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Two-dimensional lead and tin halide perovskites were prepared by intercalating the long alkyl group 1-hexadecylammonium (HDA) between the inorganic layers. We observed visible-light absorption, narrow-band photoluminescence, and nanosecond photoexcited lifetimes in these perovskites. Due to their hydrophobicity and stability even in humid air, we applied these perovskites in the decarboxylation and dehydrogenation of indoline acids using (HDA)2PbI4 or (HDA)2SnI4 as photoredox catalysts, with quantitative conversions and high yields for the former. We highlight another original application of the metal halide perovskites. NRF (Natl Research Foundation, S’pore) ASTAR (Agency for Sci., Tech. and Research, S’pore) MOE (Min. of Education, S’pore) Accepted version

Resolving the structure–property relationships of two-dimensional (2D) organic–inorganic hybrid perovskites is essential for the development of photovoltaic and photoelectronic devices. Here, pressure (0–10 GPa) was applied to 2D hybrid perovskite flakes mechanically exfoliated from butylammonium lead halide single crystals, (C4H9NH3)2PbI4, from which we observed a series of changes of the strong excitonic emissions in the photoluminescence spectra. By correlating with in situ high-pressure X-ray diffraction results, we examine successfully the relationship between structural modifications in the inorganic PbI42– layer and their excitonic properties. During the transition between Pbca (1b) phase and Pbca (1a) phase at around 0.1 GPa, the decrease in ⟨Pb–I–Pb⟩ bond angle and increase in Pb–I bond length lead to an abrupt blue shift of the excitonic bandgap. The presence of the P21/a phase above 1.4 GPa increases the ⟨Pb–I–Pb⟩ bond angle and decreases the Pb–I bond length, leading to a deep red shift of the excitonic bandgap. The total band gap narrowing of ∼350 meV to 2.03 eV at 5.3 GPa before amorphization, facilitates (C4H9NH3)2PbI4 as a much better solar absorber. Moreover, phase transitions inevitably modify the carrier lifetime of (C4H9NH3)2PbI4, where an initial 150 ps at ambient phase is prolongated to 190 ps in the Pbca (1a) phase along with enhanced photoluminescence (PL), originating from pressure-induced strong radiative recombination of trapped excitons.The onset of P21/a phase shortens significantly the carrier lifetime to 53 ps along with a weak PL emission due to pressure-induced severe lattice distortion and amorphization. High-pressure study on (C4H9NH3)2PbI4 nm-thin flakes may provide insights into the mechanisms for synthetically designing novel 2D hybrid perovskite based photoelectronic devices and solar cells. Ministry of Education (MOE) Accepted version T.T.Y., J.X.Y., and Z.X.S, gratefully acknowledge the Ministry of Education (MOE) for the following grants: AcRF Tier 1 (Reference No: RG103/16); AcRF Tier 2 (MOE2015-T2-1- 148); AcRF Tier 3 (MOE2011-T3-1-005). J.X.Y. is supported by the National Natural Science Foundation of China (Grant No. 11704185) and the Natural Science Foundation of Jiangsu Province, China (Grant No. BK20171021). T.C.S. receives funding from the Ministry of Education Academic Research Fund Tier 1 Grant RG173/16, Tier 2 Grants MOE2015-T2-2- 015 and MOE2016-T2-1-034, and from the Singapore (NRF) through the Singapore−Berkeley Research Initiative for Sustainable Energy (SinBeRISE) CREATE Program and the Competitive Research Program NRF-CRP14-2014-03. S.H.W. is supported by the National Key Basic Research Program of China (2016YFB0700700) and National Natural Science Foundation of China (51672023, 11634003, U1530401). S.J. and J.F. thank Dr. Zhongwu Wang and Dr. Ruipeng Li for their assistance and acknowledge the support from Custom Electronics Inc. and Binghamton University. CHESS was supported by the NSF award DMR-1332208.

