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The correct determination of the exciton diffusion length (LD) in novel organic photovoltaics (OPV) materials is an important, albeit challenging, task required to understand these systems. Herein, a high-throughput approach to probe LD in nonfullerene acceptors (NFAs) is reported, that builds upon the conventional photoluminescence (PL) surface quenching method using NFA layers with a graded thickness variation in combination with spectroscopic PL mapping. The method is explored for two archetypal NFAs, namely, ITIC and IT-4F, using PEDOT:PSS and the donor polymer PM6 as two distinct and practically relevant quencher materials. Interestingly, conventional analysis of quenching efficiency as a function of acceptor layer thickness results in a threefold difference in LD values depending on the specific quencher. This discrepancy can be reconciled by accounting for the differences in light in- and outcoupling efficiency for different multilayer architectures. In particular, it is shown that the analysis of glass/acceptor/PM6 structures results in a major overestimation of LD, whereas glass/acceptor/PEDOT:PSS structures give no significant contribution to outcoupling, yielding LD values of 6−12 and 8−18 nm for ITIC and IT-4F, respectively. Hence, practical guidelines for quencher choice, sample geometries, and analysis approach for the accurate assessment of LD are provided.

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.

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.

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.

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.

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.

A series of novel polythiophene-based cationic conjugated polyelectrolytes with phosphonium and ammonium pendants were prepared by modifying regioregular poly[3-(6-bromohexyl)thiophene] using a simple quaternization reaction with appropriate phosphine or amine. Partial transformation of the bromide end-groups of the polymer precursor with amine allowed adding the phosphine agent, thereby enabling quantitative transformation into a novel conjugated polyelectrolyte with phosphonium and ammonium groups in the same polymer main-chains. The properties of the synthesized polyelectrolytes in solution (water, methanol, DMSO and chloroform) were analyzed by UV/vis, luminescence, NMR, light scattering and TGA measurements. To test the prepared polyelectrolytes as potential luminescence sensors, fluorescence quenching measurements were performed in water, using K4[Fe(CN)6] and K3[Fe(CN)6] as quenchers. This study showed that polyelectrolytes containing both types of ionic pendant groups (phosphonium and ammonium) are more efficiently quenched. © 2018 Elsevier Ltd

A series of novel polythiophene-based cationic conjugated polyelectrolytes with phosphonium and ammonium pendants were prepared by modifying regioregular poly[3-(6-bromohexyl)thiophene] using a simple quaternization reaction with appropriate phosphine or amine. Partial transformation of the bromide end-groups of the polymer precursor with amine allowed adding the phosphine agent, thereby enabling quantitative transformation into a novel conjugated polyelectrolyte with phosphonium and ammonium groups in the same polymer main-chains. The properties of the synthesized polyelectrolytes in solution (water, methanol, DMSO and chloroform) were analyzed by UV/vis, luminescence, NMR, light scattering and TGA measurements. To test the prepared polyelectrolytes as potential luminescence sensors, fluorescence quenching measurements were performed in water, using K4[Fe(CN)6] and K3[Fe(CN)6] as quenchers. This study showed that polyelectrolytes containing both types of ionic pendant groups (phosphonium and ammonium) are more efficiently quenched. © 2018 Elsevier Ltd

There is an ongoing interest in organic materials due to their application in various organic electronic devices. However stability of organic materials limits their potential use. They are prone to degradation both during the working life and storage. One of the main causes is extrinsic degradation, under the influence of oxygen and moisture. This problem can be solved by encapsulation of devices. However no encapsulation is perfect. This paper presents a study of interaction of thin films of well-known organic blue emitters, namely N,N′-bis(3-methylphenyl)-N,N′-bis(phenyl)benzidine (TPD) and 4,4′-bis(2,2-diphenylvinyl)-1,1′-biphenyl (DPVBi), with UV light in air. Films of both materials are stable in vacuum, but readily degrade in the presence of oxygen. Thus, the necessary condition for interaction (degradation) is the simultaneous presence of UV light and oxygen. Chemical analysis of irradiated films by mass and infrared spectroscopy revealed presence of oxidized species (impurities). These impurities are responsible for increased morphological stability of irradiated films and quenching of photoluminescence (PL). Only small amount of impurities, 0.4 % (0.2 %) for TPD (DPVBi), causes 50 % decrease of PL. This implies a non-trivial mechanism of quenching. For both molecules it was found that distance between impurities is smaller or equal to exciton diffusion length, which is the necessary condition for quenching. Following mechanism of quenching is proposed: exciton diffuses by hopping form one host molecule (DPVBi or TPD) to another through Förster resonant energy transfer in a random walk manner. If, during its lifetime, it comes to proximity of an impurity, a PL quenching process occurs. Findings of this study are important because they show that even a small amount of oxygen that penetrates a blue emitter layer would impair luminescence efficiency of a device. Moreover, the absorption of its own radiation would additionally contribute to the rate of degradat

