Abstract
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.
Highlights
Thin film devices typically have to rely on the properties of their layers [1] being either amorphous or polycrystalline
The current investigation on grain growth with M APbBr3 layers starts by presenting the pristine layers used and their characteristics
State-of-the-art theoretical concepts represent the framework for establishing realistic expectations about the impact parameters relevant to grain growth of polycrystalline layers known from literature
Summary
Thin film devices typically have to rely on the properties of their layers [1] being either amorphous or polycrystalline. 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. With the impact of temperature and pressure at hand, we will complement the experimental findings with simulations using a capillary-based growth law [19]; the parameters required are extracted from the experiments These simulations are appropriate to mirror the grain growth observed; they reveal the prospects and the limitations of post-processing by thermal imprint in order to induce grain growth in perovskite layers, by the choice of the processing parameters and of the material. It is the basis for a direct patterning of perovskite layers in view of thin film devices and, for assessing their thermal integrity under operation
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