Polymer/Perovskite Amplifying Waveguides for Active Hybrid Silicon Photonics

Perovskite beyond solar: toward novel developments of lasers and detectors for photonic circuits

Possessing intriguing optoelectronic properties, metal halide perovskites can serve as a large-scale platform for miniaturized photonic circuits with on-chip active devices such as lasers and detectors.

Direct Fabrication of CsPbxMn1-x(Br,Cl)3 Thin Film by a Facile Solution Spraying Approach

Nowadays, Mn-doping is considered as a promising dissolution for the heavy usage of toxic lead in CsPbX₃ perovskite material. Interestingly, Mn-doping also introduces an additional photoluminescence band, which is favorable to enrich the emission gamut of this cesium lead halide. Here, a solution spraying strategy was employed for the direct preparation of CsPb_xMn_1−x(Br,Cl)₃ film through MnCl₂ doping in host CsPbBr₃ material. The possible fabrication mechanism of the provided approach and the dependences of material properties on Mn-doping were investigated in detail. As the results shown, Pb was partially substituted by Mn as expected. With the ratio of PbBr₂:MnCl₂ increasing from 3:0 to 1:1, the obtained film separately featured green, cyan, orange-red and pink-red emission, which was caused by the energy transferring process. Moreover, the combining energy of Cs, Pb, and Mn gradually red-shifted resulted from the formation of Cs-Cl, Pb-Cl and Mn-Br coordination bonding as MnCl₂ doping increased. In addition, the weight of short decay lifetime of prepared samples increased with the doping rising, which indicated a better exciton emission and less defect-related transition. The aiming of current work is to provide a new possibility for the facile preparation of Mn-doping CsPbX₃ film material.

Boosting Long-Term Stability of Pure Formamidinium Perovskite Solar Cells by Ambient Air Additive Assisted Fabrication

Due to the high industrial interest for perovskite-based photovoltaic devices, there is an urgent need to fabricate them under ambient atmosphere, not limited to low relative humidity (RH) conditions. The formamidinium lead iodide (FAPI) perovskite α-black phase is not stable at room temperature and is challenging to stabilize in an ambient environment. In this work, we show that pure FAPI perovskite solar cells (PSCs) have a dramatic increase of device long-term stability when prepared under ambient air compared to FAPI PSCs made under nitrogen, both fabricated with N-methylpyrrolidone (NMP). The T 80 parameter, the time in which the efficiency drops to 80% of the initial value, increases from 21 (in N2) to 112 days (in ambient) to 145 days if PbS quantum dots (QDs) are introduced as additives in air-prepared FAPI PSCs. Furthermore, by adding methylammonium chloride (MACl) the power conversion efficiency (PCE) reaches 19.4% and devices maintain 100% of the original performance for at least 53 days. The presence of Pb-O bonds only in the FAPI films prepared in ambient conditions blocks the propagation of α- to δ-FAPI phase conversion. Thus, these results open the way to a new strategy for the stabilization in ambient air toward perovskite solar cells commercialization.

Effect of PbSO4-Oleate Coverage on Cesium Lead Halide Perovskite Quantum Dots to Control Halide Exchange Kinetics

The selective control of halide ion exchange in metal halide perovskite quantum dots (PQDs) plays an important role in determining their band gap and composition. In this study, CsPbX₃ (X = Cl⁻, Br⁻, and I⁻) PQDs were self-assembled with PbSO₄-oleate to form a peapod-like morphology to selectively control halide ion exchange. Considering the distinct absorption and bright luminescence characteristics of these PQDs, in situ UV-Vis. absorption and fluorescence spectroscopies were employed to monitor the time-dependent band gap and compositional changes of the PQDs. We determined that the halide exchange in the capped PQDs is hindered—unlike the rapid anion exchange in noncapped PQDs—by a reduction in the halide exchange kinetic rate depending on the extent of coverage of the PQDs. Thus, we tracked the halide ion exchange kinetics between CsPbBr₃ and CsPbI₃ PQDs, depending on the coverage, using in situ UV-Vis. absorption/photoluminescence spectroscopy. We regulated the halide exchange reaction rate by varying the capping reaction temperature of the PQDs. The capping hindered the halide exchange kinetics and increased the activation energy. These results will enable the development of white LEDs, photovoltaic cells, and photocatalysts with alternative structural designs based on the divalent composition of CsPbX₃ PQDs.