The photoluminescence (PL) quenching mechanism of UV light and air-exposed amorphous thin films of 4,4'-bis(2,2-diphenylvinyl)-1,1'-biphenyl (DPVBi), a well-known hole-transport material used in organic light-emitting diodes, is studied. Thin films of DPVBi are stable when exposed to UV light in vacuum but tend to degrade if oxygen is present simultaneously. This is evident from the changes in UV-vis absorption spectra of the latter, showing that degradation rate of DPVBi films is linearly proportional to both oxygen concentration and UV light intensity. Mass spectrometry study of such films revealed a number of different oxygen-containing molecules and fragments of DPVBi thus confirming apparent photo-oxidation process. Also, DFT study of molecular DPVBi with and without oxygen was carried out, the IR spectra calculated for the lowest energy molecules found and the results are compared with the experiment. The most sensitive to photo-oxidation is DPVBi photoluminescence, which decays exponentially with respect to the concentration of photo-oxidized DPVBi molecules (impurities). The PL quantum yield of DPVBi thin film drops to a half of its original value for 0.2% of the impurities present, at which point an average distance between DPVBi molecules (the donors) and photo-oxidized DPVBi species (acceptors) is an order of magnitude larger than the separation between two adjacent molecules. This implies a need for a long-range Forster energy transfer, which we rule out based on the lack of a donor-acceptor spectral overlap. The apparent discrepancy can be removed by postulating exciton self-diffusion in DPVBi thin films, for which there is supporting evidence in existing literature.

The photoluminescence (PL) quenching mechanism of UV light and air-exposed amorphous thin films of 4,4-bis(2,2-diphenylvinyl)-1,1-biphenyl (DPVBi), a well-known hole-transport material used in organic light-emitting diodes, is studied. Thin films of DPVBi are stable when exposed to UV light in vacuum but tend to degrade if oxygen is present simultaneously. This is evident from the changes in UV-vis absorption spectra of the latter, showing that degradation rate of DPVBi films is linearly proportional to both oxygen concentration and UV light intensity. Mass spectrometry study of such films revealed a number of different oxygen-containing molecules and fragments of DPVBi thus confirming apparent photo-oxidation process. Also, DFT study of molecular DPVBi with and without oxygen was carried out, the IR spectra calculated for the lowest energy molecules found and the results are compared with the experiment. The most sensitive to photo-oxidation is DPVBi photoluminescence, which decays exponentially with respect to the concentration of photo-oxidized DPVBi molecules (impurities). The PL quantum yield of DPVBi thin film drops to a half of its original value for 0.2% of the impurities present, at which point an average distance between DPVBi molecules (the donors) and photo-oxidized DPVBi species (acceptors) is an order of magnitude larger than the separation between two adjacent molecules. This implies a need for a long-range Forster energy transfer, which we rule out based on the lack of a donor-acceptor spectral overlap. The apparent discrepancy can be removed by postulating exciton self-diffusion in DPVBi thin films, for which there is supporting evidence in existing literature. (C) 2015 Elsevier B.V. All rights reserved.

