Organic–inorganic halide perovskite single crystals possess many outstanding properties conducive for photovoltaic and optoelectronic applications. However, a clear photophysics picture is still elusive, particularly, their surface and bulk photophysics are inexorably convoluted by the spectral absorbance, defects, coexisting photoexcited species, etc. In this work, an all‐optical study is presented that clearly distinguishes the surface kinetics from those of the bulk in the representative methylammonium‐lead bromide (MAPbBr3) and ‐lead iodide (MAPbI3) single crystals. It is found that the bulk recombination lifetime of the MAPbBr3 single crystal is shortened significantly by approximately one to two orders (i.e., from ≈34 to ≈1 ns) at the surface with a surface recombination velocity of around 6.7 × 103 cm s−1. The surface trap density is estimated to be around 6.0 × 1017 cm−3, which is two orders larger than that of the bulk (5.8 × 1015 cm−3). Correspondingly, the diffusion length of the surface excited species is ≈130–160 nm, which is considerably reduced compared to the bulk value of ≈2.6–4.3 μm. Furthermore, the surface region has a wider bandgap that possibly arises from the strong lattice deformation. The findings provide new insights into the intrinsic photophysics essential for single crystal perovskite photovoltaics and optoelectronic devices.

We present the levitation and residing at particular places of human breast cancer (T47D) cells on a magnetic microsystem quickly fabricated by printing. The microsystem consists of arrays of soft magnetic elements and is put adjacent to a bulk magnet. Under a magnetic field generated by the magnet, elements are magnetized, obtaining a significant magnetic field strength and a high magnetic field gradient in the micro-space above them. The field applies a magnetic force on the cells as they are dropped to the surfaces of the elements. As a result, levitation and a stable position for each cell are observed. The levitation and position of a cell can be predicted by a theory that takes into account the magnetic, buoyant, and gravitational forces on the cells. Our theoretical and experimental results agree well with one another, and the microsystem is capable of efficient cell manipulation.

Mitzi, D. B. et al. Hybrid field-effect transistor based on a low-temperature melt-processed channel layer. Advanced materials 14, 1772-1776 (2002).