Phonon-Polaritons in Lead Halide Perovskite Film Hybridized with THz Metamaterials

In this work, we demonstrated a phonon-polariton in the terahertz (THz) frequency range, generated in a crystallized lead halide perovskite film coated on metamaterials. When the metamaterial resonance was in tune with the phonon resonance of the perovskite film, Rabi splitting occurred due to the strong coupling between the resonances. The Rabi splitting energy was about 1.1 meV, which is larger than the metamaterial and phonon resonance line widths; the interaction potential estimation confirmed that the strong coupling regime was reached successfully. We were able to tune the polaritonic branches by varying the metamaterial resonance, thereby obtaining the dispersion curve with a clear anticrossing behavior. Additionally, we performed in situ THz spectroscopy as we annealed the perovskite film and studied the Rabi splitting as a function of the films' crystallization coverage. The Rabi splitting versus crystallization volume fraction exhibited a unique power-law scaling, depending on the crystal growth dimensions.

Nano-Micro Dimensional Structures of Fiber-Shaped Luminous Halide Perovskite Composites for Photonic and Optoelectronic Applications

Perovskite nanomaterials have been revealed as highly luminescent structures regarding their dimensional confinement. In particular, their promising potential lies behind remarkable luminescent properties, including color tunability, high photoluminescence quantum yield, and the narrow emission band of halide perovskite (HP) nanostructures for optoelectronic and photonic applications such as lightning and displaying operations. However, HP nanomaterials possess such drawbacks, including oxygen, moisture, temperature, or UV lights, which limit their practical applications. Recently, HP-containing polymer composite fibers have gained much attention owing to the spatial distribution and alignment of HPs with high mechanical strength and ambient stability in addition to their remarkable optical properties comparable to that of nanocrystals. In this review, the fabrication methods for preparing nano-microdimensional HP composite fiber structures are described. Various advantages of the luminescent composite nanofibers are also described, followed by their applications for photonic and optoelectronic devices including sensors, polarizers, waveguides, lasers, light-down converters, light-emitting diode operations, etc. Finally, future directions and remaining challenges of HP-based nanofibers are presented.

Synthesis and optical applications of low dimensional metal-halide perovskites

Metal halide perovskites have received substantial attention in research communities due to their outstanding efficiency achievements in the field of photovoltaics, optoelectronics and electronics, exhibiting extraordinary optical, electrical and mechanical properties. The exceptional structural tunability enables perovskite material to possess low-dimensional form at the atomic level and extends their applications into optoelectronic and photonic fields. This review discusses the recent progress of synthetic routes and fundamental optoelectronic properties of low-dimensional metal halide perovskites. In addition, the focus is to highlight the potential applications of perovskites in various devices including solar cells, light-emitting diodes, lasers, waveguides and memory devices. Finally, outlooks and the challenges that face the development of the perovskite materials in the near future are also presented.

Inhibition of light emission from the metastable tetragonal phase at low temperatures in island-like films of lead iodide perovskites

