Synthetic exploration of halide perovskites and germanium semiconductors

Halide perovskites and germanium semiconductors are promising materials for many optical applications such as solar cells and LEDs due to their unique photophysical properties. Compositional substitution and dimensional manipulation can enhance physical or chemical properties of perovskite and germanium semiconductors which in turn promotes their performance in optoelectronic devices. In this thesis, we report the synthetic exploration of composition-control and dimensionality-control of organometal halide perovskite crystals by tuning halide-incorporation and exploiting bulky alkylammonium cations as capping ligands. We also demonstrate a systematic synthesis of all the series of mixed halide perovskite polycrystals and their low dimensional analogues. By optimizing synthetic conditions, we are able to inhibit the appearance of a reversible photoinduced PL peak derived from surface traps. We also synthesize lead-free perovskites for the environmental concerns. Lead is a heavy metal element and its potential toxicity raises concerns for environmental compatibility. To address this problem, we developed a synthetic route to antimony perovskites and germanium perovskites. Surface-bound (CH3)3Sb2I9 layers restrict the growth of CH3NH3PbI3, resulting in CH3NH3PbI3 nanocrystals. Compared to the bulk perovskites, the antimony-capped nanocrystals show stronger photoluminescence. With a direct bandgap of 1.6 eV and a corner-sharing octahedral network crystal structure that are comparable to CH3NH3PbI3, CsGeI3 is potentially promising for photovoltaic applications. To manipulate the optoelectronic properties, we doped high-spin, divalent manganese ions (Mn2+) into the octahedral Ge2+ sites of CsGeI3. Electron paramagnetic resonance (EPR) helps us better understand the local ion environment and composition of both CsGeI3 and its doped analogue (CsGe1-xMnxI3). Our results expand the lead-free halide perovskite family and set the stage for their application beyond photovoltaics to spintronics and magnetic data storage. Finally, we fabricated and characterized Ge1-xSnx alloy nanocrystals and Ge1-xSnx core/shell nanocrystals. Germanium has an indirect bandgap of 0.66 eV, which is too narrow for ideal solar cell light harvester materials and limits their absorption efficiency. By tin incorporation and quantum confinement effect, we could enhance their efficiency of solar absorption and in turn their quantum yield. We synthesized Ge1-xSnx and Ge1-xSnx/CdS core/shells in solution phase. Inclusion of tin is confirmed by X-ray diffraction and Raman peak shift. Tin alone does not result in enhanced photoluminescence intensity, however, adding an epitaxial CdS shell onto the Ge1-xSnx nanocrystals does enhance the photoluminescence up to 15ÃÂ over Ge/CdS nanocrystals with a pure Ge core. More effective passivation of surface defects—and a consequent decrease in surface oxidation—by the CdS shell as a result of improved epitaxy (smaller lattice mismatch) is the most likely explanation for the increased photoluminescence observed for the Ge1-xSnx/CdS materials. With enhanced photoluminescence in the near-infrared, Ge1-xSnx core/shell nanocrystals might be useful alternatives to other materials for energy capture and conversion applications and as imaging probes

Synthetic Development of Low Dimensional Materials

In this invited paper, we highlight some of our most recent work on the synthesis of low dimensional nanomaterials. Current graduate students and members of our group present four specific case systems: Nowotny–Juza phases, nickel phosphides, germanium-based core/shells, and organolead mixed-halide perovskites. Each system is accompanied by commentary from the student involved, which explains the motivation behind their work, as well as a protocol detailing the key experimental considerations involved in their synthesis. We trust these and similar efforts will help further advance our understanding of the broader field of synthetic nanomaterials chemistry, while, at the same time, highlighting how important this area is to the development of new materials for technologically relevant applications

Sensitivity-Enhanced 207Pb Solid-State NMR Spectroscopy for the Rapid, Non-Destructive Characterization of Organolead Halide Perovskites

Organolead halide and mixed halide perovskites (CH3NH3PbX3, CH3NH3PbX3–nYn, X and Y = Cl–, Br– or I–), are promising materials for photovoltaics and optoelectronic devices. 207Pb solid-state NMR spectroscopy has previously been applied to characterize phase segregation and halide ion speciation in mixed halide perovskites. However, NMR spectroscopy is an insensitive technique that often requires large sample volumes and long signal averaging periods. This is especially true for mixed halide perovskites, which give rise to extremely broad 207Pb solid-state NMR spectra. Here, we quantitatively compare the sensitivity of the various solid-state NMR techniques on pure and mixed halide organolead perovskites and demonstrate that both fast MAS and DNP can provide substantial gains in NMR sensitivity for these materials. With fast MAS and proton detection, high signal-to-noise ratio two-dimensional (2D) 207Pb-1H heteronuclear correlation (HETCOR) NMR spectra can be acquired in less than half an hour from only ca. 5 µL of perovskite material. Modest relayed DNP enhancements on the order of 1 to 20 were obtained for perovskites. The cryogenic temperatures (110 K) used for DNP experiments also provide a significant boost in sensitivity. Consequently, it was possible to obtain the 207Pb solid-state NMR spectrum of a 300 nm thick model thin film of CH3NH3PbI3 in 34 hours by performing solid-state NMR experiments with a sample temperature of 110 K. This result demonstrates the possibility of using NMR spectroscopy for characterization of perovskite thin films

Photophysical properties of wavelength-tunable methylammonium lead halide perovskite nanocrystals

