Hierarchical Self-Assembly of Carbon Dots into High-Aspect-Ratio Nanowires.

We report a spontaneous and hierarchical self-assembly mechanism of carbon dots prepared from citric acid and urea into nanowire structures with large aspect ratios (>50). Scattering-type scanning near-field optical microscopy (s-SNOM) with broadly tunable mid-IR excitation was used to interrogate details of the self-assembly process by generating nanoscopic chemical maps of local wire morphology and composition. s-SNOM images capture the evolution of wire formation and the complex interplay between different chemical constituents directing assembly over the nano- to microscopic length scales. We propose that residual citrate promotes tautomerization of melamine surface functionalities to produce supramolecular shape synthons comprised of melamine-cyanurate adducts capable of forming long-range and highly directional hydrogen-bonding networks. This intrinsic, heterogeneity-driven self-assembly mechanism reflects synergistic combinations of high chemical specificity and long-range cooperativity that may be harnessed to reproducibly fabricate functional structures on arbitrary surfaces.

Responsive Fluorophore Aggregation Provides Spectral Contrast for Fluorescence Lifetime Imaging.

Fluorophores experience altered emission lifetimes when incorporated into and liberated from macromolecules or molecular aggregates; this trend suggests the potential for a fluorescent, responsive probe capable of undergoing self-assembly and aggregation and consequently altering the lifetime of its fluorescent moiety to provide contrast between the active and inactive probes. We developed a cyanobenzothioazole-fluorescein conjugate (1), and spectroscopically examined the lifetime changes caused by its reduction-induced aggregation in vitro. A decrease in lifetime was observed for compound 1 in a buffered system activated by the biological reducing agent glutathione, thus suggesting a possible approach for designing responsive self-aggregating lifetime imaging probes.

Resonance Raman Spectroscopy and Imaging of Franck-Condon Vibrational Activity and Morphology in Conjugated Polymers for Solar Cells.

Vibrational reorganization influences photophysical outcomes in conjugated polymers used as active materials for optoelectronic devices. Excited state geometric rearrangements typically involve many displaced vibrations, yet most materials design schemes rely solely on pure electronic models with limited predictive capability. Although the coupling of vibrational motions to electronic processes occurs over a broad range of time scales, resolving structural displacements immediately following photon absorption can be particularly insightful for understanding the intrinsic stabilities of excited states. These Franck-Condon vibrational relaxation processes occur on time scales of <1 ps in polymers and mainly involve high-frequency skeletal motions. Establishing correlations between Franck-Condon vibrational reorganization and steady-state material properties could generate new avenues for informing materials design, which is especially important in the fast-paced field of organic photovoltaics (OPV) where seemingly elegant strategies often fail but molecular-level insights are usually lacking. The goal of this Account is to highlight relationships between molecular structure, packing, and vibrational reorganization in OPV systems, such as blends of conjugated polymers with fullerenes. Resonance Raman spectroscopy (RRS) is a sensitive probe of Franck-Condon activity in OPV materials, and signals are bolstered by large resonance enhancements and low backgrounds from quantitative fluorescence quenching. Our group has undertaken extensive RRS investigations of heterogeneous OPV materials in functioning device environments to uncover new insights of the multidimensional excited state potential energy landscape and fluctuations with local morphology. Time-dependent quantum mechanical approaches facilitate this effort by providing an intuitive theoretical framework to access dynamical perspectives of Raman transitions. Moreover, dynamics regimes of Franck-Condon excited state structural evolution can be selected simply by tuning excitation energies. This excitation detuning approach also reveals structurally and electronically distinct conformers with unique Franck-Condon signatures typically concealed under inhomogeneously broadened absorption line shapes. Interestingly, long and rich progressions of overtone and combination transitions-rare for large molecules with multiple displaced modes-are frequently resolved that exhibit strong sensitivity to the local chromophore environment. These harmonic features encode useful dynamics information by serving as internal "clocks" of Franck-Condon vibrational activity in addition to enabling quantitative estimates of mode-specific displacements. RRS attributes may be further exploited to perform noninvasive imaging of functioning OPV devices in concert with variable frequency electrical imaging probes. This approach generates direct spatial correlations between morphology-dependent Franck-Condon vibrational activity and material performance metrics (e.g., photocurrent generation) on submicrometer size scales.

Steady-State Fluorescence Signatures of Intramolecular Singlet Fission from Stochastic Predictions.