Photonic applications based on halide perovskites, namely CH3NH3PbI3 (MAPbI3), have recently attracted remarkable attention due to the high efficiencies reported for photovoltaic and light emitting devices. Despite these outstanding results, there are many temperature-, laser excitation power-, and morphology-dependent phenomena that require further research to be completely understood. In this work, we have investigated in detail the nature of exciton optical transitions and recombination dynamics below and above the orthorhombic/tetragonal ('O'-/'T'-) temperature phase transition (∼150 K) depending on the material continuity (continuous-like) or discontinuity (island-like) in MAPbI3 films. At low temperatures, continuous thin films of the perovskite can exhibit strain inhomogeneities associated with the formation of different 'T'-defective domains leading to an energy spread of states over more than 200 meV. On the other hand, a single photoluminescence line peak related to the perovskite 'O'-phase (associated with the distortion of the [PbI3]- anion) is observed in the island-like sample that we attribute to strain relaxation for this morphology. Moreover, the predominantly radiative recombination dynamics of the continuous-like sample mainly originates from nongeminate electron-hole formation of excitons in the 'O'-phase and the internal dynamics with carrier trapping levels. This observation is in strong contrast to the free exciton recombination dominantly found in the island-like sample.

Single-Exciton Amplified Spontaneous Emission in Thin Films of CsPbX3 (X = Br, I) Perovskite Nanocrystals

CsPbX₃ perovskite nanocrystals (PNCs) have emerged as an excellent material for stimulated emission purposes, with even more prospective applications than conventional colloidal quantum dots. However, a better understanding of the physical mechanisms responsible for amplified spontaneous emission (ASE) is required to achieve more ambitious targets (lasing under continuous wave optical or electrical excitation). Here, we establish the intrinsic mechanisms underlying ASE in PNCs of three different band gaps (CsPbBr₃, CsPbBr_1.5I_1.5, and CsPbI₃). Our characterization at cryogenic temperatures does not reveal any evidence of the biexciton mechanism in the formation of ASE. Instead, the measured shift toward long wavelengths of the ASE band is easily explained by the reabsorption in the PNC layer, which becomes stronger for thicker layers. In this way, the threshold of ASE is determined only by optical losses at a given geometry, which is the single-exciton mechanism responsible for ASE. Experimental results are properly reproduced by a physical model.

PbS quantum dots as additives in methylammonium halide perovskite solar cells: the effect of quantum dot capping

Colloidal PbS quantum dots (QDs) have been successfully employed as additives in halide perovskite solar cells (PSCs) acting as nucleation centers in the perovskite crystallization process. For this strategy, the surface functionalization of the QDs, controlled via the use of different capping ligands, is likely of key importance. In this work, we examine the influence of the PbS QD capping on the photovoltaic performance of methylammonium lead iodide PSCs. We test PSCs fabricated with PbS QD additives with different capping ligands including methylammonium lead iodide (MAPI), cesium lead iodide (CsPI) and 4-aminobenzoic acid (ABA). Both the presence of PbS QDs and the specific capping used have a significant effect on the properties of the deposited perovskite layer, which affects, in turn, the photovoltaic performance. For all capping ligands used, the inclusion of PbS QDs leads to the formation of perovskite films with larger grain size, improving, in addition, the crystalline preferential orientation and the crystallinity. Yet, differences between the capping agents were observed. The use of QDs with ABA capping had a higher impact on the morphological properties while the employment of the CsPI ligand was more effective in improving the optical properties of the perovskite films. Taking advantage of the improved properties, PSCs based on the perovskite films with embedded PbS QDs exhibit an enhanced photovoltaic performance, showing the highest increase with ABA capping. Moreover, bulk recombination via trap states is reduced when the ABA ligand is used for capping of the PbS QD additives in the perovskite film. We demonstrate how surface chemistry engineering of PbS QD additives in solution-processed perovskite films opens a new approach towards the design of high quality materials, paving the way to improved optoelectronic properties and more efficient photovoltaic devices.

Optical Amplification in Hollow-Core Negative-Curvature Fibers Doped with Perovskite CsPbBr3 Nanocrystals

We report a hollow-core negative-curvature fiber (HC-NCF) optical signal amplifier fabricated by the filling of the air microchannels of the fiber with all-inorganic CsPbBr3 perovskite nanocrystals (PNCs). The optimum fabrication conditions were found to enhance the optical gain, up to +3 dB in the best device. Experimental results were approximately reproduced by a gain assisted mechanism based on the nonlinear optical properties of the PNCs, indicating that signal regeneration can be achieved under low pump powers, much below the threshold of stimulated emission. The results can pave the road of new functionalities of the HC-NCF with PNCs, such as optical amplification, nonlinear frequency conversion and gas sensors.