There is an ongoing interest in organic materials due to their application in various organic electronic devices. However stability of organic materials limits their potential use. They are prone to degradation both during the working life and storage. One of the main causes is extrinsic degradation, under the influence of oxygen and moisture. This problem can be solved by encapsulation of devices. However no encapsulation is perfect. In the first part of this work a study of degradation of thin films of N,N′-bis(3-methylphenyl)-N,N′-bis(phenyl)benzidine (TPD) and 4,4′-bis(2,2-diphenylvinyl)-1,1′-biphenyl (DPVBi) under UV irradiation in air is given. Films of both materials are stable in vacuum, but readily degrade in the presence of oxygen. Thus, the necessary condition for degradation is the simultaneous presence of UV light and oxygen. Chemical analysis of irradiated films by NMR, mass and infrared spectroscopy revealed presence of oxidized species (impurities). These impurities are responsible for increased morphological stability of irradiated films and quenching of photoluminescence. Only small amount of impurities, 0.4 % (0.2 %) for TPD (DPVBi), causes 50 % decrease of photoluminescence. This implies a non-trivial mechanism of quenching. For both molecules it was found that distance between impurities is smaller or equal to exciton diffusion length, which is the necessary condition for quenching. Following mechanism of quenching is proposed: exciton diffuses by hopping form one DPVBi (TPD) to another through FRET in a random walk manner. If, during its lifetime, it comes to proximity of an impurity, a Dexter-type energy transfer occurs and PL is quenched. Findings of DPVBi study are important because they show that even a small amount of oxygen that penetrates a DPVBi layer would impair luminescence efficiency of a device. Moreover, the absorption of own radiation (for DPVBi and TPD both) would additionally contribute to the rate of degradation of a device. I

The photoluminescence (PL) quenching mechanism of UV light and air-exposed amorphous thin films of 4,4-bis(2,2-diphenylvinyl)-1,1-biphenyl (DPVBi), a well-known hole-transport material used in organic light-emitting diodes, is studied. Thin films of DPVBi are stable when exposed to UV light in vacuum but tend to degrade if oxygen is present simultaneously. This is evident from the changes in UV-vis absorption spectra of the latter, showing that degradation rate of DPVBi films is linearly proportional to both oxygen concentration and UV light intensity. Mass spectrometry study of such films revealed a number of different oxygen-containing molecules and fragments of DPVBi thus confirming apparent photo-oxidation process. Also, DFT study of molecular DPVBi with and without oxygen was carried out, the IR spectra calculated for the lowest energy molecules found and the results are compared with the experiment. The most sensitive to photo-oxidation is DPVBi photoluminescence, which decays exponentially with respect to the concentration of photo-oxidized DPVBi molecules (impurities). The PL quantum yield of DPVBi thin film drops to a half of its original value for 0.2% of the impurities present, at which point an average distance between DPVBi molecules (the donors) and photo-oxidized DPVBi species (acceptors) is an order of magnitude larger than the separation between two adjacent molecules. This implies a need for a long-range Forster energy transfer, which we rule out based on the lack of a donor-acceptor spectral overlap. The apparent discrepancy can be removed by postulating exciton self-diffusion in DPVBi thin films, for which there is supporting evidence in existing literature. (C) 2015 Elsevier B.V. All rights reserved.

The photoluminescence (PL) quenching mechanism of UV light and air-exposed amorphous thin films of 4,4'-bis(2,2-diphenylvinyl)-1,1'-biphenyl (DPVBi), a well-known hole-transport material used in organic light-emitting diodes, is studied. Thin films of DPVBi are stable when exposed to UV light in vacuum but tend to degrade if oxygen is present simultaneously. This is evident from the changes in UV-vis absorption spectra of the latter, showing that degradation rate of DPVBi films is linearly proportional to both oxygen concentration and UV light intensity. Mass spectrometry study of such films revealed a number of different oxygen-containing molecules and fragments of DPVBi thus confirming apparent photo-oxidation process. Also, DFT study of molecular DPVBi with and without oxygen was carried out, the IR spectra calculated for the lowest energy molecules found and the results are compared with the experiment. The most sensitive to photo-oxidation is DPVBi photoluminescence, which decays exponentially with respect to the concentration of photo-oxidized DPVBi molecules (impurities). The PL quantum yield of DPVBi thin film drops to a half of its original value for 0.2% of the impurities present, at which point an average distance between DPVBi molecules (the donors) and photo-oxidized DPVBi species (acceptors) is an order of magnitude larger than the separation between two adjacent molecules. This implies a need for a long-range Forster energy transfer, which we rule out based on the lack of a donor-acceptor spectral overlap. The apparent discrepancy can be removed by postulating exciton self-diffusion in DPVBi thin films, for which there is supporting evidence in existing literature.