The advent of new multichromophoric systems capable of undergoing efficient intramolecular singlet fission (iSF) has greatly expanded the range of possible motifs for multiexciton generation approaches for organic light energy harvesting materials. Transient absorption (TA) spectroscopic probes are typically used to characterize singlet fission processes that may place limitations on sensitivity and time resolution on scales comparable to the full lifespan of spin-forbidden triplets and interactions. Here, we investigate the ability of fluorescence-based spectroscopic probes to detect iSF activity in isolated dyads based on large substituted conjugated acenes (e.g., tetracene and pentacene derivatives). Photophysical models are simulated from several iSF-active dyad systems reported in the literature using a stochastic approach to assess the sensitivity of steady-state fluorescence to the presence of triplet excitons. The results demonstrate large fluctuations in expected fluorescence yields with varying excitation rate constants for systems with ΦiSF > 0.5 (assuming weak interchromophore coupling). Exciton-exciton interactions are also investigated, and we further demonstrate how treating iSF dyads stochastically (i.e., finite number of chromophores) accentuates dependences of photophysical yields with excitation rates. Last, our approach reveals the potential ability of single molecule level fluorescence spectroscopy to detect iSF activity that can aid efforts to design and optimize candidate iSF systems.

Dynamic emissive signatures of intramolecular singlet fission during equilibration to steady state revealed from stochastic kinetic simulations.

We investigate the ability of dynamic fluorescence probes to accurately track populations of multi-excitonic states in molecular dyads based on conjugated acenes capable of intramolecular singlet fission (iSF). Stochastic simulations of reported photophysical models from time-resolved spectroscopic studies of iSF dyads based on large acenes (e.g., tetracene and pentacene) are used to extrapolate population and fluorescence yield dynamics. The approach entails the use of repetitive rectangular-shaped excitation waveforms as a stimulus, with durations comparable to triplet lifetimes. We observe unique dynamics signatures that can be directly related to relaxation of multi-exciton states involved over the entire effective time of singlet fission in the presence and absence of an excitation light stimulus. In particular, time-dependent fluorescence yields display an abrupt decay followed by slower rise dynamics appearing as a prominent "dip" feature in responses. The initial fast decrease in the fluorescence yield arises from the formation of triplet pairs and separated triplets that do not produce emission resembling a complete ground state bleach effect. However, relaxation of one separated triplet allows the system to absorb, and in some cases, this increases the fluorescence yield, causing rise dynamics in the emissive response. Our approach also permits extrapolation of all multi-exciton state population dynamics up to steady state conditions in addition to the ability to explore consequences of alternative relaxation channels. The results demonstrate that it is possible to resolve unique signatures of singlet fission events from dynamic fluorescence studies, which can augment detection capabilities and extend sensitivity limits and accessible time scales.

Resolving population dynamics and interactions of multiple triplet excitons one molecule at a time.

Resolving the population dynamics of multiple triplet excitons on time scales comparable to their lifetimes is a key challenge for multiexciton harvesting strategies, such as singlet fission. We show that this information can be obtained from fluorescence quenching dynamics and stochastic kinetic modeling simulations of single nanoparticles comprising self-assembled aggregated chains of poly(3-hexylthiophene) (P3HT). These multichromophoric structures exhibit the elusive J-aggregate type excitonic coupling leading to delocalized intrachain excitons that undergo facile triplet formation mediated by interchain charge transfer states. We propose that P3HT J-aggregates can serve as a useful testbed for elucidating the presence of multiple triplets and understanding factors governing their interactions over a broad range of time scales. Stochastic kinetic modeling is then used to simulate discrete population dynamics and estimate higher order rate constants associated with triplet-triplet and singlet-triplet annihilation. Together with the quasi-CW nature of the experiment, the model reveals the expected amounts of triplets at equilibrium per molecule. Our approach is also amenable to a variety of other systems, e.g., singlet fission active molecular arrays, and can potentially inform design and optimization strategies to improve triplet harvesting yields.

Unravelling the enigma of ultrafast excited state relaxation in non-emissive aggregating conjugated polymers.