Structural characterization of bulk and nanoparticle lead halide perovskite thin films by (S)TEM techniques

Lead halide (APbX₃) perovskites, in polycrystalline thin films but also perovskite nanoparticles (NPs) has demonstrated excellent performance to implement a new generation of photovoltaic and photonic devices. The structural characterization of APbX₃ thin films using (scanning) transmission electron microscopy ((S)TEM) techniques can provide valuable information that can be used to understand and model their optoelectronic performance and device properties. However, since APbX₃ perovskites are soft materials, their characterization using (S)TEM is challenging. Here, we study and compare the structural properties of two different metal halide APbX₃ perovskite thin films: bulk CH₃NH₃PbI₃ prepared by spin-coating of the precursors in solution and CsPbBr₃ colloidal NPs synthetized and deposited by doctor blading. Both specimen preparation methods and working conditions for analysis by (S)TEM are properly optimized. We show that CH₃NH₃PbI₃ thin films grown by a one-step method are composed of independent grains with random orientations. The growth method results in the formation of tetragonal perovskite thin films with good adherence to an underlying TiO₂ layer, which is characterized by a photoluminescence (PL) emission band centered at 775 nm. The perovskite thin films based on CsPbBr₃ colloidal NPs, which are used as the building blocks of the film, are preserved by the deposition process, even if small gaps are observed between adjacent NPs. The crystal structure of CsPbBr₃ NPs is cubic, which is beneficial for optical properties due to its optimal band gap. The absorption and PL spectra measured in both the thin film and the colloidal solution of CsPbBr₃ NPs are very similar, indicating a good homogeneity of the thin films and the absence of aggregation of NPs. However, a particular care was required to avoid long electron irradiation times during our structural studies, even at a low voltage of 80 kV, as the material was observed to decompose through Pb segregation.

Interaction between Colloidal Quantum Dots and Halide Perovskites: Looking for Constructive Synergies

Colloidal quantum dots (QDs) have received extensive attention during the last few decades because of their amazing properties emerging from quantum confinement. In parallel, halide perovskites have attracted attention because of the demonstration of very high performance, especially in solar cells, light-emitting diodes (LEDs), and other optoelectronic devices. Both families of materials can be prepared in a relatively simple way, facilitating their integration. There are several examples of their interaction enhancing the properties of the final nanocomposite. Perovskites can effectively passivate QDs or act as efficient charge transporters. QDs can be used to modify the selective contacts in perovskite devices or can be used as efficient light emitters or absorbers for enhanced LEDs and photodetectors, respectively. Moreover, QDs can seed the perovskite crystal growth, improving the morphology and ultimately the solar cell performance. In addition, new advanced devices can emerge as a result of the constructive synergy between both families of materials.

Controlling the Phase Segregation in Mixed Halide Perovskites through Nanocrystal Size

Mixed halide perovskites are one of the promising candidates in developing solar cells and light-emitting diodes (LEDs), among other applications, because of their tunable optical properties. Nonetheless, photoinduced phase segregation, by formation of segregated Br-rich and I-rich domains, limits the overall applicability. We tracked the phase segregation with increasing crystalline size of CsPbBr3-x I x and their photoluminescence under continuous-wave laser irradiation (405 nm, 10 mW cm-2) and observed the occurrence of the phase segregation from the threshold size of 46 ± 7 nm. These results have an outstanding agreement with the diffusion length (45.8 nm) calculated also experimentally from the emission lifetime and segregation rates. Furthermore, through Kelvin probe force microscopy, we confirmed the correlation between the phase segregation and the reversible halide ion migration among grain centers and boundaries. These results open a way to achieve segregation-free mixed halide perovskites and improve their performances in optoelectronic devices.

