Resolved Photoluminescence Measurements Research Articles

The Effect of Antioxidants on the Stability of Perovskite Solar Cells

The passivation of defects present in the bulk and surface of the perovskite absorbing layers of solar cells is currently the subject of intense research. To address this issue, dopants can be used to minimize the density of defects that occur at the surface and grain boundaries of perovskite films. Herein, we introduced two new antioxidants (Morin and Quercetin) in the precursor solutions to passivate the defects present in the Cs0.1FA00.9PbI3 perovskite absorbing layer. The positive impact of antioxidants on the efficiency of perovskite cells has been recently reported. However, more experimental data are needed to identify the most effective passivating agent. The time resolved photoluminescence measurements revealed that the added molecules increased the photocarrier lifetime by 120-194%. Our results showed that the optimum concentrations are 2.223 mM for Morin and 0.0156 mM for Quercetin. These results demonstrate that the new molecules are effective passivating agents and have the potential to improve the device performance. The variation of the cell power conversion efficiency as function of the concentration of the antioxidants will be discussed.

Robust valley quantum coherence in synthetic 3R-phase MoS2: A material for future valley based devices

Two-dimensional layered transition metal dichalcogenides (TMDCs) offer the ideal platform to optically initialize the qubits utilizing the superposition of valley polarized photons. However, in the commonly available 2H phase TMDCs, thickness sensitive crystal inversion symmetry demands the precise control over the number of layers, which makes the study of valley physics challenging. On the other hand, 3R-polymorphic phase TMDCs are non-centrosymmetric irrespective of the number of layers and therefore retain valley degree of freedom for all the thicknesses. Here, using polarization resolved photoluminescence measurements under linearly polarized excitations, we demonstrate the coherent superposition of the valley photons (known as valley coherence) in synthetic 3R-phase MoS2 with different numbers of layers ranging from 3 layers to 11 layers. With near-resonant excitation (633 nm for AX0), the degree of linear polarization (a measure of the valley coherence) is observed up to 70% at 80 K measurement temperatures and is found to be layer thickness independent. Robust valley coherence is attributed to the high optical quality and the 3R crystal symmetry of the MoS2. Using angle resolved polarization dependent measurements, it is established that the net dipolar emission aligns with the polarization orientations of the incident laser, which signifies the valley coherence to be independent of the crystal orientation. This study paves the way toward using 3R-phase MoS2 as a key material for the development of future quantum technologies utilizing valley polarized photons.

Flexible and Red-Emissive Organic Single-Crystal Microresonator for Efficient Active Waveguides.

We fabricated a flexible and red-emissive microcrystal resonator for highly efficient optical waveguiding. The microfiber crystals of diketopyrrolopyrrole (DPP) used in this work possess a high photoluminescence (PL) quantum efficiency (ΦPL = 0.45) and exhibit a micromechanical deformation shape in the curved state. The crystals show optical fringes in their PL spectra, suggesting the existence of a naturally formed Fabry-Pérot crystal resonator owing to its flat crystal surface. The group refractive index (ng) and Rabi splitting energy (ℏΩ) indicating the coupling strength between excitons and resonator photons are large (ng = 3.7-6.0, ℏΩ = 1.38 eV), suggesting the strong confinement of waveguiding photons. Spatially resolved PL measurements revealed that the PL in a crystal fiber is efficiently waveguided, even in a curved crystal with a very small curvature radius of 11 μm. Strong photon confinement inside a crystal resonator is a plausible origin of efficient optical waveguiding.

Investigation of the Origin of High Photoluminescence Quantum Yield in Thienyl-S,S-dioxide AIEgens Oligomers by Temperature Dependent Optical Spectroscopy.

The development of organic molecules showing high photoluminescence quantum yield (PLQY) in solid state is a fundamental step for the implementation of efficient light emitting devices. In this work the origin of the high PLQY of two trimers and two pentamers having one central thiophene-S,S-dioxide unit and two and four lateral thiophene or phenyl groups, respectively, is investigated by temperature dependent photoluminescence and time resolved photoluminescence measurements. The experimental results demonstrate that the molecules with lateral phenyl rings show higher PLQY due to a weaker coupling with intramolecular vibrations-related to variations in the radiative and non-radiative decay rates-and indicate different molecular rigidity as the main factors affecting the PLQY of this class of molecules.

