Measuring <i>n</i> and <i>k</i> at the Microscale in Single Crystals of CH₃NH₃PbBr₃ Perovskite

Lead-based, inorganic–organic hybrid perovskites have shown much promise in photovoltaics, and the ability to tune their band gap makes them attractive for tandem solar cells, photodetectors, light-emitting diodes, and lasers. A crucial first step toward understanding a material’s behavior in such optoelectronic devices is determining its complex refractive index, <i>n + ik</i>; however, optically smooth films of hybrid perovskites are challenging to produce, and the optical properties of films of these materials have been shown to depend on the size of their crystallites. To address these challenges, this work reports quantitative reflectance and transmittance measurements performed on individual microcrystals of CH₃NH₃PbBr₃, with thicknesses ranging from 155 to 1907 nm. The single crystals are formed by spin-coating a film of precursor solution and then stamping it with polydimethylsiloxane (PDMS) during crystallization. By measuring crystals of varying thickness, <i>n</i> and <i>k</i> values at each wavelength (405–1100 nm) have been determined, which agree with recently reported values extracted by ellipsometry on millimeter-sized single crystals. This approach can be applied to determine the optical constants of any material that presents challenges in producing smooth films over large areas, such as mixed-halide hybrid and inorganic perovskites, and to micro- or nanoplatelets

Preparation of Organometal Halide Perovskite Photonic Crystal Films for Potential Optoelectronic Applications

Herein, a facile method for the preparation of organometal halide perovskite (OHP) thin films in photonic crystal morphology is presented. The OHP photonic crystal thin films with controllable porosity and thicknesses between 2 μm and 6 μm were prepared on glass, fluorine-doped tin oxide (FTO), and TiO₂ substrates by using a colloidal crystal of polystyrene microspheres as a template to form an inverse opal structure. The composition of OHP could be straightforwardly tuned by varying the halide anions. The obtained OHP inverse opal films possess large ordered domains with a periodic change of the refractive index, which results in pronounced photonic stop bands in the visible light range. By changing the diameter of the polystyrene microspheres, the position of the photonic stop band can be tuned through the visible spectrum. This developed methodology can be used as blueprint for the synthesis of various OHP films that could eventually be used as more effective light harvesting materials for diverse applications

Halide Perovskite 3D Photonic Crystals for Distributed Feedback Lasers

Halide perovskites are under intense investigation for light harvesting applications in solar cells. Their outstanding optoelectronic properties such as long charge carrier diffusion lengths, high absorption coefficients, and defect tolerance also has triggered interest in laser and LED applications. Here, we report on the lasing properties of 3D distributed feedback halide perovskite nanostructures prepared via an all-solution process. A colloidal crystal templating approach was developed to precisely control the hybrid halide perovskite structure on the nanoscale. The prepared CH₃NH₃PbBr₃ thin films with inverse opal morphology show narrow lasing emissions with a full width half-maximum as low as 0.15 nm and good long-term stability under pulsed laser excitation above the lasing threshold of 1.6 mJ cm^–2 in ambient atmosphere. Furthermore, lasing emission was also observed for CH₃NH₃PbI₃ inverse opals under excitation with a focused laser beam. Unlike other protocols for the fabrication of distributed feedback perovskite lasers, control of the nanostructure of hybrid halide perovskites is achieved without the use of expensive and elaborate lithography techniques or high temperatures. Therefore, the presented protocol opens a route to the low cost fabrication of hybrid halide perovskite lasers

Core–Shell CdS–Cu₂S Nanorod Array Solar Cells

As an earth-abundant p-type semiconductor, copper sulfide (Cu₂S) is an attractive material for application in photovoltaic devices. However, it suffers from a minority carrier diffusion length that is less than the length required for complete light absorption. Core–shell nanowires and nanorods have the potential to alleviate this difficulty because they decouple the length scales of light absorption and charge collection. To achieve this geometry using Cu₂S, cation exchange was applied to an array of CdS nanorods to produce well-defined CdS–Cu₂S core–shell nanorods. Previous work has demonstrated single-nanowire photovoltaic devices from this material system, but in this work, the cation exchange chemistry has been applied to nanorod arrays to produce ensemble-level devices with microscale sizes. The core–shell nanorod array devices show power conversion efficiencies of up to 3.8%. In addition, these devices are stable when measured in air after nearly one month of storage in a desiccator. These results are a first step in the development of large-area nanostructured Cu₂S-based photovoltaics that can be processed from solution

Growth and Characterization of PDMS-Stamped Halide Perovskite Single Microcrystals