Toward Metal Halide Perovskite Nonlinear Photonics

The possibility of controlling light using the nonlinear optical properties of photonic devices opens new points of view in information and communications technology applications. In this Perspective, we review and analyze the potential role of metal halide perovskites in a framework different from their usual one in photovoltaic and light-emitting devices, namely, the one where they can play as nonlinear photonic materials. We contextualize this new role by comparing the few extant results on their nonlinear optical properties to those of other known nonlinear materials. As a result of this analysis, we provide a vision of future developments in photonics that can be expected from this new perspective on metal halide perovskites.

Trap-Limited Dynamics of Excited Carriers and Interpretation of the Photoluminescence Decay Kinetics in Metal Halide Perovskites

Interpretation of the photoluminescence (PL) decay kinetics in metal halide perovskites (MHPs) is extremely important for understanding the mechanisms and control of charge recombination in these promising photovoltaic and optoelectronic materials. In this work, we give a review of current models describing the PL decay kinetics in MHP layers and nanocrystals with particular attention to the interpretation of long-lived PL decay components (hundreds of nanoseconds to microseconds). First, we analyze phenomenological photophysical models based on the rate equations, which describe the charge carrier recombination in MHP layers as an exclusively intrinsic bulk process. An important role of the carrier diffusion and nonradiative recombination on the layer surfaces is then discussed. A recently published approach is then analyzed, in the framework of which the observed long-lived components of PL decay kinetics in MHP nanocrystals are described in terms of the delayed luminescence mechanism arising due to the processes of multiple trapping and detrapping of carriers by shallow nonquenching traps. The possible origin of the shallow traps and perspectives to include the carrier trapping and detrapping processes in a model describing PL kinetics in MHP layers are discussed.

Electrochemical impedance analysis of perovskite-electrolyte interfaces

The flat band potentials and carrier densities of spin coated and sprayed MAPbI₃, FA_0.85Cs_0.15PbI₃, and MAPbBr₃ perovskite films were determined using the Mott-Schottky relation. The films developed a space charge layer and exhibited p-type conduction with a carrier concentration of ∼10¹⁶ cm^-3 for spin coated films. Electrochemical impedance spectra showed typical space charge impedance at frequencies >1 kHz, and an exceptional high capacitance at frequency <1 kHz owing to an ion diffusion component.

CH3NH3Pb1−xEuxI3 mixed halide perovskite for hybrid solar cells: the impact of divalent europium doping on efficiency and stability

The crucial role of the impact of divalent europium doping in perovskite solar cells is investigated in this work.

Operating Mechanisms of Mesoscopic Perovskite Solar Cells through Impedance Spectroscopy and J-V Modeling

The performance of perovskite solar cell (PSC) is highly sensitive to deposition conditions, the substrate, humidity, and the efficiency of solvent extraction. However, the physical mechanism involved in the observed changes of efficiency with different deposition conditions has not been elucidated yet. In this work, PSCs were fabricated by the antisolvent deposition (AD) and recently proposed air-extraction antisolvent (AAD) process. Impedance analysis and J-V curve fitting were used to analyze the photogeneration, charge transportation, recombination, and leakage properties of PSCs. It can be elucidated that the improvement in morphology of perovskite film promoted by AAD method leads to increase in light absorption, reduction in recombination sites, and interstitial defects, thus enhancing the short-circuit current density, open-circuit voltage, and fill factor. This study will open up doors for further improvement of device and help in understanding its physical mechanism and its relation to the deposition methods.

Manipulating the Net Radiative Recombination Rate in Lead Halide Perovskite Films by Modification of Light Outcoupling

Photon recycling is a fundamental physical process that becomes especially important for photovoltaic devices that operate close to the radiative limit. This implies that the externally measured radiative decay rate deviates from the internal radiative recombination rate of the material. In the present Letter, the probability of photon recycling in organic lead halide perovskite films is manipulated by modifying the underlying layer stacks. We observe recombination kinetics by time-resolved photoluminescence that is controlled by the optical design of the chosen layer structure. Quantitative simulations of decay rates and emission spectra show excellent agreement with experimental results if we assume that the internal bimolecular recombination coefficient is ∼66% radiative.

