Organic-inorganic hybrid halide perovskites are promising semiconductors with tailorable optical and electronic properties. The choice of A-site cation to support a three-dimensional (3D) perovskite structure AMX 3 (where M is a metal, and X is a halide) is limited by the geometric Goldschmidt tolerance factor. However, this geometric constraint can be relaxed in twodimensional (2D) perovskites, providing us an opportunity to understand how various the A-site cations modulate the structural properties and thereby the optoelectronic properties. Here, we report the synthesis and structures of single-crystals (BA) 2 (A)Pb 2 I 7 where BA = butylammonium, and A = methylammonium (MA), formamidinium (FA), dimethylammonium (DMA) or guanidinium (GA), a series of A-site cation varied in size. Single-crystal X-ray diffraction reveals that the MA, FA, and GA structures crystallize in the same Cmcm space group, while the DMA imposes the Ccmb space group. We observe that as the A-site cation becomes larger, the Pb−I bond continuously elongates, expanding the volume of the perovskite cage, equivalent to exerting "negative pressure" on the perovskite structures. Optical studies and DFT calculations show the Pb−I bond length elongation reduces overlap of the Pb s-and I p-orbitals and increases the optical bandgap, while Pb−I−Pb angles play a secondary role. Raman spectra show lattice softening with

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Two-dimensional (2D) halide perovskites have outstanding optoelectronic properties, and they feature a variety of organic cation spacers and cage A-site cations that can be incorporated into their structures. It has recently been reported that the Goldschmidt tolerance factor can be relaxed and expanded in iodide 2D perovskites. Bromide 2D perovskites, whose multilayer structures and optical properties are much less studied, provide a great platform for studying structure-property relationships for 2D perovskites with large A-site cations. Herein, we report the synthesis and structure of three new 2D bromide perovskites-(BA) 2 (MHy) 2 Pb 3 Br 10 (BA, butylammonium; MHy, methylhydrazinium), (BA) 2 (EA) 2 Pb 3 Br 10 (EA is ethylammonium), and (BA) 2 (DMA)Pb 2 Br 7 (DMA is dimethylammonium). We compared them with other 2D perovskites with different Asite cations but with the same spacer and layer thickness. Single-crystal structures show that the Pb-Br bonds are elongated to accommodate the large A-site cations. Additionally, the octahedra in (BA) 2 (MHy) 2 Pb 3 Br 10 and (BA) 2 (EA) 2 Pb 3 Br 10 are highly distorted, and their different stacking

High-performance and durable perovskite solar cells (PSCs) have advanced rapidly, enabled in part by the development of superior interfacial hole-transporting layers (HTLs). Here, a new series of 2,3-diphenylthieno [3,4-b]pyrazine (DPTP)-based small molecules containing bis-and tetrakis-triphenyl amino donors (1−3) was synthesized from simple, low-cost, and readily available starting materials. The matched energy levels, ideal surface topographies, high hole mobilities of 8.57 × 10 −4 cm 2 V −1 S −1 , and stable chemical structures of DPTP-4D (3) make it an effective hole-transporting material, delivering a PCE of 20.18% with high environmental, thermal, and light-soaking stability when compared to the reference HTL materials, doped Spiro-OMeTAD and PTAA in PSC n-i-p planar devices. Overall, these DPTP-based molecules are promising HTM candidates for the fabrication of stable PSCs.

Metal halide perovskite thin films have achieved remarkable performance in optoelectronic devices but suffer from spatial heterogeneity in their electronic properties. To achieve higher device performance and reliability needed for widespread commercial deployment, spatial heterogeneity of optoelectronic properties in the perovskite thin film needs to be understood and controlled. Clear identification of the causes underlying this heterogeneity, most importantly the spatial heterogeneity in charge trapping behavior, has remained elusive. Here, a multimodal imaging approach consisting of photoluminescence, optical transmission, and atomic force microscopy is utilized to separate electronic heterogeneity from morphology variations in perovskite thin films. By comparing the degree of heterogeneity in highly oriented and randomly oriented polycrystalline perovskite thin film samples, we reveal that disorders in the crystallographic orientation of the grains play a dominant role in determining charge trapping and electronic heterogeneity. This work also demonstrates a polycrystalline thin film with uniform charge trapping behavior by minimizing crystallographic orientation disorder. These results suggest that single crystals may not be required for perovskite thin film based optoelectronic devices to reach their full potential.