Singlet Exciton Lifetime Research Articles

Conjugated Polymer Nanoparticles: Photothermal and Photodynamic Capabilities According to Molecular Ordering in Their Assembly Structures

We report that conjugated polymer nanoparticles (CPNs) coated with polyethylene glycols (PEGs) exhibit photothermal and photodynamic capabilities according to molecular ordering in their assembly structures. CPN-PEGs were made using three different methods: a dispersion process of phase-separated film assemblies of a conjugated polymer and a phospholipid-conjugated PEG (CPN-I), a dispersion process of a conjugated polymer and a phospholipid followed by surface conjugation with PEGs (CPN-II), and a miniemulsification of the conjugated polymer and the phospholipid-conjugated PEG. Our findings revealed that the ordered molecular assembly structures in CPN-I and CPN-II increased intermolecular interactions and decreased the optical band gap, promoting nonradiative exciton relaxation via the energy-gap law’s internal conversion mechanism and rationalizing CPN-I’s shorter singlet exciton lifetime (13 ps). Meanwhile, CPN-III with a disordered structure generated more singlet oxygen than CPN-I and CPN-II, indicating increased triplet exciton generation upon the polaron recombination. Our findings present that the photothermal and photodynamic properties of CPNs are obviously dependent on the assembly structure order and that CPNs with an ordered assembly of conjugated backbones have a stronger photothermal effect, whereas those with a disordered structure have a better photodynamic effect.

Tracking Exciton Diffusion and Exciton Annihilation in Single Nanoparticles of Conjugated Polymers by Photon Correlation Spectroscopy

AbstractA fundamental question relating to the nature of light emission and absorption in organic semiconductors is the dimension of the domain within a bulk material responsible for the interaction of light and matter. How large can a nanoparticle become to retain the quantized nature of light emission? Excitons are only a few nanometers in size, but because they diffuse in space, they probe a much larger volume than the single molecule. When excitons meet, they may decay non‐radiatively by singlet–singlet or singlet–triplet annihilation (SSA or STA). Fluorescence photon statistics reveal whether single photons are emitted (photon antibunching) or arrive in randomly spaced packets (photon bunching), offering direct insight into excitonic mobility. Single multichain nanoparticles of ladder‐type poly(para‐phenylene) (LPPP) are examined. The effect of SSA and STA is seen in the photon antibunching and bunching, respectively, which both decrease in fidelity as the size of the nanoparticle increases. Time resolving the photon correlation measurement yields microscopic annihilation rates for SSA and STA in agreement with values obtained from bulk LPPP films. Even though triplets in LPPP are known to be highly mobile, the results show that, on the timescale of the singlet exciton lifetime, triplet diffusion is not of significance in the STA process.

Excited-state properties of Y-series small molecule semiconductors

The emergence of the Y series small molecule semiconductors, Y6 and its derivatives, have significantly improved the performance of polymer solar cells (PSCs). However, the excited-state properties of these Y-series small molecule semiconductors which are highly important for designing high-performance PSCs, need to be illustrated. In this work, the excited-state properties and electronic structures of the Y-series small molecules (Y5, Y6, Y10, N3, Y6-BO-4F, and Y6-BO-4Cl) have been systematically studied by using steady-state and time-resolved spectroscopies and quantum chemical calculations. It is shown that the influence of alkyl chains at the nitrogen atom of the pyrrole ring is weak for the electron affinities, ionization potentials, electron and hole reorganization energies and singlet exciton lifetime of Y molecules. Meanwhile, these parameters are found to be varied with the types of electron-deficient termini. Moreover, we find that Y10 and Y5 have the shortest singlet exciton lifetime in solution and the longest singlet exciton lifetime in film (~1100 ps), suggesting the engineering of electron-deficient termini can significantly influence the excited-state lifetime in solution and film. Our work could provide a guideline for designing Y-series acceptor materials for high-performance polymer solar cells.

Prolongation of the singlet exciton lifetime of nonfullerene acceptor films by the replacement of the central benzene core with naphthalene

The replacement of benzene with naphthalene in the central core of an acceptor achieved a longer singlet lifetime and a higher power conversion efficiency.

