The Bright Side of Perovskites - The Journal of Physical Chemistry

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The Bright Side of Perovskites Silvia Colella,†,‡ Marco Mazzeo,†,‡ Aurora Rizzo,‡ Giuseppe Gigli,†,‡ and Andrea Listorti*,†,‡ †

Dipartimento di Matematica e Fisica “E. De Giorgi”, Università del Salento, Via per Arnesano, 73100 Lecce, Italy Istituto di Nanotecnologia, CNR-Nanotec, c/o Campus Ecotekne via Monteroni, Lecce 73100, Italy

‡

ABSTRACT: Incubating in the rise of perovskite photovoltaic era, the advances in material design encourage further promising optoelectronic exploitations. Here, we evaluate halide perovskite envisioning light-emitting applications, with a particular focus to the role that this material can effectively play in the field, discussing advantages and limitations with respect to state of art competing players. Specific benefits derive from the use of low dimensional and nanostructured perovskites, marginally exploited in photovoltaic devices, allowing for a tuning of the excited states properties and for the obtainment of intrinsic resonating structures. Thanks to these unique properties, halide perovskite ensure a great potential for the development of high-power applications, such as lighting and lasing.

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solar cell, generating the highest voltage, will be the one where the absorbed photon flux equals the emitted one under open circuit conditions. Such a solar cell, ideally targeted for highest efficiency, is also said to be in ‘“the radiative limit”’.6,7 Those considerations particularly stands for halide perovskite, as these materials could work in a single device as either light-emitting or light harnessing layer depending on the working conditions.8,9 The electroluminescence of HPs, although at cryogenic temperatures, has been reported long before perovskite PV era in 1994.10 Nevertheless, only a recent refinement of HPs deposition processes, allowing a fine properties control, led to effective progresses in the field. On this line, the development of low dimensional (layered or colloidal nanocrystalline) perovskites, marginally exploited in PV devices, brought paramount benefits to the EL field, allowing for exciton and photon confinement.11 These premises have encouraged a decisive surge of perovskite exploitation in light-emitting devices (PeLEDs), which are critically assessed in this Perspective, with particular attention to the role that HPs can effectively play in this field. Three main targeting applications, summarized in Figure 1, are envisaged: light emitting diodes, lasers diodes, and advanced photonic devices. Thanks to HPs unique properties a development toward highpower applications, such as lighting and lasing, is likely to be observed in the coming future. HPs in Light-Emitting Diodes. PeLEDs could have a dramatic impact on light-emitting applications, as the properties of their active material, bridging between the organic and the inorganic world, could promote a progress with respect to the competing technological counterparts based on II−VI, III−V inorganic semiconductors (S-LEDs) and organic semiconductors

alide perovskites (HPs) surf an exciting, innovative wave overwhelming alternative solution processable photovoltaic (PV) technologies, toward a constructive competition with the established silicon one.1−3 This innovation originates from the unique properties of the material, which allow foreseeing similar breakthroughs in further optoelectronic fields. HPs, most extensively studied in PV in the organometal specimens (i.e., CH3NH3PbI3, CH(NH2)2PbI3), are direct band gap semiconductors, very easy to prepare, and own strong light absorption coupled to high charge carrier lifetime and impressive diffusion lengths. Despite the simple wet chemistry leading to an intrinsically disordered material-assembly process, superior optoelectronic properties, in line with ultrapure inorganic semiconductors, distinguish HPs. These materials show very high fluorescence yields associated with an easy color tunability,4,5 ideal properties envisioning light-generation by electroluminescence (EL) process in both incoherent (lightemitting diodes, or LEDs) and coherent (laser diode, or LDs) regimes. From certain perspectives, active materials for solar converting or light-emitting applications share common, necessary properties. In an ideal active component of a solar converting device, radiative recombination pathways cannot be avoided, as those are intrinsically coupled to the light absorption, whereas the detrimental nonradiative pathways have no such justification and should be minimized. The best

The best solar cell, generating the highest voltage, will be the one where the absorbed photon flux equals the emitted one under open circuit conditions.

Received: August 10, 2016 Accepted: October 14, 2016 Published: October 14, 2016 © 2016 American Chemical Society

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DOI: 10.1021/acs.jpclett.6b01799 J. Phys. Chem. Lett. 2016, 7, 4322−4334

The Journal of Physical Chemistry Letters

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material can efficiently cover the whole visible-near-infrared (vis−NIR) spectral window, due to the easy band gap tuning.13 Considering the general structure APbX3, where the A lattice site is occupied by a small monovalent cation such as cesium (Cs), methylammonium (MA), and formamidinium (FA), this variability is mainly determined by the halogen substitution/ mixing. Iodine-based active layers cover the near-infrared, mixed Br/I−the red/green region, and mixed Br/Cl−the green/blue part of the visible spectrum (Figure 2b). Despite the ease of synthesis, it is worth mentioning the lack of stability reported for the mixed halide forms, which under illumination tend to segregate in two crystalline phases with different emission properties.14 This phenomenon could potentially limit the obtainment of a wide color palette; however, because little research has focused on this topic, we expect that such a limitation could be surpassed with forthcoming material engineering. In addition, the effects of injected carriers into mixed halide perovskites over time are still unknown and should surely be taken into account because it could influence the purity and the intensity of the emission. Interestingly, most of the literature around electroluminescent devices based on halide perovskites focuses on materials bearing MA, likely because it is the most extensively studied in PV. Cs is often explored in the colloidal nanocrystals, whereas the use of other cations such as FA is still mostly unexplored in EL. Figure 2c shows the most significant PeLEDs performances, differentiated by color emission, expression of different device architectures, and structure/composition of the active layer. Several transporting and perovskite layers have been combined so far, implying that an ultimate choice for the device layout has not been determined yet. Organic small molecules, polymers or metal oxides can be effectively used as HTLs or ETLs in PeLED, depending on the specific processing path and on the target application. In all cases, a careful interface optimization is essential to minimize energetic mismatch along with nonradiative recombination thus leverage the material’s full potential. Although NIR and green PeLEDs, based on MAPbI3 and MAPbBr3 (or mixed) materials, have reached relevant performances,15−18 pivotal “blue” devices, embedding chloride perovskite are characterized by poor efficiency,19,20 attributed to nonoptimal injecting electrodes and material/interface bandtailing.5,21 In particular, the fast progress of the field led to high luminance in the green (∼104 cd/m2)16 and orange-red (103 cd/m2)22 regions; performances in line with the requirement for lighting applications (>1000 cd/m2). However at the same time there has been a very limited amount of work on “blue” perovskites, most probably due to the lack of interest toward this material for PV applications, thus in the coming future an improvement on this front is expected with the expansion of PeLED-related research. Efficiently covering the whole visible spectral range is of fundamental importance toward white light emission for indoor/outdoor lighting, indeed one of the most promising evolutions of PeLED. At the moment, the only few demonstrations of perovskite-based white emission, not yet exploited in LEDs, have been obtained using a self-assembled 2D (C6H11NH3)2PbBr4-layered perovskite23 and a mixture of different, for their dimension and chemical composition, perovskite nanocrystals inserted in an organic polymer matrix.24 Noticeably, perovskite self-assembly does not suffer but rather benefits by the presence of polymer matrix for an improved stability and for the control of material properties.25,26 In general, blending of nanocrystals in polymer hosts is among the

