ABSTRACT
Two-dimensional organic-inorganic hybrid halide perovskites possess diverse structural polymorphs with versatile physical properties, which can be controlled by order-disorder transition of the spacer cation, making them attractive for constructing semiconductor homojunctions. Here, we demonstrate a space-cation-dopant-induced phase stabilization approach to creating a lateral homojunction composed of ordered and disordered phases within a two-dimensional perovskite. By doping a small quantity of pentylammonium into (butylammonium)2PbI4 or vice versa, we effectively suppress the ordering transition of the spacer cation and the associated out-of-plane octahedral tilting in the inorganic framework, resulting in phase pining of the disordered phase when decreasing temperature or increasing pressure. This enables epitaxial growth of a two-dimensional perovskite homojunction with tunable optical properties under temperature and pressure stimuli, as well as directional exciton diffusion across the interface. Our results demonstrate a previously unexplored strategy for constructing two-dimensional perovskite heterostructures by thermodynamic tuning and spacer cation doping.
ABSTRACT
The stereochemical expression of the 6s2 lone pair on the lead atom has a significant impact on the crystal structures and physical properties of lead halide perovskites. Two-dimensional (2D) lead bromide perovskites often exhibit a broadband emission, yet the structural origin of the broadband emission has been under debate. Here, we report the synthesis and characterization of a 2D lead bromide hybrid (4-chlorophenylammonium)2PbBr4 that consists of a combination of the octahedral unit PbBr6 and the rarely observed capped octahedral unit PbBr7 through corner-sharing and edge-sharing linkages. The seven-coordination geometry indicates a strong stereo-active lone pair on the Pb2+ cation. By comparing this structure with two representative 2D perovskites, (benzylammonium)2PbBr4 and (4-aminotetrahydropyran)2PbBr4, we establish how the lone pair expression affects the local coordination geometry of the Pb2+ cation and the resulting optical and electronic properties. As the Pb-Br bond length increases, the lone pair expression leads to off-centering displacement of Pb2+ within the octahedra and then the formation of seven-coordination capped octahedra. Density functional theory calculations indicate that the off-centering distorted octahedra and capped octahedra are due to the asymmetric distribution of the Pb electrons that have both s and p orbital characteristics. Spectroscopic studies show the photoluminescence spectra evolving from narrowband emission to broadband emission with increasing LPE, as well as softer and more anharmonic lattice vibrations that facilitate exciton self-trapping. Our results demonstrate that lone pairs could be a powerful design rule for developing light emitting materials.
ABSTRACT
Tin oxide (SnO2) is the most commonly used electron transport material for perovskite solar cells (PSCs). Various techniques have been applied to deposit tin dioxide, including spin-coating, chemical bath deposition, and magnetron sputtering. Among them, magnetron sputtering is one of the most mature industrial deposition techniques. However, PSCs based on magnetron-sputtered tin oxide (sp-SnO2) have a lower open-circuit voltage (Voc) and power conversion efficiency (PCE) than those prepared by the mainstream solution method. This is mainly due to the oxygen-related defects at the sp-SnO2/perovskite interface, and traditional passivation strategies usually have little effect on them. Herein, we successfully isolate the oxygen adsorption (Oads) defects located on the surface of sp-SnO2 from the perovskite layer using a PCBM double-electron transport layer. This isolation strategy effectively suppresses the Shockley-Read-Hall recombination at the sp-SnO2/perovskite interface, which results in an increase in the Voc from 0.93 to 1.15 V and an increase in PCE from 16.66 to 21.65%. To our knowledge, this is the highest PCE achieved using a magnetron-sputtered charge transport layer to date. The unencapsulated devices maintain 92% of their initial PCE after storage in air with a relative humidity of 30-50% after 750 h. We further use the solar cell capacitance simulator (1D-SCAPS) to confirm the effectiveness of the isolation strategy. This work highlights the application prospect of magnetron sputtering in the field of perovskite solar cells and provides a simple yet effective way to tackle the interfacial defect issue.