Efficient Exciton Diffusion in Micrometer-Sized Domains of Nanographene-Based Nonfullerene Acceptors with Long Exciton Lifetimes in Blend Films with Conjugated Polymer.

Phase-separated structures in photoactive layers composed of electron donors and acceptors in organic photovoltaics (OPVs) generally exert a profound impact on the device performance. In this study, nonfullerene acceptors (NFAs) where a heteronanographene central core was furnished with branched alkoxy chains of different lengths, TACIC-EH, TACIC-BO, and TACIC-HD, were prepared to adjust the aggregation tendency and systematically probe the relationships of film structures with photophysical and photovoltaic properties. The side-chain length showed negligible effects on the absorption properties and energy levels of TACICs. In addition, regardless of the chain length, all TACIC films exhibited characteristically long singlet exciton lifetimes (1330-2330 ps) compared to those in solution (≤220 ps). Using a conjugated polymer donor, PBDB-T, the best OPV performance was achieved with TACIC-BO that contained medium-length chains, exhibiting a power conversion efficiency (PCE) of 9.92%. TACIC-HD with the longest chains showed deteriorated electron mobility due to the long insulating alkoxy groups. Therefore, the PBDB-T:TACIC-HD-based device revealed a low charge collection efficiency and PCE (8.21%) relative to the PBDB-T:TACIC-BO-based device, but their film morphologies were analogous. Meanwhile, TACIC-EH with the shortest chains showed low solubility and formed micrometer-sized large aggregates in the blend film with PBDB-T. Although the charge collection efficiency of PBDB-T:TACIC-EH was lower than that of PBDB-T:TACIC-BO, the efficiencies of exciton diffusion to the donor-acceptor interface were sufficiently high (>98%) owing to the elongated singlet exciton lifetime of TACIC-EH. The PCE of the PBDB-T:TACIC-EH-based device remained moderate (7.10%). Therefore, TACICs with the long singlet exciton lifetimes in the films provide a clear guideline for NFAs with low sensitivity of OPV device performance to the blend film structures, which is advantageous for large-scale OPV production with high reproducibility.

High Fluorescence Rate of Thermally Activated Delayed Fluorescence Emitters for Efficient and Stable Blue OLEDs.

A lack of an efficient and stable blue device is a critical factor restricting the development of organic light-emitting diode (OLED) technology that is currently expected to be overcome by employing thermally activated delayed fluorescence (TADF). Here, we investigate the TADF and electroluminescence (EL) performance of six carbazole/triphenyltriazine derivatives in different hosts. A good linearity between lg(LT50/kF2) and the EL emission wavelength is found, where LT50 is the half-life of the devices and kF is the fluorescence rate of the emitters, suggesting the dominance of the singlet exciton energy and lifetime in device stability. An indolylcarbazole/triphenyltriazine derivative (ICz-TRZ) with the capability to suppress solid-state solvation exhibits blue-shifted emission and an increased kF (1.5 × 108 s-1) in comparison to the control emitters in doped films. ICz-TRZ-based devices achieve a maximum external quantum efficiency (EQE) of 18% and an EQE of 5.5% at a very high luminance of 7 × 104 cd/m2. Ignoring the poor electrochemical stability of ICz-TRZ, the device offers an LT50 approaching 100 h under an initial luminance of 1000 cd/m2 and CIE coordinates of (0.14, 0.19). The findings in this work suggest that computer-aided design of high kF TADF emitters can be an approach to realize efficient and stable blue OLEDs.