Figure 1. Target applications foreseeing exploitation of halide perovskites in light-emitting optoelectronic and photonic devices: LEDs (white-emission and single color), lasers diodes, and advanced photonics.

(OLEDs). These two main players, due to the different properties of the materials involved, perform better at diverse working regimes. The easy-processable, cheap OLEDs, due to a certain level of maturity in terms of efficiency and stability, find application in portable electronic displays and TVs. On the other hand, S-LEDs remain superior for high-power devices due to a higher resistance to thermal stress determined by the high-carriers density typical of such applications. Finally, the achievement of coherent light emission under electrical pumping (laser diodes, LDs) remains the real chimera for organic electroluminescent materials, as this constitutes a prohibitive goal due to the very high-carriers density required, leading to material breakdown. In this scenario, PeLEDs have the potential to provide outstanding optoelectronic characteristics, even at high carriers densities, while keeping the processing cost low.12 In this particular and challenging region of functionality, thanks to their peculiar properties, halide perovskite could find their maximum expression (Figure 1), allowing outreaching limits beyond intrinsic frontier of organic players.

The achievement of coherent light emission under electrical pumping (laser diodes, LDs) remains the real chimera for organic electroluminescent materials. A typical PeLED, sketched in Figure 2a, is constituted of a perovskite-based emissive layer (EML) sandwiched between two opposite carrier transporting media, namely the electron and hole-transporting layer (ETL and HTL). Under forward voltage, the opposite charges are injected from the electrodes and funnel throughout ETL and HTL to the EML, where they radiatively recombine at the wavelength corresponding to the energy-gap of its active component. HPs as luminescent 4323



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Figure 2. (a) Sketch of a typical PeLED configuration with the (b) achievable spectral emission range covered by MAPbX3. (c) External quantum efficiency (EQE) versus emission energy of selected PeLED (numbers correspond to references). Blue dots refer to devices embedding a solution processed HP layer, whereas green dots refers to thermally deposited ones.

“all-perovskite” multilayer LED; (iv) a monolithic growing process which could be easily scalable. Further device evolution and performances improvement would need to address essential questions regarding the intrinsic material properties, the basic physical processes occurring within the perovskite materials and optoelectronic devices. In fact, different interconnected aspects impact on PeLEDs performances, among them interfaces energy mismatch between the charge carrier transport and light-emitting layers, the purity, dimension, and orientation of the crystals, and finally, the balance between opposite carriers. In the following sections, HP material properties, mainly optical and electrical, will be discussed, focusing on the challenges that need to be pursued for progress in the field. Radiative Recombination. HPs optoelectronic properties strongly depends on the material growing process, thus also on the surrounding layers and in particular on the nature of the substrate, as the latter directly drives perovskite selfassembly.32,33 At the core of a performing device is the active material, and in the case of halide perovskite, its behavior is not easy to predict and control. Both the incomplete rationalization of the physical landscape and the easy to perturb self-assembly growth generate this conundrum. Managing and maximizing the emission is a challenging task because this depends on a plethora of parameters, such as the material elemental composition, grains dimension distribution and orientation, the purity of the crystals, the distribution of defects, and the

most exploited approach for the fabrication of hybrid LEDs based on colloidal semiconductor quantum-dots (QDs),27 such as CdSe/ZnS or similar, demonstrating how the knowledge matured for the development of alternative light-emitting technologies will be useful in sustaining the first surge of PeLEDs. Being inspired by successful strategies, exploited for the obtainment of efficient white emission in organic OLEDs,28 HPs could be also employed in a multilayer-stacked architecture, in which different active layers would emit different colors. At the moment, such configuration is hardly foreseeable due to the material wet deposition methodologies. HPs are, in fact, sensitive to the solvent exposure leading to a critical stepwise growth. Thermal deposition could offer a potential solution for the deposition of multiple perovskitebased active emitting layers, although to the best of our knowledge, this particular goal has never been attempted. In exploring this, challenging difficulties would be associated with obtaining abrupt junctions due to the halide diffusion in HPs.29 This technique has already been explored, so far, but for the deposition of a single perovskite-based active layer in heterostructured architectures, both for photovoltaic of lightemitting applications.30,31 Interesting advantages are offered by thermal deposition if compared to solution processing in terms of device implementation. Indeed, it could allow for: (i) a fine control of the growing process; (ii) a higher control of EL process by the employment of multilayer architectures like in the organic light-emitting diodes;28 (iii) the fabrication of an 4324



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density of charges in the film. Importantly, once a parameter is changed (e.g., grain size) a cascade process affects the others (e.g., defect amount and distribution). The different phenomena occurring in the material at different carrier density regimes, thus of interest for the desired applications (Figure 1), are summarized in Figure 3. Under illumination as well as

