Photoluminescence of 2D and quasi-2D perovskites

In den letzten Jahren wurden Perowskite, insbesondere 2D- und Quasi-2D-Perowskite, in Bereichen wie Photovoltaik, Leuchtdioden und Lasertechnik intensiv erforscht. Diese Arbeit soll einen Einblick in die grundlegende Photophysik der angeregten Zustände dieser Materialien nach einer Anregung durch Laserlicht geben und als Anleitung zur Interpretation der emittierten Photolumineszenzspektren dienen. Zunächst werden reine 2D-Perowskite in der Ruddlesden-Popper-Phase unter Verwendung von Phenethylamin (PEA) als Spacermolekül ($PEA_{2}PbI_{4}$) und der Dion-Jacobson-Phase unter Verwendung von 1,4-Phenylendimethanamin bei verschiedenen Temperaturen und mit einer gepulsten und einer Dauerstrich-Laserquelle bei unterschiedlichen Anregungsintensitäten untersucht. Während bei beiden Materialien die exzitonische Emission bei allen gemessenen Temperaturen dominiert, werden für Temperaturen unterhalb von 140 K zusätzliche Peaks in den Emissionsspektren beobachtet. Im Ruddlesden-Popper-Material werden bei 5 K zwei Peaks mit einem Energieabstand von 40,3 meV beobachtet, wobei der Peak mit der höheren Energie den freien Exzitonen und der mit der niedrigeren Energie den gebundenen Exzitonen (gebunden durch ein Fehlstelle) zugeschrieben wird. Bei hohen Anregungsintensitäten dominiert die Emission der gebundenen Exzitonen die Gesamtemission. Dies ist darauf zurückzuführen, dass die Exziton-Exziton-Annihilierung die Emission der freien Exzitonen stark unterdrückt, während die Emission der gebundenen Exzitonen kaum beeinträchtigt wird. Dieser Effekt ist im Dion-Jacobson-Perowskit weniger ausgeprägt. In beiden Materialien kann die Emission von $PbI_{2}$-Einschlüssen bei niedrigen Temperaturen beobachtet werden. Anschließend werden die Emissionseigenschaften verschiedener Quasi-2D-Materialien untersucht. Im Gegensatz zu 2D-Perowskiten, deren angeregte Zustände nur aus Exzitonen bestehen, zeigen Quasi-2D-Perowskite Emission von Exzitonen, freien Ladungsträgern oder sogar von beiden. Durch Analyse der zeitaufgelösten Photolumineszenz, der Quanteneffizienz und der anfänglichen Photonenemissionsdichte kann die Mischung der angeregten Zustände bestimmt werden. Die Emissionseffizienz von freien Ladungsträgern nimmt mit der Anregunsintensität zu, während die von Exzitonen abnimmt. Es wird ein einfaches Modell von zwei nicht wechselwirkenden Populationen von Exzitonen und Ladungsträgern in getrennten Teilvolumina des Films vorgestellt, das alle Beobachtungen in dieser Arbeit beschreibt. Die Emissionscharakteristiken hängen stark von dem in der Perowskitlösung verwendeten Spacer-Molekül und dessen Konzentration ab. Hohe Konzentrationen von Butylamin führen zu 100 % Emission von Exzitonen, die auf 7 % Exzitonenemission und 93 % freie Ladungsträgeremission bei Perowskiten mit niedrigen Konzentrationen von 1-Naphthylmethylamin Spacermolekülen zurückgeht. Schließlich wird auf der Grundlage dieser Beobachtungen eine Messtechnik eingeführt, bei der zwei Anregungspulse mit steuerbarer Verzögerung und ein USB-Spektrometer, das die zeitintegrierten Spektren aufzeichnet, verwendet werden, um die Geschwindigkeitskonstanten sowie den Anteil der Emission von Exzitonen und freien Ladungsträgern zu ermitteln. Diese Methode ist wesentlich schneller und kostengünstiger als herkömmliche Zeitaufgelöste-Messmethoden der Photolumineszenz. Die Robustheit und Schnelligkeit dieser Methode ist potenziell sehr interessant für den Einsatz in einer Fertigungslinie für photovoltaische 3D-Perowskit-Absorberschichten und ebnet so den Weg von der Forschung zur Anwendung

Interpreting the Time-Resolved Photoluminescence of Quasi-2D Perovskites

Optical excitation of quasi-2D perovskites leads to excited-state populations of excitons, free charge carriers, or a mixture of both, depending on the type and amount of 2D spacer used. The fluence dependence of three quantities: 1) the time-resolved photoluminescence decay, 2) the photoluminescence quantum yield (PLQY) after pulsed excitation, and 3) the initial rate of photon emission, allow the mixture of excited states present to be determined. These can be described by a simple model considering noninteracting populations of excitons and charge carriers in separate subvolumes of the film. The model reproduces all unique features of the data, such as the anomalous peak of the PLQY at intermediate fluences, due to bimolecular free carrier emission gaining efficiency before exciton–exciton annihilation reduces the exciton emission efficiency. The excited state population varies from 100% excitons in films made from high concentrations of butylamine spacers to ≈7% excitons and 93% free carriers for low concentrations of 1-naphthylmethylamine spacers. The effective rates of free carrier recombination and exciton–exciton annihilation are high, often on the order of 1 × 10−9 cm3 s−1. The implications for the different excited-state populations and their dynamics in terms of device engineering are discussed