Lasers and light emitting diodes are found in numerous applications in our daily lives. The performance and applications of these diodes are very dependent on the choice of light emitting material. Ideally, these materials should be brightly emitting, solution-processible, low cost, and tunable. Among the vast variety of light emitting materials, three dimensional organic-inorganic halide perovskites are found to inherit these characteristics. As such, their light emitting and optical gain properties have been in the limelight over the past few years. On the contrary, not much attention was given to their low dimensional counterparts, which exhibit bright excitonic emission. In these low dimensional systems, the role of bound excitons cannot be neglected as they are an important species that influences light emitting and optical gain properties. In particular, this work explores the role of bound excitons in these perovskites for optical gain and white emission, which could be attractive for electrically pumped gated laser and white LEDs respectively. Bound excitons, in particular defect-bound excitons, were found to be one of the problems in limiting low temperature biexcitonic optical gain in (C6H5C2H4NH3)2PbI4 two dimensional perovskites. In this material, biexcitonic states were found to be populated through free-to-bound excitonic relaxation followed by bound-to-biexcitonic relaxation. Through these relaxation pathways, both free and bound excitonic recombinations can take place, which effectively compete withbiexcitonic population inversion build up. Theoretical modelling revealed a high biexcitonic optical gain threshold, which is consistent with the presence of strongly competing relaxation channels. Low optical damage threshold is another problem that is inherent in this material. This threshold, which is ∼1 order of magnitude lower than the theoretical biexcitonic optical gain threshold, could impede the path to achieving optical gain. On the other hand, bound excitons, in the form of self-trapped excitons, are responsible for white emission from low dimensional perovskites, such as (C6H5C2H4NH3)2PbCl4 and (NH3CH2C6H4CH2NH3)PbBr6. Notably, white emission is only observed in the latter thin films fabricated with excess PbBr2. Importantly, these excitons were found to undergo self-trapping at different locations in the two materials, with exciton self-trapping at the organic framework for the former and at the inorganic framework for the latter. Nonetheless, large exciton-phonon coupling strength, which is indicative of self-trapped excitons, was measured from both materials. Self-trapping phenomenon at the organic framework provide a new perspective on the origin of white emission in these systems, which deviates from conventional understanding of self-trapping in inorganic semiconductor systems. These two findings suggest that judicious selection of both organic and inorganic precursors are crucial for developing high efficiency white emitting perovskites. In retrospect, the presence of bound excitons in low dimensional perovskites serves as a double-edged sword, which either limits optical gain or gives rise to white emission. Doctor of Philosophy (IGS)

Resolving the structure-property relationships of two-dimensional (2D) organic-inorganic hybrid perovskites is essential for the development of photovoltaic and photoelectronic devices. Here, pressure (0-10 GPa) was applied to 2D hybrid perovskite flakes mechanically exfoliated from butylammonium lead halide single crystals, (C

Lead halide perovskite solar cells are presently the forerunner among the third generation solution-processed photovoltaic technologies. With efficiencies exceeding 20% and low production costs, they are prime candidates for commercialization. Critical insights into their light harvesting, charge transport, and loss mechanisms have been gained through time-resolved optical probes such as femtosecond transient absorption spectroscopy (fs-TAS), transient photoluminescence spectroscopy, and time-resolved terahertz spectroscopy. Specifically, the discoveries of long balanced electron–hole diffusion lengths and gain properties in halide perovskites underpin their significant roles in uncovering structure–function relations and providing essential feedback for materials development and device optimization. In particular, fs-TAS is becoming increasingly popular in perovskite characterization studies, with commercial one-box pump–probe systems readily available as part of a researcher’s toolkit. Although TAS is a powerful probe in the study of charge dynamics and recombination mechanisms, its instrumentation and data interpretation can be daunting even for experienced researchers. This issue is exacerbated by the sensitive nature of halide perovskites where the kinetics are especially susceptible to pump fluence, sample preparation and handling and even degradation effects that could lead to disparate conclusions. Nonetheless, with end-users having a clear understanding of TAS’s capabilities, subtleties, and limitations, cutting-edge work with deep insights can still be performed using commercial setups as has been the trend for ubiquitous spectroscopy instruments like absorption, fluorescence, and transient photoluminescence spectrometers. Herein, we will first briefly examine the photophysical processes in lead halide perovskites, highlighting their novel properties. Next, we proceed to give a succinct overview of the fundamentals of pump–probe spectroscopy in relation to the spectral features of halide perovskites and their origins. In the process, we emphasize some key findings of seminal photophysical studies and draw attention to the interpretations that remain divergent and the open questions. This is followed by a general description into how we prepare and conduct the TAS characterization of CH3NH3PbI3 thin films in our laboratory with specific discussions into the potential pitfalls and the influence of thin film processing on the kinetics. Lastly, we conclude with our views on the challenges and opportunities from the photophysical perspective for the field and our expectations for systems beyond lead halide perovskites. NRF (Natl Research Foundation, S’pore) MOE (Min. of Education, S’pore) Accepted version

