Water-Accelerated Photooxidation of CH3NH3PbI3 Perovskite.

Halide perovskites have the potential to disrupt the photovoltaics market based on their high performance and low cost. However, the decomposition of perovskites under moisture, oxygen, and light raises concerns about service lifetime, especially because degradation mechanisms and the corresponding rate laws that fit the observed data have thus far eluded researchers. Here, we report a water-accelerated photooxidation mechanism dominating the degradation kinetics of archetypal perovskite CH3NH3PbI3 in air under >1% relative humidity at 25 °C. From this mechanism, we develop a kinetic model that quantitatively predicts the degradation rate as a function of temperature, ambient O2 and H2O levels, and illumination. Because water is a possible product of dry photooxidation, these results highlight the need for encapsulation schemes that rigorously block oxygen ingress, as product water may accumulate beneath the encapsulant and initiate the more rapid water-accelerated photooxidative decomposition.

Synthetic Approaches for Halide Perovskite Thin Films.

Halide perovskites are an intriguing class of materials that have recently attracted considerable attention for use as the active layer in thin film optoelectronic devices, including thin-film transistors, light-emitting devices, and solar cells. The "soft" nature of these materials, as characterized by their low formation energy and Young's modulus, and high thermal expansion coefficients, not only enables thin films to be fabricated via low-temperature deposition methods but also presents rich opportunities for manipulating film formation. This comprehensive review explores how the unique chemistry of these materials can be exploited to tailor film growth processes and highlights the connections between processing methods and the resulting film characteristics. The discussion focuses principally on methylammonium lead iodide (CH3NH3PbI3 or MAPbI3), which serves as a useful and well-studied model system for examining the unique attributes of halide perovskites, but various other important members of this family are also considered. The resulting film properties are discussed in the context of the characteristics necessary for achieving high-performance optoelectronic devices and accurate measurement of physical properties.

Electron-donating functional groups strengthen ligand-induced chiral imprinting on CsPbBr3 quantum dots.

Chiral perovskite nanoparticles and films are promising for integration in emerging spintronic and optoelectronic technologies, yet few design rules exist to guide the development of chiral material properties. The chemical space of potential building blocks for these nanostructures is vast, and the mechanisms through which organic ligands can impart chirality to the inorganic perovskite lattice are not well understood. In this work, we investigate how the properties of chiral ammonium ligands, the most common organic ligand type used with perovskites, affect the circular dichroism of strongly quantum confined CsPbBr3 nanocrystals. We show that aromatic ammonium ligands with stronger electron-donating groups lead to higher-intensity circular dichroism associated with the lowest-energy excitonic transition of the perovskite nanocrystal. We argue that this behavior is best explained by a modulation of the exciton wavefunction overlap between the nanocrystal and the organic ligand, as the functional groups on the ligand can shift electron density toward the organic species-perovskite lattice interface to increase the imprinting.

Examining the Effects of Homochirality for Electron Transfer in Protein Assemblies.

Protein voltammetry studies of cytochrome c, immobilized on chiral tripeptide monolayer films, reveal the importance of the electron spin and the film's homochirality on electron transfer kinetics. Magnetic film electrodes are used to examine how an asymmetry in the standard heterogeneous electron transfer rate constant arises from changes in the electron spin direction and the enantiomer composition of the tripeptide monolayer; rate constant asymmetries as large as 60% are observed. These findings are rationalized in terms of the chiral induced spin selectivity effect and spin-dependent changes in electronic coupling. Lastly, marked differences in the average rate constant are shown between homochiral ensembles, in which the peptide and protein possess the same enantiomeric form, compared to heterochiral ensembles, where the handedness of the peptide layer is opposite to that of the protein or itself comprises heterochiral building blocks. These data demonstrate a compelling rationale for why nature is homochiral; namely, spin alignment in homochiral systems enables more efficient energy transduction.

Melting temperature suppression of layered hybrid lead halide perovskites via organic ammonium cation branching.

Hybrid organic-inorganic lead halide perovskites have attracted broad interest because of their unique optical and electronic properties, as well as good processability. Thermal properties of these materials, often overlooked, can provide additional critical information for developing new methods of thin film preparation using, for example, melt processing-i.e., making films of hybrid perovskites by solidification of a thin layer of the melt liquid. We demonstrate that it is possible to tune the melting temperature of layered hybrid lead iodide perovskites over the range of more than 100 degrees by modifying the structures of alkylammonium-derived organic cations. Through the introduction of alkyl chain branching and extending the length of the base alkylammonium cation, melting temperatures of as low as 172 °C can be achieved and high quality thin films of layered hybrid lead iodide perovskites can be made using a solvent-free melt process with no additives and in ambient air. Additionally, we show that a similar concept can be translated to the corresponding layered bromides, with slightly higher observed melting temperatures. The design rules established here can guide the discovery of new melt-processable perovskite materials for low-cost high performance devices.

Mg Doped CuCrO2 as Efficient Hole Transport Layers for Organic and Perovskite Solar Cells.

The electrical and optical properties of the hole transport layer (HTL) are critical for organic and halide perovskite solar cell (OSC and PSC, respectively) performance. In this work, we studied the effect of Mg doping on CuCrO2 (CCO) nanoparticles and their performance as HTLs in OSCs and PSCs. CCO and Mg doped CCO (Mg:CCO) nanoparticles were hydrothermally synthesized. The nanoparticles were characterized by various experimental techniques to study the effect of Mg doping on structural, chemical, morphological, optical, and electronic properties of CCO. We found that Mg doping increases work function and decreases particle size. We demonstrate CCO and Mg:CCO as efficient HTLs in a variety of OSCs, including the first demonstration of a non-fullerene acceptor bulk heterojunction, and CH3NH3PbI3 PSCs. A small improvement of average short-circuit current density with Mg doping was found in all systems.