Plasmon Mediated Near‐Field Energy Transfer From Solid‐State, Electrically Injected Excitons to Solution Phase Chromophores

AbstractAn organic diode is demonstrated that near‐field energy transfers to molecules in solution via surface plasmon polaritons, in contrast to typical far‐field excitation via absorption of traveling photons. Electrically generated excitons couple to surface plasmon modes in the cathode; the plasmons subsequently excite chromophore molecules on top of the cathode. External quantum efficiency and time resolved photoluminescence measurements are used to characterize the diode and the near‐field energy transfer process. In addition, it is shown that excited chromophores can charge‐transfer to quencher molecules, illustrating the potential of this device to be used for photochemical applications.

Investigation on guanidinium bromide incorporation in methylammonium lead iodide for enhanced efficiency and stability of perovskite solar cells

A guanidinium incorporated double cation mixed halide perovskite was investigated by including guanidinium bromide (GABr) in methylammonium lead iodide (MAPI) under various GABr concentrations. The XRD patterns with characteristic (110), (220) and (310) peaks showed an initial shift towards lower angles and then towards higher angles, indicating an initial lattice expansion and then contraction with increasing GABr content. Increasing the GABr concentration above 10% resulted in reducing the visible absorbance of the compound along with widening of the bandgap. The bandgap increased from 1.58 eV for MAPI to 2.06 eV for 80% GABr but not for 1–10% GABr. Steady state and time resolved photoluminescence measurements revealed that the 10% GABr incorporated samples exhibited higher photoluminescence emission and enhanced charge carrier lifetime than MAPI. The scanning electron microscopy images depicted typical surface passivation effect observed in the GA based additives, where an increase in grain size and low number of pinholes were observed for 10% GABr incorporated films. As an outcome of these synergistic effects, perovskite solar cells (PSCs) fabricated from 10% GABr presented a superior power conversion efficiency (PCE) of 16.70 ± 0.20% compared to the conventional MAPI (PCE = 15.35 ± 0.15%). The 10% GABr based PSCs exhibited lesser hysteresis compared to MAPI and retained > 97% of their pristine PCE after 400 h of continuous illumination, while the PCE of MAPI-based PSCs deteriorated with time.

Selectively patterned Mg-doped GaN by SiNx-driven hydrogen injection

We demonstrate a method to achieve selectively patterned Mg-doped GaN layers using hydrogen drive-in through plasma-enhanced chemical vapor deposition (PECVD) silicon nitride (SiNx) films. Activated Mg-doped GaN layers were selectively deactivated by patterned PECVD SiNx films with low-temperature annealing and showed high-resistive behavior. Spatially resolved photoluminescence measurements were used to optically verify the deactivation of Mg acceptors and showed distinct features corresponding to activated and deactivated Mg in GaN. The method suggested here provides a simple and effective method to achieve patterned Mg-doped GaN regions without thermal and plasma damage, which could cause degradation of device performance. The proposed method could provide a way to achieve future high-performance GaN lateral and vertical devices that rely on laterally patterned doping.

Interlayer Exciton Diode and Transistor.

Controlling the flow of charge neutral interlayer exciton (IX) quasiparticles can potentially lead to low loss excitonic circuits. Here, we report unidirectional transport of IXs along nanoscale electrostatically defined channels in an MoSe2-WSe2 heterostructure. These results are enabled by a lithographically defined triangular etch in a graphene gate to create a potential energy "slide". By performing spatially and temporally resolved photoluminescence measurements, we measure smoothly varying IX energy along the structure and high speed exciton flow with a drift velocity up to 2 × 106 cm/s, an order of magnitude larger than previous experiments. Furthermore, exciton flow can be controlled by saturating exciton population in the channel using a second laser pulse, demonstrating an optically gated excitonic transistor. Our work paves the way toward low loss excitonic circuits, the study of bosonic transport in one-dimensional channels, and custom potential energy landscapes for excitons in van der Waals heterostructures.