BODIPY–pyrene donor–acceptor sensitizers for triplet–triplet annihilation upconversion: the impact of the BODIPY-core on upconversion efficiency

Triplet–triplet annihilation upconversion (TTA-UC) is an important type of optical process with applications in biophotonics, solar energy harvesting and photochemistry. In most of the TTA-UC systems, the formation of triplet excited states takes place via spin–orbital interactions promoted by heavy atoms. Given the crucial role of heavy atoms (especially noble metals, such as Pd and Pt) in promoting intersystem crossing (ISC) and, therefore, in production of UC luminescence, the feasibility of using more readily available and inexpensive sensitizers without heavy atoms remains a challenge. Here, we investigated sensitization of TTA-UC using BODIPY–pyrene heavy-atom-free donor–acceptor dyads with different numbers of alkyl groups in the BODIPY scaffold. The molecules with four and six alkyl groups are unable to sensitize TTA-UC in the investigated solvents (tetrahydrofuran (THF) and dichloromethane (DCM)) due to negligible ISC. In contrast, the dyad with two methyl groups in the BODIPY scaffold and the dyad with unsubstituted BODIPY demonstrate efficient intersystem crossing (ISC) of 49–58%, resulting in TTA-UC with quantum yields of 4.7% and 6.9%, respectively. The analysis of the elementary steps of the TTA-UC process indicates that heavy-atom-free donor–acceptor dyads are less effective than their noble metal counterparts, but may equal them in the future if the right combination of solvent, donor–acceptor sensitizer structure, and new luminescent molecules as TTA-UC emitters can be found

BODIPY–pyrene donor–acceptor sensitizers for triplet–triplet annihilation upconversion: the impact of the BODIPY-core on upconversion efficiency

Triplet–triplet annihilation upconversion (TTA-UC) is an important type of optical process with applications in biophotonics, solar energy harvesting and photochemistry. In most of the TTA-UC systems, the formation of triplet excited states takes place via spin–orbital interactions promoted by heavy atoms. Given the crucial role of heavy atoms (especially noble metals, such as Pd and Pt) in promoting intersystem crossing (ISC) and, therefore, in production of UC luminescence, the feasibility of using more readily available and inexpensive sensitizers without heavy atoms remains a challenge. Here, we investigated sensitization of TTA-UC using BODIPY–pyrene heavy-atom-free donor–acceptor dyads with different numbers of alkyl groups in the BODIPY scaffold. The molecules with four and six alkyl groups are unable to sensitize TTA-UC in the investigated solvents (tetrahydrofuran (THF) and dichloromethane (DCM)) due to negligible ISC. In contrast, the dyad with two methyl groups in the BODIPY scaffold and the dyad with unsubstituted BODIPY demonstrate efficient intersystem crossing (ISC) of 49–58%, resulting in TTA-UC with quantum yields of 4.7% and 6.9%, respectively. The analysis of the elementary steps of the TTA-UC process indicates that heavy-atom-free donor–acceptor dyads are less effective than their noble metal counterparts, but may equal them in the future if the right combination of solvent, donor–acceptor sensitizer structure, and new luminescent molecules as TTA-UC emitters can be found

Scalable two-terminal all-perovskite tandem solar modules with a 19.1% efficiency

Monolithic all-perovskite tandem photovoltaics promise to combine low-cost and high-efficiency solar energy harvesting with the advantages of all-thin-film technologies. To date, laboratory-scale all-perovskite tandem solar cells have only been fabricated using non-scalable fabrication techniques. In response, this work reports on laser-scribed all-perovskite tandem modules processed exclusively with scalable fabrication methods (blade coating and vacuum deposition), demonstrating power conversion efficiencies up to 19.1% (aperture area, 12.25 cm2; geometric fill factor, 94.7%) and stable power output. Compared to the performance of our spin-coated reference tandem solar cells (efficiency, 23.5%; area, 0.1 cm2), our prototypes demonstrate substantial advances in the technological readiness of all-perovskite tandem photovoltaics. By means of electroluminescence imaging and laser-beam-induced current mapping, we demonstrate the homogeneous current collection in both subcells over the entire module area, which explains low losses (<5%rel) in open-circuit voltage and fill factor for our scalable modules

Exciton versus free carrier emission: Implications for photoluminescence efficiency and amplified spontaneous emission thresholds in quasi-2D and 3D perovskites