section), among them: surface chemical/structural perturbations,37−39 ions migration across the grains,40 or diverse defect curing phenomena,41 reason why the univocal attribution of the HPs radiative deactivation, especially for thin films, is very difficult to be done. In addition, this emission is not uniformly distributed in a film or in an isolated crystal. An example of these controversies regard the phenomena occurring ad the grain boundaries, PL quenching has been observed by DeQuillettes and co-workers via confocal microscopy on MAPbI3(Cl),42 although previous investigation reported reduced detrimental impact of the boundaries,43,44 or even a beneficial one.45 Reasonably the radiative deactivation depends on the specific kind of defect present at the boundary, with its particular chemical surrounding. Therefore, describing a standard condition at the grain boundary likely is unwise, as a comprehensive understanding of the relation between chemical/structural material organization and optoelectronic properties at the nanometric level is still lacking. Nevertheless, dedicated research will surely allow, in the next years, important progresses in the field. Related to this, some reports describe improvements in carrier lifetime and device performance from postgrowth treatments, such as exposure of the film to pyridine (C5H5N).42,46 In this line, different approaches have been explored to passivize the traps; among them, precursor engineering has been used to enhance the PeLEDs efficiency in CsPbI3-based devices, reflected in PL lifetime increases.47 Cho et al. have explored the use of an organic molecule in the dripping solvent to passivate the defects of MAPbBr3 crystals, enhancing PL lifetime.16 The reduction of the systems dimensionality, roughly below the hundred nanometer scale, induces the spatial confinement of the charges, this increase the excitonic character of the material excited states. Confined excitons possess higher binding energy and this increased tendency to merge prevents dissociation prior to radiative decay. Furthermore, with a Bohr radius spanning in these systems from 2 to 14 nm a further reduction of HPs dimensionality induce a quantum confinement, which promote a further emission increase concomitant to an excited state lifetime decrease.37 Despite an improved rationalization on the photophysical landscape characterizing these systems has been acquired in the past few years, an exhaustive comprehension of the parameters influencing radiative recombination in perovskite material is still lacking. Particularly lacking is a clear picture of the structure/chemistry influence on the optoelectronic behavior of HP at nanometric scale.48 This condition notwithstanding, a high PLQY have already been reached in HPs, both as colloidal nanocrystals (above 90%)37 or polycrystalline film (above 70%),49 underlining how such outstanding performances are accessible, the effort now have to be directed in standardize those obtainments throughout an intelligent design of the material in a plethora of conditions and device layouts.

Figure 3. Simplified scenario of excited state deactivation processes occurring in halide perovskite materials at different carrier density regimes.

under electrical pumping, the predominant population of species in a polycrystalline film of halide perovskite (i.e., MAPbI3) or in large single crystals, is constituted of free charges, whereas in isolated nanocrystals, due to the spatial confinement of the charges, the existence of a coherent excited state (exciton) is possible, drastically changing the excited state behavior. This scenario derives from the very small exciton binding energy characterizing the material and is valid in a wide range of photoexcitation regimes (with carrier densities spanning from 1016 to 1019 cm−3).34,35 The process that originates the emission of light from these materials is a traplimited electron−hole plasma bimolecular radiative recombination. In thin films, the rate of the radiative recombination process is ≈3−4 orders lower than what predicted by the Langevin model;36 however, with no other relaxation pathways available, the two-particle recombination is more favorable than three-particle Auger deactivation, in a wide window of carrier density, as the latter only dominates above 1019 cm−3.34 At low carrier densities, bimolecular radiative recombination competes with monomolecular losses originating from trap-assisted recombination. The yield of this detrimental process depends on the trap cross section, energetic depth, density, and distribution (Figure 3). Withstanding this scenario carrier recombination lifetimes measured by photoluminescence decays (TRPL), and the related emission quantum yield, are commonly taken as a hallmark of perovskite film quality, with longer decay lifetimes used as indicators of better-performing materials owing to a reduced trap population, as sketched in Figure 3. Reports in this area are, however, quite controversial because a variation in the PL decay can also be attributed to other parameters but the spatial confinement of excitons (see the next

Excitons could be spatially confined forcing holes and electrons to recombine in a highly efficient radiative process occurring within relatively short temporal regimes. Exciton Conf inement. The intrinsic HPs low exciton binding energy represents a great advantage for PV applications but it 4325



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relatively small grains (about 100 nm) promoting a spatial confinement of carriers and the record EQE to date for the green emission (8.5%), accompanied by current efficiency of 42.9 Cd/A.16 Interestingly the confinement within small grains, in a thick layer, rather than the confinement within a more uncontrollable thin layer, presents significant advantages in reducing the nonradiative losses originating from shunt paths. Besides material engineering, to confine the excitons, it is possible to play with the intrinsic chemical properties of this eclectic class of materials, for example, by reducing the dimensionality of the materials (Figure 4, left to right). In contrast to the 3D AMX3, layered perovskites of general formula (RNH3)2An−1MnX3n+1 (n = 1, pure 2D layered; n = ∞, 3D structure; and n = defined integer, quasi-2D layered structure) are formed when large organic cations are introduced in the structure not fitting the cavity between the MX6 octahedra and inducing the formation of separated inorganic metal halide layers. Natural quantum well structures shows very large exciton binding energies (above hundreds of millielectronvolts) originating from the very different dielectric constant between the two counterparts and from the reduced symmetry. The difficulties in ensuring an efficient charge transport between the layers, generally, hinder the efficiency of devices based on these materials,23,24 even though a smart design of a conceptually new mixed perovskite, comprising a series of differently quantum-sized grains that funnels photoexcitations to the lowest band gap light-emitter in the mixture, allowed for the highest EQE in NIR-LEDs to date (8.8%).15 An elegant approach toward the exciton confinement, which has not yet expressed its full potential, foresees the synthesis of either hybrid (APbX3; A = MA, FA; X = Br, I, Cl)56 or fully inorganic (CsPbX3) perovskite nanocrystals (NCs)57 to obtain solution processable perovskite material with tunable photophysical properties. These specimens, also thanks to the occurrence of an effective quantum confinement, show outstanding emission up to 90% quantum yield, with short radiative lifetimes of 1−29 ns and narrow emission line widths of 12−42 nm, reachable in larger crystals only at very high fluences.37,49 The quantum confinement of the exciton in colloidal MAPbBr3 NCs reduces the average PL lifetime linearly with particle size. Therefore, exciton binding energy increases with the size reduction,13 reaching values as large as ∼375 meV, favoring efficient radiative recombination.58 Feasibility of hybrid halide NC integration in actual LED have been demonstrated,56 and the highest efficiency with EL yield up to 3.8% for MAPbBr3 NCs was recently reported.59 Additionally, in these systems PL color-tuning can be easily obtained through both quantum size effects37,60,61 as well as by rapid halide exchange, red shifting the emission from a deep blue color (chloride) to a green color (bromide) to a red emission (iodide), exploiting the different halide ionization potentials.13,52 The synthesis of hybrid metal halide perovskite NPs has been recently extended to FAPbBr3, showing that the photophysical features can be tuned by exchanging the organic cation. Thus, in a recent work, the first FAPbBr3-based LECs was fabricated, featuring a luminance of around 1−2 cd/m2 at low driving currents.62 Although hybrid HP NC synthesis can be obtained by an extremely simple synthetic method at room temperature, replacing methylammonium with inorganic Cs could offers extra thermal stability. This would be particularly valuable given the intrinsic chemical instability of MAPbI3, which remains one of the main issues to be solved. With this aim, all-inorganic CsPbX3 (X = Cl, Br, I, and mixed Cl/Br and

could be a serious limitation for the achievement of high EL efficiencies in PeLED. In fact, the reported values of a few millielectronvolts for MAPbI3,50 could hinder radiative recombination of loosely bound charges populating the active layer, even though a low exciton binding energy promotes a good charge transport through the film, a further fundamental condition for efficient light emitting devices. To solve the issue, excitons could be spatially confined forcing holes and electrons to recombine in a highly efficient radiative process occurring within relatively short temporal regimes. This has been one of the most effective strategies to improve PeLED performances. To date, a few approaches on this line, summarized in Figure 4,

