Understanding singlet fission, a charge carrier multiplication process, and how the singlet fission and the competing processes can be manipulated with external parameters within the same material system is of considerable interest for enhancing optoelectronic device performance. Exciton polaritons, formed by strong exciton-photon coupling in organic films in microcavities, have been shown to manipulate the energy landscape that may be used to control the photophysics and photochemistry in existing singlet fission materials. To efficiently utilize exciton polariton formation to enhance singlet fission and suppress competing processes such as relaxation into low-energy trap states, it is necessary to establish how the properties of polaritons in singlet fission materials depend on molecular photophysics and microcavity configurations. We present a systematic study of strong coupling in functionalized tetracene (R-Tc), and how it affects its photophysics and photochemistry, depending on film morphology, placement in the cavity (to achieve various degrees of overlap with the cavity electric field), and cavity design. We probe cavity-coupled and uncoupled molecular populations and examine the effects of intermolecular interactions on the excited state dynamics and polariton formation and properties. By varying magnetic field, we create different excited states relaxation scenarios and determine how the polariton states participate in the competition between the singlet fission and relaxation into trap states. We observe magnetic field-enhanced emission from exciton and polariton states and cavity-suppressed emission from low-energy trap states. We also report on effects of polariton formation on photodimerization of R-Tc and discuss how concurrent studies of photochemistry and photophysics promote understanding of singlet fission and polariton formation through the evolution of excited states during photodegradation.
Singlet fission (SF) is a charge carrier multiplication process that can occur in organic semiconductors and has potential to enhance (opto)electronic device performance. We examine how SF depends on molecular packing with functionalized tetracene (R-Tc) crystals which have the same monomer properties but different crystal packings with ‘slip-stack’ (R=TES) and ‘gamma’ (R=TBDMS) packing structures. Using temperature-dependent photoluminescence spectroscopy, we find that the triplet pair state (TT) in R-Tc systems under study is non-emissive, and the PL is dominated by that from lowenergy emissive trap states in TES-Tc and from aggregate states in TBDMS-Tc, with the emissive channels competing with SF. We also study the effects of photodegradation from endoperoxide formation on R-Tc and the relationship between photodegradation and SF and find that the ‘gamma’-packed TBDMS-Tc is more photostable than the ‘slip-stacked’ TESTc derivative. To analyze SF and competitive pathways, we constructed a 4-state kinetic model to reproduce the observed PL data, which predicts maximum SF free triplet yields of 190% for TES-Tc and 185% for TBDMS-Tc at room temperature.
We present a study of photodegradation in functionalized anthraditiophene (diF TES-ADT)-based films in hybrid cavities composed of distributed Bragg reflector (DBR) bottom mirror and top silver (Ag) mirror, depending on two different DBR reflectivity stopbands. The concentration of diF TES-ADT was chosen such that the films contain molecules in “amorphous” and “aggregate” phases characterized by different optical absorption spectra. We demonstrate that the cavity photon preferentially couples to the molecules in the “amorphous” phase and observe different photodegradation, attributed to photodimerization, dynamics for cavity photon-coupled and uncoupled states. The presence of the longer lived emissive states, such as entangled triplet pair (TT) states formed in the process of singlet fission, was correlated with a faster photodimerization dynamics. The observed cavity-dependent photodimerization dynamics offers an opportunity to manipulate the reaction rates and yield with cavity characteristics.
Degradation of organic semiconductors in the presence of oxygen is one of the bottlenecks preventing their wide-spread use in optoelectronic devices. The first step towards such degradation in functionalized pentacene (Pn) derivatives is formation of endoperoxide (EPO), which can either revert back to the parent molecule or proceed to molecule decomposition. We present the study of reversibility of EPO formation through probing the photophysical properties of functionalized fluorinated pentacene (Pn-R-F8) derivatives. Experiments are done in solutions and in films both at the single molecule level and in the bulk. In solutions, degradation of optical absorption and its partial recovery after thermolysis were quantified for various derivatives depending on the solvent. At the single molecule level, low concentrations of each type of molecules were imaged in a variety of polymer matrices at 633 nm excitation at room temperature in air using wide-field fluorescence microscopy. Fluorescence time trajectories were collected and statistically analyzed to quantify blinking due to reversible EPO formation depending on the host matrix. To understand the physical changes of the molecular system, a Monte Carlo method was used to create a multi-level simulation, which enabled us to relate the change in the molecular transition rates to the experimentally measured parameters. At the bulk level, photoluminescence decay due to photobleaching and recovery due to EPO reconversion were measured for the same derivatives incorporated into various matrices. These studies provide insight into the synergistic effect of the local nanoenvironment and molecular side groups on the oxygen-related degradation and subsequent recovery which is important for development of organic electronic devices.
