Photoluminescence Mapping over Laser Pulse Fluence and Repetition Rate as a Fingerprint of Charge and Defect Dynamics in Perovskites

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

Are Shockley-Read-Hall and ABC models valid for lead halide perovskites?

Metal halide perovskites are an important class of emerging semiconductors. Their charge dynamics is poorly understood due to limited knowledge of defect physics and charge 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. Higher-power PL kinetics requires the inclusion of additional non-linear processes. The PLQY mapping developed herein is suitable for a comprehensive testing of theories and is applicable to any semiconductor.Comment: Supplementary Information available at https://cloudstore.zih.tu-dresden.de/index.php/s/t5gBPJgwZiwfRR

Expression of proteins of interest in food

PòsterJun

Repurposing Poly(3-hexylthiophene) as a Conductivity-Reducing Additive for Polyethylene-Based High-Voltage Insulation

Poly(3-hexylthiophene) (P3HT) is found to be a highly effective conductivity-reducing additive for low-density polyethylene (LDPE), which introduces a new application area to the field of conjugated polymers. Additives that reduce the direct-current (DC) electrical conductivity of an insulation material at high electric fields have gained a lot of research interest because they may facilitate the design of more efficient high-voltage direct-current power cables. An ultralow concentration of regio-regular P3HT of 0.0005 wt% is found to reduce the DC conductivity of LDPE threefold, which translates into the highest efficiency reported for any conductivity-reducing additive to date. The here-established approach, i.e., the use of a conjugated polymer as a mere additive, may boost demand in absolute terms beyond the quantities needed for thin-film electronics, which would turn organic semiconductors from a niche product into commodity chemicals

Remarkable performance recovery in highly defective perovskite solar cells by photo-oxidation

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

Small Number of Defects per Nanostructure Leads to “Digital” Quenching of Photoluminescence : The Case of Metal Halide Perovskites

Long charge carrier diffusion length and large grain size are commonly believed to be inherent properties of highly luminescent polycrystalline thin-film semiconductors. However, exactly these two properties make luminescence very susceptible to quenching by just one strongly quenching defect state if present in each grain. Moreover, when the number of quenchers per grain is small (say 1–10), it varies greatly from grain to grain, purely for statistical reasons. These fluctuations, which resemble digital signal switching, can be one of the reasons for large differences between the luminescence brightness of different grains in polycrystalline films. This and other peculiarities of photoluminescence in systems where the number of strong quenchers per grain/crystallite is small is discussed in detail using metal halide perovskites as examples

Electric field induced quenching of the fluorescence of a conjugated polymer probed at the single molecule level

Electric field (EF) induced quenching of the fluorescence of large (ca. 10(3) monomers) single chains of MEH-PPV was observed. Directly after application of the field most of the individual MEH-PPV chains were either unaffected by the field or completely quenched by it, while the number of affected molecules increased with the strength of the EF Moreover, application of the field induced fluorescence blinking. Both effects are attributed to EF-assisted formation of long-living charged species that during their lifetime act as quenchers for excitations. The same type of quenchers are thought to be responsible for blinking observed without an electric field. (c) 2005 Elsevier B.V. All rights reserved

Polydispersity of the photoluminescence quantum yield in single conjugated polymer chains

The conjugated polymer poly(2-methoxy-5-(2'-ethylhexyloxy)-1,4-pheiiylene vinylene) (MEH-PPV) was studied by a single-molecule imaging technique. A comparison of statistical distributions of fluorescence intensity with molecular weight distributions revealed that the distribution of the photoluminescence quantum yield of the single polymer chains under study is significantly asymmetric, with a polydispersity greater than or similar to 2. The result implies that there are molecules whose quantum yield is a few times higher than the ensemble quantum yield. This conclusion Suggests a possibility of a great improvement of the photoluminescence quantum yield of MEH-PPV, which is known to be several times less than 1. (c) 2005 Elsevier B.V.. All rights reserved

Photoluminescence spectra of a conjugated polymer: from films and solutions to single molecules

The purpose of this work is to address the issue of applicability of single-molecule spectroscopy (SMS) results for conjugated polymers to "bulk" samples, e.g. conjugated polymer films. Also, some apparent inconsistencies in the literature on SMS regarding the photoluminescence spectral position of conjugated polymers are discussed. We present a series of photoluminescence spectra of thin films of the conjugated polymer poly(2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylene-vinylene) (MEH-PPV) with a wide range of varying thickness. The thickness was varied from similar to 20 nm to the value corresponding to well-separated single molecules (SMS sample). The thickness variation resulted in a strong (similar to 2000 cm(-1)) blue-shift and broadening of the spectrum. The result was reproduced on isolated molecules embedded into a PMMA matrix. This effect cannot be explained by a decrease in energy transfer "freedom" alone. We performed a comprehensive comparison of presented and elsewhere published spectra of MEH-PPV polymer and oligomers in different samples: films, solutions, isolated-molecule coatings and standard SMS samples. The comparison allows that the main reason behind the blue shift is conformational disorder, which is largely dependent on the sample. We also discuss some experimental aspects of SMS, such as representativeness of detected molecules, spectral sensitivity of a setup and temperature. Together with differences in sample preparation method, these issues can explain the existing inconsistencies in the literature