Water-Induced Dimensionality Reduction in Metal-Halide Perovskites

Metal-halide perovskite materials are highly attractive materials for optoelectronic applications. However, the instability of perovskite materials caused by moisture and heat-induced degradation impairs future prospects of using these materials. Here we employ water to directly transform films of the three-dimensional (3D) perovskite CsPbBr₃ to stable two-dimensional (2D) perovskite-related CsPb₂Br₅. A sequential dissolution–recrystallization process governs this water-induced transformation under PbBr₂ rich conditions. We find that these postsynthesized 2D perovskite-related material films exhibit excellent stability against humidity and high photoluminescence quantum yield. We believe that our results provide a new synthetic method to generate stable 2D perovskite-related materials that could be applicable for light emitting device applications

Water-Induced Dimensionality Reduction in Metal-Halide Perovskites

Metal-halide perovskite materials are highly attractive materials for optoelectronic applications. However, the instability of perovskite materials caused by moisture and heat-induced degradation impairs future prospects of using these materials. Here we employ water to directly transform films of the three-dimensional (3D) perovskite CsPbBr₃ to stable two-dimensional (2D) perovskite-related CsPb₂Br₅. A sequential dissolution–recrystallization process governs this water-induced transformation under PbBr₂ rich conditions. We find that these postsynthesized 2D perovskite-related material films exhibit excellent stability against humidity and high photoluminescence quantum yield. We believe that our results provide a new synthetic method to generate stable 2D perovskite-related materials that could be applicable for light emitting device applications

Tetrathienodibenzocarbazole Based Donor–Acceptor Type Wide Band-Gap Copolymers for Polymer Solar Cell Applications

Two new donor–acceptor alternating wide band gap copolymers, <b>PTTDBC-PhQC8</b> and <b>PTTDBC-BTC12</b>, based on an electron-rich tetrathienodibenzocarbazole (<b>TTDBC</b>) donor have been designed and synthesized, wherein two additional thienyl rings are fused into a 2,7-dithienylcarbazole skeleton to reinforce the structural coplanarity and rigidity of polymers. The quinoid thiophene in <b>TTDBC</b> can endow not only a wide optical band gap (∼1.9 eV) but also a deep HOMO (∼5.3 eV) for the resulting polymers. The conventional configuration solar cells based on <b>PTTDBC-BTC12</b> exhibit a large <i>V</i>_oc of 0.91 V and a power conversion efficiency of 4.30% (with a <i>J</i>_sc of 9.27 mA cm^–2 and a FF of 0.51), while a slightly higher PCE of 4.50% can be achieved for the inverted structure devices. We believe that these wide band gap polymers have potential to be used for tandem cell applications

The Role of Surface Tension in the Crystallization of Metal Halide Perovskites

The exciting intrinsic properties discovered in single crystals of metal halide perovskites still await their translation into optoelectronic devices. The poor understanding and control of the crystallization process of these materials are current bottlenecks retarding the shift toward single-crystal-based optoelectronics. Here we theoretically and experimentally elucidate the role of surface tension in the rapid synthesis of perovskite single crystals by inverse temperature crystallization. Understanding the nucleation and growth mechanisms enabled us to exploit surface tension to direct the growth of monocrystalline films of perovskites (AMX₃, where A = CH₃NH₃⁺ or MA; M = Pb²⁺, Sn²⁺; X = Br^–, I^–) on the solution surface. We achieve up to 1 cm²-sized monocrystalline films with thickness on the order of the charge carrier diffusion length (∼5–10 μm). Our work paves the way to control the crystallization process of perovskites, including thin-film deposition, which is essential to advance the performance benchmarks of perovskite optoelectronics

Surface Restructuring of Hybrid Perovskite Crystals

Hybrid perovskite crystals have emerged as an important class of semiconductors because of their remarkable performance in optoelectronics devices. The interface structure and chemistry of these crystals are key determinants of the device’s performance. Unfortunately, little is known about the intrinsic properties of the surfaces of perovskite materials because extrinsic effects, such as complex microstructures, processing conditions, and hydration under ambient conditions, are thought to cause resistive losses and high leakage current in solar cells. We reveal the intrinsic structural and optoelectronic properties of both pristinely cleaved and aged surfaces of single crystals. We identify surface restructuring on the aged surfaces (visualized on the atomic-scale by scanning tunneling microscopy) that lead to compositional and optical bandgap changes as well as degradation of carrier dynamics, photocurrent, and solar cell device performance. The insights reported herein clarify the key variables involved in the performance of perovskite-based solar cells and fabrication of high-quality surface single crystals, thus paving the way toward their future exploitation in highly efficient solar cells

Double Charged Surface Layers in Lead Halide Perovskite Crystals

Understanding defect chemistry, particularly ion migration, and its significant effect on the surface’s optical and electronic properties is one of the major challenges impeding the development of hybrid perovskite-based devices. Here, using both experimental and theoretical approaches, we demonstrated that the surface layers of the perovskite crystals may acquire a high concentration of positively charged vacancies with the complementary negatively charged halide ions pushed to the surface. This charge separation near the surface generates an electric field that can induce an increase of optical band gap in the surface layers relative to the bulk. We found that the charge separation, electric field, and the amplitude of shift in the bandgap strongly depend on the halides and organic moieties of perovskite crystals. Our findings reveal the peculiarity of surface effects that are currently limiting the applications of perovskite crystals and more importantly explain their origins, thus enabling viable surface passivation strategies to remediate them