| 1. |
- Gerhard, Marina, et al.
(författare)
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Microscopic insight into non-radiative decay in perovskite semiconductors from temperature-dependent luminescence blinking
- 2019
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Ingår i: Nature Communications. - : Springer Science and Business Media LLC. - 2041-1723. ; 10:1
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Tidskriftsartikel (refereegranskat)abstract
- Organo-metal halide perovskites are promising solution-processed semiconductors, however, they possess diverse and largely not understood non-radiative mechanisms. Here, we resolve contributions of individual non-radiative recombination centers (quenchers) in nanocrystals of methylammonium lead iodide by studying their photoluminescence blinking caused by random switching of quenchers between active and passive states. We propose a model to describe the observed reduction of blinking upon cooling and determine energetic barriers of 0.2 to 0.8 eV for enabling the switching process, which points to ion migration as the underlying mechanism. Moreover, due to the strong influence of individual quenchers, the crystals show very individually-shaped photoluminescence enhancement upon cooling, suggesting that the high variety of activation energies of the PL enhancement reported in literature is not related to intrinsic properties but rather to the defect chemistry. Stabilizing the fluctuating quenchers in their passive states thus appears to be a promising strategy for improving the material quality.
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| 2. |
- Goetz, Katelyn P., et al.
(författare)
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Remarkable performance recovery in highly defective perovskite solar cells by photo-oxidation
- 2023
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Ingår i: Journal of Materials Chemistry C. - : Royal Society of Chemistry (RSC). - 2050-7526 .- 2050-7534. ; 11:24, s. 8007-8017
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Tidskriftsartikel (refereegranskat)abstract
- Exposure to environmental factors is generally expected to cause degradation in perovskite films and solar cells. Herein, we show that films with certain defect profiles can display the opposite effect, healing upon exposure to oxygen under illumination. We tune the iodine content of methylammonium lead triiodide perovskite from understoichiometric to overstoichiometric and expose them to oxygen and light prior to the addition of the top layers of the device, thereby examining the defect dependence of their photooxidative response in the absence of storage-related chemical processes. The contrast between the photovoltaic properties of the cells with different defects is stark. Understoichiometric samples indeed degrade, demonstrating performance at 33% of their untreated counterparts, while stoichiometric samples maintain their performance levels. Surprisingly, overstoichiometric samples, which show low current density and strong reverse hysteresis when untreated, heal to maximum performance levels (the same as untreated, stoichiometric samples) upon the photooxidative treatment. A similar, albeit smaller-scale, effect is observed for triple cation and methylammonium-free compositions, demonstrating the general application of this treatment to state-of-the-art compositions. We examine the reasons behind this response by a suite of characterization techniques, finding that the performance changes coincide with microstructural decay at the crystal surface, reorientation of the bulk crystal structure for the understoichiometric cells, and a decrease in the iodine-to-lead ratio of all films. These results indicate that defect engineering is a powerful tool to manipulate the stability of perovskite solar cells.
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| 3. |
- Gerhard, Marina, et al.
(författare)
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Heterogeneities and Emissive Defects in MAPbI3 Perovskite Revealed by Spectrally Resolved Luminescence Blinking
- 2021
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Ingår i: Advanced Optical Materials. - : Wiley. - 2195-1071. ; 9:18
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Tidskriftsartikel (refereegranskat)abstract
- The fate of excited charge carriers in metal halide perovskite semiconductors is influenced by energetic disorder and defects. Here, photoluminescence (PL) blinking is used to probe metastable nonradiative (NR) centers and the nanoscale energy landscape. Temporal activation of an NR center creates a local region with increased NR recombination. Activation and deactivation of this local PL quenching does not only lead to PL blinking, but also to fluctuations of the PL spectra, if the crystal is inhomogeneous in the sense that the PL emission spectrum is slightly different from one location to another. It resembles the spectral hole-burning technique; however, here the eliminated excited states are chosen by their spatial localization close to the quencher. In MAPbI3, PL spectral fluctuations at low temperature reveal energetic inhomogeneities on the order of 5 to 10 meV. Quenching of the main PL band is often found to correlate with an increase of the low-energetic tail of the PL spectra, which is attributed to partially radiative recombination of charges captured by the NR center. The transition energy of the NR center is found to be only ≈80 meV smaller than the bandgap, implying that the underlying defect cannot be a single mid-bandgap state.
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| 4. |
- Kiligaridis, Alexander, et al.
(författare)
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Are Shockley-Read-Hall and ABC models valid for lead halide perovskites?
