Transient Fluorescence Technique Research Articles

Energy Barriers Restrict Charge Carrier Motion in MAPI Perovskite Films

AbstractUnderstanding of charge trapping processes in halide perovskites is vital to further improve performance of perovskite optoelectronic devices such as solar cells, photodetectors, and LEDs. In this work, transient photocurrent, time‐delayed collection field and transient fluorescence techniques along with numerical simulations are combined to address charge carrier trapping processes during their lateral motion in prototypical methylammonium lead iodide perovskite films formed on interdigitated electrodes. Carrier mobility decreases on hundreds of ns timescale, and its rate depends on the motion character—it is faster when charge carriers drift in the electric field and slower when the motion is caused by diffusion only. This difference becomes particularly evident at low temperatures. Based on the time‐delayed collection field data and carrier motion modelling results, it is demonstrated that the rapid mobility decay at low temperatures is mainly caused by the energy barriers, most likely formed at crystallite boundaries. Even though these barriers are surmountable at room temperature, they still play a major role in determining carrier mobility and diffusion rates. Suggested concept of the potential barriers moves beyond the conventional understanding of carrier mobility, diffusion, and recombination processes in hybrid perovskites.

A study on film formation and fluorescence enhancement of PS latex/AgNPs composites depending on AgNPs content and annealing

In the work reported here, we investigated the characteristic change in film formation, optical, morphological and electrical properties of pyrene (P) labeled polystyrene (PS) latex/silver nanoparticles (AgNPs) (PS/AgNPs) composites by using steady state (SSF) and fast transient (FTRF) fluorescence techniques in conjunction with UV–VIS (UVV) technique. Nine different mixtures were prepared by mixing of PS latex dispersion with different amount of AgNPs in the range of 0–50 wt%. PS/AgNPs films were then prepared on glass substrates using drop casting method and drying at room temperature. After drying, film samples were separately annealed above the glass transition temperature (Tg) of PS ranging from 100 to 280 °C. Fluorescence emission spectrum, fluorescence decay curve and transmittance of these composites were measured after each annealing step as a function of AgNPs content. The emission spectrum became narrower with increasing AgNPs content due to interactions between the pyrenes and the electron plasma in nearby silver nanoparticles. Fluorescence enhancement and reduced lifetime were also observed with increasing AgNPs content in the range of 3–20 wt%. Almost a 4 fold increase in emission intensity of pyrene were observed for 10 wt% of AgNPs content film. In order to monitor the film formation process, fluorescence intensities (IP) from P and transmitted light intensity (Itr) through the films were measured after each annealing step. Below 40 wt% of AgNPs, two distinct film formation stages, which are named as void closure and interdiffusion processes, were seen in fluorescence data. However, above 40 wt%, no change was observed in IP upon annealing, whereas transparency decreased overall with increasing AgNPs content. The electrical conductivity of these composites showed very little increase with increasing AgNPs. Film morphologies were also found as consistent with optical data.

Solvent Vapour Induced Film Formation of PS/AgNPs Composites Using Spectroscopic Techniques

In this work, pyrene labeled polystyrene (PS) latex dispersions and silver nanoparticles (AgNPs) were mixed at different weight ratios of AgNPS in the range of 0-50 wt%. Powder (PS/AgNPs) films were prepared on glass substrates from these mixtures by the drop-casting method at room temperature. The fast transient fluorescence (FTRF) technique was used to monitor the film formation process of PS/AgNPs composites by measuring the fluorescence lifetimes of pyrene during vapour exposure. It was observed that pyrene lifetimes decreased as vapour exposure time, t increased obeying Stern–Volmer kinetic. The optical transmission (UVV) experiments showed that the transparency of the films decreased with increasing AgNPs content. Transparent films were produced for the composites with 0 to 20 wt % of AgNPs content. The Prager–Tirrel model was employed to FTRF and UVV data to obtain back-and-forth frequencies, ν, of the reptating PS chains during vapour induced film formation process and polymer interdiffusion was found to obey a t1/2 law. Although the conductivity increased by 3 orders of magnitude with increasing AgNPs content, the composites behaved like an insulator. The morphology of composite films also progressively changed during vapour exposure and affected by AgNPs content.