Among perovskite semiconductors, quasi-two-dimensional (2D) materials are attractive for the pursuit of electrically driven lasing given their excellent performance in light-emitting diodes (LEDs) and their recent success in continuous-wave optically pumped lasing. We investigate the spontaneous photoluminescence emission and amplified spontaneous emission (ASE) of a series of quasi-2D emitters, and their directly analogous 3D materials formed by removing the 2D organic spacer by annealing. Although the PL photoluminescence (PL) (at low optical excitation power) from quasi-2D films with high 2D spacer fractions can be much brighter than that from their 3D counterparts, the ASE thresholds of these quasi-2D materials tend to be higher. This counter-intuitive behavior is investigated through time-resolved photophysical studies, which reveal the emission in the high-spacer-content quasi-2D perovskite can be exclusively excitonic, and the exciton–exciton annihilation of quasi-2D perovskite starts to take over the exciton dynamics at a low exciton density (<10$^{16}$ cm$^{-3}$). To lower ASE thresholds in quasi-2D materials it is necessary to increase the volume fraction of thick quantum wells, which we achieve by decreasing the spacer content or by utilizing 1-naphthylmethylamine (NMA) linkers. The increase of the volume fraction of thick quantum wells correlates with an increased contribution of free carrier recombination to the emission process of the quasi-2D materials. These results suggest that material development of quasi-2D materials for gain applications should target fast free charge carrier recombination rates by engineering the well thickness and size and not maximum photoluminescence quantum yields under low power excitation

How free exciton-exciton annihilation lets bound exciton emission dominate the photoluminescence of 2D-perovskites under high-fluence pulsed excitation at cryogenic temperatures

Photoluminescence (PL) spectra of atomically thin 2D lead iodide perovskite films are shown to depend on excited-state density, especially at cryogenic temperatures. At high excited-state densities and low temperatures, free exciton (FE) emission is so suppressed by exciton-exciton annihilation (EEA) that other-normally much weaker-emissions dominate the PL spectrum, such as emission from bound excitons (BEs) or PbI2 inclusions. In the Ruddlesden-Popper perovskite with phenethylammonium (PEA) ligands (PEA2PbI4, PEPI), FE emission dominates at all temperatures at the excited-state densities reached with continuous wave excitation. At higher excited state densities reached with femtosecond pulsed excitation, the PL at temperatures under 100K is dominated by BE emission redshifted from that of FE by 40.3meV. Weak emission from PbI2 inclusions 170meV higher in energy than FE PL is also observable under these conditions. Equilibrium between BE and FE states explains why FE emission first increases with decreasing temperature from 290 until 140K and then decreases with decreasing temperature as the BEs become stable. A Dion-Jacobson (DJ) material based on 1,4-phenyl-enedimethanammonium (PDMA) supports the reduction of FE emission by EEA at cryogenic temperatures. However, in the PDMA-based DJ material, BE emission is never as pronounced. At low temperatures and high-excited state densities caused by pulsed excitation, a broad emission redshifted by 390meV from the FE dominates. Based on comparison with temperature-dependent measurements of PbI2 films, this emission is suggested to arise from PbI2 inclusions in the material. Possible avenues for improving PL at room temperature are discussed concerning these findings

Mapping the neuropsychological profile of temporal lobe epilepsy using cognitive network topology and graph theory

OBJECTIVE: Normal cognitive function is defined by harmonious interaction among multiple neuropsychological domains. Epilepsy has a disruptive effect on cognition, but how diverse cognitive abilities differentially interact with one another compared to healthy controls (HC) is unclear. This study used graph theory to analyze the community structure of cognitive networks in adults with temporal lobe epilepsy (TLE) compared with HC. METHODS: Neuropsychological assessment was performed in 100 patients with TLE and 82 HC. For each group, an adjacency matrix was constructed representing pair-wise correlation coefficients between raw scores obtained in each possible test-combination. For each cognitive network, each node corresponded to a cognitive test; each link corresponded to the correlation coefficient between tests. Global network structure, community structure and node-wise graph theory properties were qualitatively assessed. RESULTS: The community structure in patients with TLE was composed of fewer, larger, more mixed modules, characterizing three main modules representing close relationships between: 1) aspects of executive function (EF), verbal and visual memory, 2) speed and fluency, and 3) speed, EF, perception, language, intelligence, and nonverbal memory. Conversely, controls exhibited a relative division between cognitive functions, segregating into more numerous, smaller modules consisting of: 1) verbal memory, 2) language, perception and intelligence, 3) speed and fluency, and 4) visual memory and EF. Overall node-wise clustering coefficient and efficiency were increased in TLE. SIGNIFICANCE: Adults with TLE demonstrate a less clear and poorly structured segregation between multiple cognitive domains. This panorama suggests a higher degree of interdependency across multiple cognitive domains in TLE, possibly indicating compensatory mechanisms to overcome functional impairments