- 2021
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Ingår i: Nature Communications. - : Springer Science and Business Media LLC. - 2041-1723. ; 12
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Tidskriftsartikel (refereegranskat)abstract
- Metal halide perovskites are an important class of emerging semiconductors. Their charge carrier dynamics is poorly understood due to limited knowledge of defect physics and charge carrier recombination mechanisms. Nevertheless, classical ABC and Shockley-Read-Hall (SRH) models are ubiquitously applied to perovskites without considering their validity. Herein, an advanced technique mapping photoluminescence quantum yield (PLQY) as a function of both the excitation pulse energy and repetition frequency is developed and employed to examine the validity of these models. While ABC and SRH fail to explain the charge dynamics in a broad range of conditions, the addition of Auger recombination and trapping to the SRH model enables a quantitative fitting of PLQY maps and low-power PL decay kinetics, and extracting trap concentrations and efficacies. However, PL kinetics at high power are too fast and cannot be explained. The proposed PLQY mapping technique is ideal for a comprehensive testing of theories and applicable to any semiconductor.
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| 5. |
- Kiligaridis, Alexander
(författare)
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Automated Photoluminescence Experimentation for Understanding Dynamic Metal-Halide Perovskite Semiconductors
- 2023
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Metal halide perovskites have garnered significant attention for their remarkable optoelectronic properties and unique photophysical characteristics. Combined with low cost for fabrication, this new class of materials is among the most promising candidates within next generation photovoltaic technologies. However, the complex nature of metal halide perovskites' crystal structures and compositions requires precise control during synthesis to achieve desired properties. Small variations in composition can significantly impact the material's behavior, demanding meticulous experimentation and characterization. Another challenge associated with the research of perovskite metal halides is theobserver effect, a concept most often associated with quantum mechanics and refers to the alteration of a state by the act of observing it. In the context of perovskite materials, the very act of measurement or characterization can influence the material’s behavior, potentially leading to discrepancies between observed and actual properties. Photoluminescence spectroscopy is a powerful analytical technique that plays a pivotal role in unraveling the optical properties and electronic behavior of perovskite materials. The emission of light upon excitation provides insights into the material’s band structure, defect states and recombination dynamics. This thesis presents a comprehensive exploration of the convergence between photoluminescence spectroscopy and automation testing techniques, tailored to the specific requirements of metal halide perovskite materials. The integration of these two domains offers a novel approach to accelerate the characterization of the optoelectronic properties of metal halide perovskites. The integration of automation not only enhances the efficiency of experimentation but also enables the exploration of a wider parameter space. One of the examples that we will present in this work is a novel experimental methodology, incorporating photoluminescence measurements within a two-dimensional parameter space of excitation energy and laser pulse repetition frequency. We demonstrate the effectiveness of this technique in comprehensively investigating the dynamic photochemical properties of our material due to sample aging and degradation, photo-induced reversible and irreversible processes during the act of measuring.
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| 6. |
- Kiligaridis, Alexander, et al.
(författare)
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Excitation wavelength dependence of photoluminescence flickering in degraded MAPbI3 perovskite and its connection to lead iodide formation
- 2020
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Ingår i: Journal of Luminescence. - : Elsevier BV. - 0022-2313. ; 222
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Tidskriftsartikel (refereegranskat)abstract
- Metal halide perovskite semiconductors often exhibit photoluminescence blinking and flickering when luminescence of individual small nano- or even microcrystals is monitored. The nature of these fluctuations is not well understood but must be related to the presence of metastable non-radiative recombination channels and efficient charge migration in these materials. Here we report on the excitation wavelength dependence of photoluminescence flickering effect in degraded methylammonium lead iodide (MAPbI3) thin films. While the luminescence intensity is temporary stable when excited in the blue region with wavelength shorter than 530 nm, excitation with red light (wavelength longer than 530 nm) results in luminescence flickering. It is hypothesised that the wavelength dependence reflects the excitation energy dependence of the photochemical mechanism that switches non-radiative recombination channels on and off. The effect can also be related to hindered charge carrier diffusion due to their localization in the interfacial layer between MAPbI3 and PbI2 which is formed in the course of degradation.
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| 7. |
- Liu, Yanfeng, et al.
