Charge Trapping and Storage by Composite P3HT/PC60BM Nanoparticles Investigated by Fluorescence-Voltage/Single Particle Spectroscopy

Improved Superresolution Imaging Using Telegraph Noise in Organic Semiconductor Nanoparticles

Small semiconductor structures often exhibit "telegraph noise". If the number of charge carriers is small, then spontaneous changes in the number of carriers can lead to abrupt switching between two or more discrete levels, leading to burst noise or popcorn noise in transistors. We have observed similar behavior in the fluorescence of organic semiconductor nanoparticles, where typical carrier populations are often less than ∼10 carriers per nanoparticle. Spontaneous changes in the number of charges results in abrupt switching between 2 or more fluorescence intensity levels, because the charges act as highly efficient fluorescence quenchers. The equilibrium number of charges is determined by competition between a photodriven ionization process and spontaneous recombination. Doping with redox-active molecules also affects the balance. Nanoparticles of the conjugated polymer PFBT doped with the fullerene derivative PCBM, rapidly establish a fluctuating steady-state population of tens of hole polaron charge carriers, sufficient to nearly completely suppress nanoparticle fluorescence. However, fluctuations in the number of charges lead to occasional bursts of fluorescence. This spontaneous photoswitching phenomenon can be exploited for superresolution imaging. The repeated, spontaneous generation of short, intense bursts of fluorescence photons results in a localization precision of ∼0.6 nm, about 4 times better than typical resolution obtained by localization of dye molecules.

Optical properties of (Z)-2-(2-phenylhydrazinylidene)acenaphthen-1(2H)-one: a potential electron donor in organic solar cells

Electron-donating molecules play an important role in the development of organic solar cells. (Z)-2-(2-Phenylhydrazinylidene)acenaphthen-1(2H)-one (PDAK), C₁₈H₁₂N₂O, was synthesized by a Schiff base reaction. The crystal structure shows that the molecules are planar and are linked together forming `face-to-face' assemblies held together by intermolecular C-H...O, π-π and C-H...π interactions. PDAK exhibits a broadband UV-Vis absorption (200-648 nm) and a low HOMO-LUMO energy gap (1.91 eV; HOMO is the highest occupied molecular orbital and LUMO is the lowest unoccupied molecular orbital), while fluorescence quenching experiments provide evidence for electron transfer from the excited state of PDAK to C₆₀. This suggests that the title molecule may be a suitable donor for use in organic solar cells.

Surfactant-free, low band gap conjugated polymer nanoparticles and polymer:fullerene nanohybrids with potential for organic photovoltaics

Stable, aqueous dispersions of nanoparticles based on the low band gap polymers poly [2,7-(9,9-dioctyl-fluorene)-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)] (APFO-3) and poly [N-9'-heptadecanyl-2,7-carbazole-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)] (PCDTBT) were prepared, using a flexible, surfactant-free reprecipitation method, and characterized by a variety of optical techniques. Light scattering measurements indicated average nanoparticle hydrodynamic diameters of approximately 40 nm. The particles presented wide-bandwidth absorption and photoluminescence excitation spectra with high absorption cross-sections on the order of 10(-12) cm(2). Nanoparticle emission spectra were significantly red-shifted, with decreased emission quantum yields and lifetimes, consistent with increased inter-polymer chain interactions in the condensed phase. Single particle photoluminescence studies highlighted the multi-chromophoric nature of the polymer nanoparticles and confirmed their favorable photostabilities. When the nanoparticles were doped with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), the correspondence of photoluminescence emission quenching, quantum yield decreases, emission lifetime shortening, and increased non-radiative rates with increasing PCBM concentration suggested efficient photo-induced donor-to-acceptor charge transfer between the conjugated polymers and the fullerene dopants co-localized in the nanoparticle cores. Taken together, the data suggest that these surfactant-free hybrid nanomaterials may be useful for integration with future nanostructured organic photovoltaics technologies.