Intrinsically Viscoelastic Supramolecular Conjugated Polymer toward Suppressing Coffee-Ring Effect

Intrinsically Viscoelastic Supramolecular Conjugated Polymer toward Suppressing Coffee-Ring Effect

High-performance Ruddlesden-Popper two-dimensional perovskite solar cells via solution processed inorganic charge transport layers.

Two-dimensional (2D) layered halide perovskites have been shown to enable improved long-term stability in comparison to the well-known three-dimensional hybrid organic-inorganic halide perovskites. The optoelectronic properties of the 2D perovskites are strongly influenced by the chemical nature of the charge transport layer. In this work, we fabricated Ruddlesden-Popper 2D perovskite solar cells (PSCs) using solution processed inorganic NiOx and a C60 : C70 (1 : 1) mixture as the hole and electron transport layers, which significantly improved the performance of the 2D PSCs. Time resolved photoluminescence measurements indicate the shortened lifetime of excitons, which demonstrates the excellent charge extraction properties. The PSCs based on these inorganic charge transport materials (CTMs) exhibit an average power conversion efficiency (PCE) of 14.1%, which is higher than that (12.3%) of PSCs using organic CTMs of poly(3,4-ethylenedioxythiophene) : poly(styrenesulfonate) (PEDOT:PSS) and phenyl-C61-butyric acid methyl ester (PCBM). Compared with PEDOT:PSS and PCBM based cases, the PSCs using inorganic CTMs also show improved long-term stability, with the PCE degradation significantly suppressed from 20% to 12% after a measurement of 15 days. The best PSCs using NiOx and C60 : C70 show a high PCE of 14.4%, with a stable power output and negligible hysteresis.

Charge density wave activated excitons in TiSe2–MoSe2 heterostructures

Layered materials enable the assembly of a new class of heterostructures where lattice-matching is no longer a requirement. Interfaces in these heterostructures therefore become a fertile ground for unexplored physics as dissimilar phenomena can be coupled via proximity effects. In this article, we identify an unexpected photoluminescence (PL) peak when MoSe2 interacts with TiSe2. A series of temperature-dependent and spatially resolved PL measurements reveal that this peak is unique to the TiSe2–MoSe2 interface, is higher in energy compared to the neutral exciton, and exhibits exciton-like characteristics. The feature disappears at the TiSe2 charge density wave transition, suggesting that the density wave plays an important role in the formation of this new exciton. We present several plausible scenarios regarding the origin of this peak that individually capture some aspects of our observations but cannot fully explain this feature. These results therefore represent a fresh challenge for the theoretical community and provide a fascinating way to engineer excitons through interactions with charge density waves.

Interstitial Nature of Mn2+ Doping in 2D Perovskites.

Halide perovskites doped with magnetic impurities (such as the transition metals Mn2+, Co2+, Ni2+) are being explored for a wide range of applications beyond photovoltaics, such as spintronic devices, stable light-emitting diodes, single-photon emitters, and magneto-optical devices. However, despite several recent studies, there is no consensus on whether the doped magnetic ions will predominantly replace the octahedral B-site metal via substitution or reside at interstitial defect sites. Here, by performing correlated nanoscale X-ray microscopy, spatially and temporally resolved photoluminescence measurements, and magnetic force microscopy on the inorganic 2D perovskite Cs2PbI2Cl2, we show that doping Mn2+ into the structure results in a lattice expansion. The observed lattice expansion contrasts with the predicted contraction expected to arise from the B-site metal substitution, thus implying that Mn2+ does not replace the Pb2+ sites. Photoluminescence and electron paramagnetic resonance measurements confirm the presence of Mn2+ in the lattice, while correlated nano-XRD and X-ray fluorescence track the local strain and chemical composition. Density functional theory calculations predict that Mn2+ atoms reside at the interstitial sites between two octahedra in the triangle formed by one Cl- and two I- atoms, which results in a locally expanded structure. These measurements show the fate of the transition metal dopants, the local structure, and optical emission when they are doped at dilute concentrations into a wide band gap semiconductor.

