Witt et al. investigated the factors such as pressure, pressing time and temperature during the pressing process^[15]. Above 35 °C, rapid compression occurred, mainly due to two relaxation processes caused by plastic deformation and particle rearrangement. The optimal pressing conditions (100 MPa, 100 °C, 130 min) yield MAPbI₃ wafers with relative density >97%, high crystallinity, and an average size of 1.9 μm. Besides X-ray detectors, perovskite wafers can also be used in near-infrared detectors. Yu et al. made dense and smooth MAPbI₃ wafers from MAPbI₃ single crystals by hot pressing method^[20]. The near-infrared detector exhibited a responsivity of 2.1 A∙W⁻¹ (Fig. 2(d)), rise and decay time of ~239 μs and ~6.13 ms (Fig. 2(e)), and high cycling stability (Fig. 2(f)).

The same passivation strategies applied in solution engineering can also be applied to mechanical pressing methods. Yang et al. introduced a bismuth oxybromide (BiOBr) heteroepitaxial passivation layer in Cs₂AgBiBr₆ polycrystalline wafers (Figs. 2(a) and 2(b))^[14]. BiOBr initiated the epitaxial growth of Cs₂AgBiBr₆ grain boundaries, resulting in a grain size of 100 μm while passivating the grain boundary defects and eliminating the ion migration. The detector showed improved stability with a sensitivity of 250 μC/(Gy_air∙cm²) (Fig. 2(c)).

In addition to applying a stress field to the microcrystals/powder from perovskite precursor, a secondary coupling effect can be triggered. The direct densification of perovskite from powder to high-quality bulk crystals can be achieved in minutes under the dual action of a stress field and a thermal/electric field. Hu et al. prepared large MAPbI₃ wafers (diameter ~80 mm) from perovskite powder by heat-assisted pressing method^[19]. The X-ray detector with MAPbI₃ wafers has an X-ray sensitivity of 1.22 × 10⁵μC/(Gy_air∙cm²) at 10 V bias. Zheng et al. first reported an electric and mechanical field-assisted sintering technique (EM-FAST) for making perovskite wafers, which can produce high-quality bulk crystals in 5 min (Fig. 1(d))^[16]. The pressure leads to better contact between the particles, thus forming a sintered neck. The small contact area at the sintered neck leads to an increase in local pressure, which triggers grain boundary diffusion and sliding. Moreover, localized thermal concentration is induced at the neck under the application of electric field, and this surface heating triggers mass transfer and grain integration. A very dense bulk crystal was obtained by using the FAST method. The optical bandgap of FAST product (1.45 eV) is close to that of the single crystal (1.51 eV) (Fig. 1(e)). The defect density of FAST product reaches 5.4 × 10¹⁰ cm⁻³, which is close to that of the single crystal (Fig. 1(f)).

There are several reports on perovskite quasi-single crystal wafers for photodetectors, which show low defect density and good performance^[13−16]. The soft lattice of perovskite allows perovskite powder to be sufficiently deformed and densified under low pressure^{[17, 18]}. Shrestha et al. used a mechanical pressing process to make polycrystalline MAPbI₃ wafer with millimeter thickness and high crystallinity (Fig. 1(a))^[13]. They made MAPbI₃ wafers by applying a pressure of 0.3 GPa for 5 min to the microcrystals precipitated from solution. The wafer was then pressed onto PEDOT substrate under a pressure of 15 MPa for 2 min, thus obtaining an X-ray detector (Fig. 1(b)). The device exhibited a sensitivity of 2527 μC/(Gy_air∙cm²) under 70 kV_p X-ray exposure (Fig. 1(c)).

Most photodetectors are made from polycrystalline films or single crystals of perovskite^[21−23]. All efforts focus on defect passivation^{[24, 25]}, interface modification^{[26, 27]} and film formation control^[28] of polycrystalline thin films as well as crystallization engineering of single crystals. Mechanical pressing method is an easy and fast process for preparing perovskite bulk crystals. It is also necessary to achieve high adhesion between perovskite wafers and the underlying substrate. We should explore the adaptability of perovskite materials with other materials (metals^[29], carbon^[30], 2D materials^[31], etc.) to improve device performance.

Halide perovskites show excellent photovoltaic properties^[1−4]. However, the preparation of high-quality perovskite crystals remains a great challenge, which limits their applications. Perovskite materials applied to photodetectors mainly include polycrystalline thin films and single crystals. Traditional solution methods are used to prepare polycrystalline thin films, and the films are full of defects such as voids and grain boundaries^[5−7]. Compared to polycrystalline thin films, perovskite single crystals possess high crystallinity and low defect density^[8−10]. Photodetectors based on perovskite single crystals exhibit excellent performance^[11]. However, the size limitation of single crystals hinders their application in photodetectors^[12].