Exploration of photophysics and presence of Long singlet exciton diffusion length in dibenz[a,h]anthracene nanoaggregates

Disc shape nanoaggregates of dibenz[a,h]anthracene (DBAn) was synthesized following reprecipitation method. This is a nonlinearly attached polyaromatic molecules which upon aggregation shows almost three times stronger absorption, associated with the lowest absorption band. This is governed by the stronger transition dipole, resulted due to correlated interaction of individual transition dipoles of DBAn molecules, present in the aggregate. The stronger transition dipole also results faster radiative decay rate and makes the nanoaggregates 2–3 times highly emissive, as compared to its monomeric form. Time resolved emission studies indicate both excitonic and excimeric contribution in the emission spectrum. The enhanced radiative decay rate, demonstrated in this aggregate, is popularly recognized as the superradiant phenomenon of molecular assembly. Even after the presence of faster radiative decay, these aggregates are found to possess extended singlet exciton lifetime of ∼10.2 ns. The long exciton lifetime consequences elongated singlet exciton diffusion length of ∼50 nm, estimated by analysing the exciton diffusion assisted energy transfer process in tetracene doped DBAn nanoaggregates. The long exciton lifetime is expected to be advantageous for charge separation in photovoltaic devices.

Time-resolved fluorescence decay and Gaussian analysis of P3HT-derived $$\hbox {Ho}^{3+}$$- and $$\hbox {Tm}^{3+}$$-doped ZnO nanostructures

The fluorescence vibrational features of as-synthesized P3HT–ZnO: $$\hbox {Ho}^{3+}$$ and P3HT–ZnO: $$\hbox {Tm}^{3+}$$ thin films were investigated using Gaussian analysis. Relative to P3HT–ZnO: $$\hbox {Tm}^{3+}$$ film, detailed Gaussian analysis of the fluorescence spectra revealed weaker intensity exhibited in P3HT–ZnO: $$\hbox {Ho}^{3+}$$ film due to better charge transfer. Moreover, we comparatively present the Huang–Rhys factor and relaxation energy of the samples, which are calculated using relations derived from the Franck–Condon theory. Furthermore, P3HT–ZnO: $$\hbox {Ho}^{3+}$$ film exhibits lower relaxation energy as compared with P3HT–ZnO: $$\hbox {Tm}^{3+}$$ film, which implies better conjugation length. Finally, the singlet exciton lifetime of P3HT–ZnO: $$\hbox {Ho}^{3+}$$ sample was found to be shorter as compared with P3HT–ZnO: $$\hbox {Tm}^{3+}$$ , while the calculated exciton diffusion length was 6.4 and 10.3 nm, respectively.

DNA-Templated Aggregates of Strongly Coupled Cyanine Dyes: Nonradiative Decay Governs Exciton Lifetimes.

Molecular excitons are used in a variety of applications including light harvesting, optoelectronics, and nanoscale computing. Controlled aggregation via covalent attachment of dyes to DNA templates is a promising aggregate assembly technique that enables the design of extended dye networks. However, there are few studies of exciton dynamics in DNA-templated dye aggregates. We report time-resolved excited-state dynamics measurements of two cyanine-based dye aggregates, a J-like dimer and an H-like tetramer, formed through DNA-templating of covalently attached dyes. Time-resolved fluorescence and transient absorption indicate that nonradiative decay, in the form of internal conversion, dominates the aggregate ground state recovery dynamics, with singlet exciton lifetimes on the order of tens of picoseconds for the aggregates versus nanoseconds for the monomer. These results highlight the importance of circumventing nonradiative decay pathways in the future design of DNA-templated dye aggregates.

Preferential Charge Generation at Aggregate Sites in Narrow Band Gap Infrared Photoresponsive Polymer Semiconductors

AbstractInfrared organic photodetector materials are investigated using transient absorption spectroscopy, demonstrating that ultrafast charge generation assisted by polymer aggregation is essential to compensate for the energy gap law, which dictates that excited state lifetimes decrease as the band gap narrows. Short sub‐picosecond singlet exciton lifetimes are measured in a structurally related series of infrared‐absorbing copolymers that consist of alternating cyclopentadithiophene electron‐rich “push” units and strong electron‐deficient “pull” units, including benzothiadiazole, benzoselenadiazole, pyridalselenadiazole, or thiadiazoloquinoxaline. While the ultrafast lifetimes of excitons localized on individual polymer chains suggest that charge carrier generation will be inefficient, high detectivity for polymer:PC71BM infrared photodetectors is measured in the 0.6 < λ < 1.5 µm range. The photophysical processes leading to charge generation are investigated by performing a global analysis on transient absorption data of blended polymer:PC71BM films. In these blends, charge carriers form primarily at polymer aggregate sites on the ultrafast time scale (within our instrument response), leaving quickly decaying single‐chain excitons unquenched. The results have important implications for the further development of organic infrared optoelectronic devices, where targeting processes such as excited state delocalization over aggregates may be necessary to mitigate losses to ultrafast exciton decay as materials with even lower band gaps are developed.