High Efficiency Luminescent Solar Concentrator based on Organo‐Metal Halide Perovskite Quantum Dots with Plasmon Enhancement

AbstractOrgano‐metal halide perovskites (OMHPs) are currently one of the most exciting candidates for photovoltaics. However, their impact in other areas related to carrier photogeneration, such as in luminescent solar concentrators (LSCs), has been limited. OMHP thin films have demonstrated encouraging results as LSCs, but for a scalable platform with minimal losses, discrete emitters are preferable. Perovskite quantum dots (PQDs) possess higher photoluminescence quantum yield (PLQY) than their thin film counterparts, and have the added advantage of size tunability, which make them well‐suited as LSC active media. However, since PQDs are not amenable to Stokes shift modulation, large‐scale samples will suffer from increased self‐absorption (SA) losses. In this work, a facile dip‐coating approach is established to fabricating large‐scale LSCs with CH3NH3PbBr3 (methylammonium lead bromide) PQDs by leveraging their plasmonic interactions with gold nanoparticles (AuNPs) to offset SA losses. Optical characterization through emission, lifetime, and spatially resolved photoluminescence measurements provide insight into the effect of plasmon resonance on deposited PQDs, and leveraging this AuNP‐PQD coupling, 2.87% efficiency is achieved in a 100 cm2 LSC with a geometric gain of 50.

Lab-on-Microsphere-FRET-Based Multiplex Sensor Platform.

Here we report on the development and investigation of a novel multiplex assay model based on polymer microspheres (PMS) encoded with ternary AIS/ZnS quantum dots (QDs). The system was prepared via layer-by-layer deposition technique. Our studies of Förster resonance energy transfer (FRET) between the QD-encoded microspheres and two different cyanine dyes have demonstrated that the QD photoluminescence (PL) quenching steadily increases with a decrease in the QD-dye distance. We have found that the sensitized dye PL intensity demonstrates a clear maximum at two double layers of polyelectrolytes between QDs and Dye molecules on the polymer microspheres. Time resolved PL measurements have shown that the PL lifetime decreases for the QDs and increases for the dyes due to FRET. The designed system makes it possible to record spectrally different bands of FRET-induced dye luminescence with different decay times and thereby allows for the multiplexing by wavelength and photoluminescence lifetimes of the dyes. We believe that PMS encoded with AIS/ZnS QDs have great potential for the development of new highly selective and sensitive sensor systems for multiplex analysis to detect cell lysates and body fluids’ representative biomarkers.

UVB LEDs Grown by Molecular Beam Epitaxy Using AlGaN Quantum Dots

AlGaN based light emitting diodes (LEDs) will play a key role for the development of applications in the ultra-violet (UV). In the UVB region (280–320 nm), phototherapy and plant lighting are among the targeted uses. However, UVB LED performances still need to be improved to reach commercial markets. In particular, the design and the fabrication process of the active region are central elements that affect the LED internal quantum efficiency (IQE). We propose the use of nanometer-sized epitaxial islands (i.e., so called quantum dots (QDs)) to enhance the carrier localization and improve the IQE of molecular beam epitaxy (MBE) grown UVB LEDs using sapphire substrates with thin sub-µm AlN templates. Taking advantage of the epitaxial stress, AlGaN QDs with nanometer-sized (≤10 nm) lateral and vertical dimensions have been grown by MBE. The IQE of the QDs has been deduced from temperature dependent and time resolved photoluminescence measurements. Room temperature IQE values around 5 to 10% have been found in the 290–320 nm range. QD-based UVB LEDs were then fabricated and characterized by electrical and electroluminescence measurements. On-wafer measurements showed optical powers up to 0.25 mW with external quantum efficiency (EQE) values around 0.1% in the 305–320 nm range.

Structural, Optical and Electrical Characterizations of Midwave Infrared Ga-Free Type-II InAs/InAsSb Superlattice Barrier Photodetector

In this paper, a full set of structural, optical and electrical characterizations performed on midwave infrared barrier detectors based on a Ga-free InAs/InAsSb type-II superlattice, grown by molecular beam epitaxy (MBE) on a GaSb substrate, are reported and analyzed. a Minority carrier lifetime value equal to 1 µs at 80 K, carried out on dedicated structure showing photoluminescence peak position at 4.9 µm, is extracted from a time resolved photoluminescence measurement. Dark current density as low as 3.2 × 10−5 A/cm2 at 150 K is reported on the corresponding device exhibiting a 50% cut-off wavelength around 5 µm. A performance analysis through normalized spectral response and dark current density-voltage characteristics was performed to determine both the operating bias and the different dark current regimes.