Organic–inorganic hybrid perovskites with considerable dielectric differences near the phase transition are potential candidates as phase transition materials (PTMs). However, compared with traditional PTMs, which require multiple switchable channels, the hybrid perovskites so far show only switching behavior in dielectric constants. We herein report a new crystal design strategy and successful synthesis of a two-dimensional perovskite (C6H5C2H4NH3)2MnCl4. In this hybrid perovskite, the manganese chloride octahedron is a crystal field sensitive luminescent molecular system. The distortion level of MnCl64– also depends on temperature during the order–disorder phase transition. Hence, such a manganese octahedron-based perovskite can exhibit switching behaviors in both dielectric and optical properties. We observe a 14% decrease in optical absorption and 1.6 times increase in dielectric constant during the phase transition at 365 K. In addition, the characteristic photoluminescence decreases by 17% in intensity. Such a molecule-based crystal design paves a new way to explore multifunctional PTMs based on organic–inorganic perovskites. MOE (Min. of Education, S’pore) Accepted version

Layered 2D halide perovskites with their alternating organic and inorganic atomic layers that form a self‐assembled quantum well system are analogues of the purely inorganic 2D transition metal dichalcogenides. Within their periodic structures lie a hotbed of photophysical phenomena such as dielectric confinement effect, optical Stark effect, strong exciton–photon coupling, etc. Detailed understanding into the strong light–matter interactions in these hybrid organic–inorganic semiconductor systems remains modest. Herein, the intricate coherent interplay of exciton, spin, and phonon dynamics in (C6H5C2H4NH3)2PbI4 thin films using transient optical spectroscopy is explicated. New insights into the hotly debated origins of transient spectral features, relaxation pathways, ultrafast spin relaxation via exchange interaction, and strong coherent exciton–phonon coupling are revealed from the detailed phenomenological modeling. Importantly, this work unravels the complex interplay of spin–quasiparticle interactions in these layered 2D halide perovskites with large spin–orbit coupling. NRF (Natl Research Foundation, S’pore) ASTAR (Agency for Sci., Tech. and Research, S’pore) MOE (Min. of Education, S’pore) Published version

Three inorganic capping ligands (ICLs) for quantum dots (QDs), SnS4(4-), SbS4(3-) and AsS3(3-), were synthesized and the energy levels determined. Proximity between the ICL LUMO and QD conduction level governed the electronic couplings such as absorption shift upon ligand exchange, and electron transfer rate to TiO2. QD-sensitized solar cells were fabricated, using the ICL-QDs and also using QD multilayers layer-by-layer assembled by bridging coordinations, and studied as a function of the ICL ligand and the number of QD layers.

Semiconductor nanostructures (e.g., nanowires and nanobelts) hold great promise as subwavelength coherent light sources, nonlinear optical frequency converters, and all-optical signal processors for optoelectronic applications. However, at such small scales, optical second-harmonic generation (SHG) is generally inefficient. Herein, we report on a straightforward strategy using a thin Au layer to enhance the SHG from a single CdS nanobelt by 3 orders of magnitude. Through detailed experimental and theoretical analysis, we validate that the augmented SHG originates from the mutual intensification of the local fields induced by the plasmonic nanocavity and by the reflections within the CdS Fabry-Pérot resonant cavity in this hybrid semiconductor-metal system. Polarization-dependent SHG measurements can be employed to determine and distinguish the contributions of SH signals from the CdS nanobelt and gold film, respectively. When the thickness of gold film becomes comparable to the skin depth, SHG from the gold film can be clearly observed. Our work demonstrates a facile approach for tuning the nonlinear optical properties of mesoscopic, nanostructured, and layered semiconductor materials.

