Function Follows Form: Correlation between the Growth and Local Emission of Perovskite Structures and the Performance of Solar Cells

Understanding the relationship between the growth and local emission of hybrid perovskite structures and the performance of the devices based on them demands attention. This study investigates the local structural and emission features of CH₃NH₃PbI₃, CH₃NH₃PbBr₃, and CH(NH₂)₂PbBr₃ perovskite films deposited under different yet optimized conditions using X‐ray scattering and cathodoluminescence spectroscopy, respectively. X‐ray scattering shows that a CH₃NH₃PbI₃ film involving spin coating of CH₃NH₃I instead of dipping is composed of perovskite structures exhibiting a preferred orientation with [202] direction perpendicular to the surface plane. The device based on the CH₃NH₃PbI₃ film composed of oriented crystals yields a relatively higher photovoltage. In the case of CH₃NH₃PbBr₃, while the crystallinity decreases when the HBr solution is used in a single‐step method, the photovoltage enhancement from 1.1 to 1.46 V seems largely stemming from the morphological improvements, i.e., a better connection between the crystallites due to a higher nucleation density. Furthermore, a high photovoltage of 1.47 V obtained from CH(NH₂)₂PbBr₃ devices could be attributed to the formation of perovskite films displaying uniform cathodoluminescence emission. The comparative analysis of the local structural, morphological, and emission characteristics of the different perovskite films supports the higher photovoltage yielded by the relatively better performing devices.     

Photoluminescence polarization in strained GaN/AlGaN core/shell nanowires

The optical polarization properties of GaN/AlGaN core/shell nanowire (NW) heterostructures have been investigated using polarization resolved micro-photoluminescence (μ-PL) and interpreted in terms of a strain-dependent 6 × 6 k·p theoretical model. The NW heterostructures were fabricated in two steps: the Si-doped n-type c-axis GaN NW cores were grown by molecular beam epitaxy (MBE) and then epitaxially overgrown using halide vapor phase epitaxy (HVPE) to form Mg-doped AlGaN shells. The emission of the uncoated strain-free GaN NW core is found to be polarized perpendicular to the c-axis, while the GaN core compressively strained by the AlGaN shell exhibits a polarization parallel to the NW c-axis. The luminescence of the AlGaN shell is weakly polarized perpendicular to the c-axis due to the tensile axial strain in the shell.     

Photovoltaic and Amplified Spontaneous Emission Studies of High‐Quality Formamidinium Lead Bromide Perovskite Films

This study demonstrates the formation of extremely smooth and uniform formamidinium lead bromide (CH(NH₂)₂PbBr₃ = FAPbBr₃) films using an optimum mixture of dimethyl sulfoxide and N,N‐dimethylformamide solvents. Surface morphology and phase purity of the FAPbBr₃ films are thoroughly examined by field emission scanning electron microscopy and powder X‐ray diffraction, respectively. To unravel the photophysical properties of these films, systematic investigation based on time‐integrated and time‐dependent photoluminescence studies are carried out which, respectively, bring out relatively lower nonradiative recombination rates and long lasting photogenerated charge carriers in FAPbBr₃ perovskite films. The devices based on FTO/TiO₂/FAPbBr₃/spiro‐OMeTAD/Au show highly reproducible open‐circuit voltage (V_oc) of 1.42 V, a record for FAPbBr₃‐based perovskite solar cells. V_oc as a function of illumination intensity indicates that the contacts are very selective and higher V_oc values are expected to be achieved when the quality of the FAPbBr₃ film is further improved. Overall, the devices based on these films reveal appreciable power conversion efficiency of 7% under standard illumination conditions with negligible hysteresis. Finally, the amplified spontaneous emission (ASE) behavior explored in a cavity‐free configuration for FAPbBr₃ perovskite films shows a sharp ASE threshold at a fluence of 190 μJ cm^?2 with high quantum efficiency further confirming the high quality of the films.     

High degree of polarization of the near-band-edge photoluminescence in ZnO nanowires

We investigated the polarization dependence of the near-band-edge photoluminescence in ZnO strain-free nanowires grown by vapor phase technique. The emission is polarized perpendicular to the nanowire axis with a large polarization ratio (as high as 0.84 at 4.2 K and 0.63 at 300 K). The observed polarization ratio is explained in terms of selection rules for excitonic transitions derived from the k·p theory for ZnO. The temperature dependence of the polarization ratio evidences a gradual activation of the XC excitonic transition.PACS: 78.55.Cr, 77.22.Ej, 81.07.Gf.     

Photovoltaic properties of GaAsP core-shell nanowires on Si(001) substrate

We report on the growth and electro-optical studies of photovoltaic properties of GaAsP nanowires. Low density GaAsP nanowires were grown by Au assisted MOVPE on Si(001) substrates using a two step procedure to form a radial p-n junction. The STEM analyses show that the nanowires have cubic structure with the alloy composition GaAs?.??P?.?? in the nanowire core and GaAs?.??P?.?? in the shell. The nanowire ensembles were processed in the form of sub-millimeter size mesas. The photovoltaic properties were characterized by optical beam induced current (OBIC) and electronic beam induced current (EBIC) maps. Both OBIC and EBIC maps show that the photovoltage is generated by the nanowires; however, a strong signal variation from wire to wire is observed. Only one out of six connected nanowires produce a measurable signal. These strong fluctuations can be tentatively explained by the variation of the resistance of the nanowire-to-substrate connection, which is highly sensitive to the quality of the Si-GaAsP interface. This study demonstrates the importance of the spatially resolved charge collection microscopy techniques for the diagnosis of failures in nanowire photovoltaic devices.     