We present photophysical and optoelectronic properties of xylindein and optical properties of two other fungi-derived organic pigments. Photophysics of these materials is determined by the interplay of inter- and intramolecular hydrogen bonding, which was systematically explored using absorption and photoluminescence spectroscopy of xylindein in various solutions, pH buffers, and in the solid state. Amorphous xylindein films yielded a lower bound on the charge carrier mobility of 0.2-0.5 cm2=(V•s) and exhibited photocurrent upon photoexcitation in the ultraviolet and visible wavelength range. Thermal and photostability of xylindein was also characterized, and it considerably exceeded that of conventional organic semiconductors such as pentacene derivatives.
KEYWORDS: Molecules, Monte Carlo methods, Molecular interactions, Single molecule spectroscopy, Organic semiconductors, Process modeling, Organic materials, Solar cells, Excitons, Carrier dynamics, Luminescence, Polymethylmethacrylate, Molecular spectroscopy, Polymers, Data modeling
Organic semiconductors have attracted considerable attention due to their applications in low-cost (opto)electronic devices. The most successful organic materials for applications that rely on charge carrier generation, such as solar cells, utilize blends of several types of molecules. In blends, the local environment strongly influences exciton and charge carrier dynamics. However, relationship between nanoscale features and photophysics is difficult to establish due to the lack of necessary spatial resolution. We use functionalized fluorinated pentacene (Pn) molecule as single molecule probes of intermolecular interactions and of the nanoscale environment in blends containing donor and acceptor molecules. Single Pn donor (D) molecules were imaged in PMMA in the presence of acceptor (A) molecules using wide-field fluorescence microscopy. Two sample configurations were realized: (i) a fixed concentration of Pn donor molecules, with increasing concentration of acceptor molecules (functionalized indenflouorene or PCBM) and (ii) a fixed concentration of acceptor molecules with an increased concentration of the Pn donor. The D-A energy transfer and changes in the donor emission due to those in the acceptor- modified polymer morphology were quantified. The increase in the acceptor concentration was accompanied by enhanced photobleaching and blinking of the Pn donor molecules. To better understand the underlying physics of these processes, we modeled photoexcited electron dynamics using Monte Carlo simulations. The simulated blinking dynamics were then compared to our experimental data, and the changes in the transition rates were related to the changes in the nanoscale environment. Our study provides insight into evolution of nanoscale environment during the formation of bulk heterojunctions.
We present a technique to study the (dis)charging of organic semiconductor films at microscopic scales, and in various environments, using an optical tweezers-based method combined with fluorescence spectroscopy. The 1 µm silica spheres were coated with either pristine organic semiconductor or a donor-acceptor blend, trapped using optical tweezers, and their fluorescence was measured concurrently with the effective surface charge. The effective surface charge in uncoated silica spheres suspended in water was a factor of ∼70 higher as compared to that from similar spheres in a nonpolar toluene. In contrast, the coated silica spheres exhibited low effective charge densities in both environments, which is indicative of minimal interaction of organic semiconductors under study with these environments. This serves as a proof-of-principle experiment towards systematic studies of nanoscale photoinduced charge-based interactions between organic semiconductor molecules, with a resolution down to an elementary charge, and depending on the dielectric environment.
We present photophysical properties of functionalized anthradithiophene (ADT) and pentacene (Pn) derivatives, as well as charge and energy transfer properties of donor-acceptor (D/A) pairs of these derivatives. All molecules studied were fluorescent and photostable enough to be imaged on the single-molecule level in a variety of polymeric and in a functionalized benzothiophene (BTBTB) crystalline host using room-temperature wide- field epifluorescence microscopy. Flexibility of functionalization of both guest (ADT, Pn) and host (BTBTB or polymer) molecules can be used for systematic studies of nanoscale morphology and photophysics of D/A organic semiconductor bulk heterojunctions, as well as in applications relying on FRET, using single-molecule fluorescence microscopy.