(författare)
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In Situ Optical Studies on Morphology Formation in Organic Photovoltaic Blends
- 2021
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Ingår i: Small Methods. - : John Wiley & Sons. - 2366-9608. ; 5:10, s. 1-12
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Tidskriftsartikel (refereegranskat)abstract
- The efficiency of bulk heterojunction (BHJ) based organic solar cells is highly dependent on the morphology of the blend film, which is a result of a fine interplay between donor, acceptor, and solvent during the film drying. In this work, a versatile set-up of in situ spectroscopies is used to follow the morphology evolution during blade coating of three iconic BHJ systems, including polymer:fullerene, polymer:nonfullerene small molecule, and polymer:polymer. the drying and photoluminescence quenching dynamics are systematically study during the film formation of both pristine and BHJ films, which indicate that the component with higher molecular weight dominates the blend film formation and the final morphology. Furthermore, Time-resolved photoluminescence, which is employed for the first time as an in situ method for such drying studies, allows to quantitatively determine the extent of dynamic and static quenching, as well as the relative change of quantum yield during film formation. This work contributes to a fundamental understanding of microstructure formation during the processing of different blend films. The presented setup is considered to be an important tool for the future development of blend inks for solution-cast organic or hybrid electronics.
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| 8. |
- Marunchenko, Alexandr, et al.
(författare)
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Memlumor : A Luminescent Memory Device for Energy-Efficient Photonic Neuromorphic Computing
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Ingår i: ACS Energy Letters. - 2380-8195. ; , s. 2075-2082
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Tidskriftsartikel (refereegranskat)abstract
- Neuromorphic computing promises to transform the current paradigm of traditional computing toward non-von Neumann dynamic energy-efficient problem solving. To realize this, a neuromorphic platform must possess intrinsic complexity reflected in the built-in diversity of its physical operation mechanisms. We propose and demonstrate the concept of a memlumor, an all-photonic device combining memory and a luminophore, and being mathematically a full equivalence of the electrically driven memristor. Using CsPbBr3 perovskites as a material platform, we demonstrate the synergetic coexistence of memory effects within a broad time scale from nanoseconds to minutes and switching energy down to 3.5 fJ. We elucidate the origin of such a complex response to be related to the phenomena of photodoping and photochemistry activated by a tunable light input. When the existence of a history-dependent photoluminescence quantum yield is leveraged in various material platforms, the memlumor device concept will trigger multiple new research directions in both material science and photonics.
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| 9. |
- Merdasa, Aboma, et al.
(författare)
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Impact of Excess Lead Iodide on the Recombination Kinetics in Metal Halide Perovskites
- 2019
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Ingår i: ACS Energy Letters. - : American Chemical Society (ACS). - 2380-8195. ; , s. 1370-1378
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Tidskriftsartikel (refereegranskat)abstract
- Fundmental comprehension of light-induced processes in perovskites are still scarce. One active debate surrounds the influence of excess lead iodide (PbI2) on device performance, as well as optoelectronic properties, where both beneficial and detrimental traits have been reported. Here, we study its impact on charge carrier recombination kinetics by simultaneously acquiring the photoluminescence quantum yield and time-resolved photoluminescence as a function of excitation wavelength (450-780 nm). The presence of PbI2 in the perovskite film is identified via a unique spectroscopic signature in the PLQY spectrum. Probing the recombination in the presence and absence of this signature, we detect a radiative bimolecular recombination mechanism induced by PbI2. Spatially resolving the photoluminescence, we determine that this radiative process occurs in a small volume at the PbI2/perovskite interface, which is only active when charge carriers are generated in PbI2, and therefore provide deeper insight into how excess PbI2 may improve the properties of perovskite-based devices.
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| 10. |
- Rao, Shraddha M., et al.
(författare)
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Photoluminescence Mapping over Laser Pulse Fluence and Repetition Rate as a Fingerprint of Charge and Defect Dynamics in Perovskites
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Ingår i: Advanced Optical Materials. - 2195-1071.
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Tidskriftsartikel (refereegranskat)abstract
- Defects in metal halide perovskites (MHP) are photosensitive, making the observer effect unavoidable when laser spectroscopy methods are applied. Photoluminescence (PL) bleaching and enhancement under light soaking and recovery in dark are examples of the transient phenomena that are consequent to the creation and healing of defects. Depending on the initial sample composition, environment, and other factors, the defect nature and evolution can strongly vary, making spectroscopic data analysis prone to misinterpretations. Herein, the use of an automatically acquired dependence of PL quantum yield (PLQY) on the laser pulse repetition rate and pulse fluence as a unique fingerprint of both charge carrier dynamics and defect evolution is demonstrated. A simple visual comparison of such fingerprints allows for assessment of similarities and differences between MHP samples. The study illustrates this by examining methylammonium lead triiodide (MAPbI3) films with altered stoichiometry that just after preparation showed very pronounced defect dynamics at time scale from milliseconds to seconds, clearly distorting the PLQY fingerprint. Upon weeks of storage, the sample fingerprints evolve toward the standard stoichiometric MAPbI3 in terms of both charge carrier dynamics and defect stability. Automatic PLQY mapping can be used as a universal method for assessment of perovskite sample quality.
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