The photoluminescence (PL) quenching mechanism of UV light and air-exposed amorphous thin films of 4,4'-bis(2,2-diphenylvinyl)-1,1'-biphenyl (DPVBi), a well-known hole-transport material used in organic light-emitting diodes, is studied. Thin films of DPVBi are stable when exposed to UV light in vacuum but tend to degrade if oxygen is present simultaneously. This is evident from the changes in UV-vis absorption spectra of the latter, showing that degradation rate of DPVBi films is linearly proportional to both oxygen concentration and UV light intensity. Mass spectrometry study of such films revealed a number of different oxygen-containing molecules and fragments of DPVBi thus confirming apparent photo-oxidation process. Also, DFT study of molecular DPVBi with and without oxygen was carried out, the IR spectra calculated for the lowest energy molecules found and the results are compared with the experiment. The most sensitive to photo-oxidation is DPVBi photoluminescence, which decays exponentially with respect to the concentration of photo-oxidized DPVBi molecules (impurities). The PL quantum yield of DPVBi thin film drops to a half of its original value for 0.2% of the impurities present, at which point an average distance between DPVBi molecules (the donors) and photo-oxidized DPVBi species (acceptors) is an order of magnitude larger than the separation between two adjacent molecules. This implies a need for a long-range Forster energy transfer, which we rule out based on the lack of a donor-acceptor spectral overlap. The apparent discrepancy can be removed by postulating exciton self-diffusion in DPVBi thin films, for which there is supporting evidence in existing literature.

We studied degradation of 4,4′-bis(2,2′diphenyl vinyl)-1,1′-biphenyl (DPVBi), well know OLED material. Thermally evaporated thin films of DPVBi were irradiated with UV light in ambient, vacuum and under different oxygen pressures. The cause of degradation is reaction between UV excited DPVBi molecules and oxygen, via formation of singlet oxygen or electron transfer from excited DPVBi to molecular oxygen. Reaction rates depend on oxygen concentration and UV light intensity. These reactions lead to formation of oxidized species as evidenced by ASAP and MALDI-TOF mass spectroscopy. Photoluminescence quenching has two parts. One part is reversible and may imply formation of charge transfer complexes and the other is irreversible, caused by formation of oxidized species. IR and absorption spectra are studied by Density Functional Theory and results compared with the experiment.

Optical properties of self assembled quantum dots (QDs) have dominant inhomogeneous broadening due to the distribution of size and composition of the QDs in the ensemble. The carrier dynamics in such systems is affected by the neighboring dots due to interdot diffusion of carriers. In this work we investigate the intraband relaxation mechanisms in the QDs. We performed time-resolved photoluminescence (PL) quenching measurements on InAs/GaAs self assembled QDs. The samples were excited by near infrared laser pulses and the time and wavelength resolved PL was measured by a streak camera. During the decay of the PL, a THz pulse from a free electron laser, tuned to the intersublevel transition energy of the QDs, was made incident on the sample. This THz pulse induced quenching of the PL, shown by the dip in Fig.1, by re-exciting carriers to higher levels within the QDs. These carriers eventually relaxed back to the ground state (s-state) resulting in the recovery of the s-state PL intensity. Thus, the recovery of the PL was directly related to carrier relaxation dynamics in the QDs. The mechanisms involved were (i) ISL relaxation, where the excited carriers in the QDs directly fell back into the s-state of the same dot and (ii) transfer to adjacent dots via the wetting layer by multiphoton absorption or tunneling. To distinguish these two effects we performed measurements on two samples grown simultaneously but post-growth annealed at different temperatures resulting in different ISL relaxation times of 60 ps and 1.5 ns. From rate equation model fit of the measured data we found that the recovery time of the quenched PL was independent of the ISL relaxation times. This implied that the carrier relaxation within the dots was dominated by mechanisms other than ISL transition. Comparison of the amount of recovered PL signals for different emission energies showed that there was loss of carriers at higher PL energies, as showed in Fig.1(a), and gain of carriers after recove

