Halide perovskites are a compelling candidate for the next generation of clean energy harvesting technologies thanks to their low cost, facile fabrication and outstanding semiconductor properties. However, photovoltaic device efficiencies are still below practical limits and long-term stability challenges hinder their practical application. Current evidence suggests that strain in halide perovskites is a key factor in dictating device efficiency and stability. Here, we outline the fundamentals of strain within halide perovskites relevant to photovoltaic applications and rationalize approaches to characterize the phenomenon. We examine recent breakthroughs in eliminating the adverse impacts of strain, enhancing both device efficiencies and operational stabilities. Finally, we discuss further challenges and outline future research directions for placing stress and strain studies at the forefront of halide perovskite research. An extensive understanding of strain in halide perovskite is needed, which would allow effective strain management and drive further enhancements in efficiencies and stabilities of perovskite photovoltaics.

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Stereoselective coordination/insertion polymerization of the polar ortho-methoxystyrene has been achieved for the first time by using the cationic β-diketiminato rare-earth-metal species. High activity and excellent isoselectivity (mmmm>99 %) were acheived. The unmasked Lewis-basic methoxy group does not poison the Lewis-acidic metal center, but instead activates the polymerization through σ-π chelation to the active species together with the vinyl group, thus lower the coordination and activation energies as compared with those of styrene derivatives lacking the methoxy group.

A new family of syndiotactic polystyrene derivatives containing a methoxyl group was established successfully by employing a yttrium catalyst.

Styrene underwent unprecedented coordination-insertion copolymerization with naked polar monomers (ortho-/meta-/para-methoxystyrene) in the presence of a pyridyl methylene fluorenyl yttrium catalyst. High activity (1.26×10 g mol h ) and excellent syndioselectivity were observed, and high-molecular-weight copolymers (24.6×10 g mol ) were obtained. The insertion rate of the polar monomers could be adjusted in the full range of 0-100 % simply by changing the loading of the polar styrene monomer. Strikingly, the copolymers had tapered, gradient, and even random sequence distributions, depending on the position of the polar methoxy group on the phenyl ring and thus on its mode of coordination to the active metal center, as shown by tracking the polymerization process and DFT calculations.

Alternating copolymers have the clearest and most defined microstructures among manmade polymers, having been promising building blocks to access synthetic polymers able to mimic biomaterials. The most successful approaches employ donor–acceptor monomer couples, enantiomers with different substituents, as well as specially designed cyclic monomers containing various units through ionic and living radical polymerizations. Herein we report the catalytic behaviors of rare-earth metal-based catalyst systems toward the direct copolymerization of ethylene with a series of unmasked polar styrenes and nonpolar styrenes. For the copolymerization of ethylene with para-methoxystyrene, the pyridyl side-armed fluorenyl-supported yttrium catalyst was inert, while its scandium analogue displayed moderate activity to give a random copolymer; the half-sandwich fluorenyl scandium catalyst provided a gel product. In contrast, the methyl-substituted N-heterocyclic carbene (NHC) side-armed fluorenyl scandium catalyst showed the highest activity, 3.19 × 105 g molSc –1 h–1, which was 10 times higher than its analogue bearing the steric bulky trimethylphenyl-substituted NHC fluorenyl ligand, although it could not initiate any polar styrene homopolymerization. The catalytic performance was extended to the other polar styrenes, such as meta-methoxystyrenes, 6-methoxy-2-vinylnaphthalene, para-methylthiostyrene, diphenyl­(4-vinylphenyl)­phosphine, and para-(N,N-diethylamino)­styrene. All of the resultant copolymers are composed of pseudo-alternating microstructures despite polymerization conditions. In particular, when para-(N,N-dimethylamino)­styrene was used as the comonomer, a perfect alternating product was generated with an as high as 83% comonomer conversion. The relationships among the structural factors and electronics of the precursors and their catalytic performances and the resultant copolymer compositions and the sequence distributions were established.

The N,O-bidentate pyridyl functionalized alkoxy ligands 2-(6-methyl-2-pyridinyl)-1,1-dimethyl-1-ethanol (L1–H) and 2-(6-methyl-2-pyridinyl)-1,1-diphenyl-1-ethanol (L2–H) have been prepared by treatment of acetone and benzophenone with monolithiated 2,6-lutidine. Deprotonolysis of the ligands L1–H and L2–H with 1 equiv of Mg n Bu2 and ZnEt2 in toluene by releasing butane and ethane, respectively, gave the corresponding dimeric metal-monoalkyl complexes [L1Mg n Bu]2 (1), [L2Mg n Bu]2 (2), [L1ZnEt]2 (3), and [L2ZnEt]2 (4). Complexes 1–4 were characterized by 1H and 13C NMR spectroscopy analysis, and the molecular structures of 1, 3, and 4 were further confirmed by X-ray diffraction analysis. The investigation of the catalytic behavior of these complexes toward ε-caprolactone (ε-CL) and l-lactide (l-LA) polymerizations showed that the Mg-based complexes gave higher activity than those attached to zinc metal, probably owing to the greater ionic character of the magnesium metal. Remarkably, the magnesium complex 2 exhibited a striking “immortal” nature in the presence of primary alcohols where up to 500 PCL chains grew from each Mg active center when benzyl alcohol was employed, while, in particular, in the presence of triethanolamine, complex 2 also displayed an immortal mode for the polymerization of l-LA.

The homopolymerization of a polar monomer, 4-methylthiostyrene (MTS), was successfully achieved by a rare-earth metal based catalyst in the highest activity of 45.1 × 10 4 g mol Y −1 h −1 and the excellent syndioselectivity (rrrr > 99%). The polymerization was rather controllable that the resultant poly(methylthiostyrene)s (PMTS) had molecular weights comparable to the theoretic ones reaching up to 1.7 × 10 5 while the molecular weight distributions were narrow (PDI = 1.3−1.9). Moreover, the copolymerization of this polar MTS with the nonpolar styrene (St) performed fluently under various MTS-to-St ratios in a quasi-living mode. The monomer reactivity ratios were r MTS = 1.08 and r St = 0.77, following the first Markov statistics, and was close to the ideal random copolymerization. Therefore, a series of unprecedented statistical random copolymers, P(St-r-MTS)s, where the compositions were strictly closed to the monomer fed ratios, had been accessed. Strikingly, both monomer sequences remained highly syndiotactic as their homopolymers regardless of the compositions, thus endowing P(St-r-MTS)s variable glass transition temperatures and melting points. The shortest number-averaged sequence length for these copolymers P(St-r-MTS) crystallizing from the melts was n ̅ St = 5.75 for PS sequences and n ̅ MTS = 8.11 for PMTS.