Machine Vision for Interpreting Perovskite Grain Characteristics

Metal halide perovskites (MHPs) are a new class of materials with general chemical formula of ABX3, where A is CH3NH3+ (MA+), HC(NH2)2+FA+), or Cs+; B is Pb2+ or Sn2+; X is a halide anion (I–, Br–, or Cl–), as shown in Figure 1a. (1) While the standard MHPs crystal exhibit 3D structures, an even larger subclass of 2D MHPs can be formed with numerous types of organic A-cations by leveraging its rich chemistry. Over the past few years, MHPs have found promising applications in a range of optoelectronic devices including solar cells, light-emitting diodes, photodetectors and memristors. Especially, perovskite-based solar cells (PSCs) have been considered as a dominating candidate in the future of the photovoltaic industry. (2) PSCs with a simple sandwich structure (Figure 1b) have swiftly reached a certified power conversion efficiency (PCE) of 25.7%, (3) mainly because MHPs exhibit a range of favorable intrinsic properties, including tunable bandgaps, strong light absorption coefficients, high carrier mobilities and long carrier lifetimes. Regardless of the merits inherited from the chemistry and crystallography of these materials, MHPs used in PSCs are typically polycrystalline consisting of 3D aggregates of individual grains, like many other typical semiconductors. (4) It has been recognized that the interruption of long-range lattice ordering, coupled with the polycrystalline film microstructures, negatively influences a range of film properties and device performance parameters. (4) First, the lattice disorder can raise detrimental energy levels, thus influences the dynamics of photogenerated carrier transport and injection. Then, a higher chemical reactivity with internal ions and external environmental species (oxygen, moisture, etc.) may be induced by the lattice disorder, correspondingly leading to low stabilities of MHPs. Therefore, a quantitative understanding of the grain characteristics in MHP thin films is one prominent step toward elucidating the microscopic structure–property–performance relationship in PSCs and other perovskite optoelectronics.