Xiaomei Jiang

Spectroscopic studies of photoexcitations in regioregular and regiorandom polythiophene films

Using a variety of optical probe techniques we studied the steady state and transient dynamics of charged and neutral photoexcitations in thin films of poly-3-alkyl thiophene with regioregular order, which forms self-assembled lamellae structures with increased interchain interaction, as well as regiorandom order that keeps a chain-like morphology. In regiorandom polythiophene films we found that intrachain excitons with correlated photoinduced absorption and stimulated emission bands are the primary photoexcitations; they give rise to a moderately strong photoluminescence band, and long-lived triplet excitons and intrachain charged polarons. In regioregular polythiophene films, on the contrary we found that the primary photoexcitations are excitons with much larger interchain component; this results in lack of stimulated emission, vanishing intersystem crossing, and a very weak photoluminescence band. The long-lived photoexcitations in regioregular polythiophene films are interchain excitons and delocalized polarons (DP) within the lamellae, with very small relaxation energy. The characteristic properties of the DP species are thoroughly investigated as a function of the alkyl side group of the polymer backbone, film deposition conditions and solvents used, as well as at high hydrostatic pressure. The quantum interference between the low energy absorption band of the DP species and a series of photoinduced infrared active vibrations, which give rise to antiresonances that are superimposed on the electronic absorption band is studied and explained by a Fano-type interference mechanism, using the amplitude mode model.

Delocalized polarons in self-assembled poly(3-hexyl thiophene) nanocrystals

Abstract We have studied the characteristic properties of charged excitations (polarons) in thin films of regioregular poly(3-hexylthiophene) (RRPHT) by optical spectroscopies. We found that the increased interchain coupling that exists in self-assembled lamellae in these films drastically changes the properties of the polaron excitations. The traditional self-localized polaron in one-dimension delocalizes in two-dimensions, resulting in a much-reduced relaxation energy and multiple absorption bands, the most dominant being in the mid-infrared (IR) spectral range. Also it has associated IR and Raman active vibrations with reverse absorption bands [anti-resonances (AR)] compared with the continuum electronic band. The AR are attributed to the electron–phonon interaction.

Poly(3-dodedyl-2,5-thienylenevinylene)s from the Stille coupling and the Horner–Emmons reaction

Regioregular (RR) conjugated polymers are critical for electronic and optoelectronic properties of polymer based semiconducting devices. Monosubstituted polythienylvinylene (PTV), a relatively low band-gap conjugated polymer, has been reportedly synthesized using the Stille coupling reaction between 3-dodecyl-2,5-dibromothiophene and (E)-1,2-bis(tributylstannyl)ethylene. Although a small fraction of the product shows good (∼90%) regioregularity, the whole product (S-C12-PTV) consists of mostly unidentified structures. To gain better control of the polymer structure and regioregularity, a difunctionalized C12-thiophene monomer, 3-dodecyl-5-formyl-thiophen-2-ylmethyl)-phosphonic acid diethyl ester, has been polymerized in near quantitative yield via the Horner–Emmons reaction. The full (100%) regioregularity of the resulting polymer (HE-C12-PTV) has been confirmed by its 1H and 13C NMR spectra. The full regioregularity is also reflected in its strong tendency to crystallize and practically no solubility in boiling hexane, in sharp contrast to S-C12-PTV. UV-vis absorption spectroscopy, fluorescence spectroscopy, cyclovoltammetry, thermal analysis (DSC & TGA) and X-ray diffraction have been used to characterize both polymers. UV-vis absorption spectra of the HE-C12-PTV chloroform solutions of different concentrations show well-resolved vibronic structures with an absorption maximum at 577 nm and a prominent shoulder at 614 nm. The optical bandgaps are 1.80 eV in chloroform solution and 1.65 eV in film. The HOMO/LUMO energy levels of the HE-C12-PTV film were found to be at −4.98 eV and −2.88 eV, respectively. The electrochemical bandgap of the polymer in the film is estimated to be 2.1 eV. The DSC curve shows a pronounced melting peak at 205 °C. XRD study shows that a decent crystalline structure is formed without any annealing of the as-cast films.

Fabrication of organic solar array for applications in microelectromechanical systems

We have developed an innovative way to fabricate organic solar arrays for application in dc power supplies for electrostatic microelectromechanical systems devices. A solar array with 20 miniature cells interconnected in series was fabricated and characterized. Photolithography was used to isolate the individual cells and output contacts of the array, whereas the thermal-vacuum deposition is employed to make the series connections of the array. With 1mm2 for single cell and a total device area of 2.2cm2, the organic solar array based on bulk heterojunction structure of π-conjugated polymers and C60 derivative [6,6]-phenyl C61 butyric acid methyl ester produced an open-circuit voltage of 7.8V and a short-circuit current of 55μA under simulated air mass (AM) 1.5 illumination with an intensity of 132mW/cm2. The procedure described here has the full potential for use in future fabrication of microarray with the size as small as 0.01mm2.

Steady state photoinduced absorption of PbS quantum dots film

We have measured steady state photoinduced absorption (PIA) of PbS quantum dot (QD) film in an energy range of interband electronic transitions at low temperature. PIA spectra show both positive and negative PIAs corresponding to five consecutive interband transitions. The close resemblance between PIA spectrum and the second derivative of the linear optical absorption strongly suggests that these steady state PIAs may be caused by photoinduced local electric field, i.e., the linear Stark effect. We postulate this local electric field be generated by the trapped charges under laser excitations.