Interplay of Fluorescence and Phosphorescence in Organic Biluminescent Emitters.

Biluminescent organic emitters show simultaneous fluorescence and phosphorescence at room temperature. So far, the optimization of the room-temperature phosphorescence in these materials has drawn the attention of research. However, the continuous-wave operation of these emitters will consequently turn them into systems with vastly imbalanced singlet and triplet populations, which is due to the respective excited-state lifetimes. This study reports on the exciton dynamics of the biluminophore NPB (N,N′-di(1-naphthyl)-N,N′-diphenyl-(1,1-biphenyl)-4,4-diamine). In the extreme case, the singlet and triplet exciton lifetimes stretch from 3 ns to 300 ms, respectively. Through sample engineering and oxygen quenching experiments, the triplet exciton density can be controlled over several orders of magnitude, allowing us to study exciton interactions between singlet and triplet manifolds. The results show that singlet–triplet annihilation reduces the overall biluminescence efficiency already at moderate excitation levels. Additionally, the presented system represents an illustrative role model to study excitonic effects in organic materials.

Photophysical Study of DPPTT‐T/PC70BM Blends and Solar Devices as a Function of Fullerene Loading: An Insight into EQE Limitations of DPP‐Based Polymers

Diketopyrrolopyrrole (DPP)‐based polymers have been consistently used for the fabrication of solar cell devices and transistors due to the existence of intermolecular short contacts, resulting in high electron and hole mobilities. However, they also often show limited external quantum efficiencies (EQEs). In this contribution, the authors analyze the limitations on EQE by a combined study of exciton dissociation efficiency, charge separation, and recombination kinetics in thin films and solar devices of a DPP‐based donor polymer, DPPTT‐T (thieno[3,2‐b]thiophene‐diketopyrrolopyrrole copolymer) blended with varying weight fractions of the fullerene acceptor PC70BM. From the correlations between photoluminescence quenching, transient absorption studies, and EQE measurements, it is concluded that the main limitation of photon‐to‐charge conversion in DPPTT‐T/PC70BM devices is poor exciton dissociation. This exciton quenching limit is related not only to the low affinity/miscibility of the materials, as confirmed by wide angle X‐ray diffraction diffraction and transmission electron microscopy data, but also to the relatively short DPPTT‐T singlet exciton lifetime, possibly associated with high nonradiative losses. A further strategy to improve EQE in this class of polymers without sacrificing the good extraction properties in optimized blends is therefore to limit those nonradiative decay processes.

Structural Heterogeneity in the Localized Excited States of Poly(3-hexylthiophene).

Transient hole-burning and resonantly enhanced Raman spectroscopies are used to probe heterogeneities among localized singlet excitons of poly(3-hexylthiophene) in solution. Transient hole-burning spectroscopy facilitated by population dumping through wavelength-selective stimulated emission exposes inhomogeneous broadening of the exciton absorption band in the near-infrared, as reflected by correlations between stimulated emission and excited-state absorption transition energies. Dump-induced spectral diffusion of the exciton absorption band reflects structural fluctuations in the locally excited polymer. This diffusion is observed to occur slightly faster or slower than the nonequilibrium relaxation that follows direct excitation of the polymer (8-9 ps), with the time scale for diffusion varying with subpopulation: dumping across small vs large band gaps results in diffusion over 5 vs 35 ps, respectively. Furthermore, incomplete spectral relaxation of transient holes reflects that subsets of locally excited structural motifs prepared through photoexcitation cannot interchange through structural fluctuations that occur over the singlet-exciton lifetime. Raman spectra of the C═C/C-C stretching region collected in resonance at energies across the exciton absorption band exhibit frequency and intensity trends (Raman "dispersion") ascribed to variation in the local effective conjugation length. Together, results explicitly reveal heterogeneities among excitonic states associated with variations and fluctuations in local conformational order.

