Epitaxial thin film growth is the best method to obtain high-quality crystalline materials for (opto-) electronic applications. Metal halide perovskite semiconductors show high performance in polycrystalline films used for solar cells and even better electronic parameters in single crystals. The epitaxy of single crystalline layers is still in its infancy. While deposition of perovskite in a vacuum on various substrates resulted in crystallites on appropriate substrates, obtaining continuous single crystalline films has still to be shown. Here we attempt this goal by inkjet printing from precursor solutions, which in general is a very cheap and versatile technology for deposition of materials on predefined locations. This development is an advancement to the recent demonstration of epitaxy by spin casting [1], in which the formation of continuous perovskite films was discussed, but not fully demonstrated.
We found that the key towards a successful growth of epitaxial structures, which can be eventually merged to a continuous crystalline film, include (i) the appropriate choice of crystalline substrate, (ii) the surface preparation and activation, (iii) the formation of an appropriate metal halide buffer layers, (iv) the choice of humidity in the deposition ambient, (v) the substrate temperature, and the (iv) amount of deposited material. Our experimental findings are accompanied by density-function theoretical calculations of interface free-energies, predicting the advantages of applying a metal halide interlayer for epitaxial growth of perovskites upon lead chalcogenide crystalline substrates. In particular, we obtained most promising results with PbS and PbTe single crystalline substrates with [100] and [111] surface orientation, providing small lattice mismatches with the deposited methylammonium lead halide perovskite epitaxial structures. While the importance of the substrate pretreatments is confirmed by X-ray photoemission spectroscopy, the coherence between the lattices of the epitaxial perovskite structures is evidenced by X-ray pole figures and by high-resolution transmission electron microscopy. The obtained epitaxial structures include planar islands with either hexagonal or cubic shapes, whose edges are oriented according to the crystal orientation of the substrates. Increasing the amount of deposited material by increasing the ejection frequency of the inkjet printer allowed merging the individual epitaxial islands, to form quasi-continuous films, with some voids, as evidenced by electron- and optical microscopy. Most interestingly, even though the lead-chalcogenides used as substrates exhibit smaller band-gap energies as the epitaxial perovskite structures on top of them, the perovskite microstructures provide luminescence with bright intensity, making them promising for the development of future electronic devices.