Studying the temperature-dependence of singlet exciton fission by using transient fluorescence technique

During the past ten years, singlet exciton fission has been considered as an effective carrier-multiplication method to improve the energy-conversion efficiency of photovoltaic devices. However, the question of whether this process requires thermal activation is still under debate. Although different researchers have made a lot of effort, more explicit experimental results are still required in order to elucidate this basic question. In this work, highly efficient fission molecule, i.e. rubrene, was selected to be the research object. By using co-deposition technique, we fabricated four rubrene-doped films in which rubrene molecules were uniformly mixed with other inert organic molecules, forming amorphous composite solid. For each sample, steady-state photoluminescence spectra and their time-resolved fluorescence decay curves were measured at 300, 250, 200, 150, and 100 K, respectively. Upon cooling from 300 K down to 100 K, the emission features continuously sharpened, leading to a substantial increase in the signal around 575 nm. Decomposition of photoluminescence spectrum confirms the close relevance between radiative recombination of singlet excitons and molecular thermal vibration. Theoretically, based on a traditional three-state reaction model of “S 1 +S 0 ↔ 1 (TT) i ↔T 1 +T 1 ”, all measured fluorescence decay curves could be well fitted by using a set of coupled rate equations. Then the important rate constants involved in singlet exciton fission process were obtained by curve-fitting. It was found that the functional relationship between the transition rate of “S 1 +S 0 → 1 (TT) i ” process and temperature was accordant with the conventional Arrhenius law. This was in line with the endothermic nature of singlet exciton fission in rubrene material. Furthermore, according to the detailed balance condition of molecular reaction dynamics and Arrhenius laws, the experimentally determined energy difference in endothermic reaction was consistent with its theoretical value. These results are of significance for clarifying the microscopic picture and physical mechanism of exciton fission. Finally, we give some deep analysis about the variation of temperature-dependences of singlet fission processes in rubrene and tetracene materials with different morphology.

Intermolecular distance dependence of singlet exciton fission by using transient fluorescence technique

In rubrene-doped organic thin films, the average intermolecular distance between adjacent rubrene molecules was changed by controlling the doping concentration of rubrene. In rubrene: Alq3 composite films, the average distance of doped rubrene molecules was decreased from ~ 1.7 nm down to ~ 1.1 nm when the weight ratio of rubrene was increased from 14% to 50%. The variation of intermolecular distance affected the magnitude of overlap of exciton wave function between adjacent rubrene molecules, hence tuning the strength of intermolecular coupling and leading to different rate of singlet exciton fission between neighboring rubrene molecules. In experiment, the steady-state photoluminescence spectra were firstly measure. And time-resolved fluorescence decay curves of different samples were recorded at room temperature by using transient fluorescence spectroscopy. All the excited fluorescence was from doped rubrene. In theory, based on a three-state reaction model of “S1+S0 ↔ 1(TT) i ↔ T1+T1”, the dynamic process of singlet exciton fission in amorphous rubrene was appropriately described by using three coupled rate equations. All the measured transient decay curves of photoluminescence of rubrene-doped films could be well fitted by the iterative calculation of rate equations. And a detailed analysis about the critical issues which were related with curve-fitting was given. According to the fitting parameters, it was found that the rate of “S1+S0→1(TT) i ” process exponentially decreased with the increasing rubrene distance. Such a phenomenon was not reported before. This is very similar with the previous observations in the process of Dexter energy transfer in which the rate of interaction also exponentially decreased with the increasing donor-acceptor distance. In Dexter process, the energy transfer can be considered as the process of two electrons’ hopping between donor and acceptor molecules. Similarly, singlet exciton fission process also involves two electrons’ hopping between two adjacent rubrene molecules. According to this physical image, the excitation of triplet excitons and redistribution of energy of singlet exciton are accomplished by the formation of an intermediate triplet pair state. The enlarged intermolecular distance would result in exponentially decayed electron-transfer rates, thus decreasing the comprehensive rate constant of singlet fission. Since both of two processes can be explained by “double-electron transfer model”, the similarity in functional relationship could actually be regarded as the first experimental confirmation of “double-electron transfer model” for singlet exciton fission. This is of significant value for clarifying the physical mechanism of singlet fission process. Furthermore, an in-depth investigation about such an exponential relationship between the exciton fission rate and intermolecular distance will be helpful for analyzing the dynamics and key factors of singlet exciton fission process.

