Grazing incidence in solar cell research
Glancing Incidence XRD for Sample Characterisation in Perovskite Solar Cell Research
Producing simple, high performance and cost-effective designs are primary obstacles in manufacturing next-generation photovoltaics. Perovskite solar cells have shown exceptional progress in research cell efficiencies since their discovery in 2009. Additionally, the material abundance and versatility makes them a sustainable option for use in emerging photovoltaic systems.
Optimisation of the perovskite solar cell comes down to the design, morphology and crystal/defect chemistry of the light absorber (the semiconducting perovskite). Control of the perovskite crystal formation via fine-tuning the intermediate steps and perovskite conversion are central areas for improvement. Several aspects of the crystal properties may be analysed using GI XRD, where key points of interest usually include determining the phase of the material, its orientation on the substrate, presence of impurities, and changes in lattice parameters that can ultimately be used to track fluctuations in crystal formation on atomic scale. These data can in turn be compared with device efficiencies to determine the best crystal properties for achieving optimised solar cells.
Figure 1 displays some example perovskite films that were made by spin-coating the precursors in a single step and formed by dripping an anti-solvent at the end of the coating process before annealing. As these materials contain different amounts of halides in their crystal structures, changes in their lattice parameters can be compared with their measured bandgap energies to analyse their semiconducting properties. This is of particular interest as fine-tuning the light absorption edge of the perovskite allows for tandem integration with already existing photovoltaic technologies.1
Figure 1 - Examples of mixed-halide perovskite films (FAPb(IxBr1-x)3) on conductive TiO2 coated glass substrates
Figure 2 shows two GI XRD patterns of two different perovskite (CH3NH3PbI3) samples that were fabricated using two different pathways. Their relative difference in peak ratio between the (110) and (220) peaks suggests difference in preferred crystal orientation on the glass substrate they were fabricated on. The difference in perovskite crystal orientation has previously been found to influence the power conversion efficiency.2
Figure 2 - Normalised XRD patterns for two CH3NH3PbI3 perovskite samples that were fabricated using different pathways resulted in changed crystal orientation as seen by the (220) peak. The adjustment in crystal orientation also led to difference in power conversion efficiency in the solar cell device.2
1 W. Rehman et al., Energy Environ. Sci. (2017), 10, 361-369
2 A. Osherov et al., Adv. Mater. (2016), 28(48), 10757-10763
Ms Camilla Lian
Faculty of Science, SELS Chemistry
The University of Newcastle
Project: Morphology controlled perovskite solar cells developed from a metallic seed later