Coupling of quantum-well emission to waveguide–plasmon polaritons in rolled-up microtubes

We study the coupling of GaAs quantum wells to waveguide–plasmon polaritons supported by a thin InAlGaAs-based slab waveguide and a Ag grating. The hybrid photon–plasmon modes are excited in a freestanding emitter–waveguide–plasmon structure realized by rolling-up strained InAlGaAs-based layers and nanopatterned Ag structures. By varying the grating’s bar width, we tune the plasmonic resonance of the system. We observe by means of spatially, spectrally, and temporally resolved photoluminescence measurements, a coupling of the quantum-well emission to the waveguide–plasmon system for a specific grating’s bar width. Supported by finite-element simulations, we can assign the coupling to the excitation of a waveguide–plasmon polariton.

Hot exciton transport in WSe2 monolayers

We experimentally demonstrate hot exciton transport in h-BN encapsulated WSe2 monolayers via spatially and temporally resolved photoluminescence measurements at room temperature. We show that the nonlinear evolution of the mean squared displacement of the non-resonantly excited hot exciton gas is primarily due to the relaxation of its excess kinetic energy and is characterized by a density-dependent fast expansion that converges to a slower, constant rate expansion. We also observe saturation of the hot exciton gas' expansion rate at high excitation densities due to the balance between Auger-assisted hot exciton generation and the phonon-assisted hot exciton relaxation processes.

Two‐terminal Perovskite silicon tandem solar cells with a high‐Bandgap Perovskite absorber enabling voltages over 1.8 V

AbstractPerovskite silicon tandem solar cells are a promising technology to overcome the efficiency limit of silicon solar cells. Although highest tandem efficiencies have been reported for the inverted p‐i‐n structure, high‐efficiency single junction perovskite solar cells are mostly fabricated in the regular n‐i‐p architecture. In this work, regular n‐i‐p perovskite solar cells with a high‐bandgap mixed cation mixed halide absorber suitable for tandem solar cells are investigated by compositional engineering and the open‐circuit voltage is improved to over 1.12 V using a passivating electron contact. The optimized perovskite solar cell is used as a top cell in a monolithic perovskite silicon tandem device with a silicon heterojunction bottom cell allowing for voltages up to 0.725 V. The tandem solar cells with an active area of 0.25 cm2 achieve record open‐circuit voltages of up to 1.85 V and have efficiencies over 20%. Analyzing the perovskite absorber by spatially resolved photoluminescence measurements shows a homogenous and stable emission at ~ 1.7 eV which is an optimal value for tandem applications with silicon. The tandem solar cells are mainly limited due to a low current. A spectrometric characterization reveals that the perovskite solar cell is current limiting which could be improved by a thicker perovskite absorber.

A Highly Emissive Surface Layer in Mixed-Halide Multication Perovskites.

Mixed-halide lead perovskites have attracted significant attention in the field of photovoltaics and other optoelectronic applications due to their promising bandgap tunability and device performance. Here, the changes in photoluminescence and photoconductance of solution-processed triple-cation mixed-halide (Cs0.06 MA0.15 FA0.79 )Pb(Br0.4 I0.6 )3 perovskite films (MA: methylammonium, FA: formamidinium) are studied under solar-equivalent illumination. It is found that the illumination leads to localized surface sites of iodide-rich perovskite intermixed with passivating PbI2 material. Time- and spectrally resolved photoluminescence measurements reveal that photoexcited charges efficiently transfer to the passivated iodide-rich perovskite surface layer, leading to high local carrier densities on these sites. The carriers on this surface layer therefore recombine with a high radiative efficiency, with the photoluminescence quantum efficiency of the film under solar excitation densities increasing from 3% to over 45%. At higher excitation densities, nonradiative Auger recombination starts to dominate due to the extremely high concentration of charges on the surface layer. This work reveals new insight into phase segregation of mixed-halide mixed-cation perovskites, as well as routes to highly luminescent films by controlling charge density and transfer in novel device structures.