1,721,086 research outputs found
The role of a dark exciton reservoir in the luminescence efficiency of two-dimensional tin iodide perovskites
No full textWe report on the excitonic luminescence of polycrystalline films composed of two-dimensional (2D) tin iodide perovskites. By combining steady-state and time-resolved spectroscopies between room temperature and 5 K, we identify an optically inactive, dark excitonic state within the spectral fine structure at the exciton energy, ubiquitous in the optical spectra of 2D perovskites. Lying at about 10 meV below the bright exciton that is responsible for luminescence, the dark state mediates non-radiative decay of the photo-excited population at temperatures below 100 K. However, at about 100–120 K, we observe a thermally activated transfer of population from the dark state back to the bright state which results in an increase in luminescence efficiency. Based on quantitative analysis of the observed exciton dynamics, we argue that the dark state acts as a reservoir of photo-excitations at ambient temperature. By feeding the bright state at a rate that is slower than the radiative rate, the dark state mitigates the loss of photo-excited population to other non-radiative pathways. Our work provides insights into the dynamics of the inevitable dark states in 2D perovskites and their relevance in enhancing the emissivity of the technologically relevant lead-free material architectures
Vibrational Response of Methylammonium Lead Iodide: From Cation Dynamics to Phonon–Phonon Interactions
No full textThe dynamic evolution of the vibrational interactions in the prototypical CH3NH3PbI3 was studied through a comprehensive experimental and theoretical investigation with a focus on the interactions between the organic cations and the inorganic cage. To date, no clear picture has emerged on the critical and fundamental interactions between the two perovskite components, despite the relevance of phonons to the electronic properties of several classes of perovskites. For the first time, we have monitored the IR and nonresonant Raman response in the broad frequency range 30–3400 cm−1 and in the temperature interval 80–360 K. Strong changes in the energies of different vibrational modes with temperature are observed and examined in the framework of phonon–phonon interactions considering a significant anharmonic contribution to the phonon relaxation process. The vibrational relaxation of the bending modes and their reorientation activation energies identify that such mechanisms are governed by medium-to-strong hydrogen bonds in the orthorhombic phase; however, any ferroelectric ordering in the orthorhombic phase is governed mostly by dipole interactions. These changes imply that charge localization mechanisms play a primary role, and our study enriches the fundamental knowledge of phonon interactions and charge transport in CH3NH3PbI3 for the further development of optoelectronic applications
How Shallow and Deep Defects Drive Carrier Dynamics in Tin‐Iodide Perovskites
No full textTin-halide perovskites (THP) exhibit complex carrier dynamics due to the interplay between electronic doping and carrier trapping, both of which affect device performance. Evaluating the impact of trap states is challenging because the timescales of photogenerated electron recombination with dopant holes and trapping often overlap. Here, Transient Absorption Spectroscopy (TAS) is used across a broad spectral and temporal range, spanning from visible to near-infrared and from femtoseconds to microseconds, to probe both sub-bandgap and band-edge transitions, while manipulating defect and doping densities via chemical treatments. Focusing on tin triiodide perovskites, the rapid carrier recombination due to high electronic doping density is considered the main source of carrier loss. However, deep electron trap states originated by two distinct type of defects are identified: surface Sn(IV) defects and tin interstitials. Surface Sn(IV) defects play a key role in the loss of photo-generated carriers, but their density can be mitigated by the addition of SnF2, improving carrier lifetimes. Nevertheless, excessive SnF2 promotes the stabilization of tin interstitial traps, highlighting a delicate balance in defect control. Moreover, near-infrared TAS reveals sub-bandgap transitions associated with shallow traps, which contribute to band-edge repopulation within tens of picoseconds. This work disentangles the contributions of doping and trap-mediated processes to the optoelectronic mechanisms in THP, offering insights into defect management for performance optimization
Charge generation at polymer/metal oxide interface: From molecular scale dynamics to mesoscopic effects
No full textCoordinating Solvent-Assisted Synthesis of Phase-Stable Perovskite Nanocrystals with High Yield Production for Optoelectronic Applications
Full text linkInorganic perovskite nanocrystals (NCs) have shown good potential as an emerging semiconducting building block owing to their excellent optoelectronic properties. However, despite extensive studies on their structure-dependent optical properties, they still suffer severely from chemical and phase instabilities in ambient conditions. Here, we report a facile method for the synthesis of mixed halide inorganic perovskite NCs based on recrystallization in an antisolvent mixture in an ambient atmosphere, at room temperature. We introduced an alcohol-derivative solvent, as a secondary antisolvent in the solvent mixture, which crystallizes at room temperature. This mediates and facilitates the perovskite crystallization, leading to a high chemical yield and stability. We demonstrate that this secondary antisolvent establishes intermolecular interactions with lead halide salt, which successfully stabilizes the γ-dark phase of perovskite by encapsulating NCs in a solution and thin film. This allows us to produce concentrated NC solutions with a photoluminescence quantum yield of 70%. Finally, we fabricate CsPbI2Br NCs (optical bandgap 1.88 eV) solar cells, which showed a stabilized photovoltaic performance in ambient conditions, without encapsulation, showing a Voc of 1.32 V
Dynamic SEM imaging of surface photovoltages in MAPbI3 perovskites
No full textSecondary electrons (SE) imaging in Scanning Electron Microscope (SEM) is a very versatile and powerful technique to unveil the morphology and surface topology of materials and systems, over a wide range of view fields and down to the nanoscale. Actually, the SE emission yield and image contrast strongly depend on local values of the work function at material surface. In turn, work function not only depends on material composition, but also on local electric field distribution at surface. In this way, SEM imaging is a means for 2D mapping of surface voltage and charge distributions [1,2], in case morphology-related SE contrast is either negligible or it may be decoupled from charge effects.