Figure 4. Sketch summarizing the approaches to increase the exciton confinement in halide perovskite by structural (left to right) and film formation (top to down) control. The structural exciton confinement can be easily obtained by increasing the organic cation size inducing the formation of layered perovskites or by ligand-assisted synthesis of nanocolloids. In parallel, engineering the deposition techniques could allows fine control of the film thickness and of the grain sizes, finally influencing the exciton confinement.

have been explored: (i) the carriers confinement within an extra-thin (15 nm) emitting layer;4 (ii) the use of thick films constituted of small grain (around 100 nm), acting as containers for the excitons;16 (iii) the formation of perovskite 2D layered structure by intercalation of larger organic cations;51 (iv) the synthesis of colloidal perovskites nanoparticles, with defined and controlled size and shape, acting as quantum boxes.52 All these approaches show limitations and advantages, thus a good compromise needs to be found according to the specific case. For example, the adoption of extra-thin layers led to EQE of 0.1−0.4% with a maximum brightness of 364 cd/m2 in the visible region of the spectra.4 In this case, the difficulty in obtaining uniform coverage and extra-smooth film promotes nonradiative losses, limiting the EL efficiency. Solutions have been found in using polymeric compounds as buffers or hosting matrixes to cover the pinholes.53,54 Alternatively, one could foresee the employment of sequential deposition method from solution or thermal evaporation as already shown in PV55 and in fully evaporated LEDs,30 this could allow improved exciton confinement along with good substrate coverage. Recently, thick perovskite layers have also shown remarkable performances in PeLEDs owing to grain size tuning, obtained by solvent and antisolvent engineering methodologies. The approach allowed the obtainment of a uniform distribution of 4326



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based nanocrystals also showed promising values of mobilities when this is measured in solution, so for isolated particles and by terahertz spectroscopy.78 Nonetheless, these results need to be considered with great care and have to be confirmed by other measurements, most importantly on an ensemble of NCs, because the inclusion in an actual device would affect these values and a thoughtful engineering will be needed, that is, the modification of the NC surfactant. Single crystals and isolated NCs represent just the frontier of materials accessible properties; however, moving from these materials toward polycrystalline films or NC dispersion in supporting matrixes these mobilities remain impressive. The values for single perovskite crystals, in fact, are in line with the ones characterizing ultrapure optoelectronic exploited semiconductors such as silicon, CdSe, and GaAs crystal.79,80 The main reason at the ground of this behavior has to be found in the distribution of traps on the grain surface and in the nature and role of defects within the film. Electronic calculations on slabs of perovskite crystals shows no trap states in the gap either for bulk or surface electronic structure,81 suggesting a negligible impact of the grain surfaces on determining electrical losses when carriers move across perovskite grains. Analyzing the electronic levels and formation energies of various classes of defects, such as vacancies, interstitials, and antisites, demonstrates that vacancies produce either lightly perturbed states in the bands, not affecting carriers mobility, or shallow traps, whereas interstitials and antisites, associated with Pb and I, form electronic states lying deep inside the band gap. In addition, the material’s highly ionic character and the distinct size and charge of the constituting ions (MA or Cs and Pb) lead to a significant lower degree of disorder and inhomogeneity if compared to conventional semiconductors (i.e., chalcogenide materials) leading to low density of point defects within the band gap.37 In general, in semiconductors having good electronic transport parameters the density of recombination centers directly controls the diffusion length of charge carriers. In the specific case of perovskites, the density of recombination centers is determined by the growing conditions of the crystal, representing a central link between the growth chemistry and the charge transport performance. A specific example showed how an iodine-poor growing environmental, during the deposition of MAPbI3, determines an equilibrium density of trap states below 1015 cm−3. At this volume density, traps are spaced a median ∼200 lattice constants in all crystallographic directions, enabling diffusion lengths above 100 nm.81 In HP, the Fermi level positioning influences directly the density of defects thus the carries mobility, and in particular Pb, I, and MA vacancies, form shallow levels near band edges playing the role of unintentional doping sources. This implies that n-/p-type doping characteristics can be manipulated by the proper choice of atomic composition in the fabrication procedure. This peculiar behavior allows the tuning of HPs from intrinsic good p-type and moderate to good n-type. Such flexible defect properties are distinguished from conventional semiconductors and suggest potential tunability of the device layout targeting different applications and working regimes. For light-emitting applications, this behavior could be, in fact, of great advantage foreseeing heterostructures completely formed of perovskite materials. A challenging output to target would be a homogeneous interface formed by two perovskites having diverse defect chemistry (doping) thus different carriers mobilities allowing a fine-tuning of the carriers’ confinement regions.

