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.

Guiding of the phase separation of a block copolymer (BCP) by an electric field perpendicular to the substrate is investigated in order to obtain vertical structures that can provide a mask for subsequent etching. Because of practical aspects, the substrate is bare Si without any neutral brush and the process time is limited to 1 h. A polystyrene-block polymethylmethacrylate lamellar material is employed in the study. For a unique guiding of the lamellar phase, an ordering mechanism orthogonal to the electric field is introduced by the interaction with the stamp in a thermal nanoimprint process. The naturally low surface energy of the stamp shall induce the formation of lamellae along the sidewalls of linear cavities. In order to fully utilize these two ordering mechanisms, the stamp sidewalls and the electric field, the imprint process is conducted in such a way that no residual layer remains below the stamp structures and the whole BCP is accumulated inside the cavities which are just partly filled. The electrically-assisted imprint process is studied analytically, considering the capacitive effects due to the local electric field in the cavity and in particular in the BCP. In addition, a numerical simulation is performed for the actual experimental conditions to compute the electric vector field in the BCP. In this way, an extensive understanding of the situation is gained which is the basis for choosing optimal experimental conditions for electrically-assisted thermal nanoimprint. Furthermore, the ambiguity of the electric field in a thermal nanoimprint process with partly filled cavities is addressed. The field shall induce vertical phase separation but, due to instabilities, it also may induce capillary bridges that represent replication defects. An improvement of the vertical phase separation by applying an electric field as high as 25 V/μm could be identified under specific experimental conditions. However, the guiding effect within the cavities and thus the long-range order of the lamellae remained limited. This may be due to a field strength too low in the BCP; in the present configuration, higher field strengths are prohibited by an electrical breakthrough.

The quality and the stability of devices prepared from polycrystalline layers of organic–inorganic perovskites highly depend on the grain sizes prevailing. Tuning of the grain size is either done during layer preparation or in a post-processing step. Our investigation refers to thermal imprint as the post-processing step to induce grain growth in perovskite layers, offering the additional benefit of providing a flat surface for multi-layer devices. The material studied is MAPbBr3; we investigate grain growth at a pressure of 100 bar and temperatures of up to 150 °C, a temperature range where the pressurized stamp is beneficial to avoid thermal degradation. Grain coarsening develops in a self-similar way, featuring a log-normal grain size distribution; categories like ‘normal’ or ‘secondary’ growth are less applicable as the layers feature a preferential orientation already before imprint-induced grain growth. The experiments are simulated with a capillary-based growth law; the respective parameters are determined experimentally, with an activation energy of Q ≈ 0.3 eV. It turns out that with imprint as well the main parameter relevant to grain growth is temperature; to induce grain growth in MAPbBr3 within a reasonable processing time a temperature of 120 °C and beyond is advised. An analysis of the mechanical situation during imprint indicates a dominance of thermal stress. The minimization of elastic energy and surface energy together favours the development of grains with (100)-orientation in MaPbBr3 layers. Furthermore, the experiments indicate that the purity of the materials used for layer preparation is a major factor to achieve large grains; however, a diligent and always similar preparation of the layer is equally important as it defines the pureness of the resulting perovskite layer, intimately connected with its capability to grow. The results are not only of interest to assess the potential of a layer with respect to grain growth when specific temperatures and times are chosen; they also help to rate the long-term stability of a layer under temperature loading, e.g. during the operation of a device.

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The copy of structures in the same tone as the original asks at least for a double replication. Each replication generation will suffer from shrinkage of the replication material used, due to curing or thermal contraction. The impact of shrinkage is addressed in a basic study by simulation and experiment. The main replication materials investigated are OrmoStamp and SU-8. Presently, the preparation of anisotropic adhesion elements with hierarchical structures is investigated. The structures are micrometer-sized isolated pads with nanometer-sized self-aligned ripples (laser-induced periodic surface structures) on top. The initial structures are available as isolated photoresist patterns on Si. The double replication of this polymeric master raises questions with respect to its chemical and mechanical stability. The authors report an optimized replication process with an OrmoStamp intermediate template and a final replication in an elastomer. The anisotropic character of the so prepared hierarchical elastomeric adhesion elements is demonstrated by measurement.