Impact of Pressure and Temperature on the Compaction Dynamics and Layer Properties of Powder-Pressed Methylammonium Lead Halide Thick Films

Abstract

While halide perovskite X-ray detectors based on single crystals could achieve extraordinary sensitivities, detectors based on polycrystalline thick films lag behind in efficiency. This is unfortunate since the processing methods for producing polycrystalline thick films, especially by pressure treatment of powders, are suitable for upscaling. Here, we investigate in detail the pressing of readily prepared powders of methylammonium lead halide perovskites MAPbI3 and MAPbBr3 to thick layers. By time-dependent pressure measurements, we monitor the occurring compaction dynamics, identifying two relaxation processes with different timescales. When pressing at elevated temperatures from room temperature (RT) to 100 °C, the pressure relaxations change drastically. While the layer properties such as relative density and surface roughness only improve to a certain degree by increasing the pressure at RT, we observe relative densities >97%, considerable reduction in surface roughness, and a significant increase in grain size with tempered pressing. Analyses regarding time-dependent pressure relaxations of tempered pressing allow attributing the dynamics to a sintering process, where we find the sinter onset to be surprisingly low at about 30 °C, mainly independent of the applied pressure (10–100 MPa). Our results will allow for an improved and more targeted powder processing of halide perovskite thick films as they are promising candidates for efficient X-ray detectors.

Recent Advances and Perspectives on Powder‐Based Halide Perovskite Film Processing

“…Under optimized pressing conditions (100 MPa, 100 °C and longpress time of ≈130 min), it was possible to produce 1 mm thick MAPbI 3 layers with relative densities of >97% with obvious grain growth, resulting in an average grain size of 1.9 µm. [113] Hong et al also used the pressing approach and pressed mechanochemically synthesized CsSnBr 1.5 Cl 1.5 powder into 2 mm thick pellets. [16] By evaporating gold electrodes on the pellet, they realized a proof-of-concept photodetector, measuring a change of the photocurrent by about one order of magnitude under illumination using a 445 nm LED.…”

“…In the case of powder pressing it is easy to achieve different degrees of compaction, as the relative density of the layer depends on the applied pressure, temperature, and pressing time. [113] Recently, Sheikh et al took advantage of this and used a 0.6 mm thick powder pressed pellet with a relative density of 82% as an efficient ammonium sensor with low response (0.3 s) and recovery (3.6 s) times at ammonium levels as low as 10 ppm. [120]…”

“…b,c) Reproduced with permission. [ 113 ] Copyright 2020, American Chemical Society. d) Adapted with permission.…”

“…Recently, we systematically investigated the influence of pressure (Figure 7b), pressing time and temperature on the compaction dynamics occurring during the pressing of perovskite powders. [ 113 ] Using transient pressure relaxation measurements, we identified different compaction processes, namely a faster, initially dominant particle rearrangement dynamic and subsequently increasing plastic deformation dynamics of the powder particles on longer time scales (Figure 7c). Furthermore, we found that at temperatures of >≈35 °C a faster compression dynamic, presumably due to a sintering effect, occurs mainly independent of the pressure level (10–100 MPa).…”

“…Under optimized pressing conditions (100 MPa, 100 °C and long‐press time of ≈130 min), it was possible to produce 1 mm thick MAPbI 3 layers with relative densities of >97% with obvious grain growth, resulting in an average grain size of 1.9 µm. [ 113 ]…”