Br/I halides) NCs have been developed by hot-injection method.37 In general, inorganic CsPbX3 perovskites show improved stability but suffer for a reduced solubility; thus, they can be hardly deposited from common solvents, contrarily to the hybrid MAPbX3 counterparts. The possibility to make colloidally stable CsPbX3 NC suspensions endows all-inorganic perovskites with solution processability. In addition, at room temperature, only the nonemissive orthorhombic polymorph of bulk CsPbX3 perovskites has been obtained,63 whereas CsPbX3 NCs crystallize in the emissive cubic phase. This fact could be due to the combined effect of the high synthesis temperature and contributions from the surface energy. Disparate properties amenable to light emitting devices and several advantages, when compared to conventional colloidal NCs, have been demonstrated for HP colloids. Namely, low threshold amplified spontaneous and stimulated emission,58 single photon emission, exciton fine structure,64 reduced self-absorption and Förster energy transfer, no variation of PL peak position in the temperature range 25−100 °C, and substantial blinking suppression.65 In addition, nonlinear absorption properties have been observed, opening the path for nonlinear photonic and advanced optoelectronic devices.66 The combination of improved chemical stability and optical properties makes CsPbX3 NCs attractive for light emitting applications, particularly in the blue range, where conventional semiconductor NCs generally photodegrade. Examples of CsPbBr3 NC-based LEDs have been reported in multilayer architecture combined with polymers and organic molecules for green emitting67 and in multicolor OLED device, with EQE up to 5.7%, through NC cross-linking.22 However, the presence of relatively long and insulating capping ligands, ensuring colloidal stability and solution processability, could hinder the efficient inter-NC charge transport.68,69 Replacing these long ligands with shorter molecules or atomic species is the key strategy to design efficient CsPbX3 NC-based LEDs70 With this aim, a recent work from Sargent et al. proposes the fabrication of highly stable films of CsPbX3 NCs capped with a halide ion pair (e.g., didodecyl dimethylammonium bromide), a relatively short ligand that facilitates carrier transport and enables highly efficient PeLEDs.71 In addition, the inclusion of such NC into alternative bulk perovskite matrixes could be one of the breakthroughs of the filed toward white light emission.72 Charge Transport Management. In a light-emitting device, the charge-transport characteristics of the active material and the surrounding layers play a key role in the determination of the device external quantum efficiency. Charge mobility, their correct balance, and diffusion length determine the condition for high performances devices. Similar concentrations for both charge carriers, in a specific region of the device, are necessary to maximize its efficiency; this makes ambipolar materials with good transport properties for both electrons and holes the most suitable candidates. Indeed, this is one of the most attractive properties of metal halide perovskites, which fostered the impressive advancement of solar converting devices. HPs are characterized by a free-charge-carrier-dominated transport35 with long and balanced charge-carrier diffusion lengths.73,74 Single-crystal hybrid systems (either MAPbI3 or MAPbBr3) display high charge carrier mobilities, in the order on tens to hundreds cm2·V−1·s−1, varying with preparation and mobility characterization method.75,76 The replacement of the organic cation with an inorganic one leads to a further increase of charge mobility, up to 1000 cm2·V−1·s−1 for CsPbBr3, due to an improved material crystallinity.77 Finally, fully inorganic Cs4327



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cavity laser with very low thresholds of 320 nJ cm−289 have been reported. Finally, a lasing threshold as low as 220 nJ cm−2 has been obtained for a single crystal perovskite nanowire being an intrinsic resonator with outstanding quality factors of 360011 overcoming the typical performances of state-of-the-art GaAs− AlGaAs nanowire lasers. Ultimately, optically pumped lasing has been obtained also from single crystal nanowires of FAPbI3 and FAPbBr3, with low lasing thresholds of several microjoules per square centimeter and high quality factors of about 1500− 2300, interestingly exceeding the stability under pulsed laser excitation of MA-based counterparts.90 When designing and fabricating a LD, the integration of a LED structure into a resonator and a careful engineering of both optical and electrical characteristics are required. Indeed, the achievement of population inversion, thus stimulated emission by electrical injection, can be strongly limited by the optical losses due to the cavity feedback and electrical leakage accompanying interfacial energy and physical/mechanical mismatches. Understanding the impact of the laser architecture in introducing these leakages is crucial as they may increase the current density threshold making lasing conditions ineffective. Because the lowest thresholds in optically pumped devices have been obtained in edge-emitting waveguide laser configuration,11 which enables a long interaction with the gain medium, this could represent a natural choice for a pivotal perovskite-based laser diode design. This typical architecture is depicted in Figure 1, consisting of a gain medium sandwiched between thicker n-type electron transport and p-type hole transport cladding layers with lower refractive indices. In this layout, the guided mode can leak outside the high-index active part due to the poor optical confinement and overlaps with the absorbing metallic electrodes, thus resulting in an threshold about hundred times larger than that of the free-metal system.91 The typical injection electrodes so far used in PeLED architectures can introduce plasmonic effects, in the case of silver electrodes, or waveguiding effects, in the case of ITO layers. It is possible to carefully design the device geometry so that the overlap between the optical mode and the electrodes is reduced as far as possible, either by keeping the mode away from the metal electrode92 using thick transport organic layers or by replacing the metal electrode with thin transparent conductive oxide layers.93 The use of organic transport layers, often chosen for PeLEDs, can be problematic for lasers application because of the low mobilities of amorphous organic semiconductors (in the range 10−5−10−2 cm2·V−1·s−1) that, at high current flow, could unbalance the carrier transport and cause heat losses. This problem can be partially solved by electrically doping the organic transport layers, improving and balancing conductivity;30 however, this would not solve the heat management, which probably remains the main obstacle of hybrid organic/inorganic heterostructures. It is important at this stage to assess how far from achieving electrically pumped lasing PeLEDs are today. Considering an edge-emitting laser diode employing oxide electrodes (Figure 5b), with the simplest ideal ITO/perovskite/ITO waveguide structure, it is possible to estimate the current density threshold, considering only the optical losses due to the materials and the cavity. For this particular architecture, the total cavity losses αT is due to the optical absorption coefficient of the active material (α0) and the reflectivity (R) of material/ air interface at the edges of the cavity

A further peculiarity of the material is related to its softness, HPs undergo structural evolution due to ion migration under applied biases, in particular when involving organic cation.82 This could be a fundamental obstacle for the achievement of high carrier densities as those will be likely associated with high biases. However, specific strategies could be foresaid to limit the issue, among them pulsed switching biases4 or alternative transistor-like layouts, allowing spatial confinement of the carriers.83 High Carriers Density toward Lasing. Commercially available laser diodes are still exclusively based on inorganic semiconductors: those are usually assembled on hard and fragile materials, through an energy demanding processing. This is also due to the failure of organic materials to fulfill the requirements of electrically pumped lasers,84 leaving an important gap in materials for optoelectronics. Halide perovskites simultaneously satisfy the conditions needed for electrically pumped lasing: (i) a large optical gain, exceeding 104 cm−1; (ii) low nonradiative decay rates; (iii) low electronic trap densities; (iv) large and balanced charge-carrier mobility; (v) large free carrier density, above 1017 cm−1; (vi) potential good thermal resistance at high current injection.85 In addition, the simple tuning of the band gap offers the possibility to fabricate lasers with a wide color palette, an interesting advantage if compared to inorganic semiconductor. As the absorption and the stimulated emission coefficients have the same value, a strong absorbing material can efficiently gain in a lasing process. Indeed HPs possess a sharp optical absorption with reduced Urbach tail, characterized by a large oscillator strength (104 cm−1).12 Defect density as low as 1016 cm−3, commonly measured for polycrystalline perovskite films, leads to low nonradiative decay rates,49 reflected in high PL efficiencies.86 These traps can be filled easily at low current density, likely not constituting a limiting factor for the population inversion. Large charge-carrier mobilities up to 4000 cm2·V−1·s−1 are reported for isolated nanocrystals;78 those limit values are comparable to the state of the art inorganic semiconductors and are greatly above organic materials limits. In addition, other perovskite characteristics are advantageous to achieve lasing. In fact, due to the minimal Stokes shift (less than 20 meV) of these materials, the energy lost to heat during the down conversion may be limited. However, related to this, self-absorption can cause optical losses; thus, a good balance between these two effects has to be considered.