We explored relationships between photophysical processes and solar cell characteristics in solution-processable bulk heterojunctions (BHJs), in particular: (1) polymer donor:fullerene acceptor:small-molecule (SM) nonfullerene acceptor, (2) polymer donor:SM donor:SM nonfullerene acceptor, and (3) SM donor:SM nonfullerene or fullerene acceptor. Addition of a nonfullerene SM acceptor to “efficient” polymer:fullerene BHJs led to a reduction in power conversion efficiency (PCE), mostly due to decreased charge photogeneration efficiency and increased disorder. By contrast, addition of an SM donor to “inefficient” polymer:SM nonfullerene acceptor BHJs led to a factor of two to three improvement in the PCE, due to improved charge photogeneration efficiency and transport. In most blends, exciplex formation was observed and correlated with a reduced short-circuit current (Jsc) without negatively impacting the open-circuit voltage (Voc). A factor of ∼5 higher PCE was observed in SM donor:fullerene acceptor BHJs as compared to SMBHJs with the same SM donor but nonfullerene acceptor, due to enhanced charge carrier photogeneration in the blend with fullerene. Our study revealed that the HOMO and LUMO energies of molecules comprising a blend are not reliable parameters for predicting Voc of the blend, and an understanding of the photophysics is necessary for interpreting solar cell characteristics and improving the molecular design of BHJs.
An exciting application of optical tweezers is the measurement of the surface charge on a trapped particle, as well
as its time evolution with a single charge resolution. We report on an optical tweezer-based method to measure
the effective surface charge on an organic semiconductor film at microscopic scales, which offers opportunities for
investigations of ion and electron transfer between organic molecules and surrounding medium. Effective charge
densities of 13±5 elementary charges per μm2were observed in anthradithiophene-coated silica microspheres suspended in water, with a more than an order of magnitude reduction in charge densities upon replacing water
with the 50% wt/wt glycerol/water mixture.
We performed numerical simulations of transient photocurrents in organic thin films, in conjunction with experiments. This enabled us to quantify the contribution of multiple charge generation pathways to charge carrier photogeneration, as well as extract parameters that characterize charge transport, in functionalized anthradithiophene (ADT-TES-F) films prepared using two different deposition methods: drop casting on an untreated substrate and spin casting on a pentauorobenzenethiol (PFBT)-treated substrate. These deposition methods yielded polycrystalline lms with considerably larger grain sizes in the case of the spin cast lm. In both drop cast and spin cast films, simulations revealed two competing charge photogeneration pathways: fast charge generation on a picosecond (ps) or sub-ps time scale with efficiencies below 10%, and slow charge generation, on the time scale of tens of nanoseconds, with efficiencies of 11-12% in drop cast and 50-60% in spin cast films, depending on the applied electric field. The total charge photogeneration efficiency in the spin cast sample was 59-67% compared to 14-20% in the drop cast sample, whereas the remaining 33-41% and 80-86%, respectively, of the absorbed photon density did not contribute to charge carrier generation on these time scales. The spin cast film also exhibited higher hole mobilities, lower trap densities, shallower traps, and lower charge carrier recombination, as compared to the drop cast lm. As a result, the spin cast lm exhibited higher photocurrents despite a considerably lower lm thickness (and thus reduced optical absorption and cross section of the current flow).
We present a study of optical, photoluminescent (PL), and photoconductive properties of small-molecule D/A bulk heterojunctions of functionalized fluorinated anthradithiophene (ADT-R-F) and pentacene (Pn-R-F8) derivatives. We chose one of the ADT derivatives, ADT-TES-F, which exhibits a 2D “brick-work” .π-stacking, as the donor, and ADT-TIPS-F (2D “brick-work”), ADT-TSBS-F (1D “sandwich-herringbone”), Pn-TIPS-F8 (2D “brick-work”), or Pn-TCHS-F8 (1D “sandwich-herringbone”), as acceptors. We measured PL and photoconductivity at time scales from sub-nanoseconds to many seconds after photoexcitation, at various acceptor concentrations, under various experimental conditions. The choice of acceptors enabled us to distinguish between effects of the LUMO energy offsets between the donor and acceptor molecules and those of the molecular packing in the acceptor domains on exciton and charge carrier dynamics.
We present optical, photoluminescent (PL), and photoconductive properties of functionalized anthradithiophene
(ADT) derivatives and their composites. Solution-deposited ADT films exhibit charge carrier mobilities of over
1.5 cm2/(Vs), high PL quantum yields, and high photoconductivity. We show that molecular arrangement
and intermolecular interactions significantly contribute to (opto)electronic properties of guest-host thin films
of these pi-stacked materials. Specifically, the formation of aggregates plays an important role in establishing
efficient conduction pathways in ADT derivatives dispersed in a host matrix. In addition, the extent and nature
of aggregation and the resulting changes in PL and photoconductive behavior can be effectively manipulated
through different film fabrications techniques. Furthermore, energy transfer and exciplex formation has been
shown to occur between two different ADT derivatives, which also alters the photoconductive response of the
systems. We explore the dependence of the photoconductive response of such guest-host systems on excitation
wavelength.
