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Understanding hybrid halide perovskite thin layers to improve solar cell efficiency
Full field diffraction X-ray microscopy has been used to study the microstructure of hybrid halide perovskite solar cells, namely texture and strain. A deeper insight into the microstructure is essential to be able to control it; and could ultimately lead to methods to engineer it in order to improve solar cell performance.
Hybrid halide perovskite (HHP) solar cells are the newest addition to photovoltaic technologies developed over the past 10 years, with the efficiencies of these devices having increased to reach values close to those of silicon- based solar cells, at around 25.5%. However, a number of issues need to be addressed in order to increase the performances of these devices. The effect of the presence of strain in the perovskite layer is still under debate , while the orientation of the perovskite lattice, or texture, can also affect its physical and mechanical properties . In this work, X-ray diffraction (XRD) and full-field diffraction X-ray microscopy (FFDXM) experiments were carried out at beamline ID01 to study the structure of HHP thin layers at the microscale, in order to understand the mechanisms governing strain and texture with the perspective of finding ways to engineer the material for higher performance.
Using in-situ XRD, the strain experienced by a hybrid halide perovskite (MAPbI3) thin layer deposited on a glass substrate covered by an inorganic oxide was monitored as it cooled down to room temperature after crystallisation at 100°C. Figure 60 shows the evolution of the lattice parameters of the MAPbI3 film, compared to parameters obtained with bulk MAPbI3 powder. The data show that the MAPbI3 layer is mainly relaxed on the substrate,
Fig. 60: Evolution of the measured out-of-plane and calculated in-plane lattice parameters of MAPbI3 thin layers during controlled cooling from 100°C to room temperature. The open symbols display the lattice parameters of the unstrained reference sample: MAPbI3 powder and the dotted lines indicate the cubic-tetragonal phase transition temperature in the bulk material (57°C).
contradicting the commonly accepted hypothesis that strain is present in MAPbI3 layers synthesised above room temperature due to the large mismatch in the thermal expansion coefficients of the perovskite and its substrate. These results demonstrate that there is no direct correlation between this mismatch and the presence of strain in the film. Grain boundaries might play a key role in the strain relaxation, more particularly combined with the high ion mobility observed in this material, which has been shown to favour the mitigation of lattice defects. However further
Fig. 61: a) Measurement setup for FFDXM on ID01. b) Obtained rocking curves for both [hh0]-oriented grains (a-domains)
and [00l]-oriented grains (c-domains) and resulting summed micrographs. c) Correlation map of a- and c-domains obtained from rocking curve measurements. d) Schematic explaining the
conclusion of twin domains. Purple and green colours correspond to c- and a-domains respectively.