Metal halide perovskites are the first solution processed semiconductors that can compete in their functionality with conventional semiconductors, such as silicon. Over the past several years, perovskite semiconductors have reported breakthroughs in various optoelectronic devices, such as solar cells, photodetectors, light emitting and memory devices, and so on. Until now, perovskite semiconductors face challenges regarding their stability, reproducibility, and toxicity. In this Roadmap, we combine the expertise of chemistry, physics, and device engineering from leading experts in the perovskite research community to focus on the fundamental material properties, the fabrication methods, characterization and photophysical properties, perovskite devices, and current challenges in this field. We develop a comprehensive overview of the current state-of-the-art and offer readers an informed perspective of where this field is heading and what challenges we have to overcome to get to successful commercialization.

Colorless, transparent, and environmentally stable polymer films with large luminescence Stokes shift are in demand for down-conversion of solar UV radiation. To develop such a polymer film that exhibits phosphorescence at room-temperature with a large Stokes shift and excellent thermal stability, a semi-aromatic polyimide (DBrBP-PI) was newly synthesized using a biphenyltetracarboxylic dianhydride substituted with two bromine (Br) atoms. Thin films of DBrBP-PI were completely colorless because of effective suppression of the aggregation of the PI chains due to the steric effects of the Br-substituted dianhydride structure. The PI films exhibited bright green phosphorescence at 512 nm (Φ = 0.02) with a very large Stokes shift of 12 215 cm⁻¹ as well as prompt blue fluorescence at around 408 nm when excited at 315 nm at 298 K. The glass transition and the 5 wt% loss temperatures of DBrBP-PI were 325 °C and 427 °C, respectively. The phosphoresce intensity of the DBrBP-PI film was significantly enhanced under vacuum (Φ = 0.22) and at low temperatures (Φ = 0.76 at 77 K) owing to suppression of triplet state quenching by atmospheric oxygen and dumping of the molecular motions of the PI chains in the solid film. The suggested material design concept shows a way towards creation of stable and efficient down-converters for UV radiation.

Annealing of TQ1:N2200 photovoltaic blends reduces geminate charge recombination, without compromising charge extraction, leading to higher photocurrents and device efficiency.

Due to their high thermal and environmental stability, polyimides (PIs) are one of the most attractive candidates for novel highly fluorescent polymers, though photophysical studies of PIs are challenging owing to their poor solubility in common solvents. To overcome these problems, we have synthesized and examined a series of low molecular weight model imide compounds: substituted N-cyclohexylphthalimides with alicyclic amino groups at the 3 or 4-positions of the benzene rings (x-NHPIs). Their photophysical properties were systematically investigated by steady-state UV/Visible absorption, fluorescence, and time-resolved fluorescence techniques. In solution, unsubstituted N-cyclohexylphthalimide (NHPI) showed almost no emission, while x-NHPIs exhibited enhanced fluorescence emission depending on the solvent polarity. Analysis of the solvatochromism of the x-NHPIs via Lippert–Mataga plots indicated the generation of large dipole moments in the excited singlet states originating from the intramolecular charge-transfer (ICT) states. The significant difference in the fluorescence quantum yields (Φ) between the 3-substituted (3Pi and 3Pyr) and 4-substituted NHPIs (4Pi and 4Pyr) strongly suggests that the former form a twisted ICT (TICT) state, whereas the latter form a planar ICT (PICT) state when excited. 4-Substituted NHPIs also show high fluorescence yields in the crystalline state. A particularly large Φ value was obtained for the 4Pi crystal, which we explain by the large intermolecular distances and the arrangement of molecules minimizing intermolecular interactions as well as the small non-radiative deactivation rate. These facts clearly demonstrate that the introduction of an alicyclic amino group into NHPI at the 4-position enhances the fluorescence quantum yields significantly, which suggests a new pathway for the development of novel, highly fluorescent PIs.

Charge carrier generation and decay in $m$-LPPP polymer films were examined by means of femtosecond transient absorption spectroscopy in the time window of $100\mathrm{fs}–15\mathrm{ns}$. Two modes of polaron formation with distinct behavior were identified, impurity induced in the absence of an external electric field and electric field induced in pristine film. While field induced charge generation is relatively slow, occurring throughout the excited state lifetime, the rate of impurity induced charge generation is much faster and depends on excitation wavelength; it occurs on the several hundred femtosecond time scale under excitation within the main absorption band, but excitation into the red wing of the absorption band results in charge generation within less than $100\mathrm{fs}$. Polaron decay through geminate electron-hole recombination occurs with widely distributed lifetimes, from $\sim 0.8\mathrm{ns}$ to microseconds; the polarons characterized by the shortest decay time have a redshifted absorption spectrum (as compared to more long-lived polarons) and are attributed to tightly bound polaron pairs.