Time Resolved Thermodynamic Studies of Ligand Binding/release to Sol-Gel Encapsulated Horse Heart Myoglobin

The successful confinement of proteins in solid state materials allows for a multitude of applications in the study of protein dynamics, as well as, advances in biotechnologies. The importance of bulk solvent and restricted space on the conformational dynamics of proteins can be identified by encapsulating the biomolecules (peptides, proteins, enzymezes, etc) in environments where both can be regulated. In addition, the affect of encapsulation on ligand binding and preservation of reactivity once encapsulated are of equal importance. With the use of Photoacoustic calorimetry (PAC) and Photothermal beam deflection (PBD) methods along with picosecond-millisecond transient absorption and fluorescence techniques we are investigating enthalpy, molar volume changes, and kinetics associated with with CO dissociation and rebinding to horse heart myoglobin encapsulated in sol-gels. Preliminary PAC data suggest that the dissociation of CO from HHMb results in four kinetic phases: <7ns, ∼125ns, ∼260ns, and ∼600ns. We will discuss the differences in molar volume and enthalpy changes associated with the conformational dynamics of these events in sol-gels relative to native protein in aqueous solution.

Reordering of polystyrene gel due to multiple swelling in organic vapor: Fast transient fluorescence technique study

Reordering of disc-shaped polystyrene (PS) gels due to multiple swelling, under organic vapor was studied by using Fast Transient Fluorescence (FTRF) technique. Disc-shaped polystyrene gels were prepared by free radical copolymerization (FRC) of styrene (S) with ethylene glycol dimethacrylate (EGDM) as a crosslinker. Pyrene (P) was introduced as a fluorescence probe during polymerization. Swelling experiments were performed by using P doped PS gels under chloroform vapor. After each swelling step, gels were left to dry in an oven at 30 °C for consecutive reswelling experiments. Decay curves of P were measured and pyrene lifetimes, τ, were determined. It was observed that τ values decreased as swelling recycles were repeated. It was observed that after the fifth swelling step, two different regimes appeared in the swelling processes in PS gels. Swelling time constant, τ c, and cooperative diffusion coefficients, D c, were determined by using Li–Tanaka equation for each swelling step in both regimes. It was observed that D c values decreased up to tenth swelling step and then remained unchanged in both regimes.

Film formation from pure and mixed latices; transient fluorescence study

A fast transient fluorescence technique was used to study latex film formation induced by organic solvent vapor. Mixtures of pyrene (P)- and naphthalene (N)-labeled and/or pure naphthalene-labeled latex films were prepared separately from poly(methyl methacrylate) (PMMA) particles. Then these pure and mixed latex films were exposed to vapor of various chloroform–heptane mixtures in seven different experiments. In both films, fluorescence lifetimes from N were monitored during vapor-induced film formation. It was observed that N lifetimes decreased as the vapor exposure time is increased. A Stern–Volmer kinetic analysis was used for low quenching efficiencies to interpret the decrease in N lifetimes. A Prager–Tirrell model was employed to obtain back-and-forth frequencies, ν, of reptating PMMA chains during latex film formation induced by solvent vapor. In both pure and mixed latex films, ν values were found to be correlated with the chloroform content in the vapor mixture. It was observed that polymer interdiffusion obeyed a t1/2 law during film formation.

