Formation of a Photovoltaic Material from Precursor to Crystal

Lead halide perovskites—compounds with crystal structures characterized by octahedral cages of halogen atoms (iodine, chlorine, or bromine) surrounding lead (Pb) atoms—have emerged as promising light-absorbing photovoltaic materials. Such compounds have power-conversion efficiencies (PCEs) that have skyrocketed from 3.8% to 22.1% in just seven years. Despite this remarkable progress, a detailed understanding of how the materials’ morphology evolves during preparation is critically important to further improvements in material quality and device performance.

At ALS Beamline 7.3.3, researchers observed the formation of lead iodide perovskite thin films in real time, from the liquid precursor stage (in which thin films were “printed” via an integrated slot-die process) through drying and crystallization. Grazing-incidence x-ray diffraction (GIXD), Fourier-transform infrared spectroscopy (FTIR), and optical microscopy studies were performed at the beamline over relevant temperature and time scales. Based on the results, a model of crystal growth via the intermediate formation of [PbI₆]^4– nanoparticles was proposed. Furthermore, the observation of concentric ring patterns in the crystal domains was explained via a “crystallization-depletion” mechanism—a competition between the linear growth of the crystals and the periodic depletion of crystallizable material. The slot-die-printed samples were also fabricated into solar cells at the Molecular Foundry and tested, performing comparably to samples made using traditional spin-coating production techniques.

With these methods, the researchers were able to monitor perovskite evolution from nanoscale to mesoscale, offering new perspectives on how best to tune crystal growth to make high-quality perovskite thin films for a wide variety of optoelectronic device applications.