Optical properties of self assembled quantum dots (QDs) have dominant inhomogeneous broadening due to the distribution of size and composition of the QDs in the ensemble. The carrier dynamics in such systems is affected by the neighboring dots due to interdot diffusion of carriers. In this work we investigate the intraband relaxation mechanisms in the QDs. We performed time-resolved photoluminescence (PL) quenching measurements on InAs/GaAs self assembled QDs. The samples were excited by near infrared laser pulses and the time and wavelength resolved PL was measured by a streak camera. During the decay of the PL, a THz pulse from a free electron laser, tuned to the intersublevel transition energy of the QDs, was made incident on the sample. This THz pulse induced quenching of the PL, shown by the dip in Fig.1, by re-exciting carriers to higher levels within the QDs. These carriers eventually relaxed back to the ground state (s-state) resulting in the recovery of the s-state PL intensity. Thus, the recovery of the PL was directly related to carrier relaxation dynamics in the QDs. The mechanisms involved were (i) ISL relaxation, where the excited carriers in the QDs directly fell back into the s-state of the same dot and (ii) transfer to adjacent dots via the wetting layer by multiphoton absorption or tunneling. To distinguish these two effects we performed measurements on two samples grown simultaneously but post-growth annealed at different temperatures resulting in different ISL relaxation times of 60 ps and 1.5 ns. From rate equation model fit of the measured data we found that the recovery time of the quenched PL was independent of the ISL relaxation times. This implied that the carrier relaxation within the dots was dominated by mechanisms other than ISL transition. Comparison of the amount of recovered PL signals for different emission energies showed that there was loss of carriers at higher PL energies, as showed in Fig.1(a), and gain of carriers after recove

Functional dye molecules, such as porphyrins, attached to CdSe quantum dots (QDs) through anchoring meso-pyridyl substituents, form quasi-stable nanoassemblies. This fact results in photoluminescence (PL) quenching of the QDs both due to Forster resonance energy transfer (FRET) and the formation of non-radiative surface states under conditions of quantum confinement (non-FRET). The formation process is in competition with the ligand dynamics. At least two timescales are found for the formation of the assemblies: 1) one faster than 60 s attributed to saturation of empty attachment sites and 2) one slower than 600 s, which is attributed to a reorganisation of the tri-n-octylphosphine oxide (TOPO) ligand shell. Finally, this process results in almost complete exchange of the TOPO shell by porphyrin dye molecules. Following a SternVolmer analysis, we established a microscopic description of PL quenching and assembly formation. Based on this formalism, we determined the equilibrium constant for assembly formation between QDs and the pyridyl-functionalised dye molecules to be K approximate to 10(5)-10(7) M-1, which is several orders of magnitude larger than that of the TOPO ligands. Our results give additional insights into the non-FRET PL quenching processes involved and show that the QD surface is inhomogeneous with respect to the involved attachment and detachment processes. In comparison with other methods, such as NMR spectroscopy, the advantage of our approach is that ligand dynamics can be investigated at extremely low ratios of dye molecules to QDs., Funding Agencies|Volkswagen foundation|I/79435|German Science Foundation (DFG, Graduate College)|829|INTAS|R.03-05-4540|BRFFI|Phi10CO-005|German Academic Exchange Service (DAAD)|325|RISE

Carrier dynamics and relaxation processes in self assembled quantum dots (QDs) are of fundamental interest due to their influences on the efficiency and performances of optoelectronic devices [1]. The intersublevel relaxation mechanisms influence the temporal response of the photoluminescence (PL) [2]. It is therefore interesting to study the carrier relaxation in a series of QD samples where the intersublevel separation varies, resulting in different relaxation times. In this paper we present our work on time-resolved PL quenching measurements on QD ensembles using terahertz pulses, to study the effect of carrier redistribution on PL. Interband quasi-resonant excitation was done by a Ti-Sapphire laser and the PL emission was measured using a spectrometer coupled to a streak camera. Terahertz pulses were obtained from a Free Electron Laser (FEL) synchronized to the Ti:Sapphire laser. The FEL wavelengths were tuned to excite intersublevel transitions in the QDs (ranging from 23 meV to 15 meV) which caused partial depletion of the electronic ground state resulting in quenching of the interband PL. The samples studied consisted of a series of self-assembled InAs/GaAs QDs for which the intersublevel relaxation times varied from few ps to ns, as a result of thermal annealing [3]. Simultaneous time and wavelength resolved measurements enabled us to study the carrier redistribution by the terahertz pulse and their dynamics. PL measurements were done with and without FEL pulses. Figure 1 shows the calculated difference of two such streak camera images measured for a typical QD sample. The blue regions show the PL quenching dip caused by the FEL excitation for two interband transition energies corresponding to the QD ensemble. The data was fitted using exponential functions convoluted with a Gaussian system response. From the fitting parameters the PL quenching depth and recovery times were extracted. The recovery times were found to be significantly shorter than the intersu