Solution-Grown CsPbBr3 /Cs4 PbBr6 Perovskite Nanocomposites: Toward Temperature-Insensitive Optical Gain

With regards to developing miniaturized coherent light sources, the temperature-insensitivity in gain spectrum and threshold is highly desirable. Quantum dots (QDs) are predicted to possess a temperature-insensitive threshold by virtue of the separated electronic states; however, it is never observed in colloidal QDs due to the poor thermal stability. Besides, for the classical II-VI QDs, the gain profile generally redshifts with increasing temperature, plaguing the device chromaticity. Herein, this paper addresses the above two issues simultaneously by embedding ligands-free CsPbBr₃ nanocrystals in a wider band gap Cs₄ PbBr₆ matrix by solution-phase synthesis. The unique electronic structures of CsPbBr₃ nanocrystals enable temperature-insensitive gain spectrum while the lack of ligands and protection from Cs₄ PbBr₆ matrix ensure the thermal stability and high temperature operation. Specifically, a color drift-free stimulated emission irrespective of temperature change (20-150 °C) upon two-photon pumping is presented and the characteristic temperature is determined to be as high as ≈260 K. The superior gain properties of the CsPbBr₃ /Cs₄ PbBr₆ perovskite nanocomposites are directly validated by a vertical cavity surface emitting laser operating at temperature as high as 100 °C. The results shed light on manipulating optical gain from the advantageous CsPbBr₃ nanocrystals and represent a significant step toward the temperature-insensitive frequency-upconverted lasers.

Two-Dimensional CH3NH3PbI3 Perovskite Nanosheets for Ultrafast Pulsed Fiber Lasers

Even though the nonlinear optical effects of solution processed organic-inorganic perovskite films have been studied, the nonlinear optical properties in two-dimensional (2D) perovskites, especially their applications for ultrafast photonics, are largely unexplored. In comparison to bulk perovskite films, 2D perovskite nanosheets with small thicknesses of a few unit cells are more suitable for investigating the intrinsic nonlinear optical properties because bulk recombination of photocarriers and the nonlinear scattering are relatively small. In this research, we systematically investigated the nonlinear optical properties of 2D perovskite nanosheets derived from a combined solution process and vapor phase conversion method. It was found that 2D perovskite nanosheets have stronger saturable absorption properties with large modulation depth and very low saturation intensity compared with those of bulk perovskite films. Using an all dry transfer method, we constructed a new type of saturable absorber device based on single piece 2D perovskite nanosheet. Stable soliton state mode-locking was achieved, and ultrafast picosecond pulses were generated at 1064 nm. This work is likely to pave the way for ultrafast photonic and optoelectronic applications based on 2D perovskites.

Methylammonium Lead Bromide Perovskite Battery Anodes Reversibly Host High Li-Ion Concentrations

Ions migrate through the hybrid halide perovskite lattice, allowing for a variety of electrochemical applications as perovskite-based electrodes for batteries. It is still unknown how extrinsic defects such as lithium ions interact with the hybrid perovskite structure during the charging process. It is shown here that Li⁺ intake/release proceeds by topotactic insertion into the hybrid perovskite host, without drastic structural alterations or rearrangement. Even the perovskite electronic band structure remains basically unaltered upon cycling. The occurrence of conversion or alloying reactions producing metallic lead is discarded. Stable specific capacity ∼200 mA h g^-1 is delivered, which entails outstanding Li-ion molar concentration, x in Li_xCH₃NH₃PbBr₃, approaching 3. Slight distortions of the perovskite lattice upon cycling explain the highly reversible Li⁺ intercalation reaction that also exhibits an excellent rate capability.