M-plane core-shell InGaN/GaN multiple-quantum-wells on GaN wires for electroluminescent devices

Nonpolar InGaN/GaN multiple quantum wells (MQWs) grown on the {11-00} sidewalls of c-axis GaN wires have been grown by organometallic vapor phase epitaxy on c-sapphire substrates. The structural properties of single wires are studied in detail by scanning transmission electron microscopy and in a more original way by secondary ion mass spectroscopy to quantify defects, thickness (1-8 nm) and In-composition in the wells (～16%). The core-shell MQW light emission characteristics (390-420 nm at 5 K) were investigated by cathodo- and photoluminescence demonstrating the absence of the quantum Stark effect as expected due to the nonpolar orientation. Finally, these radial nonpolar quantum wells were used in room-temperature single-wire electroluminescent devices emitting at 392 nm by exploiting sidewall emission.     

Nanometer scale spectral imaging of quantum emitters in nanowires and its correlation to their atomically resolved structure

We report the spectral imaging in the UV to visible range with nanometer scale resolution of closely packed GaN/AlN quantum disks in individual nanowires using an improved custom-made cathodoluminescence system. We demonstrate the possibility to measure full spectral features of individual quantum emitters as small as 1 nm and separated from each other by only a few nanometers and the ability to correlate their optical properties to their size, measured with atomic resolution. The direct correlation between the quantum disk size and emission wavelength provides evidence of the quantum confined Stark effect leading to an emission below the bulk GaN band gap for disks thicker than 2.6 nm. With the help of simulations, we show that the internal electric field in the studied quantum disks is smaller than what is expected in the quantum well case. We show evidence of a clear dispersion of the emission wavelengths of different quantum disks of identical size but different positions along the wire. This dispersion is systematically correlated to a change of the diameter of the AlN shell coating the wire and is thus attributed to the related strain variations along the wire. The present work opens the way both to fundamental studies of quantum confinement in closely packed quantum emitters and to characterizations of optoelectronic devices presenting carrier localization on the nanometer scale.     

Intra-species chromosome-length polymorphism in Geotrichum candidum revealed by pulsed field gel electrophoresis

Geotrichum candidum is an ascomycetous anamorph yeast-like fungus found in various habitats. It is a component of the natural flora of milk and is used as a maturing agent for both soft and hard cheeses. This microorganism displays phenotypic variability and may act as an opportunist pathogen, causing geotrichosis. Cytological analysis of G. candidum strain ATCC 204307 showed this strain to have eight chromosomes. We prepared chromosomal DNA from 13 strains of G. candidum differing in habitat and morphotype. We used pulsed field gel electrophoresis (PFGE) in two sets of conditions to determine the size of the chromosomal DNA molecules. The strains investigated had five to eight chromosomes, 0.6 to 4.5 Mb in size. We estimated genome size in these 13 strains to be between 11 and 19 Mb. Pulsed-field gel electrophoresis profiles showed a high degree of polymorphism, indicating considerable variability between strains. Genome size and the presence of large chromosomes appeared to be correlated with morphotype. Strains with a mold-like or intermediate morphotype tended to have larger genomes than strains with a yeast-like morphotype did.     

Insights about the Absence of Rb Cation from the 3D Perovskite Lattice: Effect on the Structural,Morphological, and Photophysical Properties and Photovoltaic Performance

Efficiencies >20% are obtained from the perovskite solar cells (PSCs) employing Cs⁺ and Rb⁺ based perovskite compositions; therefore, it is important to understand the effect of these inorganic cations specifically Rb⁺ on the properties of perovskite structures. Here the influence of Cs⁺ and Rb⁺ is elucidated on the structural, morphological, and photophysical properties of perovskite structures and the photovoltaic performances of resulting PSCs. Structural, photoluminescence (PL), and external quantum efficiency studies establish the incorporation of Cs⁺ (x < 10%) but amply rule out the possibility of Rb‐incorporation into the MAPbI₃ (MA = CH₃NH₃ ⁺) lattice. Moreover, morphological studies and time‐resolved PL show that both Cs⁺ and Rb⁺ detrimentally affect the surface coverage of MAPbI₃ layers and charge‐carrier dynamics, respectively, by influencing nucleation density and by inducing nonradiative recombination. In addition, differential scanning calorimetry shows that the transition from orthorhombic to tetragonal phase occurring around 160 K requires more thermal energy for the Cs‐containing MAPbI₃ systems compared to the pristine MAPbI₃. Investigation including mixed halide (I/Br) and mixed cation A‐cation based compositions further confirms the absence of Rb⁺ from the 3D‐perovskite lattice. The fundamental insights gained through this work will be of great significance to further understand highly promising perovskite compositions.