Carrier dynamics and relaxation processes in self assembled quantum dots (QDs) are of fundamental interest due to their influences on the efficiency and performances of optoelectronic devices [1]. The intersublevel relaxation mechanisms influence the temporal response of the photoluminescence (PL) [2]. It is therefore interesting to study the carrier relaxation in a series of QD samples where the intersublevel separation varies, resulting in different relaxation times. In this paper we present our work on time-resolved PL quenching measurements on QD ensembles using terahertz pulses, to study the effect of carrier redistribution on PL. Interband quasi-resonant excitation was done by a Ti-Sapphire laser and the PL emission was measured using a spectrometer coupled to a streak camera. Terahertz pulses were obtained from a Free Electron Laser (FEL) synchronized to the Ti:Sapphire laser. The FEL wavelengths were tuned to excite intersublevel transitions in the QDs (ranging from 23 meV to 15 meV) which caused partial depletion of the electronic ground state resulting in quenching of the interband PL. The samples studied consisted of a series of self-assembled InAs/GaAs QDs for which the intersublevel relaxation times varied from few ps to ns, as a result of thermal annealing [3]. Simultaneous time and wavelength resolved measurements enabled us to study the carrier redistribution by the terahertz pulse and their dynamics. PL measurements were done with and without FEL pulses. Figure 1 shows the calculated difference of two such streak camera images measured for a typical QD sample. The blue regions show the PL quenching dip caused by the FEL excitation for two interband transition energies corresponding to the QD ensemble. The data was fitted using exponential functions convoluted with a Gaussian system response. From the fitting parameters the PL quenching depth and recovery times were extracted. The recovery times were found to be significantly shorter than the intersu

Carrier dynamics and relaxation processes in self assembled quantum dots (QDs) are of fundamental interest due to their influences on the efficiency and performances of optoelectronic devices [1]. The intersublevel relaxation mechanisms influence the temporal response of the photoluminescence (PL) [2]. It is therefore interesting to study the carrier relaxation in a series of QD samples where the intersublevel separation varies, resulting in different relaxation times. In this paper we present our work on time-resolved PL quenching measurements on QD ensembles using terahertz pulses, to study the effect of carrier redistribution on PL. Interband quasi-resonant excitation was done by a Ti-Sapphire laser and the PL emission was measured using a spectrometer coupled to a streak camera. Terahertz pulses were obtained from a Free Electron Laser (FEL) synchronized to the Ti:Sapphire laser. The FEL wavelengths were tuned to excite intersublevel transitions in the QDs (ranging from 23 meV to 15 meV) which caused partial depletion of the electronic ground state resulting in quenching of the interband PL. The samples studied consisted of a series of self-assembled InAs/GaAs QDs for which the intersublevel relaxation times varied from few ps to ns, as a result of thermal annealing [3]. Simultaneous time and wavelength resolved measurements enabled us to study the carrier redistribution by the terahertz pulse and their dynamics. PL measurements were done with and without FEL pulses. Figure 1 shows the calculated difference of two such streak camera images measured for a typical QD sample. The blue regions show the PL quenching dip caused by the FEL excitation for two interband transition energies corresponding to the QD ensemble. The data was fitted using exponential functions convoluted with a Gaussian system response. From the fitting parameters the PL quenching depth and recovery times were extracted. The recovery times were found to be significantly shorter than the intersu

Time and wavelength resolved spectroscopy requires optical sources emitting very short pulses and a fast detection mechanism capable of measuring the evolution of the output spectrum as a function of time. We use table-top Ti:sapphire lasers and a free-electron laser (FEL) emitting ps pulses as excitation sources and a streak camera coupled to a spectrometer for detection. One of the major aspects of this setup is the synchronization of pulses from the two lasers which we describe in detail. Optical properties of the FEL pulses are studied by autocorrelation and electro-optic sampling measurements. We discuss the advantages of using this setup to perform photoluminescence quenching in semiconductor quantum wells and quantum dots. Carrier redistribution due to pulsed excitation in these heterostructures can be investigated directly. Sideband generation in quantum wells is also studied where the intense FEL pulses facilitate the detection of the otherwise weak nonlinear effect.