High pressure (HP) can drive the direct sintering of nanoparticle assemblies for Ag/Au, CdSe/PbS nanocrystals (NCs). Instead of direct sintering for the conventional nanocrystals, this study experimentally observes for the first time high-pressure-induced comminution and recrystallization of organic-inorganic hybrid perovskite nanocrystals into highly luminescent nanoplates with a shorter carrier lifetime. Such novel pressure response is attributed to the unique structural nature of hybrid perovskites under high pressure: during the drastic cubic-orthorhombic structural transformation at ≈2 GPa, (301) the crystal plane fully occupied by organic molecules possesses a higher surface energy, triggering the comminution of nanocrystals into nanoslices along such crystal plane. Beyond bulk perovskites, in which pressure-induced modifications on crystal structures and functional properties will disappear after pressure release, the pressure-formed variants, i.e., large (≈100 nm) and thin (10 nm) perovskite nanoplates, are retained and these exhibit simultaneous photoluminescence emission enhancing (a 15-fold enhancement in the photoluminescence) and carrier lifetime shortening (from ≈18.3 ± 0.8 to ≈7.6 ± 0.5 ns) after releasing of pressure from 11 GPa. This pressure-induced comminution of hybrid perovskite NCs and a subsequent amorphization-recrystallization treatment offer the possibilities of engineering the advanced hybrid perovskites with specific properties.

Hybrid two-dimensional (2D) lead halide perovskites have been employed in optoelectronic applications, including white-light emission for light-emitting diodes (LEDs). However, until now, there have been limited reports about white-light-emitting lead halide perovskites with experimental insights into the mechanism of the broadband emission. Here, we present white-light emission from a 2D hybrid lead chloride perovskite, using the widely known phenethylammonium cation. The single-crystal X-ray structural data, time-resolved photophysical measurements, and density functional theory calculations are consistent with broadband emission arising from strong exciton–phonon coupling with the organic lattice, which is independent of surface defects. The phenethylammonium lead chloride material exhibits a remarkably high color rendering index of 84, a CIE coordinate of (0.37,0.42), a CCT of 4426, and photostability, making it ideal for natural white LED applications. NRF (Natl Research Foundation, S’pore) ASTAR (Agency for Sci., Tech. and Research, S’pore) MOE (Min. of Education, S’pore) Accepted version

A new spin on perovskites untwined: Ultrafast optical switching and tuning of spin-energy states in layered halide perovskites., Ultrafast spin manipulation for opto–spin logic applications requires material systems that have strong spin-selective light-matter interaction. Conventional inorganic semiconductor nanostructures [for example, epitaxial II to VI quantum dots and III to V multiple quantum wells (MQWs)] are considered forerunners but encounter challenges such as lattice matching and cryogenic cooling requirements. Two-dimensional halide perovskite semiconductors, combining intrinsic tunable MQW structures and large oscillator strengths with facile solution processability, can offer breakthroughs in this area. We demonstrate novel room-temperature, strong ultrafast spin-selective optical Stark effect in solution-processed (C6H4FC2H4NH3)2PbI4 perovskite thin films. Exciton spin states are selectively tuned by ~6.3 meV using circularly polarized optical pulses without any external photonic cavity (that is, corresponding to a Rabi energy of ~55 meV and equivalent to applying a 70 T magnetic field), which is much larger than any conventional system. The facile halide and organic replacement in these perovskites affords control of the dielectric confinement and thus presents a straightforward strategy for tuning light-matter coupling strength.

Perovskite semiconductors are actively investigated for high performance solar cells. Their large optical absorption coefficient and facile solution-based, low-temperature synthesis of thin films make perovskites also a candidate for light-emitting devices across the visible and near-infrared. Specific to their potential as optical gain medium for lasers, early work has demonstrated amplified spontaneous emission and lasing at attractively low thresholds of photoexcitation. Here, we take an important step toward practically usable perovskite lasers where a solution-processed thin film is embedded within a two-dimensional photonic crystal resonator. We demonstrate high degree of temporally and spatially coherent lasing whereby well-defined directional emission is achieved near 788 nm wavelength at optical pumping energy density threshold of 68.5 ± 3.0 μJ/cm(2). The measured power conversion efficiency and differential quantum efficiency of the perovskite photonic crystal laser are 13.8 ± 0.8% and 35.8 ± 5.4%, respectively. Importantly, our approach enables scalability of the thin film lasers to a two-dimensional multielement pixelated array of microlasers which we demonstrate as a proof-of-concept for possible projection display applications. MOE (Min. of Education, S’pore) Accepted version

