Search for: All records

The melting properties and kinetics of glass formation in 2D perovskites can be finely tuned using isomeric organic cations bearing distinct substitutional functional group positions, resulting in enhancement of glass-crystalline switching speed. 

Hybrid lead-halide perovskites have been studied extensively for their promising optoelectronic properties and prospective applications including photovoltaics, solid-state lighting, and radiation detection. Research into these materials has also been aided by the simple and low temperature synthetic conditions involved in solution-state deposition/crystallization or melt processing techniques. However, concern over lead toxicity has plagued the field since its early days. One of the most promising routes to mitigating toxicity in hybrid perovskite materials is substituting isoelectronic Bi(III) for Pb(II). Various methods have been developed to allow pnictide-based systems to capture properties of the Pb(II) analogs, but the ability to melt process extended hybrid pnictide-halide materials has not been investigated. In this work, we prepare a series of 1D antimony and bismuth-iodide hybrid materials employing tetramethylpiperazinium (TMPZ)-related cations. We observe, for the first time, the ability to melt hybrid pnictide-halide materials for both Sb(III) and Bi(III) systems. Additionally, we find that Sb(III) analogs melt at lower temperatures, and attribute this observation to structural changes induced by the increased stereochemical activity of the Sb(III) lone pair coupled with reduction in effective dimensionality due to steric interactions with the organic cations. Finally, we demonstrate the ability to melt process phase pure thin films of (S-MeTMPZ)SbI5. 

Earth-abundant Cu2BaSnS4-xSex (CBTSSe) represents a recent alter- native for Cu2ZnSn(S,Se)4 for solar energy conversion with a lower level of disorder and band tailing. We report the heterogeneous excited-state and trap-state pattern in different solution-processed CBTSSe films using ultrafast two-color pump-probe diffuse reflec- tance microscopic imaging. The spectroscopy/microscopy method can visualize and correlate the microscopic compositional and elec- tronic variations (i.e., trap states) in real space with time-resolved photophysics. Heterogeneity patterns in TAM images show that some grains exhibit a positive excited-state absorption (ESA) signal, while others show negative ground-state bleaching (GSB). Our re- sults visualize that film processing, such as air annealing and Na addition, has a clear influence on the heterogeneity of the excited- state pattern. Importantly, we report stable charge carrier over 100 ps. We applied the image principal component and histogram for quantitative analysis of TAM images to deconvolute and visu- alize the contribution and fingerprints of minority free carriers and sub-band-gap trapped carriers. 

While crystalline 2D metal halide perovskites (MHPs) represent a well-celebrated semiconductor class, with ensuing applications in the fields of photovoltaics, emitters, and sensors, the recent discovery of glass formation in an MHP opens many new opportunities associated with reversible glass-crystalline switching, with each state offering distinct optoelectronic properties. However, the previously reported [S-(−)-1-(1-naphthyl)ethylammonium]2PbBr4 perovskite is a strong glass former with sluggish glass-crystal transformation timescales, pointing to a need for glassy MHPs with a broader range of compositions and crystallization kinetics. Herein we report glass formation in low melting temperature 1-MeHa2PbI4 (1-MeHa = 1-methyl-hexylammonium) using ultrafast calorimetry, thereby extending the range of MHP glass formation across a broader range of organic (fused ring to branched aliphatic) and halide (bromide to iodide) compositions. The importance of a slight loss of organic and hydrogen iodide components from the MHP in stabilizing the glassy state is elucidated. Furthermore, the underlying kinetics of glass-crystal transformation, including activation energies, crystal growth rate, Angell plot, and fragility index is studied using a combination of kinetic, thermodynamic, and rheological modeling techniques. An inferred fast crystal growth rate of 0.21 m/s for 1-MeHa2PbI4 shows promise toward suitability in extended application spaces, for example in metamaterials, nonvolatile memory, and optical and neuromorphic computing devices. 

1-Methylhexylammonium tin iodide yields the lowest reported melting temperature ( T m = 142 °C) to date among lead-free hybrid perovskite semiconductors. Molecular branching near the organic ammonium group coupled with tuning of metal/halogen character suppresses T m and facilitates effective melt-based deposition of films with 568 nm absorption onset.