We investigate the time-resolved photoluminescence (PL) dynamics of an undoped GaAs/AlGaAs multiple quantum well under mid-infrared (MIR) irradiation. A time-delayed MIR laser pulse from a free electron laser, tuned to the intersubband transition energy of the quantum well, induces temporal quenching of the PL intensity with subsequent recovery. The experimental data can be accurately described by a simple rate-equation model, which accounts for the cooling of the non-radiative states to radiative states. By performing polarization sensitive measurements, we are able to discriminate the contributions of free-carrier absorption from that of intersubband absorption, where the latter is about 50 times more efficient.

Carrier dynamics and relaxation processes in self assembled quantum dots (QDs) are of fundamental interest due to their influences on the efficiency and performances of optoelectronic devices [1]. The intersublevel relaxation mechanisms influence the temporal response of the photoluminescence (PL) [2]. It is therefore interesting to study the carrier relaxation in a series of QD samples where the intersublevel separation varies, resulting in different relaxation times. In this paper we present our work on time-resolved PL quenching measurements on QD ensembles using terahertz pulses, to study the effect of carrier redistribution on PL. Interband quasi-resonant excitation was done by a Ti-Sapphire laser and the PL emission was measured using a spectrometer coupled to a streak camera. Terahertz pulses were obtained from a Free Electron Laser (FEL) synchronized to the Ti:Sapphire laser. The FEL wavelengths were tuned to excite intersublevel transitions in the QDs (ranging from 23 meV to 15 meV) which caused partial depletion of the electronic ground state resulting in quenching of the interband PL. The samples studied consisted of a series of self-assembled InAs/GaAs QDs for which the intersublevel relaxation times varied from few ps to ns, as a result of thermal annealing [3]. Simultaneous time and wavelength resolved measurements enabled us to study the carrier redistribution by the terahertz pulse and their dynamics. PL measurements were done with and without FEL pulses. Figure 1 shows the calculated difference of two such streak camera images measured for a typical QD sample. The blue regions show the PL quenching dip caused by the FEL excitation for two interband transition energies corresponding to the QD ensemble. The data was fitted using exponential functions convoluted with a Gaussian system response. From the fitting parameters the PL quenching depth and recovery times were extracted. The recovery times were found to be significantly shorter than the intersu

Optical properties and interdot transfer dynamics of trapped carriers in InGaAs quantum dots (QDs) are investigated. Time resolved photoluminescence (PL) was measured for time-delayed interband and intraband excitations. Terahertz activated luminescence (TAL) from trapped carriers having lifetimes of ~250 ns at 8 K, was observed. Spectral shift of the TAL with respect to the PL showed the trionic nature of the PL in the n-doped QDs. With increasing terahertz excitation intensity, the TAL increased and reached saturation. The activation energy associated with the trapped carrier decay was quite close to the intersublevel transition energy (~20 meV) indicating trapping in the QDs.

The importance of the interracial properties in organic light emitting devices (OLED) is well recognized. We have studied the formation of interfaces that occurs in OLEDs using mainly surface/interface analytical techniques in a well controlled ultra high vacuum (UHV) environment. The results have shown that microscopic surface and interface properties are intimately related to the device characteristics and performance. Specifically, the metal electrode material is observed to quench strongly the luminescence of the organic material in the interface region. Proper treatments of the interface may at least partially recover the quenched luminescence. The implications of these results in the design and operation of organic light-emitting devices are discussed.

The importance of the interracial properties in organic light emitting devices (OLED) is well recognized. We have studied the formation of interfaces that occurs in OLEDs using mainly surface/interface analytical techniques in a well controlled ultra high vacuum (UHV) environment. The results have shown that microscopic surface and interface properties are intimately related to the device characteristics and performance. Specifically, the metal electrode material is observed to quench strongly the luminescence of the organic material in the interface region. Proper treatments of the interface may at least partially recover the quenched luminescence. The implications of these results in the design and operation of organic light-emitting devices are discussed.