Singlet Exciton Lifetimes in Conjugated Polymer Films for Organic Solar Cells.

The lifetime of singlet excitons in conjugated polymer films is a key factor taken into account during organic solar cell device optimization. It determines the singlet exciton diffusion lengths in polymer films and has a direct impact on the photocurrent generation by organic solar cell devices. However, very little is known about the material properties controlling the lifetimes of singlet excitons, with most of our knowledge originating from studies of small organic molecules. Herein, we provide a brief summary of the nature of the excited states in conjugated polymer films and then present an analysis of the singlet exciton lifetimes of 16 semiconducting polymers. The exciton lifetimes of seven of the studied polymers were measured using ultrafast transient absorption spectroscopy and compared to the lifetimes of seven of the most common photoactive polymers found in the literature. A plot of the logarithm of the rate of exciton decay vs. the polymer optical bandgap reveals a medium correlation between lifetime and bandgap, thus suggesting that the Energy Gap Law may be valid for these systems. This therefore suggests that small bandgap polymers can suffer from short exciton lifetimes, which may limit their performance in organic solar cell devices. In addition, the impact of film crystallinity on the exciton lifetime was assessed for a small bandgap diketopyrrolopyrrole co-polymer. It is observed that the increase of polymer film crystallinity leads to reduction in exciton lifetime and optical bandgap again in agreement with the Energy Gap Law.

Near-Infrared Absorbing Thienoisoindigo-Based Copolymers for Organic Photovoltaics

We present bulk heterojunction organic photovoltaic (OPV) cells of novel electron accepting moiety, dithienoketopyrrole-based π-conjugated copolymers. An analog of isoindigo, thus so-called thienoisoindigo, was coupled to benzodithiophene, cyclopentadithiophene, and fluorene, affording a low bandgap and an appropriate LUMO level (−3.8 eV) for n-type methanofullerene (PCBM). A laser and Xe-flash-time-resolved microwave conductivity was extensively applied for guiding the processing conditions (p/n blend ratio, solvent, and thermal annealing), giving clearly defined optimized parameters. OPV performances are discussed in-depth in conjunction with atomic force microscope, grazing incidence X-ray diffraction, modulation of light intensity, field-effect transistor, and femtosecond transient absorption spectroscopy. We found that excessively shortened singlet exciton lifetime of polymer (<1 ps) competes with the photocurrent generation, which may arise from its near-infrared absorbing nature. The maximized power conversion efficiency was 1.4%, and there is still room for boosting the performance by rational design of counter donor unit and solubilizing substituents.

Femtosecond Dynamics of Excitons and Hole-Polarons in Composite P3HT/PCBM Nanoparticles

The dynamics of charge separation in aqueous suspensions of regioregular P3HT nanoparticles containing PCBM were investigated for the first time using femtosecond transient absorption spectroscopy. This investigation is supported by the recently reported use of regioregular P3HT/PCBM nanoparticles as charge trapping and storage devices. In this study, the presence of excited-state and charge-separated species, including singlet excitons, polymer polarons and free charges, generated in rr-P3HT/PCBM nanoparticles was identified through visible pump and visible/near-infrared probe femtosecond transient absorption spectroscopy at a range of electron acceptor concentrations. The decrease of the singlet exciton lifetime by charge transfer to PCBM is well described by a one-dimensional diffusion model with a P3HT domain size of approximately 5 nm for 5-50 wt % PCBM. This model also indicates that bimolecular recombination is the dominant charge recombination mechanism at 20 wt % PCBM and above.

