This review provides an overview of factors affecting film morphology and how together with device architecture they impact perovskite cell performance.

Solution-processed organic-inorganic hybrid perovskites are promising emitters for next-generation optoelectronic devices. Multiple-colored, bright light emission is achieved by tuning their composition and structures. However, there is very little research on exploring optically active organic cations for hybrid perovskites. Here, unique room-temperature phosphorescence from hybrid perovskites is reported by employing novel organic cations. Efficient room-temperature phosphorescence is activated by designing a mixed-cation perovskite system to suppress nonradiative recombination. Multiple-colored phosphorescence is achieved by molecular design. Moreover, the emission lifetime can be tuned by varying the perovskite composition to achieve persistent luminescence. Efficient room-temperature phosphorescence is demonstrated in hybrid perovskites that originates from the triplet states of the organic cations, opening a new dimension to the further development of perovskite emitters with novel functional organic cations for versatile display applications.

2,4-Trifluoromethylquinoline (TFMAQ) derivatives that have amine (1), methylamine (2), phenylamine (3), and dimethylamine (4) substituents at the 7-position of the quinoline ring were prepared and crystallized. Six crystals including the crystal polymorphs of 2 (crystal GB and YG) and 3 (crystal B and G) were obtained and characterized by X-ray crystallography. In solution, TFMAQ derivatives emitted relatively strong fluorescence (lambda(max)(f)=418-469 nm and Φ(f)(s)=0.23-0.60) depending on the solvent polarity. From Lippert-Mataga plots, Δμ values in the range of 7.8-14 D were obtained. In the crystalline state, TFMAQ derivatives emitted at longer wavelengths (lambda(max)(f)=464-530 nm) with lower intensity (Φ(f)(c)=0.01-0.28) than those in n-hexane solution. The polymorphous crystals of 2 and 3 emitted different colors: 2, lambda(max)(f)=470 and 530 nm with Φ(f)(c)=0.04 and approximately 0.01 for crystal GB and YG, respectively; and 3, lambda(max)(f)=464 and 506 nm with Φ(f)(c)=0.28 and approximately 0.28 for crystal B and G, respectively. In both crystal polymorphs of 2 and 3, crystals GB and G showed emission color changes by heating/melting/cooling cycles that were representative. By following the color changes in heating at the temperature below the melting point with X-ray diffraction measurements and X-ray crystallography, the single-crystal-to-single-crystal transformations from crystal GB to YG for 2 and from crystal B to G for 3 were revealed.

Crystal polymorphs of 1,8-naphthyridine derivative, being anti and syn conformers, show a reversible transformation from anti to syn by heating and from syn to anti by grinding with the alteration of emittance intensity, and notably, thermal transformation from anti to syn conformer took place in single-crystal-to-single-crystal (SC-to-SC) form, which was confirmed by a single crystal X-ray crystallography.

Phenyl-7-amino-2,4-trifluoromethylquinoline derivatives (R = Me (1), Et (2), isopropyl (3), and Ph (4)) were prepared as a new type of fluorophore responsive to external stimuli. 1, 2, 3, and 4 were obtained as single crystals including three crystal polymorphs (1α, 1β, and 1γ) of 1 and two (2α and 2β) of 2. In 4, a phase transition from 4 173 and 4 90 between 173 and 90 K was observed. The solid-state emission showed a red shift by 30−58 nm compared with the emission in n-hexane, and their emission properties depended on the molecular arrangements. The modes of molecular arrangements for 1α, 1β, and 1γ were a slipped parallel (SP), head-to-tail γ-type herringbone (HT-γ-HB), and head-to-head γ-type herringbone (HH-γ-HB); those for 2α and 2β were HT-γ-HB and head-to-tail dimer (HT-dimer), and that for 3 was head-to-tail columnar (HTC). 4 173 and 4 90 were similar HT-γ-HB. The crystal-to-crystal transformations from 1γ to 1β and from 2β to 2α were observed by heating and grinding the crystal, respectively, with emittance changes. After melting, on cooling, all crystals formed supercooled liquid (SCL) and then glass states. In the SCL state, molecules were amorphous and were quickly crystallized by a mechanical stimulus such as scratching. By taking advantage of the difference of emitting intensity between the SCL and the crystal states for 1, "writing" and "erasing" of a letter with scratching and heating, respectively, were demonstrated.