Structural Diversity in White-Light-Emitting Hybrid Lead Bromide Perovskites

Hybrid
organic–inorganic halide perovskites are under intense
investigations because of their astounding physical properties and
promises for optoelectronics. Lead bromide and chloride perovskites
exhibit intrinsic white-light emission believed to arise from self-trapped
excitons (STEs). Here, we report a series of new structurally diverse
hybrid lead bromide perovskites that have broad-band emission at room
temperature. They feature Pb/Br structures which vary from 1D face-sharing
structures to 3D corner- and edge-sharing structures. Through single-crystal
X-ray diffraction and low-frequency Raman spectroscopy, we have identified
the local distortion level of the octahedral environments of Pb2+ within the structures. The band gaps of these compounds
range from 2.92 to 3.50 eV, following the trend of “corner-sharing
< edge-sharing < face-sharing”. Density functional theory
calculations suggest that the electronic structure is highly dependent
on the connectivity mode of the PbBr6 octahedra, where
the edge- and corner-sharing 1D structure of (2,6-dmpz)3Pb2Br10 exhibits more disperse bands and smaller
band gap (2.49 eV) than the face-sharing 1D structure of (hep)­PbBr3 (3.10 eV). Using photoemission spectroscopy, we measured
the energies of the valence band of these compounds and found them
to remain almost constant, while the energy of conduction bands varies.
Temperature-dependent PL measurements reveal that the 2D and 3D compounds
have narrower PL emission at low temperature (∼5 K), whereas
the 1D compounds have both free exciton emission and STE emission.
The 1D compound (2,6-dmpz)3Pb2Br10 has the highest photoluminescence quantum yield of 12%, owing to
its unique structure that allows efficient charge carrier relaxation
and light emission.

有机-无机杂化卤化物钙钛矿（hybrid organic–inorganic halide perovskites）因其优异的物理性质及在光电子领域的应用前景而受到广泛深入研究。溴化铅与氯化铅钙钛矿展现出本征白光发射，该现象被认为源自自陷激子（self-trapped excitons，STEs）。本文报道了一系列结构多样的新型杂化溴化铅钙钛矿，此类材料在室温下具有宽带发射特性。它们的Pb/Br结构维度从一维面共享结构，延伸至三维角共享与边共享结构。通过单晶X射线衍射与低频拉曼光谱，我们明确了结构内Pb²⁺八面体配位环境的局部畸变程度。这些化合物的带隙范围为2.92至3.50 eV，遵循“角共享 < 边共享 < 面共享”的变化趋势。密度泛函理论（density functional theory）计算表明，材料的电子结构高度依赖于PbBr₆八面体的连接方式：(2,6-二甲基吡嗪)₃Pb₂Br₁₀的边共享与角共享一维结构，其能带更弥散，且带隙（2.49 eV）小于(hep)PbBr₃的面共享一维结构（3.10 eV）。利用光电子能谱，我们测定了此类化合物的价带能级，发现其价带能级基本保持恒定，而导带能级则存在变化。变温光致发光测试表明，二维与三维化合物在低温（约5 K）下具有更窄的光致发光发射峰，而一维化合物则同时存在自由激子发射与自陷激子发射。一维化合物(2,6-dmpz)₃Pb₂Br₁₀的光致发光量子产率最高，达到12%，这得益于其独特的结构，能够实现高效的载流子弛豫与光发射。