We investigate a class of non-emissive conjugated polymers with very short excited state lifetimes believed to undergo singlet fission and relaxation to mid-gap forbidden excited states. Poly(3-decylthieneylenvinylene) (P3DTV) and its heavy atom analog, poly(3-decylseleneylenvinylene) (P3DSV), are strongly aggregating conjugated polymers that experience large excited state displacements along multiple vibrational modes. We demonstrate this Franck-Condon vibrational activity effectively disperses excitation energy into multiple non-radiative channels that can be explained using a simple, two-state potential energy surface model. Resonance Raman spectroscopy is sensitive to early Franck-Condon vibrational activity and we observe rich harmonic progressions involving multiple high frequency CC backbone symmetric stretching motions (∼1000-1600 cm-1) in both systems reflecting mode-specific excited state geometrical displacements. Transient absorption spectra confirm that efficient non-radiative processes dominate excited state relaxation dynamics which are confined to π-stacked aggregated chains. Surprisingly, we found little influence of the heteroatom consistent with efficient vibrational energy dissipation. Our results highlight the importance of aggregation and multi-dimensional Franck-Condon vibrational dynamics on the ability to harvest excitons, which are not usually considered in materials design and optimization schemes.

Giant PbSe/CdSe/CdSe Quantum Dots: Crystal-Structure-Defined Ultrastable Near-Infrared Photoluminescence from Single Nanocrystals.

Toward a truly photostable PbSe quantum dot (QD), we apply the thick-shell or "giant" QD structural motif to this notoriously environmentally sensitive nanocrystal system. Namely, using a sequential application of two shell-growth techniques-partial-cation exchange and successive ionic layer adsorption and reaction (SILAR)-we are able to overcoat the PbSe QDs with sufficiently thick CdSe shells to impart new single-QD-level photostability, as evidenced by suppression of both photobleaching and blinking behavior. We further reveal that the crystal structure of the CdSe shell (cubic zinc-blende or hexagonal wurtzite) plays a key role in determining the photoluminescence properties of these giant QDs, with only cubic nanocrystals sufficiently bright and stable to be observed as single emitters. Moreover, we demonstrate that crystal structure and particle shape (cubic, spherical, or tetrapodal) and, thereby, emission properties can be synthetically tuned by either withholding or including the coordinating ligand, trioctylphosphine, in the SILAR component of the shell-growth process.

Effect of a heavy heteroatom on triplet formation and interactions in single conjugated polymer molecules and aggregates.

Triplet formation and interactions with emissive singlet excitons are investigated in poly(3-hexylselenophene) (P3HS) using single molecule spectroscopy. P3HS is a heavy atom analog of the more commonly studied poly(3-hexylthiophene) (P3HT), a benchmark polymer for solar cells. P3HS tends to aggregate strongly which necessitates dilution to ultra-low levels within a solid inert host in order to resolve photophysical responses of single chains. Fluorescence excitation intensity modulation is performed on isolated P3HS chains using a sequence of rectangular pulses of varying intensities to probe the presence of spin-forbidden triplet excitons. Triplet population dynamics originating from singlet-triplet and triplet-triplet interactions appear as quenching of the initial fluorescence intensity to steady-state levels on characteristic time scales of ∼1-10 µs. Over 80% of all molecules studied display significant fluorescence intensity modulation (quenching depths >50%) indicative of efficient intersystem crossing and large triplet occupancies. Because triplets are highly localized and singlet-triplet and triplet-triplet annihilation rate constants are comparable to those of intersystem crossing, multiple triplets are present at any given time on single P3HS chains. Triplet lifetimes were estimated to be ∼4 µs (upper limit) determined from recovery to the ground electronic singlet state in the absence of light and, surprisingly, triplets vanish at the onset of P3HS aggregation. This result was unexpected since P3HS triplet formation takes place on time scales <30 ps making this process competitive with most accessible non-radiative deactivation pathways.

Synthesis, lanthanide coordination chemistry, and liquid-liquid extraction performance of CMPO-decorated pyridine and pyridine N-oxide platforms.

