Activation Energy of Organic Cation Rotation in CH₃NH₃PbI₃ and CD₃NH₃PbI₃: Quasi-Elastic Neutron Scattering Measurements and First-Principles Analysis Including Nuclear Quantum Effects

The motion of CH₃NH₃⁺ cations in the low-temperature phase of the promising photovoltaic material methylammonium lead triiodide (CH₃NH₃PbI₃) is investigated experimentally as well as theoretically, with a particular focus on the activation energy. Inelastic and quasi-elastic neutron scattering measurements reveal an activation energy of ∼48 meV. Through a combination of experiments and first-principles calculations, we attribute this activation energy to the relative rotation of CH₃ against an NH₃ group that stays bound to the inorganic cage. The inclusion of nuclear quantum effects through path integral molecular dynamics gives an activation energy of ∼42 meV, in good agreement with the neutron scattering experiments. For deuterated samples (CD₃NH₃PbI₃), both theory and experiment observe a higher activation energy for the rotation of CD₃ against NH₃, which results from the smaller nuclear quantum effects in CD₃. The rotation of the NH₃ group, which is bound to the inorganic cage via strong hydrogen bonding, is unlikely to occur at low temperatures due to its high energy barrier of ∼120 meV

Growth Mechanism and Surface State of CuInS₂ Nanocrystals Synthesized with Dodecanethiol

Ternary metal chalcogenide nanocrystals (NCs) offer exciting opportunities as novel materials to be explored on the nanoscale showing optoelectronic properties tunable with size and composition. CuInS₂ (CIS) NCs are the most widely studied representatives of this family as they can be easily prepared with good size control and in high yield by reacting the metal precursors (copper iodide and indium acetate) in dodecanethiol (DDT). Despite the widespread use of this synthesis method, both the reaction mechanism and the surface state of the obtained NCs remain elusive. Here, we perform in situ X-ray diffraction using synchrotron radiation to monitor the pre- and postnucleation stages of the formation of CIS NCs. SAXS measurements show that the reaction intermediate formed at 100 °C presents a periodic lamellar structure with a characteristic spacing of 34.9 Å. WAXS measurements performed after nucleation of the CIS NCs at 230 °C demonstrate that their growth kinetics depend on the degree of precursor conversion achieved in the initial stage at 100 °C. NC formation requires the cleavage of S–C bonds. We reveal by means of combined 1D and 2D proton and carbon NMR analyses that the generated dodecyl radicals lead to the formation of a new thioether species R–S–R. The latter is part of a ligand double layer, which consists of dynamically bound dodecanethiolate ligands as well as of head-to-tail bound R–S–R molecules. This ligand double layer and a high ligand density (3.6 DDT molecules per nm²) are at the origin of the apparent difficulty to functionalize the surface of CIS NCs obtained with the DDT method

Toward Efficient Solid-State p‑Type Dye-Sensitized Solar Cells: The Dye Matters

Photoelectrochemical devices based on p-type nanostructured semiconducting materials show strong potentialities for various applications, such as photovoltaics and photocatalysis. While only one study was reported on the use of the reference dye P1 for solid-state p-type dye-sensitized solar cells (DSSC), in this work we have systematically investigated two diketopyrrolopyrrole (DPP) derivatives as sensitizers for solid-state p-type DSSC based on NiO and [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) as solid-state electron transporter material. We report on the performance in solid-state p-type DSSC of a simple DPP dye bearing a thienyl carboxylic acid as the binding group and a parent compound substituted by a pyromellitimide (PYRO) playing the role of a secondary inner electron acceptor. By focusing on the dye/PCBM interface, we specifically show using transient photoluminescence measurements that the presence of a secondary electron acceptor unit can efficiently favor the formation of the (dye+/PCBM-) state, owing to its significant reducing ability and lifetime of the charge separated state. As a consequence, using these DPP derivatives leads to unprecedented photocurrents up to 0.45 mA cm^–2, which are 10 times larger than previously reported values for the system based on P1. Our analysis also demonstrates the strong correlation between the ability of the dyes to efficiently generate charge carriers and the resulting photocurrents