Organic-inorganic perovskites have shown promise as high-performance absorbers in solar cells, first as a coating on a mesoporous metal oxide scaffold and more recently as a solid layer in planar heterojunction architectures. Here, we report transient absorption and photoluminescence-quenching measurements to determine the electron-hole diffusion lengths, diffusion constants, and lifetimes in mixed halide (CH3NH3PbI(3-x)Cl(x)) and triiodide (CH3NH3PbI3) perovskite absorbers. We found that the diffusion lengths are greater than 1 micrometer in the mixed halide perovskite, which is an order of magnitude greater than the absorption depth. In contrast, the triiodide absorber has electron-hole diffusion lengths of ~100 nanometers. These results justify the high efficiency of planar heterojunction perovskite solar cells and identify a critical parameter to optimize for future perovskite absorber development.

No abstract

The topology of a conjugated molecule plays a significant role in controlling both the electronic properties and the conformational manifold that the molecule may explore. Fully π-conjugated molecular nanorings are of particular interest, as their lowest electronic transition may be strongly suppressed as a result of symmetry constraints. In contrast, the simple Kasha model predicts an enhancement in the radiative rate for corresponding linear oligomers. Here we investigate such effects in linear and cyclic conjugated molecules containing between 6 and 42 butadiyne-linked porphyrin units (corresponding to 600 C-C bonds) as pure monodisperse oligomers. We demonstrate that as the diameter of the nanorings increases beyond ∼10 nm, its electronic properties tend toward those of a similarly sized linear molecule as a result of excitation localization on a subsegment of the ring. However, significant differences persist in the nature of the emitting dipole polarization even beyond this limit, arising from variations in molecular curvature and conformation.

No abstract

This work describes the unexpected alignment of a common perylene bisimide fluorescent dye (Lumogen Red305) in a host liquid crystal matrix. The negative dichroic fluorophore orients with the primary absorption/emission dipole, corresponding to the physical long axis of the perylene bisimide core, perpendicular to the director of a host liquid crystal. A second absorption dipole, which lies perpendicular to the primary dipole and corresponding to the physical short axis of the perylene core, lies parallel to the host liquid crystal. Individual illumination of the two absorbing optical transition dipole moments of the Red305 dye results in a single linearly polarized emission. When Red305 is combined with a second positive dichroic Coumarin‐type fluorophore that aligns in the conventional manner in a liquid crystal host, that is, with an absorption dipole along the physical long axis of the fluorophore and the director of the host liquid crystal, advanced light management is possible, such as electrically switchable colors and directing emission light to different edges in a luminescent solar concentrator device.