Syntheses for a set of new ligands containing one or two carbamoylmethylphosphine oxide (CMPO) fragments appended to pyridine and pyridine N-oxide platforms are described. Molecular mechanics analyses for gas phase lanthanide-ligand interactions for the pyridine N-oxides indicate that the trifunctional NOPOCO molecules, 2-{[Ph2P(O)][C(O)NEt2]C(H)}C5H4NO (7) and 2-{[Ph2P(O)][C(O)NEt2]CHCH2}C5H4NO (8), and pentafunctional NOPOP'O'COC'O' molecules, 2,6-{[Ph2P(O)][C(O)NEt2]C(H)}2C5H3NO (9) and 2,6-{[Ph2P(O)][C(O)NEt2]CHCH2}2C5H3NO (10), should be able to adopt, with minimal strain, tridentate and pentadentate chelate structures, respectively. As a test of these predictions, selected lanthanide coordination chemistry of the N-oxide derivatives was explored. Crystal structure analyses reveal the formation of a tridentate NOPOCO chelate structure for a 1:1 Pr(III) complex containing 7 while 8 adopts a mixed bidentate/bridging monodentate POCO/NO binding mode with Pr(III). Tridentate and tetradentate chelate structures are obtained for several 1:1 complexes of 9 while a pentadentate chelate structure is observed with 10. Emission spectroscopy for one complex, [Eu(9)(NO3)3], in methanol, shows that the Eu(III) ion resides in a low-symmetry site. Lifetime measurements for methanol and deuterated methanol solutions indicate the presence of four methanol molecules in the inner coordination sphere of the metal ion, in addition to the ligand, with the nitrate anions most likely dissociated. The solvent extraction performance of 7-10 in 1,2-dichloroethane for Eu(III) and Am(III) in nitric acid solutions was analyzed and compared with the performance of 2,6-bis(di-n-octylphosphinoylmethyl)pyridine N-oxide (TONOPOP'O') and n-octyl(phenyl)-N,N-diisobutylcarbamoylmethylphosphine oxide (OPhDiBCMPO) measured under identical conditions.

Resonance Raman overtones reveal vibrational displacements and dynamics of crystalline and amorphous poly(3-hexylthiophene) chains in fullerene blends.

Resonance Raman spectra of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester blend thin films display progressions of overtone and combination bands (up to two harmonics) involving the dominant symmetric C=C backbone stretching mode of P3HT that encode excited state vibrational displacements and dynamics information. Contributions from both crystalline (aggregated) and amorphous (unaggregated) P3HT domains are resolved and intensities are analyzed using the time-dependent theory of spectroscopy. Raman spectra, excitation profiles, and absorption spectra are simulated with the same parameters using a single electronic state description for each P3HT form. Time-dependent wavepacket overlaps expose vibrational coherence on sub-100 fs timescales, which is usually difficult to extract from conventional ultrafast pump-probe spectra and transients of polymer∕fullerene blends. The results demonstrate the potential of simpler CW resonance Raman approaches to uncover excited state geometry changes and early vibrational dynamics from distinct morphological forms in polymer∕fullerene blends.

Encapsulation of poly(3-hexylthiophene) J-aggregate nanofibers with an amphiphilic block copolymer.

Poly(3-hexylthiophene) (P3HT) nanofibers (NF) displaying J-aggregate exciton coupling behavior are encapsulated with the amphiphilic block copolymer (BCP), poly(3-hexyl-thiophene)-block-poly(ethylene-glycol), (PHT(20)-b-PEG(108)). Encapsulation results in the formation of hierarchical superstructures, and the BCP coating is expected to exert a mild chemical pressure on the periphery of the NFs. Photoluminescence from encapsulated NF superstructures show line shape distortions due to self-absorption of the 0-0 transition which is consistent with preservation of J-aggregate character (intrachain order). Detailed resonance Raman spectra of encapsulated BCP-NF structures show no discernible changes in the P3HT aggregation state, and overtone and combination bands involving the symmetric stretching C═C (~1450 cm(-1)) and C-C (~1380 cm(-1)) backbone modes are observed. These features permit quantitative estimates of vibrational mode-specific excited state structural displacements using a time-dependent Raman intensity analysis which is not possible from conventional vibronic analysis of optical lineshapes.

Resonance Raman studies of excited state structural displacements of conjugated polymers in donor/acceptor charge transfer complexes.

Resonance Raman spectra of poly(2-methoxy-5-(3'-7'-dimethyloctyloxy)-1,4-phenylenevinylene) (MDMO-PPV) and small molecule acceptor blend charge transfer (CT) complexes reveal long and detailed progressions of overtone and combination bands. These features are sensitive to the specific MDMO-PPV/acceptor interactions and enable quantitative calculations of vibrational mode specific displacements of the polymer CT complex.

Unique Degradation Signatures of Organic Solar Cells with Nonfullerene Electron Acceptors.