COMPARISION OF CRITICAL PARAMETERS OF POLYMERIZATION AND GELATION PROCESSES: A FAST TRANSIENT FLUORESCENCE STUDY

The fast transient fluorescence technique was used to study free-radical polymerization and crosslinking copolymerization of styrene. Ethylene glycol dimethacrylate and Pyrene were used as crosslinking agent and fluorescence probe, respectively. The fluorescence lifetimes of Pyrene from its decay traces were measured and used to monitor the vitrification processes at various temperatures. Changes in the viscosity due to polymer formation dramatically enhanced the fluorescent yield of pyrene molecules. This effect is used to monitor the polymerization and crosslinking copolymerization of Styrene as a function of time, at various temperatures. The results are interpreted in the view of static percolation theory. The critical exponents β and γ for vitrified fraction and average degree of polymerization were found to be 0.39±0.012; 1.62±0.036 and 0.39±0.004; 1.69±0.064 in agreement with percolation results for polymerization and gelation processes, respectively. Activation energies for polymerization and gelation (ΔEP and ΔEG) were measured and found to be 112.0±4 and 86.9±4 kJ mol-1, respectively.

Small molecule desorption from a swelling polymeric glass in polymer solution: Energy transfer method

Direct energy transfer (DET) method conjunction with fast transient fluorescence (FTRF) technique were used to study small molecule desorption from a swelling poly(methyl methacrylate) (PMMA) glass in polymer solution, which is consist of naphthalene (N)-labeled PMMA chains in chlorofrom–heptane mixture. Desorption coefficients, D d of pyrene (P) desorbing from the swelling PMMA glass prior to dissolution were measured and found to be 8.3 × 10 −8 and 1.3 × 10 −5 cm 2 s −1 in short and long time regions, respectively.

Universal behaviour of glass transition exponents in various polymeric systems

The fast transient fluorescence (FTRF) technique was used to study the critical exponents during glass transition in free-radical cross-linking copolymerization (FCC). Methyl methacrylate (MMA), ethyl methacrylate (EMA) and various combinations of MMA with EMA were used during FCC experiments. Pyrene (Py) was used as a fluorescence probe and its fluorescence lifetimes from its decay traces were measured during glass transition. Changes in the viscosity of the pre-gel solutions due to glass formation dramatically increased the Py fluorescent lifetimes, which were used to study the glass transition of MMA, EMA and their mixtures as a function of time, at various temperatures and monomer concentrations. The results were interpreted in the view of percolation theory. The critical exponents, β and γ, were measured near the glass transition point and found to be around 0.37 ± 0.015 and 1.69 ± 0.05, respectively, in all systems studied, which are in good agreement with the static percolation results.

Comparison of gel swelling under organic vapor and in organic solvent

A Fast Transient Fluorescence Technique (FTRF) was employed for studying swelling of disc shaped poly (methyl methacrylate) (PMMA) gels, which were prepared by free radical copolymerization of methyl (methacrylate) (MMA) using various ethylene glycol dimethacrylate (EGDM) contents at 60 ∘C. Pyrene (P) was introduced as a fluorescence probe during polymerization. Swelling experiments were performed by using P doped PMMA gels under chloroform vapor and in chloroform at room temperature. P lifetimes in and out of the gel were measured from fluorescence decay traces during in-situ swelling experiments. It was observed that P lifetimes in the gel decreased as swelling proceeds. An equation is derived for low quenching efficiencies to interpret the behavior of P lifetimes during swelling. The Li‐Tanaka equation was used to determine the cooperative, Dc diffusion coefficients for the gels made at various crosslinker contents. It is observed that Dc values decrease as the crosslinker content is increased both in chloroform vapor and in chloroform.