We present optical, photoluminescent (PL), and photoconductive properties of functionalized anthradithiophene
(ADT) and benzothiophene (BTBTB) derivatives and their composites. Solution-deposited ADT films exhibit
charge carrier mobilities of over 1.5 cm2/Vs, high PL quantum yields, and high photoconductivity at room
temperature. We show molecular arrangement and intermolecular interactions significantly contribute to the
(opto)electronic properties of thin films of these pi-stacked materials. In addition, these properties can be
effectively manipulated through the addition of guest molecules to a host material. In particular, exciton and
charge carrier dynamics can be varied using a competition between photoinduced charge and energy transfer
in a guest-host system. To better understand these processes at a molecular level, we apply single-molecule
fluorescence spectroscopy (SMFS) to probe the effects of intermolecular interactions on the molecular properties.
Specifically, we demonstrate that ADT molecules exhibit high enough quantum yields and photostability to be
imaged on a single-molecule level at room temperature. Moreover, we show that stability of single ADT molecules
immobilized in a solid-state matrix are comparable to those of the best fluorophores utilized in SMFS.
We report on the effects of introducing guest molecules into a functionalized anthradithiophene (ADT) host
on the photoluminescent (PL) and photoconductive properties of solution-deposited thin films. An addition
of 0.1 wt % of an ADT derivative with cyano end groups (ADT-TIPS-CN) to a fluorinated ADT derivative
(ADT-TES-F) resulted in the near complete quenching of the fluorescence spectrum of the ADT-TES-F host
with an enhancement in the fluorescence spectrum of the ADT-TIPS-CN guest. A markedly longer PL lifetime
was noted in films containing 10% ADT-TIPS-CN guest molecules compared to both pristine ADT-TES-F and
ADT-TIPS-CN films. Stronger temperature dependence of the PL quantum yield was obtained in ADT-TIPSCN/
ADT-TES-F films at low ADT-TIPS-CN concentrations than in films of pristine material, with PL decreasing
with increasing temperature. Significant changes in the photoexcited charge carrier dynamics were observed on
nanosecond time-scales after 400 nm 100 fs pulsed photoexcitation upon adding ADT-TIPS-CN guest molecules
to the ADT-TES-F host. In contrast, no considerable change in the photocurrent was detected under continuous
wave 532 nm excitation for guest molecule concetrations up to 1% of ADT-TIPS-CN in ADT-TES-F.
The optical, fluorescent, and photoconductive properties of
solution-processable functionalized pentacene and
anthradithiophene (ADT) derivatives are presented. Considerable fluorescence quantum yields of - 70-75% and ~ 40-50% were observed in several ADT derivatives in toluene solutions and in thin films, respectively. Using
conventional wide-field fluorescence microscopy, ADT derivatives were successfully imaged in the polymethylmethacrylate
(PMMA) matrix on a single molecule level, at 532 nm at room temperature. All films exhibited fast
charge carrier photogeneration upon 100 fs 400 nm excitation and power-law decay dynamics of the transient
photocurrent over many orders of magnitude in time. In
solution-deposited ADT thin films, effective charge
carrier mobilities calculated from the space-charge-limited currents reached ~ 0.1 cm2/Vs. In the same films,
bulk photoconductive gains of up to 130 were observed at 532 nm continuous wave (cw) excitation with light
intensity of 0.58 mW/cm2 at the applied electric field of 4 × 104 V/cm. Effects of metal-organic interfaces on
the dark current and transient and cw photocurrent are also discussed.
We present a comprehensive study of ultrafast time-resolved photconductivity in pentacene and functionalized pentacene single crystals and thin films measured using optical pump-terahertz probe technique. By investigating the wavelength and temperature dependence of the transient photoconductivity, we reveal a sub-picosecond wavelength-independent charge carrier photogeneration and band-like charge transport in both single crystal and thin film samples. The amplitude and decay dynamics of the photoconductivity transients are correleated with the morphology of the films, assessed by atomic force and electron microscopy.
We examine ultrafast photoconductivity in functionalized pentacene single crystals using optical-pump terahertz-probe techniques. The 0.5 ps rise time observed in the photoconductive transients, which is limited by the response time of the terahertz pulse setup, suggests that mobile charge carriers are a primary photoexcitation. The peak of the photoconductive signal increases as the temperature decreases due to higher carrier mobilities at lower temperatures. A lower limit for the carrier mobility of 1.6 cm2/Vs at 10 K and 0.2 cm2/Vs at room temperature is obtained. We further show that the absorption edge near the pump excitation wavelength of 800 nm remains temperature independent, and is therefore not a contributing factor in our observation of larger transient signals at lower temperatures. After an initial fast decay, a power-law decay is observed in the tail of the transient response from 2 to 600 ps. The dependence of the photoconductive response on the pump fluence and the electric field amplitude of the terahertz pulse are examined.