Ultrafast dynamics of singlet-singlet and singlet-triplet exciton annihilation in poly(3-2′-methoxy-5′octylphenyl)thiophene films

Singlet-singlet (S-S) and singlet-triplet (S-T) exciton annihilation was studied in poly(3-2(')-methoxy-5(')octylphenyl)thiophene films. For the S-S exciton annihilation studies, transient absorption spectroscopy at excitation laser pulse fluences of 1.2x10(13)-4.4x10(14) photons/cm(2) and 2.5 kHz pulse repetition rate was applied. The obtained kinetics demonstrate a typical nonexponential character with intensity-dependent amplitudes and lifetimes. In time-resolved fluorescence experiments, low excitation pulse fluences of 1.6x10(9)-2.2x10(12) photons/cm(2) at high repetition rates of 0.4, 0.8, 4, and 81 MHz lead to S-T exciton annihilation as a result of triplet exciton accumulation. S-T annihilation kinetics results in monoexponential decay of the fluorescence kinetics and manifests itself as a decrease of the singlet exciton lifetime. The calculated time-independent S-S and S-T exciton annihilation rates strongly support the conclusion that the processes are controlled by the interchain diffusion of singlet excitons. Despite the low efficiency of S-T annihilation compared to that of S-S annihilation, it has a substantial effect on the singlet exciton lifetime due to a relatively long triplet lifetime (60 mu s). Thus, even optical excitation with low fluence at high pulse repetition rate creates a significant concentration of triplet states that efficiently quenches singlet excitons. (Less)

Nanosecond QFRS study of photoluminescence in amorphous semiconductors

The paper is concerned with the study of the photoluminescence (PL) lifetime distribution of a-Si : H and a-Ge : H, down to the nanosecond (ns) region using a newly developed dual-phase, double lock-in (DPDL) quadrature frequency-resolved spectroscopy (QFRS). The DPDL-QFRS results show no indication of a peak in the ns region, and the PL lifetime distribution is basically double-peaked with slow (≈ms) and fast (≈μs) components, as reported earlier. The double-peak lifetime, a blue-shift of the fast component spectrum against the slow one, a dependence of the radiative transition rate on the PL emission energy and a fast lifetime (∼μs, this being much longer than singlet-exciton lifetime normally (∼ns)) can be well explained by invoking self-trapped excitons. A singlet–triplet exchange energy ∼40 meV is deduced for a-Si : H. The dependence of the lifetime distribution on the generation rate is discussed on the basis of the non-geminate recombination, where non-geminate pairs prevail over excitons under strong photoexcitation conditions.

Influence of the degree of conjugation on excited state lifetimes in phenylene-based materials

Phenylene-based materials are of particular interest for applications in organic luminescent devices. To gain deeper insight into the emissive properties of this class of materials, we have systematically studied the relation between the degree of conjugation and singlet exciton lifetimes. Two classes of phenylenes are investigated by time resolved frequency domain spectroscopy: oligo- para-phenylenes (OPPs) with n-alkyl substitutions at the central phenylene ring of the molecule and ladder-type-oligo- para-phenylenes (LOPPs) featuring rigid backbones with fixed interring twist angles. Additional quantum yield (QY) measurements allow to assess the intrinsic lifetime of the radiative decay. The experimental data are compared to the results of quantum-chemical calculations yielding an excellent agreement for both the classes of materials.

Influence of the Chemical Structure on the Luminescence Properties of Organic Dye Molecules

ABSTRACTIn this contribution we compare experimental investigations (of photoluminescence, absorption and modulation spectroscopy) for a number of novel dye molecules to quantum-chemical simulations. The investigated materials contain phenylene-, phenylenevinylene-, naphthylene- and anthrylene units linked either by saturated or non saturated bonds. In the first part of the paper we give a short overview of the principal optical properties of the investigated molecules including a discussion of exciton localization effects. The latter can be accomplished by studying the geometry modifications in the excited state relative to the ground state. To do so, we couple the Austin Model 1 (AM1) approach to a multi-electron configuration interaction technique (MECI). The optical spectra are subsequently obtained from the Intermediate Neglect of Differential Overlap (INDO) Hamiltonian combined with a Single Configuration Interaction (SCI) approach. In the main section of this contribution we show the results of singlet exciton lifetime measurements performed with a modulation technique. An excellent agreement was found between the experimental data and quantum-chemical simulations for the transition dipole moments