The importance of the interracial properties in organic light emitting devices (OLED) is well recognized. We have studied the formation of interfaces that occurs in OLEDs using mainly surface/interface analytical techniques in a well controlled ultra high vacuum (UHV) environment. The results have shown that microscopic surface and interface properties are intimately related to the device characteristics and performance. Specifically, the metal electrode material is observed to quench strongly the luminescence of the organic material in the interface region. Proper treatments of the interface may at least partially recover the quenched luminescence. The implications of these results in the design and operation of organic light-emitting devices are discussed.

In this paper, we review several aspects of molecular recognition (based on non-covalent binding interactions) occurring between meso-pyridyl substituted tetrapyrrole extra-ligands and chemical dimers of tetrapyrrolic macrocycles containing central Zn ions and spacers of various nature and flexibility. Experimental results obtained by us earlier are analyzed using a novel approach (based on steady-state absorption/fluorescence measurements) for the evaluation of complexation constants KC for the formation of porphyrin triads. It was found that KC values [KC ~ (0.5 – 70) × 10⁶ M⁻¹] show noticeable dependence on the structural parameters of the interacting subunits as well as on the solvent nature. The same self-assembly approach has been used to attach meso-pyridyl substituted porphyrins to the surface of semiconductor CdSe/ZnS quantum dots (QD). It was comparatively found that in contrast to self-assembled porphyrin triads, the formation of “QD-porphyrin” nanoassemblies takes place in competition with surface stabilizing tri-n-octyl phosphine oxide (TOPO) ligand molecules and attached porphyrin molecules. It manifests in a temporal dynamics of QD photoluminescence caused by ligand exchange, TOPO layer reorganization, QD surface reconstruction, solvent properties. It was shown that the sensitivity of QD surface morphology to attached organic ligands (e.g. porphyrins) provides an opportunity to control the dynamics and pathways of the exciton relaxation in “QD-dye” nanoassemblies by changing the structure and electronic properties of these ligands.

Dye molecules with pyridyl side substituents (porphyrins and heterocyclic perylene diimides) coordinatively attached to semiconductor CdSe/ZnS quantum dots (QDs) surface form quasi-stable “QD-Dye” nanocomposites of various geometry in the competition with capping molecules (tri-n-octyl phosphine oxide or long chain amines) exchange. This results in photoluminescence (PL) quenching of the QDs both due to Foerster resonance energy transfer and formation of non-radiative surface states. QD surface is inhomogeneous with respect to the involved attachment and detachment processes. The formation of “QD-Porphyrin” nanocomposites is realized at least two time scales (60 and 600 s), which is attributed to a reorganisation of tri-n-octylphosphine oxide capping shell. In a low temperature range of 220÷240 K related changes in QD absorption and emission reveal a phase transition of the capping shell (tri-n-octyl phosphine oxide and amine). In “QD-Dye” nanocomposites, this phase transition is enhanced considerably by only a few attached dye molecules and has impact on the QD core structure followed by changes of PL quenching and exciton-phonon coupling. A combination of ensemble and single molecule spectroscopy of “QD-Dye” nanocomposites reveals that few or even only one attached dye molecule change the surface distribution and energy of dye related surface trap states considerably.

Substrates containing zinc oxide and different amounts of gadolinium on the surface of the zinc oxide nano particles for biosensorics were obtained for the first time by the standard sol-gel technique. This is a new material has not yet been extensively studied. In this paper, we present the results of the photoluminescent response of the zinc oxide surface when it is modified by various amounts of gadolinium during the classical sol-gel method of the surface obtaining. Aspects of creating a modified surface of zinc oxide are highlighted. The used concentrations of gadolinium vary from 1 to 33% caused both quenching and ignition of photoluminescence at the wavelength of 356 nm upon excitation into the absorption band of protein 280 nm. The photoluminescence intensity of zinc oxide without doping is 40 relative value units, the addition of 8% Gd increases the photoluminescence by 7 times (280 r.u.), the addition of 31% Gd decreases the photoluminescence by 40 times compared to undoped zinc oxide. The resulting possibility of increased luminescence when the doping of the films with gadolinium willing, apparently, to increase the ratio “signal/background”, and therefore increase the sensitivity of the biosensor with fluorescent registration. © 2018 TANGER Ltd. All Rights Reserved.