Recently, halide perovskites have attracted considerable attention for optoelectronic applications, but further progress in this field requires a thorough understanding of the fundamental properties of these materials. Studying perovskites in their single-crystalline form provides a model system for building such an understanding. In this work, a simple solution-processed method combined with PDMS (polydimethyl­siloxane) stamping was used to prepare thin single microcrystals of halide perovskites. The method is general for a broad array of materials including CH₃NH₃PbBr₃, CH₃NH₃PbCl₃, CH₃NH₃Pb­(Br_0.5Cl_0.5)₃, CH₃NH₃Pb­(Br_0.75Cl_0.25)₃, CsPbBr₃, Cs₃Bi₂Br₉, and Cs₃Bi₂I₉. Electron backscatter diffraction (EBSD) was used to investigate the microstructure of the crystals. In order to characterize the microcrystals of CH₃NH₃PbBr₃ electrically, the crystals were grown on prefabricated electrodes creating single-crystal devices contacted from the back. This back-contacted platform circumvents the incompatibility between halide perovskites and the aqueous chemistry used in standard microfabriation processes. It also allows <i>in situ</i> characterization of the perovskite crystal while it operates as a microscopic solar cell

Epitaxially Aligned Cuprous Oxide Nanowires for All-Oxide, Single-Wire Solar Cells

As a <i>p</i>-type semiconducting oxide that can absorb visible light, cuprous oxide (Cu₂O) is an attractive material for solar energy conversion. This work introduces a high-temperature, vapor-phase synthesis that produces faceted Cu₂O nanowires that grow epitaxially along the surface of a lattice-matched, single-crystal MgO substrate. Individual wires were then fabricated into single-wire, all-oxide diodes and solar cells using low-temperature atomic layer deposition (ALD) of TiO₂ and ZnO films to form the heterojunction. The performance of devices made from pristine Cu₂O wires and chlorine-exposed Cu₂O wires was investigated under one-sun and laser illumination. These faceted wires allow the fabrication of well-controlled heterojunctions that can be used to investigate the interfacial properties of all-oxide solar cells

Carrier Diffusion Lengths in Hybrid Perovskites: Processing, Composition, Aging, and Surface Passivation Effects

Carrier Diffusion Lengths in Hybrid Perovskites: Processing, Composition, Aging, and Surface Passivation Effect

Perovskite Nanowire Extrusion

The defect tolerance of halide perovskite materials has led to efficient optoelectronic devices based on thin-film geometries with unprecedented speed. Moreover, it has motivated research on perovskite nanowires because surface recombination continues to be a major obstacle in realizing efficient nanowire devices. Recently, ordered vertical arrays of perovskite nanowires have been realized, which can benefit from nanophotonic design strategies allowing precise control over light propagation, absorption, and emission. An anodized aluminum oxide template is used to confine the crystallization process, either in the solution or in the vapor phase. This approach, however, results in an unavoidable drawback: only nanowires embedded inside the AAO are obtainable, since the AAO cannot be etched selectively. The requirement for a support matrix originates from the intrinsic difficulty of controlling precise placement, sizes, and shapes of free-standing nanostructures during crystallization, especially in solution. Here we introduce a method to fabricate free-standing solution-based vertical nanowires with arbitrary dimensions. Our scheme also utilizes AAO; however, in contrast to embedding the perovskite inside the matrix, we apply a pressure gradient to extrude the solution from the free-standing templates. The exit profile of the template is subsequently translated into the final semiconductor geometry. The free-standing nanowires are single crystalline and show a PLQY up to ∼29%. In principle, this rapid method is not limited to nanowires but can be extended to uniform and ordered high PLQY single crystalline perovskite nanostructures of different shapes and sizes by fabricating additional masking layers or using specifically shaped nanopore endings

Phase-Selective Cation-Exchange Chemistry in Sulfide Nanowire Systems

As a cation-deficient, <i>p</i>-type semiconductor, copper sulfide (Cu_2–<i>x</i>S) shows promise for applications such as photovoltaics, memristors, and plasmonics. However, these applications demand precise tuning of the crystal phase as well as the stoichiometry of Cu_2–<i>x</i>S, an ongoing challenge in the synthesis of Cu_2–<i>x</i>S materials for a specific application. Here, a detailed transformation diagram of cation-exchange (CE) chemistry from cadmium sulfide (CdS) into Cu_2–<i>x</i>S nanowires (NWs) is reported. By varying the reaction time and the reactants’ concentration ratio, the progression of the CE process was captured, and tunable crystal phases of the Cu_2–<i>x</i>S were achieved. It is proposed that the evolution of Cu_2–<i>x</i>S phases in a NW system is dependent on both kinetic and thermodynamic factors. The reported data demonstrate that CE can be used to precisely control the structure, composition, and crystal phases of NWs, and such control may be generalized to other material systems for a variety of practical applications