Optical absorption is known to excite photovoltage distributions at semiconductor and insulator surfaces. In principle, therefore, SEM can also be a contactless means to probe photovoltages in shallow layers, by coupling an optical pump beam inside the analysis chamber. Optically excited charge distributions act to modify electrical potentials at surface proximity, which in turn affect the collection of SE contrast signal.
Dynamically evolving phenomena can be probed by standard sampling of the photoexcited SE contrast over time. As for optically induced phenomena evolving on faster time scales (up to the kHz range), these can be accessed by using modulated optical and electron beams, properly synchronized.
We report on the assembling of laser-assisted dynamic SEM apparatus and on the imaging of evolving surface photovoltages in semiconducting methylammonium lead tri-iodede (MaPbI3) perovskites,an emerging material in hybrid-organic photovoltaics.
Our experiments show the generation of a local field due to laser pumping in the visible region, revealed by an evolution in the SE contrast map.
These results are consistent with the photo activation and dynamics of trap states inside the material band gap3. A spectral dependence of the contrast signal on the choice of optical pumping wavelength is noticed. The SE contrast distribution evolves on the second time scale. However, under the high vacuum conditions – as those in the SEM chamber- we observe a permanence of the local field up to several hours.
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[2] L. Xu et al., “Secondary Electron Microscopy Dopant Contrast Image (SEMDCI) for Laser Doping”, IEEE J.PHOTOVOLTAICS, 3, 762, (2013)
[3] Motti, S. G. , et al. Photoinduced Emissive Trap States in Lead Halide Perovskite Semiconductors, ACS Energy Lett. 2016, 1, 726−730, DOI: 10.1021/acsenergylett.6b0035
Real-time dynamical imaging of light induced photo-voltages in hybrid halide perovskites by Scanning Electron Microscopy
No full textSecondary electron (SE) Surface-sensitive imaging detection in Scanning Electron Microscopy (SEM) probes space-charge field distributions and potentials at surfaces and interfaces [1,2]. Recently, its combination with optical excitation has opened wider scenarios [3,4,5,6]. In this work we present the combination of a customized SEM with laser light excitation of the sample under test in a Light-Assisted SEM configuration (LASEM). LASEM relies on the optically induced local modification of SE yield to provide real-time mapping of photo-voltages and charge dynamics, and qualifies as a complementary approach to near-field probe microscopies [7] and nonlinear photoemission spectroscopies [8]. We applied LASEM to thin films of Metal Ammonium Lead Triiodide perovskite (MAPbI3), an outstanding light-sensitive material in solar light harvesting and photovoltaics [9], and also appealing as an active material for light generation [10]. MAPbI3 is excited by illumination at above-bandgap photon energy, while keeping electron beam dose below damaging threshold. Fig. 1 reports temporal evolution of LASEM contrast under illumination and after light removal. A contrast reversal is evident, as the illumination is turned off. The system evolves in dark over several hours and a near complete recovery occurs within days. LASEM contrast pattern depends on the geometry of SE collection, as proven by varying angular orientation of the sample with respect to the in-column detector, (Fig. 2), consistently with the hypothesis of an optically excited charge field at surface. Ray-tracing numerical simulation of SE flight and detection supports the hypothesis. The contrast dynamics, is consistent with the evolution of optically induced structural modifications in MAPbI3, as also experimentally observed, by using complementary techniques [11,12]. As above demonstrated, LASEM is potentially a versatile technique which can be extensively applied to photo-sensitive materials. A real-time temporal resolution in the millisecond range can be envisaged.
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[4] Najafi, E.,et al. Science, 347 (6218) ,164 (2015).
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[9] Stranks, S. D., et al. Science 342(2013), 341 (2014).
[10] D’Innocenzo V. et al., J. Am. Chem. Soc., 136 (51), 17730 (2014)
[11] DeQuilettes D. W., et al., Nature Communications 7, 11683 (2016)
[12] Gottesman, R., Journal of Phys.Chem. Letters 6, 2332 (2015)
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