Decreasing Radiative Recombination Coefficients via an Indirect Band Gap in Lead Halide Perovskites

High absorption coefficients imply also high radiative recombination coefficients due to detailed balance. It is therefore worthwhile to investigate whether a combination of two transitions with different absorption coefficients (such as a direct and an indirect band gap) could be used to reduce radiative recombination while at the same time retaining the high absorption coefficient. We show here that a longer radiative lifetime helping charge collection can indeed be achieved, while an increase in open-circuit voltage by adding an indirect band gap below the direct one is impossible. We also show that the absorption coefficient in CH₃NH₃PbI₃ could indeed consist of a direct and an indirect contribution; however, the indirect one seems to dominate luminescence and therefore radiative recombination. Thus, the condition that the direct gap is mainly responsible for absorption and emission would not be valid for CH₃NH₃PbI₃. Therefore, we would not expect any benefit of an indirect gap in the radiative limit. However, there may be benefits for charge collection but not open-circuit voltage if nonradiative recombination is dominant.

Enhancement of the Performance of Perovskite Solar Cells, LEDs, and Optical Amplifiers by Anti-Solvent Additive Deposition

The efficiency of perovskite optoelectronic devices is increased by a novel method; its suitability for perovskite solar cells, light-emitting diodes, and optical amplifiers is demonstrated. The method is based on the introduction of organic additives during the anti-solvent step in the perovskite thin-film deposition process. Additives passivate grain boundaries reducing non-radiative recombination. The method can be easily extended to other additives.

Role of Intrinsic Ion Accumulation in the Photocurrent and Photocapacitive Responses of MAPbBr3 Photodetectors

We studied steady state and transient photocurrents in thin film and single-crystal devices of MAPbBr₃, a prototype organic-inorganic hybrid perovskite. We found that the devices' capacitance is abnormally large, which originates from accumulation of large densities of Pb²⁺ and Br^- in the active perovskite layer. Under applied bias, these ions are driven toward the opposite electrodes leading to space-charge fields close to the metal/perovskite interfaces. The ion accumulation, in turn, causes photocurrent reversal polarity that depends on the history of the applied bias and excitation photon energy with respect to the optical gap. Furthermore, the large capacitive response dominates the transient photocurrent and, therefore, obscures the weaker contribution from the photocarriers' drift. We show that these properties depend on the ambient conditions in which the measurements are performed. Understanding these phenomena may lead to better control over the stability of perovskite photodetectors for visible light.

Organic-inorganic perovskite plasmonic nanowire lasers with a low threshold and a good thermal stability

Plasmonic nanolasers have ushered in a paradigm of deep sub-wavelength coherent optical sources with ultrafast dynamics that exploit the strong confinement capabilities of metals. Although these devices are usually associated with higher thresholds due to absorption in metals, the high gain inorganic II-VI and III-V semiconductor materials have allowed the realization of plasmonic nanolasers operating under ambient conditions. In this work, we introduce single-crystalline lead halide perovskite (CH₃NH₃PbI₃) nanowires as an organic-inorganic semiconducting gain material to the plasmonic laser community. We demonstrate plasmonic laser action using a hybrid geometry whereby the perovskite nanowires are placed on a silver substrate with an insulating spacer layer. We report relatively low threshold operation under ambient conditions (13.5 μJ cm^-2), and the devices work well even at temperatures up to 43.6 °C. The demonstration highlights the high optical gain achievable in perovskite materials and thus provides a solution to high gain materials for plasmonic devices.

Nanoimprinted distributed feedback lasers of solution processed hybrid perovskites

Hybrid perovskite materials have considerable potential for light emitting devices such as LEDs and lasers. We combine solution processed CH₃NH₃PbI₃ perovskite with UV nanoimprinted polymer gratings to fabricate distributed feedback (DFB) lasers. The lead acetate deposition route is shown to be an effective method for fabricating low-loss waveguides (loss coefficient ~6 cm-1) and highly compatible with the polymer grating substrates. The nanoimprinted perovskite exhibited single-mode band-edge lasing, confirmed by angle-dependent transmission measurements. Depending on the excitation pulse duration the lasing threshold shows a value of 110 μJ/cm2 under nanosecond pumping and 4 μJ/cm2 under femtosecond pumping. We demonstrate further that this laser has excellent stability with a lifetime of 10⁸ pulses.