The simple tuning of the band gap offers the possibility to realize lasers with a wide color palette, an interesting advantage if compared to inorganic semiconductor. Very different gain values have been achieved for single crystals, nanocrystals, and thin films, mirroring the variability of perovskite properties in their different specimens, Figure 5a. High optical gain of about 3200 ± 830 cm−1 and optical threshold near 16 μJ cm−2 have been reported for atomic layer deposited MAPbI3 films under pulsed conditions,87 located well within the typical values of the competitors, Figure 5a. ASE thresholds of 7.6 μJ cm−2 under pulse durations of 5 ns, respectively,88 and a perovskite distributed-feedback (DFB) 4328



Perspective

Figure 5. (a) Typical material gain achievable in different classes of materials.96,87 (b) Schematic representation of a simple ideal laser diode (LD) constituted by a perovskite layer (200 nm) embedded between two ITO thin layers (20 nm). The thickness of perovskite has been selected in order to have solely one guiding mode (black line), whereas the thicknesses of ITO layers reduce the waveguide effect, allowing for an excessive dispersion of the optical mode. The refractive index profile (red lines) shows that owing to the higher refractive index of the perovskite layer, the mode is well confined into the gain material. (c) External quantum efficiency against the current density injected into the device. The red line represents the limit that separates the incoherent emission from lasing one under electrical injection in an LD configuration as in (b), for a perovskite MAPbI3 emitting at 780 nm and an αT = 5000 cm−1 and an optical confinement factor Γ of 85%. This line has been extracted by eq 3, considering an outcoupling factor of 20% and a threshold of 1.78 × 1014 (αT/Γ) cm−3 (see the text). The blue line represents the same situation for an organic gain material made of Alq3:DCM297 with a αT about 50 cm−1 and Γ of 40%. Red dots: maximum quantum efficiency and relative current density of several HPLEDs as reported in literature. The horizontal stripes correspond to the current density thresholds typical of double heterostructure (DHS), quantum wells (QW), and quantum dots (QD) inorganic laser diodes.

α T = α0 −

1 ln(R ) L

For a typical NIR emission of MAPbI3, with a gain peak wavelength of 780 nm, a Δλ of 2 nm, a τrad of about 10 ns,5 Nth results in a value of 1.78 × 1014 (αT/Γ) cm−3. For the proposed ideal configuration, the cavity losses due to the facets are negligible (50 cm−1) compared to the material absorption coefficient at 780 nm, about 5000 cm−1.98 Therefore, the threshold density Nth is ∼1018 cm−3, in line with previous estimations;5 noticeably, at these densities, bimolecular recombination still holds, as previously discussed. In this simulation, an ideal situation where facets, as well as the layer mismatch and electrical to photon conversion, are perfect, has been considered. Rough facets, higher surface roughness caused by perovskite grains, poor interfacial energy and mechanical mismatch, or the use of metallic electrodes, can cause a further dramatic increase in the threshold current density, implying loss processes due to high carrier densities such as Auger recombination. It has been demonstrated, however, that lifetimes of Auger processes are slower compared to amplified spontaneous emission in HPs.11 This peculiar behavior distinguishes HPs from organic materials, in which several exciton quenching processes, such as triplet−triplet or singletpolaron absorption, take place at high carrier densities.97 The threshold density is related to the injected current density Jth

(1)

where L is the length of the waveguide. The threshold density of carriers (Nth), for a defined lasing wavelength (λ), leading to lasing will be94 2 Nth = 8πcncav

τradΔλ ⎛ αT ⎞ ⎜ ⎟ λ4 ⎝ Γ ⎠

(2)

where ncav is the cavity effective refractive index, c is the speed of light, Γ is the optical confinement factor within the active layer, τrad is the radiative spontaneous emission time, and Δλ derives from the gain line-width. As clearly shown by eq 2, improving the optical coupling or reducing the τrad and Δλ results in a reduced threshold. In perovskites, the reduction of the natural radiative lifetime is possible pursuing the exciton confinement as previously discussed. A simple optical simulation for a device architecture embedding 20 nm ITO electrodes and 200 nm of HP active material (Figure 5b), gives a Γ of about 85% for the single mode traveling from one facet to the opposite one. Noticeably Γ is higher than organics in the specific case of using HPs as their refractive index is above 295 (then above ITO), leading to strong optical confinement within the cavity. 4329