Studying of the critical exponents around the glass transition in bulk polymerization of ethyl methacrylate by using fluorescence techniques

The glass transition during bulk polymerization was studied in free-radical crosslinking copolymerization (FCC) of ethyl methacrylate (EMA), using both the steady-state fluorescence (SSF) and the fast transient fluorescence (FTRF) techniques. Pyrene (Py) was used as a fluorescence probe. Changes in the viscosity of the pregel solutions due to gel formation dramatically enhance the fluorescent yield of Py molecules. The reaction time at which the Py intensity and lifetime exhibit a sudden increase corresponds to the reaction time at which the rate of polymerization becomes maximum resulting from the gel effect. This effect is used to study the gelation of EMA, as a function of time, at various crosslinker concentrations and different temperatures. The results were interpreted in the view of percolation theory. The gel fraction, β and weight average degree of polymerization, γ exponents β = 0.37 ± 0.01, γ = 1.71 ± 0.04 and β = 0.36 ± 0.002, γ = 1.687 ± 0.01 are found in agreement with percolation results for SSF and FTRF measurements, respectively.

Fast transient fluorescence technique for studying sol–gel phase transition in polymeric mixtures

Fast transient fluorescence (FTRF) technique was used to study the sol–gel phase transition in free-radical crosslinking copolymerization of methyl methacrylate (MMA), ethyl methacrylate (EMA) and their mixtures with ethylene glycol dimethacrylate (EGDM). Fluorescence lifetimes of pyrene (Py) from its decay traces were monitored during sol–gel phase transitions. Consumption of MMA and/or EMA in pregel solutions due to gel formation dramatically increased the Py fluorescence lifetimes, in which were used to study the gelation of MMA, EMA and their mixtures as a function of time, at various temperatures and monomer concentrations. Monomer consumption rates, k r were measured during sol–gel transition process by using Stern–Volmer model. Gelation activation energies Δ E G were determined and found to be 14 and 10 kcal mol −1 for MMA and EMA respectively.

Fast transient fluorescence method for measuring swelling and drying activation energies of a polystyrene gel

Abstract The time-resolved, fast transient fluorescence (TRF) technique which uses a strobe master system (SMS), was employed for studying swelling and drying of disc-shaped polystyrene gels. Disc-shaped gels were prepared by free-radical crosslinking copolymerization of styrene and ethylene glycol dimethacrylate. Pyrene was introduced as a fluorescence probe during polymerization and lifetimes, τ of pyrene were measured during in situ swelling and drying processes. Chloroform was used as an organic vapor agent to induce gel swelling at various temperatures. It was observed that τ values decreased as swelling is proceeded. Li–Tanaka equation was used to determine the swelling time constant, τc and cooperative diffusion coefficients, Dc for the swelling processes. It is observed that lifetimes, τ of pyrene increased during drying process and an empirical equation was introduced to determine the desorption coefficient, D for drying at various temperatures. The activation energies, ΔE were measured for the swelling and drying processes and found to be 80.0 and 33.5 kJ mol−1, respectively.

Cover Picture: Macromol. Chem. Phys. 4/2004

Cover: The cover shows the mobility of pyrene end‐capped polystyrene (Py‐PSt) trapped in swollen polystyrene (PSt) gel. Py‐PSt chain was produced by atom transfer radical polymerization (ATRP). Swelling experiments were performed in real time by monitoring pyrene lifetimes, outside (τ1) and inside (τ2) of PSt gel, by using fast transient fluorescence technique (inset in the Figure). The graph shows the variation of solvent uptake, W, of PSt gel against swelling time, ts.Further details can be found in the Full Paper by M. Erdogan, Ö. Pekcan,* Y. Yagcι on page 456.