Single step deposition of an interacting layer of a perovskite matrix with embedded quantum dots

Hybrid lead halide perovskite (PS) derivatives have emerged as very promising materials for the development of optoelectronic devices in the last few years. At the same time, inorganic nanocrystals with quantum confinement (QDs) possess unique properties that make them suitable materials for the development of photovoltaics, imaging and lighting applications, among others. In this work, we report on a new methodology for the deposition of high quality, large grain size and pinhole free PS films (CH3NH3PbI3) with embedded PbS and PbS/CdS core/shell Quantum Dots (QDs). The strong interaction between both semiconductors is revealed by the formation of an exciplex state, which is monitored by photoluminescence and electroluminescence experiments. The radiative exciplex relaxation is centered in the near infrared region (NIR), ≈1200 nm, which corresponds to lower energies than the corresponding band gap of both perovskite (PS) and QDs. Our approach allows the fabrication of multi-wavelength light emitting diodes (LEDs) based on a PS matrix with embedded QDs, which show considerably low turn-on potentials. The presence of the exciplex state of PS and QDs opens up a broad range of possibilities with important implications in both LEDs and solar cells.

Nanowire Lasers of Formamidinium Lead Halide Perovskites and Their Stabilized Alloys with Improved Stability

The excellent intrinsic optoelectronic properties of methylammonium lead halide perovskites (MAPbX3, X = Br, I), such as high photoluminescence quantum efficiency, long carrier lifetime, and high gain coupled with the facile solution growth of nanowires make them promising new materials for ultralow-threshold nanowire lasers. However, their photo and thermal stabilities need to be improved for practical applications. Herein, we report a low-temperature solution growth of single crystal nanowires of formamidinium lead halide perovskites (FAPbX3) that feature red-shifted emission and better thermal stability compared to MAPbX3. We demonstrate optically pumped room-temperature near-infrared (∼820 nm) and green lasing (∼560 nm) from FAPbI3 (and MABr-stabilized FAPbI3) and FAPbBr3 nanowires with low lasing thresholds of several microjoules per square centimeter and high quality factors of about 1500-2300. More remarkably, the FAPbI3 and MABr-stabilized FAPbI3 nanowires display durable room-temperature lasing under ∼10(8) shots of sustained illumination of 402 nm pulsed laser excitation (150 fs, 250 kHz), substantially exceeding the stability of MAPbI3 (∼10(7) laser shots). We further demonstrate tunable nanowire lasers in wider wavelength region from FA-based lead halide perovskite alloys (FA,MA)PbI3 and (FA,MA)Pb(I,Br)3 through cation and anion substitutions. The results suggest that formamidinium lead halide perovskite nanostructures could be more promising and stable materials for the development of light-emitting diodes and continuous-wave lasers.

Light-Induced Space-Charge Accumulation Zone as Photovoltaic Mechanism in Perovskite Solar Cells

We fabricated formamidinium lead iodide perovskite solar cell for analysis of the photovoltaic mechanism based on the interpretation of the capacitance variation under illumination. It was shown that the low-frequency capacitance increases proportional to incident light intensity, and in addition it increases proportional to absorber thickness. Furthermore, the voltage dependence of capacitance is exponential with slope 1/2 (thermal energy). We conclude that the large photovoltage and capacitance are associated with electronic accumulation zone at the interface with the metal oxide contact. While this type of accumulation capacitance is common in many devices as transistors, the perovskite solar cell shows a singular behavior in that under light the electronic carrier accumulation grows unlimited by another series capacitance, reaching values as large as 10 mF cm(-2) at one sun illumination.

Highly stabilized perovskite solar cell prepared using vacuum deposition

With proper optimization, high efficiency solar cells are fabricated with the perovskite layers prepared using a one-step solution process and vacuum deposition methods and the devices prepared by vacuum deposition show significantly better ambient and thermal stability.