We investigate the degradation phenomena of organic solar cells based on nonfullerene electron acceptors (NFA) using intensity-modulated photocurrent spectroscopy (IMPS). Devices composed of NIR absorbing blends of a polymer (PTB7) and NFA molecules (COi8DFIC) were operated in air for varying periods of time that display unusual degradation trends. Light aging (e.g., ∼3 days) results in a characteristic first quadrant (positive phase shifts) degradation feature in IMPS Nyquist (Bode) plots that grow in amplitude and frequency with increasing excitation intensity and then subsequently turns over and vanishes. By contrast, devices aged and operated in air for longer times (>5 days) display poor photovoltaic performance and have a dominant first quadrant IMPS component that grows nonlinearly with excitation intensity. We analyze these degradation trends using a simple model with descriptors underlying the first quadrant feature (i.e., trap lifetime and occupancy). The results indicate that the quasi first-order recombination rate constant, krec, is significantly slower in addition to lower trap densities in devices exhibiting light aging effects that are overcome by increasing carrier densities (viz. excitation intensity). By contrast, larger trap densities and distributions coupled with larger krec values are found to be responsible for the continuous growth of the first quadrant with light intensity. We believe that defect formation and charge recombination at device contact interfaces is chiefly responsible for performance degradation, which offers several directions for materials and device optimization strategies to minimize long-term detrimental factors.

Observation of the missing mode effect in a poly-phenylenevinylene derivative: effect of solvent, chain packing, and composition.

Optical emission spectra of poly[2-methoxy-5-[3('),7(')-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV) in dilute solutions exhibit a vibronic progression interval (∼1225 cm(-1)) that does not correspond to any ground state vibrational mode frequency. This phenomenon is assigned as the missing mode effect (MIME) in which five key displaced polymer backbone vibrational modes in the range of 800-1600 cm(-1) contribute to the MIME interval. Emission spectra are calculated by analytically solving the time-dependent Schrödinger equation using estimates of mode-specific vibrational displacements determined independently from preresonance Raman intensities. Emission spectra of MDMO-PPV thin films and nanoparticles are measured and lineshapes show an increase of the MIME frequency to ∼1340 cm(-1) in addition to changes in vibronic intensity distributions and energies. Composite blend thin films consisting of MDMO-PPV and a fullerene derivative (1:1 w/w) exhibit a substantially larger MIME interval (∼1450 cm(-1)) that arises from an increase in polymer chain planarity. This structural change is most apparent from large decreases of the excited state displacement of an out-of-plane C-H bending mode (961 cm(-1)) that becomes forbidden in the planar structure.

Resonance chemical imaging of polythiophene/fullerene photovoltaic thin films: mapping morphology-dependent aggregated and unaggregated C=C Species.

Resonance Raman spectroscopic imaging is introduced as a physical probe to identify and spatially map morphology-dependent variations of intra- and interchain interactions and order in poly-3-hexylthiophene (P3HT) and [6,6]-phenyl-C(61)-butyric acid methyl ester (PCBM) photovoltaic blend thin films. Absorption spectra and C=C symmetric stretching Raman modes of P3HT/PCBM blend films show contributions from two distinct species that are assigned as aggregated and unaggregated P3HT chains with characteristic Raman frequencies of approximately 1450 (I(C=C)(agg)) and approximately 1470 cm(-1) (I(C=C)(un)), respectively. Upon thermal annealing of blend films, the relative concentrations of I(C=C)(agg) and I(C=C)(un) species (R = I(C=C)(agg)/I(C=C)(un)) changes on average from 0.79 +/- 0.20 (as-cast) to 2.45 +/- 0.77 (annealed). It is proposed that R values report on the relative densities of states (DOS) of aggregated and unaggregated species, and resonance Raman imaging is then used to spatially map morphology-dependent variations of R values and uncover subclassifications of these species. From both R and frequency dispersion resonance Raman images of I(C=C)(agg) and I(C=C)(un) species, four distinct types of P3HT chains are identified and mapped in annealed P3HT/PCBM blend thin films: (i) highly aggregated/crystalline, (ii) partially aggregated, (iii) interfacial, and (iv) unaggregated/PCBM rich. The change in aggregation upon annealing is attributed to an increase in planarity of the P3HT chains that is determined from the ratios of C=C/C-C symmetric stretching mode intensities.

Population dynamics of multiple triplet excitons revealed from time-dependent fluorescence quenching of single conjugated polymer chains.