Fast Transient Fluorescence Technique for Studying Homopolymer Mobility in a Swelling Gel

Summary: The mobility of pyrene end-capped polystyrene (Py-PSt) trapped in swollen polystyrene gels (PSt) was investigated. PSt gels were prepared by free radical crosslinking copolymerization (FCC) of styrene (St) and ethylene glycol dimethacrylate (EGDM). The pyrene end-capped polystyrene was produced by atom transfer radical polymerization (ATRP). After drying, these disc-shaped PSt gels were left in a toluene solution of Py-PSt of various molecular weights. During this process, Py-PSt chains were trapped in the gel. These swollen gels were re-dried in air and then immersed in pure toluene solution for monitoring the mobility of Py-PSt chains in and out of the gel. These reswelling experiments were performed at room temperature in real time by monitoring pyrene lifetimes outside (τ1) and inside (τ2) of the PSt gel by using in-situ fast transient fluorescence (FTRF) measurements. It was observed that τ2 values decrease as swelling proceeds; however, τ1 values stay constant during swelling. The Li-Tanaka equation was employed to produce the swelling parameters. The swelling time constant, τc was found to increase as the crosslinker density of the gels and the molecular weight, of Py-PSt chains were increased. It was observed that the collective diffusion coefficient, Dc decreased by increasing the molecular weight by obeying Dc ≈ M−1 law. Position of the PSt gel in the fluorescence quartz cell before (a) and after (b) swelling. I0 and Ip are the excitation and the emission intensities.

A fluorescence study on critical exponents during sol‐gel phase transition in complex monomeric systems

AbstractMethyl methacrylate (MMA), ethyl methacrylate (EMA) and various combinations of MMA with EMA were used during FCC experiments. Pyrene (Py) was introduced as a fluorescence probe and fluorescence lifetimes from its decay traces were measured during sol‐gel phase transitions. The fast transient fluorescence (FTRF) technique was used to study the critical exponents during sol‐gel phase transition in free‐radical crosslinking copolymerization (FCC). The results were interpreted in the view of percolation theory. The critical exponents of gel fraction, β and weight average degree of polymerization, γ were measured near the point of gel effect and found to be around 0.37 ± 0.015 and 1.69 ± 0.05 in all systems studied respectively.

Experimental determination of gelation and swelling parameters using time‐resolved fluorescence technique

AbstractA fast transient fluorescence (FTRF) technique was used to study the sol–gel transition during gelation of methyl methacrylate (MMA) and ethylene glycol dimethacrylate (EGDM). Pyrene (Py) was used as a fluorescence probe for the in situ polymerization experiments. Swelling of this cylindrical poly(methyl methacrylate) (PMMA) gel in chloroform was monitored using FTRF technique. Fluorescence lifetimes of Py from its decay traces were measured and used to monitor both the gelation and swelling processes. Stern–Volmer kinetics were employed to determine the quenching rate constant before the onset of gelation. MMA consumption rate was measured during gelation process using the Stern–Volmer model. A Li–Tanaka equation was introduced to determine the cooperative diffusion coefficient Dc, which was found to be around 10−5 cm2 s−1 during the swelling of PMMA gel in chloroform. © 2003 Wiley Periodicals, Inc. Adv Polym Techn 22: 238–245, 2003; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/adv.10052

Ffast Transient Fluorescence Technique to Study Critical Exponents at the Glass Transition

The fast transient fluorescence (FTRF) technique was used to study critical exponents at the glass transition in free-radical crosslinking copolymerization (FCC) for two different monomeric systems, methyl methacrylate (MMA) and styrene (S). Pyrene (Py ) was used as a fluorescence probe. The fluorescence lifetimes of Py from its decay traces were measured and used to monitor the gelation process. Changes in the viscosity of the pregel solutions due to glass formation dramatically enhance the fluorescent yield of aromatic molecules. This effect is used to study the glass transition upon gelation of MMA and S monomeric systems as a function of time, at various temperatures and crosslinker concentrations. The results are interpreted in the view of percolation theory. The gel fraction and weight average degree of polymerization exponents β and γ are found to be 0.37 ± 0.02 and 1.66 ± 0.07 in agreement with percolation results.