High Resolution Phonon-assisted Quasi-resonance Fluorescence Spectroscopy.

High resolution optical spectroscopy methods are demanding in terms of either technology, equipment, complexity, time or a combination of these. Here we demonstrate an optical spectroscopy method that is capable of resolving spectral features beyond that of the spin fine structure and homogeneous linewidth of single quantum dots (QDs) using a standard, easy-to-use spectrometer setup. This method incorporates both laser and photoluminescence spectroscopy, combining the advantage of laser line-width limited resolution with multi-channel photoluminescence detection. Such a scheme allows for considerable improvement of resolution over that of a common single-stage spectrometer. The method uses phonons to assist in the measurement of the photoluminescence of a single quantum dot after resonant excitation of its ground state transition. The phonon's energy difference allows one to separate and filter out the laser light exciting the quantum dot. An advantageous feature of this method is its straight forward integration into standard spectroscopy setups, which are accessible to most researchers.

Polymer Side Chain Modification Alters Phase Separation in Ferroelectric-Semiconductor Polymer Blends for Organic Memory.

Side chain modification of a semiconducting polythiophene changes the resulting phase separation length scales when blended with a ferroelectric polymer for use in organic ferroelectric resistive switches. The domain size of the semiconducting portion of blends of poly[3-(ethyl- 5-pentanoate)thiophene-2,5-diyl] (P3EPT) and poly(vinylidene fluoride-co-trifluoroethylene) (PVDF-TrFE) in thin film blends are smaller than previously reported and easily controllable in size through simple tuning of the weight fraction of the semiconducting polymer. Furthermore, P3EPT has a relatively high degree of crystallinity and bimodal crystallite orientations, as probed by wide-angle X-ray scattering. Resistive switches fabricated from blends of P3EPT and PVDF-TrFE show memristive switching behavior over a wide range of polythiophene content and good ON/OFF ratios.

Spectroscopic imaging of photopotentials and photoinduced potential fluctuations in a bulk heterojunction solar cell film.

We present spatially resolved photovoltage spectra of a bulk heterojunction solar cell film composed of phase-separated poly(9,9'-dioctylfluorene-co-benzothiadiazole) (F8BT) and poly(9,9'-dioctylfluorene-co-bis-N,N'-(4-butylphenyl)-bis-N,N'-phenyl-1,4-phenylenediamine) (PFB) polymers prepared on ITO/PEDOT:PSS and aluminum substrates. Over both PFB- and F8BT-rich domains, the photopotential spectra were found to be proportional to a linear combination of the polymers' absorption spectra. Charge trapping in the film was studied using photopotential fluctuation spectroscopy, in which low-frequency photoinduced electrostatic potential fluctuations were measured by observing noise in the oscillation frequency of a nearby charged atomic force microscope cantilever. Over both F8BT- and PFB-rich regions, the magnitude, distance dependence, frequency dependence, and illumination wavelength dependence of the observed cantilever frequency noise are consistent with photopotential fluctuations arising from stochastic light-driven trapping and detrapping of charges in F8BT. Taken together, our findings suggest a microscopic mechanism by which intermixing of phases leads to charge trapping and thereby to suppressed open-circuit voltage and decreased efficiency in this prototypical bulk heterojunction solar cell film.