Perspective

and to the internal quantum efficiency ηint by the following equation: Jth =

ed Nth ηintτrad

carriers mobility, is one of the most challenging objectives of future research. Photonics. Some of the unique properties HPs suggest further interesting phenomena when coupling the material with optical microcavities (Figure 1). Indeed, strong coupling regimes have been obtained for fluorophenethylamine hybrid layered perovskite in resonating structures. Strong coupling is an interesting phenomenon that could be exploited targeting the coherent light emission throughout the realization of low-threshold LDs via polariton (a quantum superposition between exciton and photon states) Bose Einstein Condensation (BEC).102 The main difference between polariton and conventional lasers is that in the former case the coherence origins from the condensate, which emits by spontaneous radiative decay, whereas in latter stimulated radiative emission is required. This could importantly reduce lasing thresholds in polariton lasers with respect to conventional ones (Figure 5c). BEC is a process in which bosonic particles begin to occupy the system ground state once overcoming critical parameters such as temperature and polariton density threshold. For an ideal bosonic gas (in three dimensions), the critical condition for BEC occurs when nλD3 = 2.62, where n is the density of the bosons and λD = (2πℏ2/(mkBT))1/2 is the de Broglie wavelength, in which m is the mass of the boson, kB is Boltzmann’s constant, and T is the temperature. This criterion qualitatively says that when the bosons’ density is high enough and the de Broglie wavelength is sufficiently large, such that their wave functions overlap, it is possible to achieve condensation. The mass dependence of the de Broglie wavelength shows that in order to achieve a condensation at room temperature, the mass of the bosons must be low. Interestingly, HPs possess a very low effective electron and hole mass (0.18−0.23 me; 0.21−0.29 me, respectively),103 values that further decreases when moving toward fully inorganic ones (CsPbI3, 0.16 me; 0.07me), resulting in low polariton mass.104 Till now BECs with polaritons have been realized in both inorganic (CdTe, GaAs, GaN, and ZnO)105,106 as well as in organic107,108 materials deposited in high quality factor FabryPérot resonators. GaN, ZnO, and organic compounds show a large binding energy interfering with the RT exciton dissociation, thus reducing the exciton-polariton density necessary to trigger the BEC. Perovskites show similar high energy binding up to 60 meV in the bulk and above 100 meV in NCs. This is crucial as their large energy binding comparable to high-cost GaN (23 meV) along with their superior energy binding compared to GaAs (4 meV) depict HPs interesting candidates to reach condensation at room temperature for the so-called polariton lasers. Nevertheless, this has never been attempted but all these considerations indicate that a deep study of polariton mechanisms in the perovskites-cavity devices can be of extreme interest for the realization of LD based on BEC effects. Concluding Remarks. Halide perovskites will play a prominent role in light-emitting devices development, allowing a progress toward high-power applications, such environmental illumination and lasing devices. The peculiarity of the material needs however to be finely mastered for achieving reproducible properties and intelligent device conception. The huge variability of HPs properties, related to the difficult to control manufacturing processes, still represents a major limit that makes critical an exhaustively evaluation of the material whole potential. Another important limitation is represented by the inherent instability under working conditions of hybrid halide

(3)

where d is the recombination section of the active layer, which is considered approximately corresponding to the whole perovskite active material thickness. Using Nth previously calculated and an ideal internal quantum efficiency of 100%, the required threshold current density for a Fabry−Perot laser is about 320 A/cm2, which is much higher than the best current density achieved in the typical PeLED configuration so far reported, 1.8 A/cm2 (Figure 5c).99 If one takes the best reported lasing threshold derived for a nanowire single crystal with a high quality factor, reducing the gain line-width by an order of magnitude,11 the value of the current threshold importantly reduce and becomes about 32 A/cm2. In Figure 5c, the maximum external quantum efficiency vs current density of the best PeLEDs have been reported. The blue and red lines in figure are the theoretical optical losses calculated at threshold normalized to optical confinement ratios (αT/Γ). The typical optical losses shown by an organic cavity structure in the edgeemitting diode configuration, embedding host:guest organic active materials (50 cm−1), are lower than the losses reported in PeLEDs. The main reason is due to the large shift between the absorption peak wavelength of host and the gain emission peak wavelength of the diluted lasing dye guest, resulting in a poor absorption coefficient a0 (hundreds times lower). Figure 5c depicts the actual situation of PeLEDs if related to an ideal LD based on this material and allows a rationalization of the guidelines toward electrically pumped lasing. The reported devices show, at the maximum quantum efficiency, current density far from the required threshold. Reducing losses from the resonator would have only a marginal impact on this scenario, as the absorption of the perovskite material is the main factor moving the lasing threshold in the high currents regions. A not yet attempted path to reduce absorption coefficient a0 in perovskite containing cavities would foresee the host:guest approach, in which an absorbing matrix would transfer the energy to low concentrated perovskite lasing NCs, thus minimizing the self-absorption. At the moment, the obtainment of high carriers density of at least to 1017 cm−3 and beyond100 is the final goal for this class of devices. Nevertheless, several problems arise when targeting high carrier densities in HP materials: first, material resistance to heat generation is reasonably reduced if compared to inorganic semiconductors, although it is above the one of organic material.85 Further study is needed to address the issue, that is, an evaluation of fully inorganic perovskite heat resistance is still lacking. To increase free carrier densities, the most promising path is the doping of perovskite materials. Recently, trivalent cation incorporation into MAPbI3 has been achieved with Bi3+, Au3+, or In3+, inducing a charge concentration (∼1019 cm−3),101 demonstrating that controlled doping can be a valuable direction. The challenge derives from the requirement that incorporated dopants should adjust in an octahedral coordination and satisfy compositional tolerance factors. Most importantly, strongly connected to this development is the impact that dopants could have on the outstanding optical and electrical properties of the material. In fact, those are, as discussed, strongly dependent on the electronic trap population. Achieving a high concentration of free carriers, holding high radiative yields and balanced high 4330