Deep localized distortion of alternating bonds and reduced transport of charged carriers in conjugated polymers under photoexcitation

In a real bulk heterojunction polymer solar cell, after exciton separation in the heterojunction, the resulting negatively-charged carrier, a polaron, moves along the polymer chain of the acceptor, which is believed to be of significance for the charged carrier transport properties in a polymer solar cell. During the negative polaron transport, due to the external light field, the polaron, which is re-excited and induces deep localization, also forms a new local distortion of the alternating bonds. It is revealed that the excited polaron moves more slowly than the ground-state polaron. Furthermore, the velocity of the polaron moving along the polymer chain is crucially dependent on the photoexcitation. With an increase in the intensity of the optical field, the localization of the excited polaron will be deepened, with a decrease of the polaron's velocity. It is discovered that, for a charged carrier, photoexcitation is a significant factor in reducing the efficiency during the charged carrier transport in polymer solar cells. Mostly, the deep trapping effect of charged carrier in composite conjugated polymer solar cell presents an opportunity for the future application in nanoscale memory and imaging devices.

Optical Pumping of Poly(3-hexylthiophene) Singlet Excitons Induces Charge Carrier Generation

The dynamics of high-energy excitons of poly(3-hexylthiophene) (P3HT) are shown to consist of torsional relaxation and exciton dissociation to form free carriers. In this work, we use pump-push-probe femtosecond transient absorption spectroscopy to study the highly excited states of P3HT in solution. P3HT excitons are generated using a pump pulse (400 nm) and allowed to relax to the lowest-lying excited state before re-excitation using a push pulse (900 or 1200 nm), producing high-energy excitons that decay back to the original excited state with both subpicosecond (0.16 ps) and picosecond (2.4 ps) time constants. These dynamics are consistent with P3HT torsional relaxation, with the 0.16 ps time constant assigned to ultrafast inertial torsional relaxation. Additionally, the signal exhibits an incomplete recovery, indicating dissociation of high-energy excitons to form charge carriers due to excitation by the push pulse. Our analysis indicates that charge carriers are formed with a yield of 11%.

Appearance of perfect amorphous linear bulk polyethylene under applied electric field and the analysis by radial distribution function and direct tunneling effect

Without melting flow, linear ultrahigh molecular weight polyethylene (UHMWPE) provided X-ray intensity curve from only amorphous halo at 129.0 °C (surface temperature, Ts arisen by Joule heat) lower than the conventionally known melting point 145.5 °C on applying electric field to UHMWPE-nickel-coated carbon fiber (NiCF) composite. Such surprising phenomenon was analyzed by simultaneous measurements of X-ray intensity, electric current, and Ts as a function of time. The calculated radial distribution function revealed the amorphous structure with disordered chain arrangement. The appearance of such amorphous phase was arisen by the phenomenon that the transferring electrons between overlapped adjacent NiCFs by tunneling effect struck together with X-ray photons and some of the transferring electron flown out from the gap to UHMWPE matrix collided against carbon atoms of UHMWPE. The impact by the collision caused disordering chain arrangement in crystal grains.

Tuning aggregation of poly(3-hexylthiophene) within nanoparticles

Nanoparticles derived from π-conjugated polymers have gained widespread attention as active layer materials in various organic electronics applications. The optoelectronic, charge transfer, and charge transport properties of π-conjugated polymers are intimately connected to the polymer aggregate structure. Herein we show that the internal aggregate structure of regioregular poly(3-hexylthiophene) (P3HT) within polymer nanoparticles can be tuned by solvent composition during nanoparticle fabrication through the miniemulsion process. Using absorption spectra and single-NP photoluminescence decay properties, we show that a solvent mixture consisting of a low boiling good solvent and a high boiling marginal solvent results in polymer aggregate structure with a higher degree of uniformity and structural order. We find that the impact of solvent on the nature of P3HT aggregation within nanoparticles is different from what has been reported in thin films.