy can easily overcome the phase transition energy barrier and spontaneously transform into the low-energy hexagonal phase. In efficient and stable perovskite photovoltaic devices, it is necessary to suppress the transition from cubic phase to hexagonal phase.

Figure 2. FAPbI3Schematic diagram of crystal structure (a): cubic alpha phase connected at the top; (b) Hexagonal delta phase with coplanar connection

3、 Application of XRD in Perovskite Thin Film Materials

XRD (X-ray diffraction) is a core characterization method for the study of perovskite thin film materials. By accurately measuring the position and intensity of diffraction peaks, the crystal phase of perovskite (such as photoactive alpha phase and inactive delta phase) can be quickly identified, the purity of phases can be quantified, and the phase transition kinetics under annealing, light irradiation, and other conditions can be tracked; By combining peak shape analysis (such as full width at half maximum and strain shift), the grain size, lattice stress, and defect density of the thin film can also be evaluated, providing key structural basis for optimizing the preparation process (such as solvent engineering and additive control), and playing an irreplaceable role in improving the efficiency and stability of perovskite photovoltaic devices.

4、 Application Cases

(1) Sample/Preparation

This experiment used the FRINGE EV desktop X-ray diffractometer from Suzhou Langsheng Scientific Instrument Co., Ltd. to detect the perovskite samples (powders, hundred nanometer level films) provided by a certain company.

Figure 3. Image of the tested perovskite thin film sample

(2) Test parameter settings

(3) Test spectrum

Figure 4. Diffraction pattern of PVK powder

Figure 5. Diffraction pattern of PVK film-1

Figure 6. Diffraction pattern of PVK film-2

Figure 7. Diffraction pattern of PVK film-3

Figure 8. Diffraction pattern overlay and crystal structure qualitative results of FAPbI3 perovskite PVK sample

(4) Test results

The desktop X-ray diffractometer FRINGE EV of Langsheng Science was used to test the perovskite sample, and the diffraction pattern is shown in Figure 7-10. The superimposed diffraction pattern is shown in Figure 11, and the diffraction peaks at 14.08 ° (110), 28.24 ° (220), and 31.62 ° (310) are characteristic peaks of the perovskite phase, which can well reflect the formation of perovskite crystals in all samples. In addition, the PVK powder diffraction pattern shows that it is a pure perovskite α - FAPbI3However, a small peak appeared at 10.19 ° in PVK film-1, attributed to δ - FAPbI3The (010) crystal plane. PVK film-2 and PVK film-3 can clearly observe an additional small peak at 12.8 °, which can be determined as unreacted PbI2The (001) crystal plane. At the same time, three PVK film samples all exhibited very obvious diffraction peaks of FTO glass, namely SnO2The (110), (101), and (200) crystal planes are due to the PVK film being loaded onto FTO glass, and the diffraction signal of FTO glass is also collected.

V. Conclusion

XRD is a very important characterization method in materials science research. The desktop X-ray diffractometer of Langsheng Science can provide strong data support for material research and development, process adjustment, quality control, and other aspects of perovskite thin films.