Perspective

(4) Tan, Z.-K.; Moghaddam, R. S.; Lai, M. L.; Docampo, P.; Higler, R.; Deschler, F.; Price, M.; Sadhanala, A.; Pazos, L. M.; Credgington, D.; Hanusch, F.; Bein, T.; Snaith, H. J.; Friend, R. H. Bright LightEmitting Diodes Based on Organometal Halide Perovskite. Nat. Nanotechnol. 2014, 9, 687−692. (5) Xing, G.; Mathews, N.; Lim, S. S.; Yantara, N.; Liu, X.; Sabba, D.; Grätzel, M.; Mhaisalkar, S.; Sum, T. C. Low-Temperature SolutionProcessed Wavelength-Tunable Perovskites for Lasing. Nat. Mater. 2014, 13, 476−480. (6) Tvingstedt, K.; Malinkiewicz, O.; Baumann, A.; Deibel, C.; Snaith, H. J.; Dyakonov, V.; Bolink, H. J. Radiative Efficiency of Lead Iodide Based Perovskite Solar Cells. Sci. Rep. 2014, 4, 6071. (7) Tress, W.; Marinova, N.; Inganäs, O.; Nazeeruddin, M. K.; Zakeeruddin, S. M.; Graetzel, M. Predicting the Open-Circuit Voltage of CH 3 NH 3 PbI 3 Perovskite Solar Cells Using Electroluminescence and Photovoltaic Quantum Efficiency Spectra: The Role of Radiative and Non-Radiative Recombination. Adv. Energy Mater. 2015, 5, 1400812. (8) Jaramillo-Quintero, O. A.; Sanchez, R. S.; Rincon, M.; Mora-Sero, I. Bright Visible-Infrared Light Emitting Diodes Based on Hybrid Halide Perovskite with Spiro-OMeTAD as a Hole-Injecting Layer. J. Phys. Chem. Lett. 2015, 6, 1883−1890. (9) Gil-Escrig, L. N.; Longo, G.; Pertegás, A.; Roldá N-Carmona, C.; Soriano, A.; Sessolo, M.; Bolink, H. J. Efficient Photovoltaic and Electroluminescent Perovskite Devices. Chem. Commun. 2015, 51, 569−571. (10) Era, M.; Morimoto, S.; Tsutsui, T.; Saito, S. Organic-Inorganic Heterostructure Electroluminescent Device Using A Layered Perovskite Semiconductor (c6h5c2h4nh3)2pbl4. Appl. Phys. Lett. 1994, 65, 676−678. (11) Zhu, H.; Fu, Y.; Meng, F.; Wu, X.; Gong, Z.; Ding, Q.; Gustafsson, M. V.; Trinh, M. T.; Jin, S.; Zhu, X.-Y. Lead Halide Perovskite Nanowire Lasers with Low Lasing Thresholds and High Quality Factors. Nat. Mater. 2015, 14, 636−642. (12) Stranks, S. D.; Snaith, H. J. Metal-Halide Perovskites for Photovoltaic and Light-Emitting Devices. Nat. Nanotechnol. 2015, 10, 391−402. (13) Zhang, F.; Zhong, H.; Chen, C.; Wu, X. G.; Hu, X.; Huang, H.; Han, J.; Zou, B.; Dong, Y. Brightly Luminescent and Color-Tunable Colloidal CH3NH3PbX3 (X = Br, I, Cl) Quantum Dots: Potential Alternatives for Display Technology. ACS Nano 2015, 9, 4533−4542. (14) Hoke, E. T.; Slotcavage, D. J.; Dohner, E. R.; Bowring, A. R.; Karunadasa, H. I.; McGehee, M. D. Reversible Photo-Induced Trap Formation in Mixed-Halide Hybrid Perovskites for Photovoltaics. Chem. Sci. 2015, 6, 613−617. (15) Yuan, M.; Quan, L. N.; Comin, R.; Walters, G.; Sabatini, R.; Voznyy, O.; Hoogland, S.; Zhao, Y.; Beauregard, E. M.; Kanjanaboos, P.; Lu, Z.; Kim, D. H.; Sargent, E. H. Perovskite Energy Funnels for Efficient Light-Emitting Diodes. Nat. Nanotechnol. 2016, 11, 872. (16) Cho, H.; Jeong, S.-H.; Park, M.-H.; Kim, Y.-H.; Wolf, C.; Lee, C.-L.; Heo, J. H.; Sadhanala, A.; Myoung, N.; Yoo, S.; Im, S. H.; Friend, R. H.; Lee, T.-W. Overcoming the Electroluminescence Efficiency Limitations of Perovskite Light-Emitting Diodes. Science 2015, 350, 1222−1225. (17) Wang, J.; Wang, N.; Jin, Y.; Si, J.; Tan, Z.-K.; Du, H.; Cheng, L.; Dai, X.; Bai, S.; He, H.; Ye, Z.; Lai, M. L.; Friend, R. H.; Huang, W. Interfacial Control Toward Efficient and Low-Voltage Perovskite Light-Emitting Diodes. Adv. Mater. 2015, 27, 2311−2316. (18) Yu, J. C.; Kim, D. B.; Jung, E. D.; Lee, B. R.; Song, M. H. HighPerformance Perovskite Light-Emitting Diodes via Morphological Control of Perovskite Films. Nanoscale 2016, 8, 7036−7042. (19) Kumawat, N. K.; Dey, A.; Kumar, A.; Gopinathan, S. P.; Narasimhan, K. L.; Kabra, D. Band Gap Tuning of CH3NH3Pb(Br1− XClX)3 Hybrid Perovskite for Blue Electroluminescence. ACS Appl. Mater. Interfaces 2015, 7, 13119−13124. (20) Sadhanala, A.; Ahmad, S.; Zhao, B.; Giesbrecht, N.; Pearce, P. M.; Deschler, F.; Hoye, R. L. Z.; Gödel, K. C.; Bein, T.; Docampo, P.; Dutton, S. E.; De Volder, M. F. L.; Friend, R. H. Blue-Green Color Tunable Solution Processable Organolead Chloride−Bromide Mixed

perovskites, especially due to the ionic nature of organic species. The degradation processes in MAPbI3 upon exposure to thermal, moisture, and mechanical conditions require a thoughtful strategy aiming at improved material stability. Several approaches have been explored in the PV field, although their effectiveness in light emitting devices has not been tested. So far, the use of alternative cations such as FA or, in particular, the inorganic Cs constituted a path to combine excellent PLQY with improved thermal and chemical resistance. Despite these limitations, PeLEDs development is already following an exponential trend such that actual LEDs limitations could be rapidly overwhelmed. Superior material optical and electrical properties are at the base of this rise, which echoes the impressive escalation of perovskite-based solar cells. In PeLEDs, benefits derive from the use of low dimensionality perovskites, marginally exploited in PV devices, comprising layered, or nanostructured materials. The use of these materials allows engineering of the exciton properties and the creation of intrinsic resonating structures; in addition, it would also represent an advancement in terms of stability, being intrinsically more stable than the 3D counterparts. Along with exciton confinement, electrical doping of perovskite material could allow the achievement of high carrier density required for lasing. Aspiring to reach such application, particular attention needs to be dedicated to the impact that dopants could have on the outstanding optical and electrical properties of the material. In fact, those are strongly dependent on the electronic trap population; therefore, the achievement of a high concentration of free carriers, holding high radiative yields and balanced high carriers mobility, is one of the most challenging objectives of the coming field research.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions

(S.C. and M.M.) These authors contributed equally. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the project Beyond-Nano (Project Number: PON03_00362). We acknowledge Fabrizio Mariano and Armando Genco for fruitful discussions. We acknowledge Progetto di ricerca PON MAAT (Project Number: PON02_00563_3316357). A.R. gratefully acknowledges SIR (project number RBSI14FYVD, CUP: B82I15000950008). SC. and A.L. acknowledge Regione Puglia and ARTI for founding FIRFuture in Research projects “PeroFlex” project no. LSBC6N4 and” HyLight” project no. GOWMB21.

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