Upgrading of methylammonium lead halide perovskite layers by thermal imprint

Abstract

The manufacturing of devices from methylammonium-based perovskites asks for reliable and scalable processing. As solvent engineering is not the option of choice to obtain homogeneous layers on large areas, our idea is to ‘upgrade’ a non-perfect pristine layer by recrystallization in a thermal imprint step (called ‘planar hot pressing’) and thus to reduce the demands on the layer formation itself. Recently, imprint has proven both its capability to improve the crystal size of perovskite layers and its usability for large area manufacturing. We start with methylammonium lead bromide layers obtained from a conventional solution-based process. Acetate is used as a competitive lead source; even under perfect conditions the resulting perovskite layer then will contain side-products due to layer formation besides the desired perovskite. Based on the physical properties of the materials involved we discuss the impact of the temperature on the status of the layer both during soft-bake and during thermal imprint. By using a special imprint technique called ‘hot loading’ we are able to visualize the upgrade of the layer with time, namely a growth of the grains and an accumulation of the side-products at the grain boundaries. By means of a subsequent vacuum exposition we reveal the presence of non-perovskite components with a simple inspection of the morphology of the layer; all experiments are supported by X-ray and electron diffraction measurements. Besides degradation, we discuss recrystallization and propose post-crystallization to explain the experimental results. This physical approach towards perovskite layers with large grains by post-processing is a key step towards large-area preparation of high-quality layers for device manufacturing.

Relevance of processing parameters for grain growth of metal halide perovskites with nanoimprint

“…The precursor solution contains 504 mg methylammonium bromide, MABr (purity 99.999%, Dyesol) and 488 mg lead acetate, Pb(AcO) 2 (purity 98%, TCI). For comparison, also a lead acetate trihydrate was used (purity 99.999%, Sigma-Aldrich) applying an adapted recipe as investigated in detail earlier [21]. The components were mixed together in 1 ml anhydrous dimethylformamide, DMF (purity 99.99%, Sigma Aldrich), and the solution was stirred for 24 h at 60 °C.…”

“…1a; when a flat stamp (here a piece of silicon) is used, we call this 'planar hot pressing', PHP. When the imprint stack is loaded, the pressure is applied first; this assures a tight contact between stamp and sample and seals the perovskite from the ambient atmosphere, thus preventing the layer from degradation [21]. Then, the temperature is raised and held constant for the imprint time, followed by a cool down to room temperature where the imprint stack is unloaded [23].…”

“…under inert atmosphere. Our investigation concentrates on thermal imprint as the post-processing step [20][21][22][23][24] to induce grain growth with polycrystalline layers of metal halide perovskites.…”

“…One of the questions to be answered by the present investigation is to what extent these concepts can be transferred/applied, on the one hand, to polycrystalline layers of organic-inorganic metal halide perovskites and, on the other hand, to thermal imprint as the post-processing technique. It has already been shown that thermal imprint in fact induces grain growth in perovskite layers [13,20,21,33]. Earlier work [21] has even demonstrated that thermal imprint is comparable to annealing under nitrogen as the layer is tightly confined between stamp and substrate during imprint due to the pressure applied; the stamp acts as a cover that shields the layer from the environment, thus enabling processing under ambient air at elevated temperature without layer degradation.…”

“…It has already been shown that thermal imprint in fact induces grain growth in perovskite layers [13,20,21,33]. Earlier work [21] has even demonstrated that thermal imprint is comparable to annealing under nitrogen as the layer is tightly confined between stamp and substrate during imprint due to the pressure applied; the stamp acts as a cover that shields the layer from the environment, thus enabling processing under ambient air at elevated temperature without layer degradation. Furthermore, after imprint the perovskite surface reproduces the flat or structured surface of the stamp.…”