Precise morphological control in perovskite films is key to high performance photovoltaic and light emitting devices. However, a clear understanding of the interplay of morphological effects from substrate/perovskite antisolvent treatments on the charge dynamics is still severely lacking. Through detailed ultrafast optical spectroscopy, we correlate the morphology-kinetics relationship in a combination of substrate/film treated samples (i.e., plasma-cleaned vs piranha-etched substrates and solvent (toluene)-engineered (or toluene anti-solvent treated) perovskite films). Our findings reveal that toluene-dripped treatment has a more pronounced influence on the morphology of perovskite films prepared on plasma-cleaned substrates over those on piranha-etched substrates. Surprisingly, the highly effective toluene-dripping/washing approach reported in the literature increases the surface trap densities of perovskite films. Despite the marked improvements in the surface morphology of the toluene-dripped films, there is only a slight improvement in the carrier relaxation lifetimes – likely due to the competition between the morphology improvements and the increased surface traps densities. In addition, the injection of photoexcited holes to spiro-OMeTAD from toluene-dripped films on piranha-etched substrates is inhibited, possibly due to a realignment of the energy bands. Nonetheless, piranha-etching of the substrates could possibly offer an approach to improve the balance between the electron and hole diffusion lengths in the perovskite film. Importantly, our findings would help unravel the complex relationship of substrate/film treatments on the morphology and charge kinetics in perovskite thin films. NRF (Natl Research Foundation, S’pore) ASTAR (Agency for Sci., Tech. and Research, S’pore) MOE (Min. of Education, S’pore) Accepted version

The family of solution-processed tin-based perovskites is demonstrated as a new and superior near-infrared gain medium. Due to the large electron–hole bimolecular recombination associated with tin and the reduced trap density with SnF2 treatment, these lead-free “green” perovskites yield stable coherent light emission extending to ≈1 μm at strikingly low thresholds. NRF (Natl Research Foundation, S’pore) ASTAR (Agency for Sci., Tech. and Research, S’pore) MOE (Min. of Education, S’pore) Accepted version

Semiconductors are ubiquitous gain media for coherent light sources. Solution-processed three-dimensional (3D) halide perovskites (e.g., CH3NH3PbI3) with their outstanding room temperature optical gain properties are the latest members of this family. Their two-dimensional (2D) layered perovskite counterparts with natural multiple quantum well structures exhibit strong light–matter interactions and intense excitonic luminescence. However, despite such promising traits, there have been no reports on room temperature optical gain in 2D layered perovskites. Herein, we reveal the challenges towards achieving amplified spontaneous emission (ASE) in the archetypal (C6H5C2H4NH3)2PbI4 (or PEPI) system. Temperature-dependent transient spectroscopy uncovers the dominant free exciton trapping and bound biexciton formation pathways that compete effectively with biexcitonic gain. Phenomenological rate equation modeling predicts a large biexciton ASE threshold of ∼1.4 mJ cm−2, which is beyond the damage threshold of these materials. Importantly, these findings would rationalize the difficulties in achieving optical gain in 2D perovskites and provide new insights and suggestions for overcoming these challenges. NRF (Natl Research Foundation, S’pore) MOE (Min. of Education, S’pore) ASTAR (Agency for Sci., Tech. and Research, S’pore) Published version

Perovskite (CH 3 NH 3 PbI 3 ) films possessing optical quality were prepared by solution-based spin-casting at room temperature. We report near infrared lasing with well-defined spatially coherent output from second-order surface-emitting distributed feedback grating structure with perovskite active media.

We report observations of coherent phonon oscillations in solution-processed polycrystalline stibnite $({\mathrm{Sb}}_{2}$${\mathrm{S}}_{3})$ photovoltaic thin films using transient absorption spectroscopy. Detailed optical spectroscopy correlated by extensive first-principles lattice dynamics calculations indicates that the coherent ${B}_{3g}$ longitudinal optical phonon mode with a frequency of 63.74 \ifmmode\pm\else\textpm\fi{} 0.05 ${\mathrm{cm}}^{\ensuremath{-}1}$ (or 1.911 \ifmmode\pm\else\textpm\fi{} 0.002 THz) at room temperature is generated via the impulsive stimulated-Raman-scattering mechanism. These strong electron-phonon interactions indicate a dominant energy-loss channel in these materials that could impose a fundamental limit on their solar energy conversion efficiency.