Room-temperature Substrate Integration of Thick Halide Perovskite Single Crystal for X-ray Detection

Halide perovskite has optoelectrical properties that are very suitable for direct X-ray detector application such as high stopping power against X-ray, tunable bandgap, high resistivity, large mobility-lifetime product and cheap fabrication cost. Halide perovskite as a photon-absorber in X-ray detector can be prepared in three different forms—polycrystalline thin film, polycrystalline wafer and single crystal. For hard X-ray imaging such as medical computed tomography (CT), the thickness of an X-ray photo-absorber needs to be in the range of several hundreds of µm’s to several mm’s. Halide perovskite thin films can be readily prepared with a solution process, however, their thickness is limited to several microns at maximum. A thick polycrystalline wafer can be fabricated but a strong electric field (&gt; 100 V/mm) is often required for the operation owing to the presence of grain boundaries, which raises current noise. Halide perovskite single crystal (PSC) possesses a high potential as X-ray absorbing layer because it can be easily fabricated to a millimeter thick layer while maintaining a superior mobility-lifetime product due to the absence of grain boundaries.<br/>Most of PSCs grown by a solution process have free-standing (substrate-free) form because a substrate provides nucleation sites and results in the growth of polycrystalline perovskite. As a target dimension of an X-ray absorber increases, it becomes more difficult to monolithically grow a single crystal on a substrate. A millimeter scale free-standing single crystal can be directly applied to either a lateral-structure detector where both anode and cathode locate on same plane or a vertical-structure detector where anode and cathode are formed on both sides of the crystal. For high resolution pixelated imaging, however, thin-film transistor (TFT) arrays must be integrated with the photo-absorber. In other words, the free-standing PSC should be attached to a TFT array substrate.<br/>In this study, we have adopted an ionic liquid, methylammonium acetate (MAAc) to integrate inverse-temperature crystallized PSC onto an indium-tin-oxide (ITO) substrate. Hydrated MAAc, which exists as liquid at room temperature, chemically dissolves the interfacial part of methylammonium lead bromide (MAPbBr₃) into liquid and provides a strongly bonding between PSC and ITO after the solidification of the melted interfacial MAPbBr₃. The integration via MAAc is conducted at room temperature and therefore induced no crack during the process, unlike the integration using a low melting temperature metal (such as Gallium alloy) which involves heating over the melting temperature of the metal and cooling. PSC/ITO bonded by MAAc showed an adhesion force of 164 kPa which can hold a weight of 1.67 kg per 1 cm². This is the first quantitative measured adhesion force of PSC on any substrate. The PSC/ITO interface showed excellent ohmic contact which verifies that the interface is not only mechanically integrated but also electrically well-connected. A single pixel detector with 2 cm x 2 cm PSC integrated with MAAc showed a superior sensitivity of 9 x 10⁴ µC Gy_air^-1cm^-2 which is the highest value among substrate-integrated direct X-ray detectors. Multiple PSCs are integrated onto a TFT panel for array imaging forming active area of 8 cm x 8 cm (~10³ x 10³ pixel), which is the largest area ever reported for X-ray detector based on PSC. In addition, the MAAc integration effectively preserved electrical pixelation of the TFT due to its resistive nature unlike metallic integration. This study provided a strategy for integrating millimeter thick, large area (&gt; 64 cm²) PSC onto various substrates with the demonstration of high performance devices, which is essential towards the commercialization of halide perovskite radiation detectors.