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.

The Bethe–Salpeter equation for the electron–hole correlation function is the state-of-the-art formalism for optical and core spectroscopy in condensed matter. Solutions of this equation yield the full dielectric response, including both the absorption and the inelastic scattering spectra. Here, we present an efficient implementation within the all-electron full-potential code exciting , which employs the linearized augmented plane-wave (L)APW+LO basis set. Being an all-electron code, exciting allows the calculation of optical and core excitations on the same footing. The implementation fully includes the effects of finite momentum transfer which may occur in inelastic x-ray spectroscopy and electron energy-loss spectroscopy. Our implementation does not require the application of the Tamm–Dancoff approximation that is commonly employed in the determination of absorption spectra in condensed matter. The interface with parallel linear-algebra libraries enables the calculation for complex systems. The capability of our implementation to compute, analyze, and interpret the results of different spectroscopic techniques is demonstrated by selected examples of prototypical inorganic and organic semiconductors and insulators.

The development of novel materials for vacuum electron sources in particle accelerators is an active field of research that can greatly benefit from the results of ab initio calculations for the characterization of the electronic structure of target systems. As state-of-the-art many-body perturbation theory calculations are too expensive for large-scale material screening, density functional theory offers the best compromise between accuracy and computational feasibility. The quality of the obtained results, however, crucially depends on the choice of the exchange–correlation potential, v _xc. To address this essential point, we systematically analyze the performance of three popular approximations of v _xc [PBE, strongly constrained and appropriately normed (SCAN), and HSE06] on the structural and electronic properties of bulk Cs₃Sb and Cs₂Te as representative materials of Cs-based semiconductors employed in photocathode applications. Among the adopted approximations, PBE shows expectedly the largest discrepancies from the target: the unit cell volume is overestimated compared to the experimental value, while the band gap is severely underestimated. On the other hand, both SCAN and HSE06 perform remarkably well in reproducing both structural and electronic properties. Spin–orbit coupling, which mainly impacts the valence region of both materials inducing a band splitting and, consequently, a band-gap reduction of the order of 0.2 eV, is equally captured by all functionals. Our results indicate SCAN as the best trade-off between accuracy and computational costs, outperforming the considerably more expensive HSE06.

In a combined theoretical and experimental investigation the optical excitations of three polymorphs of crystalline pentacene are characterized in detail.

Time-dependent density-functional theory (TDDFT) often successfully reproduces excitation energies of finite systems, already in the adiabatic local-density approximation (ALDA). Here we show for prototypical molecular materials, i.e., oligothiophenes, that ALDA largely fails, and we explain why this is so. By comparing TDDFT with an in-depth analysis based on many-body perturbation theory, we demonstrate that correlation effects have a crucial impact on the energies and character of the optical excitations, not only for molecules of increasing length and in a crystalline environment but even for isolated small molecules. We argue that only high-level methodologies, which explicitly include correlation effects, can reproduce optical spectra of molecular materials with equal accuracy from gas phase to crystal structures.