Finally, we show some preliminary results of transient photocurrent measurements on contact-biased samples using a fast oscilloscope with a system rise time of about 50 ps.
A new class of fluorophores has been identified that can be imaged at the single-molecule level and offer additional beneficial properties such as a significant ground state dipole moment, moderate hyperpolarizability, and sensitivity to local rigidity. These molecules contain an amine donor and a dicyanodihydrofuran (DCDHF) acceptor linked by a conjugated unit (benzene, thiophene, alkene, styrene, etc.) and were originally designed to deliver both high polarizability anisotropy and dipole moment as nonlinear optical chromophores for photorefractive applications. Surprisingly, we have found that these molecules are also well-suited for single-molecule fluorescence imaging in polymers and other reasonably rigid environments. We report the bulk (ensemble) and single-molecule photophysical properties measured for six dyes in this new class of single-molecule reporters, with absorption maxima ranging from 486 to 614 nm.
Since the first observation of the photorefractive (PR) effect in polymers, extensive efforts have been directed toward understanding the physics of the PR process in these systems, as well as optimizing polymer composites and glasses for various applications. Despite remarkable progress both in elucidating the mechanisms and processes contributing to the PR effect and in designing organic materials with high gain and diffraction efficiency, simultaneously attaining high refractive index modulation, fast dynamics, and good thermal properties in one material remains a challenge. Monolithic glasses represent an attractive class of PR organic materials since they possess large nonlinearities and minimal inert volume, which enhances the performance without stability problems. In this paper, we present a complete study of monolithic glasses based on a promising new class of chromophores (containing 2-dicyanomethylen-3-cyano-5,5-dimethyl-2,5-dihydrofuran, abbreviated as DCDHF-derivatives). We describe thermal, photoconductive, orientational, and photorefractive properties of these materials in both red and near infrared wavelength regions. By studying the temperature dependence of various parameters, we analyze the factors that affect photorefractivity in DCDHF-based materials.
Derivatives of 2-dicyanomethylen-3-cyano-2,5-dihydrofuran (DCDHF) have been synthesized by different methods to be used as photorefractive (PR) chromophores. Structure modifications were performed on the donor, acceptor and conjugated π-system for improving properties such as glass formation. Structure-property relationships important for PR applications are discussed from the results of studies including UV-Vis, electrochemistry and DSC.
We present a theoretical and experimental study of both photoconductivity and photorefraction (PR) in several PVK- based photorefractive composites. We used the modified Schildkraut and Buettner's model of space-charge formation in photorefractive polymers that includes both deep and shallow traps. The dynamic equations have been solved semi- empirically using independent measurements of photoconductive properties to predict photorefractive dynamics. Dependence of the dynamics on charge generation, mobility, trap density, acceptor density, ionized acceptor density, as well as their associated rates is examined. The magnitude of the fast time constant of photorefractive development is successfully predicted. The model has also been found to qualitatively predict the reduction in speed due to deep trap filling and ionized acceptor growth. By choosing chromophores with different ionization potentials and by varying the chromophore concentrations, we study the influence of the chromophore ionization potential on the photoelectric and PR properties and reveal the nature of deep traps in the composites and their contribution to both photoconductivity and PR dynamics. Effects of plasticizer components were also investigated.
This paper describes the development and optimization of chiral, non-polar media with large second-order nonlinear optical responses. We employ molecular engineering, quantum- mechanical sum-over-states theory, and measurements of molecular hyperpolarizability by means of Kleinman-disallowed hyper-Rayleigh scattering in order to understand molecular properties. Then we analyze the appropriate arrangement of the chromophores that produce an optimum axial nonlinear optical medium. Chromophores with large Kleinman disallowed traceless symmetric second rank tensor hyper-polarizabilities (beta) can be aligned so as to result in large susceptibilities, (chi) (2), in structures that lack polar order. We found that (Lambda) -shaped chromophores with C2v or similar symmetry are good candidates for these materials as they can exhibit large second-rank components of the hyperpolarizability tensor. A wide variety of techniques can be used to fabricate bulk materials belonging to the chiral non-polar symmetry groups, D(infinity ) and D2. The microscopic chromophore alignment schemes that optimize the NLO response in such materials are deduced from general symmetry consideration for both molecules and bulk.
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