The advent of multiple exciton harvesting schemes and prolonging exciton lifetimes to improve performance attributes of solar cells based on conjugated organic materials presents some interesting challenges that must be overcome in order to realize the full potential of these strategies. This is especially important for applications involving multi-chromophoric conjugated polymers where interactions between multiple spin-forbidden triplet excitons can be significant and are mediated by chain conformation. We use single molecule spectroscopic techniques to investigate interactions between multiple triplet excitons and emissive singlets by monitoring time-dependent fluorescence quenching on time scales commensurate with the triplet lifetime. Structurally related conjugated polymers differing by heteroatom substitution were targeted and we use a stochastic photodynamic model to numerically simulate the evolution of multi-exciton populations following photoexcitation. Single chains of poly(3-hexylthiophene) (P3HT) exhibit longer-lived triplet dynamics and larger steady-state triplet occupancies compared to those of poly(3-hexylselenophene) (P3HS), which has a larger reported triplet yield. Triplet populations evolve and relax much faster in P3HS which only becomes evident when considering all kinetic factors governing exciton population dynamics. Overall, we uncover new guidelines for effectively managing multi-exciton populations and interactions in conjugated polymers and improving their light harvesting efficiency.

Large Excited-State Conformational Displacements Expedite Triplet Formation in a Small Conjugated Oligomer.

Intersystem crossing in conjugated organic molecules is most conveniently viewed from pure electronic perspectives; yet, vibrational displacements may often drive these transitions. We investigate an alkyl-substituted thienylene-vinylene dimer (dTV) displaying efficient triplet formation. Steady-state electronic and Raman spectra display large Stokes shifts (∼4000 cm-1) involving high-frequency skeletal symmetric stretching modes (∼900-1600 cm-1) in addition to large displacements of low-frequency torsional motions (∼300-340 cm-1). Transient absorption spectroscopy reveals the emergence of distorted singlet (S1) and triplet signatures following initial vibrational relaxation dynamics that dominate spectral dynamics on time scales > 100 ps, with the latter persisting on time scales up to ca. 7 µs. Potential energy surfaces calculated along the dominant displaced out-of-plane torsional mode reveal shallow energy barriers for entering the triplet manifold from S1. We propose that dTV is a good model system for understanding vibrational contributions to intersystem crossing events in related polymer systems.

Morphological Contributions to Interfacial Charge Trapping and Nongeminate Recombination in Polymer Solar Cells Revealed by UV Light Soaking.

Nongeminate charge recombination occurs over a broad range of time scales in polymer solar cells and represents a serious loss channel for the performance and lifetime of devices. Multiple factors influence this process, including changes in morphology and formation of permanent defects, but individual contributions are often difficult to resolve from conventional experiments. We use intensity modulated photocurrent/photovoltage spectroscopy (IMPS/IMVS) to investigate nongeminate charge recombination in blends of poly[2,6-(4,4-bis-(2-ethylhexyl)-4 H-cyclopenta [2,1- b;3,4- b']dithiophene)- alt-4,7(2,1,3-benzothiadiazole)] (PCPDTBT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) solar cells. PCPDTBT/PCBM devices are exposed to varying doses of UV light resonant with PCBM to induce small perturbations in the thin film morphology, namely local heating. IMPS/IMVS sweeps display signatures unique to degradation, that is, photocurrent and photovoltage leading the excitation light modulation appearing as positive phase shifts or 1st quadrant features in Bode and Nyquist representations, respectively. We assign this component to interface charging at purified PCPDTBT/PCBM phase boundaries that trap mobile charges and facilitate nongeminate recombination. Time- and frequency-domain drift-diffusion simulations are then used to model the perturbed photocurrent responses that show good agreement with experiments. Trap occupancies and their impact of photocurrent production are investigated using variable background (dc) excitation light intensities revealing increases of the 1st quadrant component in devices irradiated for longer times. No evidence of chemical degradation was observed from molecular spectroscopy and imaging experiments, and we conclude that morphological changes are chiefly responsible for larger nongeminate charge recombination yields as devices age. Lastly, we propose that the 1st quadrant IMPS/IMVS is a universal signature of morphology-related degradation, although its relative contribution may vary between material systems.

Size-dependent spectroscopic properties of conjugated polymer nanoparticles.

This paper is focused on how the spectroscopic properties of conjugated polymers evolve in the size range between single polymer chains and the bulk material. The measurements used single-particle spectroscopy techniques and include both static and dynamic measurements. The main observation of this work is that the spectroscopic properties of MEH-PPV evolve rapidly as a function of nanoparticle size and achieve bulk-like properties for nanoparticles greater than 10 nm in size. Nanoparticles were assembled by a reprecipitation technique and characterized by fluorescence emission spectroscopy. The physical origin of the size-dependent spectroscopic properties is assigned to the distance dependence of four main processes: electronic energy transfer between blue and red sites, triplet-triplet annihilation, singlet exciton quenching by triplets, and singlet exciton quenching by hole polarons.