Large photocurrent response and external quantum efficiency in biophotoelectrochemical cells incorporating reaction center plus light harvesting complexes.

Bacterial photosynthetic reaction centers (RCs) are promising materials for solar energy harvesting, due to their high ratio of photogenerated electrons to absorbed photons and long recombination time of generated charges. In this work, photoactive electrodes were prepared from a bacterial RC-light-harvesting 1 (LH1) core complex, where the RC is encircled by the LH1 antenna, to increase light capture. A simple immobilization method was used to prepare RC-LH1 photoactive layer. Herein, we demonstrate that the combination of pretreatment of the RC-LH1 protein complexes with quinone and the immobilization method results in biophotoelectrochemical cells with a large peak transient photocurrent density and photocurrent response of 7.1 and 3.5 μA cm(-2), respectively. The current study with monochromatic excitation showed maximum external quantum efficiency (EQE) and photocurrent density of 0.21% and 2 μA cm(-2), respectively, with illumination power of ∼6 mW cm(-2) at ∼875 nm, under ambient conditions. This work provides new directions to higher performance biophotoelectrochemical cells as well as possibly other applications of this broadly functional photoactive material.

Photoexcitations in regio-regular and regio-random polythiophene films

The steady state dynamics of the long-lived photoexcitations in thin films of redo-regular (RR-P31HT) that form lamellae and regio-random poly-3-alkyl thiophene (RRa-P3HT) that form glassy films have been studied by photo-induced absorption (PA) and photo-induced absorption detected magnetic resonance(PADMR) spectroscopies . In RRa-P3HT films we found a moderately strong photoluminescence band, long-lived triplet excitons and some intrachain charged polarons. On the contrary, the long-lived photoexcitations in RR-P3HT films are interchain excitons and delocalized polarons (DP) in the lamellae, with very small relaxation energy. The quantum interference between the DP absorption band and a series of photoinduced infrared active vibrations (IRAV) that gives rise to antiresonances is studied and explained by a Fano-type interference mechanism.

Generation and recombination kinetics of optical excitations in poly(3-dodecylthienylenevinylene) with controlled regioregularity.

Regioregularity in π-conjugated polymers is known to have desirable effects on chain organization in the solid state leading to improved characteristics for electronic device applications. In this report, we studied the photophysics of a new series of π-conjugated polymers, namely, poly(3-dodecylthienylenevinylene), with controlled regioregularity (PTV-CR), using the steady state photoinduced absorption (PIA), doping-induced absorption (DIA), thermally modulated absorption (TMA), and photoluminescence (PL) techniques in a broad spectral range, as well as Grazing incidence X-ray diffraction (GIXRD). PTV-CR has a low band gap of 1.65-1.70 eV from the most random PTV-5:5 to regular PTV-0:10. We have found that regioregularity affects optical properties of PTV-CR in different ways than in the case of well-studied poly(3-alkylthiophene). We discovered that even the most random PTV-5:5 shows submicrometer crystalline order, and there is no correlation between regioregularity and crystallinality. We found that PTV is intrinsically dark due to the reverse order of 1B(u) and 2A(g) excitons, with PL quantum efficiency ranging from 10(-5) to 2 × 10(-4). Spectroscopic data confirm the existence of long-lived neutral (triplets) and charged (polaron) excitations, and thus PTV is a unique nondegenerate ground state π-conjugated polymer which is intrinsically nonluminescent. The recombination mechanism and lifetime of such long-lived excitations are investigated through the pump intensity and pump frequency dependencies as well as temperature dependence of the sample. Increasing regioregularity is seen to reduce the thermal responsivity and intersystem crossing in these films. The novel, intrachain character of these effects and their implications for use of PTV derivatives for photovoltaic application are discussed.

FTIR studies of charged photoexcitations in regio-regular and regio-random poly(3-alkylthiophene) films

Abstract We have studied the low energy photo-induced absorption (PA) band of photo-generated polarons in a variety of poly(3-alkylthiophene) (P3AT) films which including regio-regular and regio-random stereo orders, different length of alkyl side group, and various film preparation techniques. In addition to the traditional one-dimensional (1D) polaron PA bands, we observed quasi two-dimensional excitations (DP) in regio-regular P3AT films, where the polymer chains self-organize to form 2D lamella structures. Due to increased interlayer and interchain coupling, the two plaron energy levels split by 2 Δ (≈2 t ⊥ ), resulting in two allowed optical transitions where the lower energy band (DP 1 ) dominates and red-shifts into the mid infrared range. The DP 1 band blue-shifts when the length of the alkyl side group increases, or when using lower boiling point solvent, or when preparing films at higher temperature.

SYNTHESIS OF PbS NANOCRYSTAL/FUNCTIONALIZED CONDUCTING POLYMERS FOR PLASTIC SOLAR CELLS

Lead-based nanocrystals (NCs) are promising materials for high-efficiency solar cells since they are able to generate multiexcitons with high efficiency. One complication of utilizing these NCs is the insulating ligands capping their surfaces. In this paper, we have successfully developed and characterized a phosphonate-functionalized poly-3-hexylthiophene (POP3HT-50) and used it in the direct synthesis of PbS NCs within the polymeric host matrix without extraneous ligands. Devices made of POP3HT-50/PbS nanocomposites show an order of magnitude improvement in η when compared to that reported for a P3HT/PbS device (η = 0.011% versus 0.001%). The improved performance is consistent with better electronic contact between PbS NCs and POP3HT-50.