We present the time-correlated luminescence of isolated nanocrystals of five methylammonium lead mixed-halide perovskite compositions (CH3NH3PbBr3−xIx) that were synthesized with varying iodide and bromide anion loading. All analyzed nanocrystals had a spherical morphology with diameters in the range of 2 to 32 nm. The luminescence maxima of CH3NH3PbBr3−xIx nanocrystals were tuned to wavelengths ranging between 498 and 740 nm by varying the halide loading. Both CH3NH3PbI3 and CH3NH3PbBr3 nanocrystals exhibited no luminescence intermittency for more than 90% of the 250 s analysis time, as defined by a luminescence intensity three standard deviations above the background. The mixed halide CH3NH3PbBr0.75I0.25, CH3NH3PbBr0.50I0.50, and CH3NH3PbBr0.25I0.75 nanocrystals exhibited luminescence intermittency in 18%, 4% and 26% of the nanocrystals, respectively. Irrespective of luminescence intermittency, luminescence intensities were classified for each nanocrystal as: (a) constant, (b) multimodal, (c) photobrightening, and (d) photobleaching. Based on their photophysics, the CH3NH3PbBr3−xIx nanocrystals can be expected to be useful in a wide-range of applications where low and non-intermittent luminescence is desirable, for example as imaging probes and in films for energy conversion devices

Persistent Dopants and Phase Segregation in Organolead Mixed-Halide Perovskites

Organolead mixed-halide perovskites such as CH3NH3PbX3–aX′a (X, X′ = I, Br, Cl) are interesting semiconductors because of their low cost, high photovoltaic power conversion efficiencies, enhanced moisture stability, and band gap tunability. Using a combination of optical absorption spectroscopy, powder X-ray diffraction (XRD), and, for the first time, 207Pb solid state nuclear magnetic resonance (ssNMR), we probe the extent of alloying and phase segregation in these materials. Because 207Pb ssNMR chemical shifts are highly sensitive to local coordination and electronic structure, and vary linearly with halogen electronegativity and band gap, this technique can provide the true chemical speciation and composition of organolead mixed-halide perovskites. We specifically investigate samples made by three different preparative methods: solution phase synthesis, thermal annealing, and solid phase synthesis. 207Pb ssNMR reveals that nonstoichiometric dopants and semicrystalline phases are prevalent in samples made by solution phase synthesis. We show that these nanodomains are persistent after thermal annealing up to 200 °C. Further, a novel solid phase synthesis that starts from the parent, single-halide perovskites can suppress phase segregation but not the formation of dopants. Our observations are consistent with the presence of miscibility gaps and spontaneous spinodal decomposition of the mixed-halide perovskites at room temperature. This underscores how strongly different synthetic procedures impact the nanostructuring and composition of organolead halide perovskites. Better optoelectronic properties and improved device stability and performance may be achieved through careful manipulation of the different phases and nanodomains present in these materials

Nanosecond, Time-Resolved Shift of the Photoluminescence Spectra of Organic, Lead-Halide Perovskites Reveals Structural Features Resulting from Excess Organic Ammonium Halide

The effort to drive solution-based perovskite solar cells towards higher efficiency has been considerable, reaching over 24%. Such progress has been made possible by the low-energy barrier to crystallization. The low-energy barrier in the reverse direction, however, also renders them susceptible to dissociation from heat, moisture, and photoexcitation. Consequently, studies that provide information on the stability of perovskites are of considerable importance. It has been reported that perovskite crystals formed using different stoichiometries of the organic precursors and metal halide are equivalent. Our findings, however, suggest that the difference in reaction pathways affects the quality of the final crystal and that changes in morphology and the production of any defects can lead to differences in behavior under illumination. Here, we present photoluminescence spectra subsequent to nanosecond photoexcitation of perovskites synthesized under various conditions. Our results indicate that the presence of excess precursors (i.e., CH3NH3X, X= I and surfactant) gives rise to an ~20-nanosecond relaxation time with which the photoluminescence spectrum achieves its equilibrium value. This relaxation is absent in bulk, polycrystalline material. This is, to our knowledge, the first report of the ~20-ns relaxation time, which we attribute to cation migration. These structural changes are not detectable subsequent to photoexcitation by x-ray diffraction, nor are they detectable by in situ x-ray diffraction during photoexcitation

Synthetic Control of the Photoluminescence Stability of Organolead Halide Perovskites

An optimized synthetic procedure for preparing photostable nanocrystalline methylammonium lead halide materials is reported. The procedure was developed by adjusting the lead halide to methylammonium/octylammonium halide precursor ratio. At a high precursor ratio (1:3), a blue-shifted photoinduced luminescence peak is measured at 642 nm for CH3NH3PbI3 with 0.01 to 12 mJ pulsed-laser irradiation. The appearance of this peak is reversible over 300 min upon blocking the irradiation. In order to determine if the peak is the result of a phase change, in situ x-ray diffraction measurements were performed. No phase change was measured with an irradiance that causes the appearance of the photoinduced luminescence peak. Luminescence microscpectroscopy measurements showed that the use of a lower precursor ratio (1:1.5) produces CH3NH3PbI3 and CH3NH3PbBr3 perovskites that are stable over 4 min of illumination. Given the lack of a measured phase change, and the dependence on the precursor ratio, the photoinduced luminesce peak may derive from surface trap states. The enhanced photostability of the resulting perovskite nanocrystals produced with the optimized synthetic procedure supports their use in stable optoelectronic devices