Lead‐free halide double perovskites with diverse electronic structures and optical responses, as well as superior material stability show great promise for a range of optoelectronic applications. However, their large bandgaps limit their applications in the visible light range such as solar cells. In this work, an efficient temperature‐derived bandgap modulation, that is, an exotic fully reversible thermochromism in both single crystals and thin films of Cs2AgBiBr6 double perovskites is demonstrated. Along with the thermochromism, temperature‐dependent changes in the bond lengths of AgBr (RAgBr) and BiBr (RBiBr) are observed. The first‐principle molecular dynamics simulations reveal substantial anharmonic fluctuations of the RAgBr and RBiBr at high temperatures. The synergy of anharmonic fluctuations and associated electron–phonon coupling, and the peculiar spin–orbit coupling effect, is responsible for the thermochromism. In addition, the intrinsic bandgap of Cs2AgBiBr6 shows negligible changes after repeated heating/cooling cycles under ambient conditions, indicating excellent thermal and environmental stability. This work demonstrates a stable thermochromic lead‐free double perovskite that has great potential in the applications of smart windows and temperature sensors. Moreover, the findings on the structure modulation‐induced bandgap narrowing of Cs2AgBiBr6 provide new insights for the further development of optoelectronic devices based on the lead‐free halide double perovskites.
Effective passivation and stabilization of both the inside and interface of a perovskite layer are crucial for perovskite solar cells (PSCs), in terms of efficiency, reproducibility, and stability. Here, the first formamidinium lead iodide (δ-FAPbI ) polymorph passivated and stabilized MAPbI PSCs are reported. This novel MAPbI /δ-FAPbI structure is realized via treating a mixed organic cation MA FA PbI perovskite film with methylamine (MA) gas. In addition to the morphology healing, MA gas can also induce the formation of δ-FAPbI phase within the perovskite film. The in situ formed 1D δ-FAPbI polymorph behaves like an organic scaffold that can passivate the trap state, tunnel contact, and restrict organic-cation diffusion. As a result, the device efficiency is easily boosted to 21%. Furthermore, the stability of the MAPbI /δ-FAPbI film is also obviously improved. This δ-FAPbI phase passivation strategy opens up a new direction of perovskite structure modification for further improving stability without sacrificing efficiency.
Solution-processed organic−inorganic halide perovskite (OIHP) thin films typically contain fine, randomly oriented grains and a high-density grain-boundary network, which are unfavorable for key film functions including charge transport and environmental stability. Here, we report a new chemical route for achieving CH 3 NH 3 PbI 3 (MAPbI 3 ) OIHP thin films comprising large, uniaxially oriented grains and an ultralowdensity grain-boundary network. This route starts with a new metastable liquid-state precursor phase, MAPbI 3 ·MACl· xCH 3 NH 2 , which converts to metastable MAPbI 3 ·MACl and then to MAPbI 3 OIHP upon stepwise release of volatile CH 3 NH 2 and MACl. Perovskite solar cells made via this route show high power conversion efficiency of up to 19.4%, with significantly enhanced environmental stability.
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